Kurzgesagt – In a Nutshell

Sources – Pregnancy

We thank the following experts for their critical reading and scientific input:

Victoria Male

Associate Professor in Reproductive Immunology
Department of Metabolism, Digestion and Reproduction, Imperial College London

Petra Arck
Professor for Feto-Maternal Medicine
University Medical Center Hamburg-Eppendorf

Jelmer Prins

Program Leader, Faculty of Medical Sciences/UMCG, Senescence, Stem Cells, and Inflammation

Faculty of Medical Sciences/UMCG, Translational Immunology Groningen, University of Groningen

Ai Ing Lim

Assistant Professor of Molecular Biology

Department of Molecular Biology, Princeton University

– Our story begins with an army of tens of millions sperm cells that immediately face death. They are on a tight timer before they run out of energy and die. But this is the least of their problems right now.

There are many different numbers for the current average sperm counts but the details of it are beyond our scope here, it's a topic on its own. So we went with a rounded up number that we followed from a few sources, one of which we added below.

#Barlow PW. Why so many sperm cells? Not only a possible means of mitigating the hazards inherent to human reproduction but also an indicator of an exaptation. Commun Integr Biol. 2016

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4988455/

Quote: “First, from the numerical point of view, the critical number of spermatozoa necessary, in a single ejaculate, to effect fertilization needs to be ascertained. The World Health Organization (WHO) guidelines on fertility indicate that the minimum total sperm count (TSC) required to reach a threshold probability of fertilization is between 18 and 29 × 10^6, the 95% CI around the 2.5th centile value of 23 × 10^6.19 The upper value for this range, 29 × 10^6, can be called the ‘critical TSC’. Nevertheless, the TSC of normal healthy males, including those of proven fertility, varies considerably. An average ejaculate of volume 3.0–3.2 ml may contain between 46−259 × 10^6 sperm,20 38−751 × 10^6,21 or 52−837 × 10^6,22 with an extreme limit of 1000 × 10^6.19 Some of this numerical variation can be attributed to variation in the body mass and diet of the males studied.21 There is also the important question of the proportion of sperm which are motile and of normal morphology as opposed to the proportion which are inert and mis-shapen. The motile fraction of the TSC ranges from to 65 to 75%.19,20,22,23 ”

Sperm is not yet mature before it reaches the female reproductive tract. It has to go through a series of changes to mature. This is called capacitation and only capacitated sperm can fertilize the egg. Through this process, sperm basically change movement from linear to hyperactive, and acquire capabilities to fuse with the membrane of the egg. Not all sperm get capacitated and after capacitation the clock starts ticking quite fast for them.

#UCSF Center for Reproductive Health. Conception: How It Works. Retrieved March 2025.

https://crh.ucsf.edu/about-fertility/conception

Quote: “Following ejaculation, the semen forms a gel which provides protection for the sperm from the acidic environment of the vagina. The gel is liquefied within 20-30 minutes by enzymes from the prostate gland. This liquefaction is important to free the sperm so transportation may occur. The seminal plasma is left in the vagina. The protected sperm with the greatest motility travel through the layers of cervical mucus that guard the entrance to the uterus. During ovulation, this barrier becomes thinner and changes its acidity creating a friendlier environment for the sperm. The cervical mucus acts as a reservoir for extended sperm survival. Once the sperm have entered the uterus, contractions propel the sperm upward into the fallopian tubes. The first sperm enter the tubes minutes after ejaculation. The first sperm, however, are likely not the fertilizing sperm. Motile sperm can survive in the female reproductive tract for up to 5 days.”

#Eisenbach and Giojalas. Sperm guidance in mammals — an unpaved road to the egg. Nat Rev Mol Cell Biol. 2006

https://pubmed.ncbi.nlm.nih.gov/16607290/

Quote: “The percentage of capacitated spermatozoa is low (~10% in humans)4–6 and, therefore, the number of spermatozoa that can reach and fertilize the egg is small. The chances that such low numbers of spermatozoa will successfully reach the egg by coincidence, without a guidance mechanism, are very slim.”

#Orr, Teri J. et al. Sperm storage Current Biology, 2012.

https://www.cell.com/current-biology/fulltext/S0960-9822(11)01252-8

Quote: “I didn't want to ask, but can humans store sperm? Humans can maintain viable sperm in the female's reproductive tract for about seven days. This is a fairly short period of sperm storage, and may be due more to the natural longevity of human sperm rather than the active storage by the female. For this to be considered short-term sperm storage, sperm must be unable to live equally long in a similar environment outside the female's reproductive tract.”

#Basic Medical Key. Transport of Gametes and Fertilization. Retrieved March 2025.

https://basicmedicalkey.com/transport-of-gametes-and-fertilization/
Quote: “On ejaculation, the spermatozoa rapidly pass through the ductus deferens and become mixed with fluid secretions from the seminal vesicles and prostate gland. Prostatic fluid is rich in citric acid, acid phosphatase, zinc, and magnesium ions, whereas fluid of the seminal vesicle is rich in fructose (the principal energy source of spermatozoa) and prostaglandins. The 2 to 6 mL of ejaculate (semen, or seminal fluid) typically consists of 40 to 250 million spermatozoa mixed with alkaline fluid from the seminal vesicles (60% of the total) and acid secretion (pH 6.5) from the prostate (30% of the total). The pH of normal semen ranges from 7.2 to 7.8. Despite the numerous spermatozoa (>100 million) normally present in an ejaculate, a number as small as 25 million spermatozoa per ejaculate may be compatible with fertility.

In the female reproductive tract, sperm transport begins in the upper vagina and ends in the ampulla of the uterine tube, where the spermatozoa make contact with the ovulated egg. During copulation, the seminal fluid is normally deposited in the upper vagina (see Fig. 2.3), where its composition and buffering capacity immediately protect the spermatozoa from the acid fluid found in the upper vaginal area. The acidic vaginal fluid normally serves a bactericidal function in protecting the cervical canal from pathogenic organisms. Within about 10 seconds, the pH of the upper vagina is increased from 4.3 to as much as 7.2. The buffering effect lasts only a few minutes in humans, but it provides enough time for the spermatozoa to approach the cervix in an environment (pH 6.0 to 6.5) optimal for sperm motility.”

– Like all bodily openings, the female reproductive tract system is a fortress defending against invaders.

Similar to other surfaces that come in contact with anything external, like the lungs or the gut, the female reproductive tract (FRT) is covered with a protective mucus layer. It has to hit a tight immune balance between avoiding pathogens and allowing sperm. The following paper is an extensive review on the immune responses along this mucus layer across different parts of the FRT.

#Monin L, Whettlock EM, Male V. Immune responses in the human female reproductive tract. Immunology. 2020

https://pmc.ncbi.nlm.nih.gov/articles/PMC7218661/pdf/IMM-160-106.pdf

Quote: “Mucosal barriers are critical interfaces between the host and the environment. Over the past decades, much effort has been dedicated to understanding immune protection in the intestinal and pulmonary mucosae, but our grasp of immune mechanisms in the female reproductive tract (FRT) has lagged behind. Although this partly reflects a climate in which research into women’s health has been underfunded,1 it is also a result of the not inconsiderable difficulties of studying the FRT.

The immune system encounters vastly different challenges in the various compartments of the FRT, and as a result differs between the lower (vagina and ectocervix) and upper (uterus and endocervix) FRT. Like the gut, the vagina is populated by a commensal flora, and here the immune system must allow the growth of beneficial microbes while preventing that of pathogens. The cervix acts as a gatekeeper, preventing the entry of microbes into the uterus, while permitting the passage of sperm. Finally, the immune system in the uterus faces the challenge of allowing the fetus, which is immunologically distinct from its mother, to co-exist with her for 9 months, while simultaneously eliminating any pathogens that may enter. Every site within the FRT must mediate a fine balance between protection from pathogens and maintenance of tissue integrity and function, allowing fertilization, implantation and pregnancy to occur.”

– The future mom’s body has set up deadly barriers and puts the sperm through a brutal process that kills almost all of them and ideally selects the strongest and fittest one.

Ovaries release one egg at a time so letting millions of sperm live and reach the egg would not make much sense when only one is needed at the end. The female reproductive tract (FRT) acts as a series of selective filters to eliminate the not so potent sperm. To be able to reach the egg, sperm need to navigate the FRT through chemotaxis and thermotaxis. The ones that are not able to follow the chemical cues from FRT get lost and die.

#Eisenbach and Giojalas. Sperm guidance in mammals — an unpaved road to the egg. Nat Rev Mol Cell Biol. 2006

https://pubmed.ncbi.nlm.nih.gov/16607290/
Quote: “The percentage of capacitated spermatozoa is low (~10% in humans)4–6 and, therefore, the number of spermatozoa that can reach and fertilize the egg is small. The chances that such low numbers of spermatozoa will successfully reach the egg by coincidence, without a guidance mechanism, are very slim.”

#Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Fertilization. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26843/

Quote: “Although many sperm can bind to an egg, normally only one fuses with the egg plasma membrane and injects its nucleus and other organelles into the egg cytoplasm. If more than one sperm fuses—a condition called polyspermy—multipolar or extra mitotic spindles are formed, resulting in faulty segregation of chromosomes during cell division; nondiploid cells are produced, and development usually stops. Two mechanisms can operate to ensure that only one sperm fertilizes the egg. In many cases, a rapid depolarization of the egg plasma membrane, which is caused by the fusion of the first sperm, prevents further sperm from fusing and thereby acts as a fast primary block to polyspermy. But the membrane potential returns to normal soon after fertilization, so that a second mechanism is required to ensure a longer-term, secondary block to polyspermy. This is provided by the egg cortical reaction.”

#Rahban R, Nef S. CatSper: The complex main gate of calcium entry in mammalian spermatozoa. Mol Cell Endocrinol. 2020 https://www.researchgate.net/publication/343186718_CatSper_The_complex_main_gate_of_calcium_entry_in_mammalian_spermatozoa
Quote: “A: The female reproductive tract is composed of different regions and the sperm cells must constantly adapt to different environments throughout their journey from the vagina to the ampulla. The vagina is the site of deposition of semen and where sperms begin to migrate out of the seminal plasma and into the cervical mucus. Out of the millions of spermatozoa deposited in the vagina, only an estimated number of one hundred capacitated sperms will meet the egg. Capacitation is mainly induced by exposure of sperms to high concentrations of bicarbonate (HCO3􀀀 ) and serum albumin. After this first selection process, most of the sperm cohort continues their way up to the uterus where sperm transport is facilitated via peristaltic contractions and rheotaxis. As they approach the egg at the ampulla, sperms are exposed to warmer temperature at the site of fertilization (37 ◦C at the ampulla), a guiding mechanism referred to as thermotaxis. B: High concentrations of progesterone secreted by the cumulus cells forms a gradient that guides the sperm when in close proximity to the egg through chemotaxis. Progesterone, as well as prostaglandins, activate CatSper leading to a rapid transient increase in intracellular calcium concentration [Ca2+]i. CatSper activation is implicated in hyperactivated motility characterized by a star shaped, non-progressive trajectory in aqueous media. This will facilitate sperm penetration through the extracellular matrix of cumulus cells by mechanical shear forces. Sperms will then release their acrosomal content to digest the Zona Pellucida during the acrosome reaction and finally fertilize the egg.”

– The first deadly obstacle is a highly acidic environment guarded by hundreds of thousands of guard cells. The acidity kills millions, especially of the weaker sperm within the first half hour, although they have a bit of protection: Seminal fluid is alkaline and so it makes this first obstacle a little less deadly.

The acidic environment of the vagina is a defense against infection since most bacteria grow in neutral conditions around pH 7. Though phagocytosis of sperm gets rid of more or less the useless ones, this was shown to not to influence fertility. Dead or moribund cells, in this case sperm, have to be cleaned up like elsewhere in the body.

#Monin L, Whettlock EM, Male V. Immune responses in the human female reproductive tract. Immunology. 2020

https://pmc.ncbi.nlm.nih.gov/articles/PMC7218661/pdf/IMM-160-106.pdf

Quote: “In contrast with the upper reproductive tract, which is lined by a monolayer of columnar epithelial cells, the ectocervix and vagina are lined by protective layers of non-keratinized stratified squamous epithelium. In addition to the physical barrier that a stratified epithelium constitutes, chemical and biological barriers form a first line of defence, with mucus and antimicrobial peptides protecting the vagina from pathogens.4,5 The epithelial layer may also play a role in the success of human immunodeficiency virus (HIV) containment using antiretroviral therapy, as FRT epithelial cells, as well as the underlying fibroblasts, can deliver and store antiretrovirals, promoting sustained protection of vaginal CD4+ T-cells from viral infection.6”

#UCSF Center for Reproductive Health. Conception: How It Works. Retrieved March 2025.

https://crh.ucsf.edu/about-fertility/conception

#S.S. Suarez, A. A. Pacey, Sperm transport in the female reproductive tract, Human Reproduction Update. 2006. https://www.researchgate.net/publication/7496330_Sperm_transport_in_the_female_reproductive_tract

Quote: “Human semen coagulates, but it forms a loose gel rather than the compact fibrous plug seen in rodents. The coagulate forms within about a minute of coitus and then is enzymatically degraded in ½ to 1 h (Lilja and Lundwall, 1992). The predominant structural proteins of the gel are the 50 kDa semenogelin I and the 63 kDa semenogelin II, as well as a glycosylated form of semenogelin II, all of which are secreted primarily by the seminal vesicles (Lilja, 1985). The gel is degraded by prostate-specific antigen (PSA), a serine protease secreted by the prostate gland (Watt et al., 1986). It has been proposed that this coagulum serves to hold the sperm at the cervical os (Harper, 1994) and that it protects sperm against the harsh environment of the vagina (Lundwall et al., 2003).
Seminal gels are not fully successful at holding sperm at the cervical os. In cattle, several studies have demonstrated loss of sperm from the vagina after mating or insemination (reviewed by Hawk, 1987). The fate of spermatozoa that are ejaculated or inseminated into the vagina, but that do not enter the cervix, has not been studied extensively in humans. However, in a 5 year study of 11 female volunteers Baker and Bellis (1993) examined the characteristics of sperm loss from the vagina following coitus (‘flowback’). They found that flowback occurred in 94% of copulations with the median time to the emergence of ‘flowback’ of 30 min (range 5–120 min). Furthermore they estimated that a median of 35% of spermatozoa were lost through flowback but that in 12% of copulations almost 100% of the sperm inseminated were eliminated. This suggests that less than 1% of sperm might be retained in the female reproductive tract and this supports the notion that only a minority of sperm actually enter cervical mucus and ascend higher into the female reproductive tract.”

Quote: “The vagina is open to the exterior and thus to infection, especially at the time of coitus; therefore, it is well equipped with antimicrobial defenses. These defenses include acidic pH and immunological responses and can damage sperm as well as infectious organisms. To enable fertilization to take place, both the female and the male have adopted mechanisms for protecting sperm.
In humans, semen is deposited at the external os of the cervix so that sperm can quickly move out of the vagina (Sobrero and MacLeod, 1962). Human sperm must contend, however briefly, with the acidic pH of vaginal fluid. The vaginal pH of women is normally five or lower, which is microbicidal for many sexually transmitted disease pathogens. Evidence indicates that the acidity is maintained through lactic acid production by anaerobic lactobacilli that feed on glycogen present in shed vaginal epithelial cells (Boskey et al., 2001). Lowering pH with lactic acid has been demonstrated to immobilize bull sperm (Acott and Carr, 1984; Carr et al., 1985).

The pH of seminal plasma ranges from 6.7 to 7.4 in common domestic species (Roberts, 1986) and has the potential to neutralize vaginal acid. Vaginal pH was measured by radio-telemetry in a fertile human couple during coitus. The pH rose from 4.3 to 7.2 within 8 s of the arrival of semen; whereas, no change was detected when the partner used a condom (Fox et al., 1973). Vaginal washings of women with high levels of detectable seminal antigens had a median pH of 6.1, whereas the median pH of washings lacking detectable antigens was 3.7 (Bouvet et al., 1997).”

#Lykke MR, Becher N, Haahr T, Boedtkjer E, Jensen JS, Uldbjerg N. Vaginal, Cervical and Uterine pH in Women with Normal and Abnormal Vaginal Microbiota. Pathogens. 2021

https://pmc.ncbi.nlm.nih.gov/articles/PMC7909242/#abstract1

Quote: “This study demonstrated a pronounced pH gradient in the female genital tract. In nonpregnant women with NVM, the median pH was 3.9 in the lower vagina, 5.7 in the upper vagina, and not less than 7.7 in the upper uterine cavity. Furthermore, it was remarkable that the pH in the lower part of the cervical canal decreased from 6.3 in early pregnancy to 5.4 at term, nearly a 10-fold increase in the H+ concentration. Finally, the effect of AVM on the pH was significant only in the lower vagina, much less marked and not statistically significant in the upper vagina, and not measurable at all in the cervical canal or the uterine cavity.”

#Lin YP, Chen WC, Cheng CM, Shen CJ. Vaginal pH Value for Clinical Diagnosis and Treatment of Common Vaginitis. Diagnostics (Basel). 2021
https://pmc.ncbi.nlm.nih.gov/articles/PMC8618584/#abstract1

Quote: “The vagina serves as an outside-communicating channel with the functions of draining menstruation and childbirth delivery. The vagina normally has unique flora that sustains the internal physical and chemical environment. The presence of normal flora relies on maintenance of various components of the ecosystem, which is in dynamic equilibrium [1]. Based on several published articles, the normal vaginal pH for women of childbearing age ranges from 3.8 to 5.0, which is moderately acidic [2,3]. The normal vagina is covered by a thin layer of transparent liquid, commonly known as vaginal fluid. Many factors may lead to changes or imbalances in the vaginal pH value, including vaginal infections, aging, sexual activity, and vaginal douching [4].”

– The survivors reach the next obstacle, a treacherous maze full of protein nets where millions more find a brutal end getting stuck and lost.

After passing through vagina, sperm reach the cervical canal, a few centimeters large canal right before the uterine cavity. Here cervical mucus mediates sperm migration, both through its density, which would not favor sperm with bad motility, or also immune responses, like neutrophils which clean up the misguided sperm.

#S.S. Suarez, A. A. Pacey, Sperm transport in the female reproductive tract, Human Reproduction Update. 2006. https://www.researchgate.net/publication/7496330_Sperm_transport_in_the_female_reproductive_tract

Quote: “Cervical mucus presents a greater barrier to abnormal sperm that cannot swim properly or that present a poor hydrodynamic profile than it does to morphologically normal, vigorously motile sperm and is thus thought as one means of sperm selection (Hanson and Overstreet, 1981; Barros et al., 1984; Katz et al., 1990, 1997). The greatest barrier to sperm penetration of cervical mucus is at its border, because here the mucus microarchitecture is more compact (Yudin et al., 1989). Components of seminal plasma may assist sperm in penetrating the mucus border. More human sperm were found to enter cervical mucus in vitro when an inseminate was diluted 1:1 with whole seminal plasma than when it was diluted with Tyrode’s medium, even though the sperm swam faster in the medium (Overstreet et al., 1980).
[...]

Like the vagina, the cervix can mount immune responses. In rabbits and humans, vaginal insemination stimulates the migration of leukocytes, particularly neutrophils and macrophages, into the cervix as well as into the vagina (Tyler, 1977; Pandya

and Cohen, 1985). Neutrophils migrate readily through midcycle human cervical mucus (Parkhurst and Saltzman, 1994). In rabbits, neutrophils were found to heavily infiltrate cervices within a ½ h of mating or artificial insemination (Tyler, 1977).”

#Zambrano, Fabiola et al. “Leukocytes coincubated with human sperm trigger classic neutrophil extracellular traps formation, reducing sperm motility.” Fertility and sterility. 2016.

https://www.sciencedirect.com/science/article/pii/S001502821661302X

Quote: “Alongside phagocytosis, PMN have several other potent effector mechanisms to combat and eventually kill pathogens and spermatozoa, such as the oxidative burst activity resulting in the production of reactive oxygen species (ROS), the release of antimicrobial peptides/proteins, and the formation of neutrophil extracellular traps (NETs). NETs are released after PMN cell death process, reported as NETosis, and are primarily situated in the extracellular space. NETosis has been shown to be an NADPH oxidase (NOX)-dependent mechanism, which ultimately leads to the extrusion of a mixture of nuclear as well as cytoplasmic granule contents resulting in the formation of DNA-rich web-like structures that are adorned with histones (H1, H2A/H2B, H3, H4) and granular effector molecules, such as neutrophil elastase (NE), pentraxin, lactoferrin, MPO, and others 5, 6.” [PMN: Polymorphonuclear neutrophils]

#Ralf Henkel. Sperm preparation: state-of-the-art—physiological aspects and application of advanced sperm preparation methods. Asian Journal of Andrology. 2012.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3735088/pdf/aja2011133a.pdf

– Now if a woman is in the part of her menstrual cycle where she is ovulating, her body helps the sperm out a bit, by releasing chemicals that guide the way and being a bit less hostile. Luckily for our brave sperm this is the case today.

Around the time of ovulation, the cervical mucus becomes thinner and more watery, which facilitates sperm transport. This is not the only mechanism that helps with sperm migration and many other hormone mediated guiding mechanisms help out sperm until it reaches the egg.

#Nakano et al. Insights into the role of cervical mucus and vaginal pH in unexplained infertility. 2015 Medical Express 2(2).

https://www.scielo.br/j/medical/a/qjRg5mV765Dvs5tYjCtBwyC/

Quote: “The anatomical and functional structure of the human cervix facilitates the performance of these aforesaid functions, but the production of mucus is probably the most important one. Throughout the menstrual cycle, the cervix changes in size and texture. Just prior to ovulation and as a result of the rise in estrogen levels, the cervix swells and softens, while its external os dilates. Also, during this time, the cervix secretes more abundant, slippery, clear and stretchy mucus, which exudes from cervix into the vagina, thus facilitating the entrance of sperm into the uterine cavity.2,48 In the periovulatory period more than 96% of the cervical mucus is water, thus conferring the mucus high spinnbarkeit and pronounced ferning capacity; as such, sperm penetrability is highest at this time.46 After ovulation, progesterone induces the cervix to harden, close and secrete thicker mucus, which acts as a plug, preventing bacteria and sperm from entering the uterus and making fertilization very unlikely.48
[...]
It is generally believed that another potentially important feature of human cervical mucus is its ability to restrict migration of abnormal spermatozoa, thus acting as a "filter" that eliminates deficient sperm.21,87,88 It has been shown that abnormal sperm have a poorer hydrodynamic profile compared with morphologically normal motile sperm.7,21,87,88 Moreover, sperm movement is probably influenced by the interaction between the mucus and the surface properties of the sperm head; for instance, sperm antibodies on the sperm head may inhibit sperm movement through the mucus.89”

#Lacroix G, Gouyer V, Gottrand F, Desseyn JL. The Cervicovaginal Mucus Barrier. Int J Mol Sci. 2020

https://pmc.ncbi.nlm.nih.gov/articles/PMC7663572/

Quote: “Proteomic analysis has identified specific proteins in the cervical mucus of the preovulatory, ovulatory, and postovulatory phases [33]. The composition and viscoelasticity of CVM change during the menstrual cycle. Variations in CVM viscoelasticity allow the identification of the fertile window, which can be used to help promote pregnancy because, near the time of ovulation, the cervical mucus becomes waterier and more permeable to spermatozoa [30,34]. These changes appear to be influenced mainly by variations in mucin concentration and mucin-associated O-glycans because cervical mucus production increases in midcycle and is accompanied by modification of mucin glycosylation [35,36,37].

[...]

Cervicovaginal mucus has multiple important functions. It protects the vaginal epithelium from repetitive friction during sexual intercourse, helps to ensures fertility [42], and stops or selectively restricts sperm transport within the female reproductive tract only during the ovulation phase through modulation of the size of the pores in the GFM network [30,32,43].”

#Zierden HC, DeLong K et al. Cervicovaginal mucus barrier properties during pregnancy are impacted by the vaginal microbiome. Front Cell Infect Microbiol. 2023

https://pmc.ncbi.nlm.nih.gov/articles/PMC10103693/

Quote: “During the normal 28-day menstrual cycle, CVM [cervicovaginal mucus] properties change based on fluctuating levels of progesterone and estrogen (Moreno-Escallon et al., 1982). As estrogen increases, and the body prepares for ovulation, the water content of cervical mucus increases and the average pore size increases, to facilitate sperm transport through mucus (Mihm et al., 2011). As progesterone increases in the luteal phase, mucus thickens, and the effective pore size decreases (Moreno-Escallon et al., 1982; Mihm et al., 2011).”

#UCSF Center for Reproductive Health. Conception: How It Works. Retrieved March 2025.

https://crh.ucsf.edu/about-fertility/conception

Quote: “The protected sperm with the greatest motility travel through the layers of cervical mucus that guard the entrance to the uterus. During ovulation, this barrier becomes thinner and changes its acidity creating a friendlier environment for the sperm. The cervical mucus acts as a reservoir for extended sperm survival. Once the sperm have entered the uterus, contractions propel the sperm upward into the fallopian tubes. The first sperm enter the tubes minutes after ejaculation. The first sperm, however, are likely not the fertilizing sperm. Motile sperm can survive in the female reproductive tract for up to 5 days.”

#Fitzpatrick JL, Willis C, Devigili A, Young A, Carroll M, Hunter HR, Brison DR. Chemical signals from eggs facilitate cryptic female choice in humans. Proc Biol Sci. 2020

https://pmc.ncbi.nlm.nih.gov/articles/PMC7341926/

Quote: “Here, we assess if follicular fluid, a source of sperm chemoattractants [16], differentially regulates sperm behaviour to reinforce pre-mating mate choice decisions and mediate fertilization success in humans. Human sperm respond to chemoattractants present in the follicular fluid surrounding eggs (most likely progesterone [5], although this remains a source of ongoing debate) by altering their swimming behaviour to orient towards, and accumulate in, follicular fluid [16]. Sperm behavioural responses can differ among follicular fluids, such that follicular fluids from different females exhibit variation in their ability to attract sperm from the same male [16]. Moreover, females producing follicular fluid that was better at causing an accumulation response in sperm also produce eggs that achieved higher fertilization rates in clinical IVF cycles [16].”

– Only a couple hundred - less than 0.0001% of the original sperm - pass through into the uterine cavity, where two tunnels branch out. Here mucus helps transport the stronger sperm to move on, while weeding out the weaker ones.

#Eisenbach, M., Giojalas, L. Sperm guidance in mammals — an unpaved road to the egg. Nat Rev Mol Cell Biol 7, 276–285 (2006).

https://doi.org/10.1038/nrm1893

Quote: “For fertilization to occur in mammals, ejaculated spermatozoa must reach the egg, which, following ovulation, has moved from the ovary into the Fallopian tube. Until not too long ago, the common belief was that, in mammals, following ejaculation into the female genital tract, large numbers of spermatozoa ‘race’ towards the egg and compete to fertilize it. This dogma, as well as conflicting results in the literature, instilled the idea that the guidance of sperm to the egg was superfluous in mammals. The ‘competitive-race model’ dismantled when it became clear that, in fact, few of the ejaculated spermatozoa (in humans, only ~1 of every million spermatozoa) succeed in entering the Fallopian tubes1–3. Furthermore, the number of spermatozoa that can fertilize the egg is even smaller. Spermatozoa must undergo a process of ripening, known as capacitation, and only capacitated spermatozoa can penetrate the cumulus layer that surrounds the egg, bind to the sperm receptor on the egg coat, and undergo the acrosome reaction that enables sperm penetration through the egg coat and then fusion with the egg (see REF. 4 for a review).”

Quote: “Rapid transport of sperm through the uterus by myometrial contractions can enhance sperm survival by propelling them past the immunological defenses of the female. As is the case in the vagina and cervix, coitus induces a leukocytic infiltration of the uterine

cavity, which reaches a peak several hours after mating in mice (Austin, 1957). The leukocytes are primarily neutrophils and have been observed phagocytizing uterine sperm in mice, rats and rabbits (Austin, 1957; Bedford, 1965). This phagocytosis was observed several hours after insemination and therefore might be directed primarily against damaged sperm. However, normal sperm may also be attacked, particularly in vaginal inseminators like humans, because their sperm have lost much of the immune protection afforded by seminal plasma constituents (Suarez and Oliphant, 1982; Dostal et al., 1997).

When sperm first enter the uterus, they outnumber the leukocytes. As time passes, the leukocytes begin to outnumber the sperm. Also, as sperm lose protective seminal plasma coating, they may become more susceptible to leukocytic attack. At some point, even undamaged sperm may fall victim to the leukocytes. Probably, to ensure fertilization, sperm should pass through the uterine cavity before significant numbers of leukocytes arrive.”

#Holt and Fazeli. Sperm Transport and Selection in Mammals. Encyclopedia of Reproduction (Second Edition) Volume 2, 2018, Pages 269-275.
https://www.sciencedirect.com/science/article/abs/pii/B9780128012383651978

Quote: “After copulation, spermatozoa in the mammalian ejaculate make their way through the female reproductive tract and eventually reach the vicinity of the ovulated eggs within the oviduct. This journey, known generally as “sperm transport,” involves several physiological processes acting in concert. Ejaculation itself forces spermatozoa to interact directly with the mucus produced by the cervix. In many species, including humans (Yudin et al., 1989), cervical mucus consists of a fine network of microscopic fibers and the spermatozoa have to penetrate and pass through this mesh-like network powered by their own motility in order to reach the uterus. Thus the cervical mucus represents an initial selective barrier for spermatozoa, and many with inadequate motility cannot make any further progress. Transport of those spermatozoa that reach the uterus is assisted by muscle contractions, pushing the spermatozoa towards the next major barrier; the utero-tubal junction (UTJ). This is a narrow and tortuous, mucus-filled, tubular structure that controls access to the oviducts (also known as the Fallopian tubes) and, in fact, prevents the entry of most spermatozoa. It is now known that the UTJ is another important barrier, blocking the progress of many spermatozoa towards the oocyte and ensuring that only a small and privileged subpopulation of spermatozoa are ever allowed even to approach oocytes.”

#Gamete Transport. Reference Module in Biomedical Sciences. 2014.

https://www.sciencedirect.com/science/article/abs/pii/B9780128012383054271

Quote: “There are two main modes of sperm transport through the cervix. One is a phase of initial rapid transport, by which some spermatozoa can reach the uterine tubes within 5–20 min of ejaculation. Such rapid transport relies more on muscular movements of the female reproductive tract than on the motility of the spermatozoa themselves. These early-arriving sperm, however, appear not to be as capable of fertilizing an egg as do those that have spent more time in the female reproductive tract. The second, slow phase of sperm transport involves the swimming of spermatozoa through the cervical mucus (traveling at a rate of 2–3 mm h− 1), their storage in cervical crypts, and their final passage through the cervical canal as much as 2–4 days later.

Relatively little is known about the passage of spermatozoa through the uterine cavity, but the contraction of uterine smooth muscle, rather than sperm motility, seems to be the main intrauterine transport mechanism. At this point, the spermatozoa enter one of the uterine tubes. According to some more recent estimates, only several hundred spermatozoa enter the uterine tubes, and most enter the tube containing the ovulated egg.”

#Bruce M. Carlson. Chapter 14 - The Reproductive System. The Human Body Linking Structure and Function 2019, Pages 373-396

https://www.sciencedirect.com/science/article/abs/pii/B9780128042540000144

Quote: “Of the huge numbers of sperm that enter the female reproductive tract, almost all fail to reach the uterine tubes. The unsuccessful sperm are removed by the infiltration of white blood cells into the cavities of the vagina, cervix, and uterus. These cells, along with certain immunoglobulins, inactivate and degrade foreign invaders, in this case, the excess sperm. Fortunately, the uterine tubes are not subject to this sort of cellular infiltration.

The openings of the uterine tubes into the uterus (uterotubal junction) represent another barrier to sperm transport. With two uterine tubes and usually only one ovulated egg, any spermatozoon that enters the empty uterine tube is automatically doomed to reproductive failure. Roughly 10,000 or fewer sperm cells of the millions in the ejaculate enter the correct tube. These sperm cells collect in the lower part of the uterine tube and attach to the epithelium of the tube for about 24 hours.”

– The mom’s immune cells pick off and devour every cell that trails behind or takes the wrong turn. And then finally, maybe just a few dozen sperm reach their goal, the mighty, giant egg.

#Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Fertilization. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK26843/

Quote: “Of the 300,000,000 human sperm ejaculated during coitus, only about 200 reach the site of fertilization in the oviduct. There is evidence that chemical signals released by the follicle cells that surround the ovulated egg attract the sperm to the egg, but the nature of the chemoattractant molecules is unknown. Once it finds an egg, the sperm must first migrate through the layer of follicle cells and then bind to and cross the egg coat—the zona pellucida. Finally, the sperm must bind to and fuse with the egg plasma membrane. To become competent to accomplish these tasks, ejaculated mammalian sperm must normally be modified by conditions in the female reproductive tract, a process called capacitation, which requires about 5–6 hours in humans. Capacitation is triggered by bicarbonate ions (HCO3–) in the vagina, which enter the sperm and directly activate a soluble adenylyl cyclase enzyme in the cytosol. The cyclase produces cyclic AMP (discussed in Chapter 15), which helps to initiate the changes associated with capacitation. Capacitation alters the lipid and glycoprotein composition of the sperm plasma membrane, increases sperm metabolism and motility, and markedly decreases the membrane potential (that is, the membrane potential moves to a more negative value so that the membrane becomes hyperpolarized).”

Quote: “Like the vagina, the cervix can mount immune responses. In rabbits and humans, vaginal insemination stimulates the migration of leukocytes, particularly neutrophils and macrophages, into the cervix as well as into the vagina (Tyler, 1977; Pandya and Cohen, 1985). Neutrophils migrate readily through midcycle human cervical mucus (Parkhurst and Saltzman, 1994). In rabbits, neutrophils were found to heavily infiltrate cervices within a ½ h of mating or artificial insemination (Tyler, 1977). Interestingly, it was discovered that if female rabbits were mated to a second male during the neutrophilic infiltration induced by an earlier mating, sperm from the second male were still able to fertilize (Taylor, 1982). Thus, although the cervix is capable of mounting a leukocytic response, and neutrophils may migrate into cervical mucus, the leukocytes may not present a significant barrier to sperm. It has been demonstrated that neutrophils will bind to human sperm and ingest them only if serum that contains both serological complement and complement-fixing anti-sperm antibodies is present (D’Cruz et al., 1992). This can happen in vivo if the female somehow becomes immunized against sperm antigens. Altogether, the evidence indicates that leukocytic invasion serves to protect against microbes that accompany sperm and does not normally present a barrier to normal motile sperm, at least not shortly after coitus.

Immunoglobulins, IgG and IgA, have been detected in human cervical mucus. Secretory IgA is produced locally by plasma cells in subepithelial connective tissue. The amount secreted increases in the follicular phase but then decreases at about the time of ovulation (Kutteh et al., 1996). The immunoglobulins provide greater protection from microbes at the time when the cervical mucus is highly hydrated and offers the least resistance to penetration. However, when there are antibodies present that recognize antigens on the surface of ejaculated sperm, infertility can result (Menge and Edwards, 1993).

Complement proteins are also present in cervical mucus (Matthur et al., 1988), along with regulators of complement activity (Jensen et al., 1995). Thus, there is a potential for antibody-mediated destruction of sperm in the cervical mucus as well as leukocytic capture of sperm. Some anti-sperm antibodies are not complement-activating; however, they can still interfere with movement of sperm through cervical mucus by physical obstruction (Menge and Edwards, 1993; Ulcova-Gallova, 1997).”

– 10,000 times bigger than them, it is an industrial scale monster loaded with nutrients and nearly 100,000 mitochondria, the powerhouse of the cell – 50 times more than an average cell.

The egg is the largest cell in the human body, it is almost at the limit of the smallest thing that our eyes can see, whereas the sperm is the smallest cell. The 10,000 times difference that we refer to here is by volume.

Kitasaka H, Konuma Y, Tokoro M, Fukunaga N, Asada Y. Oocyte cytoplasmic diameter of ≥130 μm can be used to determine human giant oocytes. F S Sci. 2022

https://linkinghub.elsevier.com/retrieve/pii/S2666-335X(21)00089-6

Quote: “The morphological characteristics and quality of giant oocytes (GOs) are not yet well understood, although there are several reports, including the following studies. Usually,

the diameter of human oocytes is approximately 110 µm (12, 13), and the cytoplasmic diameter of GOs is larger than that of normal oocytes; moreover, there have been reports that the volume of GOs is approximately twice that of normal oocytes (14) and that prophase I has 2 nuclei (15).”

#Sunanda P, Panda B, Dash C, Padhy RN, Routray P. An illustration of human sperm morphology and their functional ability among different group of subfertile males. Andrology. 2018

https://onlinelibrary.wiley.com/doi/10.1111/andr.12500

Quote: “Human spermatozoon is the only highly differentiated haploid cell with a unique structural organization that gains motility to travel inside female reproductive tract and has the potential to fertilize an egg. They are produced in millions by the process of spermatogenesis involving a series of complex events, namely repeated cell divisions from the pool of germ cells in testis, chromatin condensation, acrosomal cap formation, cytoplasmic extrusion, and tail formation. A typical human spermatozoon has a distinct structure with an oval-shaped head (3–5 μm length and 2–3 μm width), a midpiece (7–8 μm), and a tail (45 μm) (Sedo et al., 2012).”

#Babayev E, Seli E. Oocyte mitochondrial function and reproduction. Curr Opin Obstet Gynecol. 2015

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590773/

Quote: “There are about 100,000 mitochondria per fully-grown human oocyte. Mitochondria in mammalian oocytes are spherical with little cristae [2]. These mitochondria are transcriptionally and bioenergetically silent and this functional state seems to be evolutionarily conserved [12,13], especially in immature eggs, which have slow ATP production [14]. This quiescent state is believed to be important to keep the number of mitochondrial DNA mutations to minimum, as these mutations will then be passed down to the embryo [13]. After fertilization, oocyte mitochondria undergo structural changes and resemble their somatic counterparts by the time of blastocyst formation [7▪].”

The number of mitochondria per cell changes depending on the energy demands of that cell and can range from zero to hundreds of thousands. We took a liver cell for comparison.

#Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Mitochondrion.
https://www.ncbi.nlm.nih.gov/books/NBK26894/
Quote: ”Mitochondria are large enough to be seen in the light microscope, and they were first identified during the nineteenth century. Real progress in understanding their function, however, depended on procedures developed in 1948 for isolating intact mitochondria. For technical reasons, many of these biochemical studies have been performed with mitochondria purified from liver; each liver cell contains 1000–2000 mitochondria, which in total occupy about one-fifth of the cell volume.”

– But the egg is not easily conquered – the survivors get closer and try to get in until the egg decides to accept a single one of them.

The oocyte is covered with a special thick-coat membrane called “zona pellucida”. It is believed that this membrane is there to allow species specific fertilization. For example, human sperm can fertilize hamster eggs that lack zona pellucida. So it might be an obstacle, but it’s one for a good reason.

The sperm has the enzymes to fuse with the zona pellucida and then fertilize the egg. This is called Acrosome Reaction. However, this does not happen on its own and is actually triggered by the egg as the sperm gets closer to the egg.

#Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Fertilization.

https://www.ncbi.nlm.nih.gov/books/NBK26843/

Quote: “Once a capacitated sperm has penetrated the layer of follicle cells, it binds to the zona pellucida (see Figure 20-21). The zona usually acts as a barrier to fertilization across species, and removing it often eliminates this barrier. Human sperm, for example, will fertilize hamster eggs from which the zona has been removed with specific enzymes; not surprisingly, such hybrid zygotes fail to develop. Zona-free hamster eggs, however, are sometimes used in infertility clinics to assess the fertilizing capacity of human sperm in vitro (Figure 20-30).”

“Figure 20-30 Scanning electron micrograph of a human sperm contacting a hamster egg

The zona pellucida of the egg has been removed, exposing the plasma membrane, which contains numerous microvilli. The ability of an individual's sperm to penetrate hamster eggs is used as an assay of male fertility; penetration of more than 10–25% of the eggs is considered to be normal. (Courtesy of David M. Phillips.)”

– For a few days it rapidly divides and grows to a couple of hundred cells, while traveling down to the uterus, the part of the female reproductive system where it will try to make its new home. Here it begins to divide into two specialised teams of cells - one will eventually become the baby. The other cells are trophoblasts and their job is to turn into a temporary organ inside the mother: the placenta, that will make pregnancy possible and eventually die.

Trophoblasts are the first cells to differentiate and eventually give rise to the placenta.

#Gauster M, Moser G, Wernitznig S, Kupper N, Huppertz B. Early human trophoblast development: from morphology to function. Cell Mol Life Sci. 2022
https://pubmed.ncbi.nlm.nih.gov/35661923/
Quote: “In the human, fertilization of the oocyte by a sperm forms a diploid zygote and is the earliest developmental stage. The zygote does not possess any implantation competencies and hence undergoes three rounds of cleavage divisions within days 2–3 post fertilization, which, in contrast to conventional cell divisions, are not accompanied by a significant overall cell growth during the interphase. Hence, the early embryo does not increase in total volume during this stage of development. The cells arising from cleavage divisions are referred to as blastomeres and start to arrange in a compact cluster of cells called morula (Fig. 1).”

Quote: “This cell positioning is considered a prerequisite for the first lineage formation, guiding the differentiation of internalized cells into the pluripotent inner cell mass lineage that gives rise to all embryonic cells and tissues. Those cells remaining on the surface undergo apico-basal polarization and differentiate into the trophectoderm that is the precursor of a specific subset of cells in the extra-embryonic tissues after implantation, the trophoblast residing in the placenta and the fetal membranes. The polarization process and tight sealing of the trophectoderm enable formation of the blastocoel, a fluid-filled cavity that is generated when fluid is pumped through the water transporter aquaporin 3 (AQP3) of the polarized surface of the trophectoderm into intercellular spaces between trophectoderm cells and cells of the inner cell mass (a process also referred to as cavitation). This way, initial accumulation of fluid leads to swelling of the embryo to almost 10 times its original volume [116], and the increasing pressure eventually breaks down cell–cell contacts and pushes the inner cell mass into one quadrant of the arising blastocyst at 5 days post fertilization [49]. The region where the inner cell mass is attached to the trophectoderm is called the embryonic pole, which is the crucial site for interaction of the blastocyst with the endometrium.”

#Blastocyst. Cleveland Clinic. 2022. Retrieved March 2025.

https://my.clevelandclinic.org/health/body/22889-blastocyst
Quote: “A blastocyst is a hollow ball of cells. The cells form two layers. The inner layer is about three to four cells thick and the outer layer is about one cell thick. The cells in a blastocyst divide rapidly. A mature blastocyst may contain as many as 200 to 300 cells.”

https://embryology.med.unsw.edu.au/embryology/index.php?title=Foundations_Practical_-_Week_1_and_2

#Khan YS, Ackerman KM. Embryology, Week 1. [Updated 2023 Apr 17].
https://www.ncbi.nlm.nih.gov/books/NBK554562/

Quote: “Blastocyst Formation: By the blastocyst stage (approximately day five after fertilization), the embryo has reached 50 to 150 cells and starts to strain at the confines of the zona pellucida. This is due to cell division but also active pumping of fluid by outer cell mass cells into the inner space of the blastocyst, which forms a cavity or blastocoel. The filling of this space with fluid expands the blastocyst; thus, the term expanded blastocyst. Before the creation of this fluid space, the embryo is referred to as non-expanded. Expansion functions to thin and eventually rupture the zona pellucida to permit the blastocyst to hatch from the zona pellucida. Blastocyst expansion also functions to allow a large amount of fluid entering the space to shift a grouping of cells off to one side of the hollowed interior. This cellular mass is a grouping of embryonic stem cells with unrestricted developmental potential termed the inner cell mass (ICM) within the blastocyst. The ICM has the cells that will give rise to actual fetal cells. The other cells that surround and protect the ICM and that line the inner side of the zona pellucida are the trophectoderm cells, which give rise to the fetal part of the placenta. Thus, hallmarks of successful blastocyst formation are the fluid-filled blastocoele, the ICM, and the fully differentiated trophectoderm-derived trophoblast.”

#Lawless L, Qin Y, Xie L, Zhang K. Trophoblast Differentiation: Mechanisms and Implications for Pregnancy Complications. Nutrients. 2023
https://pmc.ncbi.nlm.nih.gov/articles/PMC10459728/
Quote: “Proper development of the placenta relies on intricate processes of cell differentiation. As trophoblast cells differentiate into their terminal cell types, critical events, such as spiral artery remodeling and vascularization, take place, enabling the formation of a functional placenta. Spiral artery remodeling involves the transformation of maternal arteries into high-velocity, low-resistance vessels by invasive trophoblast cells, facilitating sufficient blood flow into the placenta. Concurrently, placental vascularization establishes a specialized network of fetal capillaries that interact with the maternal circulation to extract vital nutrients and oxygen necessary for fetal growth.”

– In any case, the following days are the most dangerous for the potential new life.

It is not a sure shot for all fertilized eggs. Even though it is difficult to estimate the ratio in nonclinical conditions and the numbers vary, as high as half of the embryos are believed to fail to implant.

#Muter et al. Human embryo implantation. Development (2023)

https://journals.biologists.com/dev/article/150/10/dev201507/310869/Human-embryo-implantation

Quote: “Human reproduction has been described as disappointingly inefficient (Evers, 2002; Macklon et al., 2002). This statement appears justifiable as only 40-60% of all conceptions survive to birth in young healthy women (Hertig and Bernischke, 1967; Zinaman et al., 1996; Norwitz et al., 2001; Jarvis, 2016). Once the embryo embeds in the endometrium, human chorionic gonadotrophin (hCG) rises in maternal blood and urine, thus allowing for robust estimates of the incidence of pregnancy loss. There is remarkable agreement among many studies that one in three embryos perish after implantation (Wilcox et al., 1999; Wang et al., 2003; Foo et al., 2020; Zinaman et al., 1996). More than half of pregnancy losses occur so early that they escape detection with little or no discernible impact on maternal reproductive fitness beyond increasing the likelihood of conception in subsequent cycles (Wang et al., 2003). The remaining losses present as clinical miscarriages, 90-95% of which occur in the first 12 weeks of pregnancy (Fig. 1) (Magnus et al., 2019). Estimates of the rates of embryo attrition prior to implantation are challenging and dependent on variables that determine the likelihood of fertilisation, including the frequency of intercourse during the fertile window of the menstrual cycle (Dunson et al., 2002; Jarvis, 2016). Nevertheless, with conception rates per cycle of 40% or less in young women who are trying to conceive, ‘normal’ rates of pre-implantation embryo loss between 20 and 40% appear reasonable estimates (Fig. 1).”

– Like a tennis ball rolling through syrup the young embryo moves along the uterus and tries to take a hold.

There are a multitude of mechanisms that facilitate implantation with dozens of molecules like adhesion molecules, cytokines, growth factors and more. Cell adhesion molecules are expressed on the surface of the invasive trophoblast, and these molecules interact with ligands expressed by the extracellular matrix of the decidua in a temporal and spatial way. In the window of implantation, the mother increases the number of these molecules.

The following paper is a review on this topic.

#Kim SM, Kim JS. A Review of Mechanisms of Implantation. Dev Reprod. 2017
https://pmc.ncbi.nlm.nih.gov/articles/PMC5769129/
Quote: ”Implantation is defined as the process by which the embryo attaches to the endometrial surface of the uterus and invades the epithelium and then the maternal circulation to form the placenta. Before the initiation of implantation, however, both embryo and endometrium should embark on an elaborated process in a time- and location-specific manner. The crosstalk between a receptive uterus and a competent blastocyst can only occur during a limited time span, known as the “window of implantation” (Psychoyos, 1986; Ma et al., 2003). The window of endometrial receptivity is restricted to days 16-22 of a 28-day normal menstrual cycle, 5-10 days after the luteinizing hormone (LH) surge (Navot et al., 1991). The uterus then continues into the non- receptive period for the remaining cycle as the late luteal phase until menstruation ensues.”

#Blanco-Breindel MF, Singh M, Kahn J. Endometrial Receptivity. [Updated 2023 Jun 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Retrieved March 2025

https://www.ncbi.nlm.nih.gov/books/NBK587449/

Quote: “Endometrial receptivity describes the intricate process undertaken by the uterine lining to prepare for the implantation of an embryo. While embryo development and endometrial preparation are concurrent yet independent processes, their synchronization is critical to the success of embryo apposition, adhesion, invasion, and further ongoing pregnancy.[1] The accepted definition of endometrial receptivity is "the period of endometrial maturation during which the trophectoderm of the blastocyst can attach to the endometrial epithelial cells and subsequently invade the endometrial stroma and vasculature." [2] The limited period of optimal endometrial receptivity in which the endometrium is ready to receive an embryo, paired with an embryo's readiness to the implant, is commonly referred to as the "window of implantation" and is generally detected between days 20 and 24 of a normal 28-day menstrual cycle.[2][3]

Many molecular pathways involve hormones, adhesion molecules, cytokines, and growth factors acting in concert to create a synchronous window of implantation. When synchrony is lost or receptivity is not achieved, the consequence is early pregnancy loss or infertility.[4]”

We unfortunately cannot cover it all, but we refer to one of the most studied molecules here: L-selectin. This adhesion molecule is found on blastocyst (also on some immune cells where it helps tether them to epithelial cells in the site of inflammation). It sticks to special carbohydrate groups on the endometrial endothelium which are upregulated during the time of implantation. This can help keep the blastocyst in place until more sticky molecules like integrin come into action. Some researchers suggest that the slippery MUC1 is downregulated both by embryo and the mother at the site of implantation and in the window of implantation. This exposes the carbohydrate groups for the L-selectin to bind and opens up a sticky patch for the otherwise rolling embryo on the slippery roads of the uterus. As put by the authors of the paper who discovered L-selectins “It’s like a tennis ball rolling across a surface covered in syrup. The embryo’s journey along the uterine wall is arrested by the sticky interaction.”

#EurekAlert. Scientists discover what makes human embryo attach to uterus. 2003.
https://www.eurekalert.org/news-releases/716615

Quote: “The team found that about six days after fertilization, molecules on the embryo’s surface interact with molecules on the mother’s uterine wall to create the sticky environment – the same combination of molecules known to stop the movement of disease-fighting leukocytes migrating through blood vessels, and allow them to attach to the blood vessel walls in areas of inflammation.

“It’s like a tennis ball rolling across a surface covered in syrup,” said Susan Fisher, PhD, UCSF professor of stomatology, anatomy and pharmaceutical chemistry and senior author of the SCIENCE report. “The embryo’s journey along the uterine wall is arrested by the sticky interaction.””

#Johnson GA, Burghardt RC, Bazer FW, Seo H, Cain JW. Integrins and their potential roles in mammalian pregnancy. J Anim Sci Biotechnol. 2023 https://jasbsci.biomedcentral.com/articles/10.1186/s40104-023-00918-0

Quote: “Perhaps nowhere else in the mammalian body do the apical domains of the surface epithelium of one organ physically attach to the apical domains of the surface epithelium of another organ. This arrangement is unique to pregnancy. Thus, it is not surprising that attachment of the conceptus to the uterine LE to initiate implantation is highly synchronized and requires reciprocal secretory and physical interactions between a developmentally competent conceptus and the uterus during a restricted period of the uterine cycle termed the “window of receptivity” [34]. Interactions between the apical surfaces of the uterine LE and trophoblast progress from a non-adhesive or pre-contact phase to an apposition phase and conclude with adhesion. Conceptus attachment first requires the removal of mucins from the glycocalyx of the uterine LE that sterically inhibit adhesion. The removal of these mucins allows for direct physical interactions between a mosaic of carbohydrates and lectins at the apical surfaces of the opposing uterine LE and conceptus trophoblast cells which contribute to initial attachment of the trophoblast to the uterine LE [35, 36]. These low affinity contacts are then strengthened by a repertoire of adhesive interactions between integrins and ECM molecules that appear to be the dominant contributors to stable adhesion for implantation [37,38,39] (Fig. 6 [39,40,41,42]).”
Quote: “Perhaps nowhere else in the mammalian body do the apical domains of the surface epithelium of one organ physically attach to the apical domains of the surface epithelium of another organ. This arrangement is unique to pregnancy. Thus, it is not surprising that attachment of the conceptus to the uterine LE to initiate implantation is highly synchronized and requires reciprocal secretory and physical interactions between a developmentally competent conceptus and the uterus during a restricted period of the uterine cycle termed the “window of receptivity” [34]. Interactions between the apical surfaces of the uterine LE and trophoblast progress from a non-adhesive or pre-contact phase to an apposition phase and conclude with adhesion. Conceptus attachment first requires the removal of mucins from the glycocalyx of the uterine LE that sterically inhibit adhesion. The removal of these mucins allows for direct physical interactions between a mosaic of carbohydrates and lectins at the apical surfaces of the opposing uterine LE and conceptus trophoblast cells which contribute to initial attachment of the trophoblast to the uterine LE [35, 36]. These low affinity contacts are then strengthened by a repertoire of adhesive interactions between integrins and ECM molecules that appear to be the dominant contributors to stable adhesion for implantation [37,38,39] (Fig. 6 [39,40,41,42]).”

– An intense chemical dialog between two living things begins, the embryo releases dozens of chemicals that announce its presence and asks the guard cells of the uterine wall to allow it to please, please, please attach itself so it can survive. The uterus responds with dozens of hormones and immune signals itself. If it is satisfied with the quality of the chemical conversation its cells allow it. If it doesn’t the embryo will be lost and die.

Implantation is a highly collaborative process mostly mediated by the immune system of the mother. It is a tightly controlled invasion otherwise almost looking like a tumor invasion. The endometrium undergoes a series of changes, concerted mainly by the hormone progesterone, during the limited window of implantation, making it more likely for the embryo to attach. This is also referred to as endometrial receptivity. Endometrium also acts as a sensor for embryo quality, as more recent research suggests, but exact mechanisms are not yet fully known. It is also partly due to the difficulty of studying this period in vivo, which has made the first weeks of development almost a blackbox.

#Lacconi et al. When the Embryo Meets the Endometrium: Identifying the Features Required for Successful Embryo Implantation. Int J Mol Sci. 2024 Feb

https://www.mdpi.com/1422-0067/25/5/2834

Quote: “While embryo quality can be assessed using morphological and molecular parameters, the evaluation of proper endometrial competency/receptivity is more challenging. Due to its role in limiting embryo implantation, the concept of the endometrium as the guardian of pregnancy has been proposed [13]. Indeed, the embryo can efficiently implant in any tissue, independent of the stage of the cycle, with great invasion ability, while in the endometrium, it can only implant during a short period of time called the window of implantation (WOI) or window of endometrial receptivity [14,15]. In a 28-day normal cycle, the WOI occurs around 6–10 days after the LH surge and lasts about 3–6 days [14,16,17]. In cases of artificial cycles, the WOI occurs 4–7 days after the administration of progesterone [2]. The WOI is finely regulated by a plethora of factors, which include hormones, such as estrogen and progesterone, cytokines, and growth and immunomodulatory factors, all driving a series of morphological and molecular changes fundamental for a correct blastocyst-endometrial dialogue.”

#Kao LC, Tulac S, Lobo S, Imani B, Yang JP, Germeyer A, Osteen K, Taylor RN, Lessey BA, Giudice LC. Global gene profiling in human endometrium during the window of implantation. Endocrinology. 2002

https://pubmed.ncbi.nlm.nih.gov/12021176/
Quote: “Implantation in humans is a complex process that is temporally and spatially restricted. Over the past decade, using a one-by-one approach, several genes and gene products that may participate in this process have been identified in secretory phase endometrium. Herein, we have investigated global gene expression during the window of implantation (peak E2 and progesterone levels) in well characterized human endometrial biopsies timed to the LH surge, compared with the late proliferative phase (peak E2 level) of the menstrual cycle. Tissues were processed for poly(A) RNA and hybridization of chemically fragmented, biotinylated cRNAs on high density oligonucleotide microarrays, screening for 12,686 genes and expressed sequence tags. After data normalization, mean values were obtained for gene readouts and fold ratios were derived comparing genes up- and down-regulated in the window of implantation vs. the late proliferative phase. Nonparametric testing revealed 156 significantly (P < 0.05) up-regulated genes and 377 significantly down-regulated genes in the implantation window.”

#Moffett A, Colucci F. Uterine NK cells: active regulators at the maternal-fetal interface. J Clin Invest. 2014

https://pubmed.ncbi.nlm.nih.gov/24789879/

Quote: “The placenta is embedded within the decidua, the maternal component of the maternal-fetal interface. The decidua only exists during pregnancy and originates from the endometrial lining of the uterus (the endometrium). At the conclusion of pregnancy (parturition), the decidua is shed, to be rebuilt only upon subsequent pregnancy. However, signs of predecidualization can be observed within the nonpregnant human endometrium halfway through the luteal phase (around days 23 to 25 of the menstrual cycle), including increased prominence of the spiral arterioles, differentiation of endometrial fibroblast-like cells into enlarged and granulated decidual stromal cells, and an influx of leukocytes (1). Timing is critical for pregnancy to occur. Implantation must take place before this predecidualization because this thickening of the endometrium is not amenable for implantation.”

#Zhang S, Lin H, Kong S, Wang S, Wang H, Wang H, Armant DR. Physiological and molecular determinants of embryo implantation. Mol Aspects Med. 2013

https://pmc.ncbi.nlm.nih.gov/articles/PMC4278353/

Quote: “Successful implantation requires synchronization between the acquisition of implantation competency by the blastocyst and a receptive state in the uterine endometrium (Dey et al., 2004;Tranguch et al., 2005b; Wang and Dey, 2006). These two events are precisely regulated by maternal hormones, in particular, ovarian estrogen and progesterone (Conneely et al., 2002; Curtis Hewitt et al., 2002). Molecular and genetic evidence indicates that ovarian hormones together with locally produced signaling molecules, including cytokines, growth factors, homeobox transcription factors, lipid mediators and morphogen genes, function through autocrine, paracrine and juxtacrine interactions to specify the complex process of implantation (Dey et al., 2004). However, the hierarchical landscape of the molecular signaling pathways that govern embryo-uterine interactions during early pregnancy remains to be explored in depth.”

#Kim SM, Kim JS. A Review of Mechanisms of Implantation. Dev Reprod. 2017

https://pmc.ncbi.nlm.nih.gov/articles/PMC5769129/

Quote: “During early pregnancy, fetal trophoblast cells invade the uterus and penetrate the basement membrane, a property that is characteristic of malignant cells. However, unlike tumor invasion, trophoblast invasion of the uterus should be under strict control confining the placenta and within the time constraint of a pregnancy. Limitation of trophoblastic invasion is attributed to the balance of activating and inhibiting growth factors, cytokines, and enzymes. Decidual cells produce plasminogen activator inhibitor-1 (PAI-1) which is the major inhibitor of uPA (Schatz et al., 1995; Simón et al., 1996a). The tissue inhibitors of MMPs (TIMPs) tightly regulate the activities of MMPs. Decidual transforming growth factor (TGF)-β plays a major regulatory role in limitation of human trophoblast invasion by up-regulating both TIMPs and PAI-1 (Karmakar & Das, 2002). In addition, TGF-β provides antiproliferative signals to differentiate from invasive and proliferative cytotrophoblasts into non-invasive and multinucleated syncytiotrophoblasts at the human fetal-maternal interface (Graham et al., 1992). Decorin, a decidua-derived TGF-β binding proteoglycan, negatively regulates proliferation, migration, and invasiveness of human extravillous trophoblast cells in a TGFβ-independent manner (Iacob et al., 2008).”

#James JL, Lissaman A, Nursalim YNS, Chamley LW. Modelling human placental villous development: designing cultures that reflect anatomy. Cell Mol Life Sci. 2022

https://pubmed.ncbi.nlm.nih.gov/35753002/

Quote#1: “Within several hours of attachment, the blastocyst begins to invade into the decidua (Fig. 1). This is initially driven by proliferation and diferentiation of the polar trophectoderm, and by 8 dpc two primitive trophoblast lineages are evident; the primitive cytotrophoblast and the primitive syncytium (Fig. 1) [6]. The primitive syncytium is multinucleated and forms an expanding mass that enables invasion of the embryo via the expression of proteases that facilitate extracellular matrix (ECM) remodelling and degradation [12, 13]. By 9 dpc, the primitive syncytium expands to surround almost the entire implanted blastocyst, and the decidual epithelium heals over the top of the implantation site (Fig. 1) [6].”

Quote#2: “Villus development begins around 12 dpc, with proliferation of primitive cytotrophoblasts, forming expanding projections into the overlying primitive syncytium (Fig. 1) [6]. Around 14 dpc, these projections are invaded by cells from the extraembryonic mesoderm, forming the core of secondary villi [6, 14]. At the same time, primitive cytotrophoblasts continue to expand outwards from the tips of these initial villi, connecting to form the ‘cytotrophoblast shell’ that surrounds the villi, and takes over expansion of

the early placenta from the primitive syncytium [6]. Between 15 and 17 dpc, the mesenchymal core develops primitive endothelial tubes, laying the first foundations of the vascular network that will be required for the uptake of oxygen and nutrients, and these villi are then termed tertiary villi [6, 14]. Thus, in contrast to many other organs and tissues (including cancers) in which blood vessels develop first and tissue development follows, in placental development, villus growth precedes vascularisation [15, 16]. By the 4th week of gestation (28 dpc), the basic villous structure of the placenta is evident, consisting of a mesenchymal core containing fetal blood vessels, placental macrophages (Hofbauer cells), pericytes and connective tissue, surrounded by a bilayer of cytotrophoblasts and syncytiotrophoblast [6]. This mature syncytiotrophoblast is quite different to the primitive syncytium and is not invasive, but rather acts as the interface between the maternal circulation and the placenta.”

– This is where we begin to see that the interests of both living beings no longer completely align anymore. Pregnancy is a huge energy investment so the mother’s body will eject the embryo if it doesn’t see it as viable.

Generally for a pregnancy to reach the term, a healthy embryo, a receptive uterus and the crosstalk between the two are required. Recent research points out that the endometrium might be acting as a biosensor to evaluate the fitness of the embryo. Human embryos have a high rate of aneuploidy, having the wrong number of chromosomes, due to commonplace errors in the early cell divisions in the embryo. This can hinder communication between the uterus and mother’s immune cells and embryo, leading to miscarriages. Since aneuploidy can lead to severe health conditions if it reaches term, it is of great importance that the endometrium can sense the fitness of the embryo.

#Macklon NS, Brosens JJ. The human endometrium as a sensor of embryo quality. Biol Reprod. 2014

https://pubmed.ncbi.nlm.nih.gov/25187529/

Quote: “Similar to its previously proposed role in the bovine species [47], the decidualized human endometrium is emerging as an active gatekeeper to implantation in the human. This novel model requires further validation but addresses a number of imperatives characteristic of human reproduction. The remarkable prevalence of aneuploidy in human embryos demands maternal investment in effective means of preventing invasive but chromosomally chaotic embryos from establishing a clinical pregnancy destined to fail. Menstruation, a process triggered by spontaneous decidualization of the endometrium in an embryo-independent manner and almost unique to humans among mammalian species, may have emerged as a strategy for early detection and active rejection of developmentally abnormal embryos that have breached the luminal epithelium. At the same time, cyclic regeneration of the endometrium provides a mechanism to continuously rebalance the receptivity and selectivity traits of the endometrium, thus increasing the likelihood of reproductive success.”

#Hassold T, Hunt P. To err (meiotically) is human: the genesis of human aneuploidy. Nat Rev Genet. 2001 https://www.researchgate.net/publication/12049418_To_ERR_meiotically_is_human_The_genesis_of_human_aneuploidy

Quote: “Dosage imbalance of whole chromosomes typically results in inviability. So, it is not surprising that, in most organisms, meiotic non-disjunction is a rare occurrence. In the yeast Saccharomyces cerevisiae, for example, the likelihood of an individual chromosome mal-segregating during meiosis is as low as 1 in 10,000 (for example, see REF. 1). Similarly, in Drosophila melanogaster, estimates of X-chromosome non-disjunction in the female

germ line range from ~1 in 1,700 to ~1 in 6,000 (REF. 2) and autosomal non-disjunction is probably as rare3. In mammals, the frequency of meiotic errors seems to be higher; nevertheless, in the organism that has been best studied (the mouse), the overall incidence of aneuploidy (trisomy or monosomy) among fertilized eggs does not exceed 1–2%(REF. 4).

Our species provides a notable exception to this general rule. An estimated 10–30% of fertilized human eggs have the ‘wrong’ number of chromosomes, with most of

these being either trisomic or monosomic. This has profound clinical consequences: approximately one-third of all miscarriages are aneuploid, which makes it the leading known cause of pregnancy loss and, among conceptions that survive to term, aneuploidy is the leading genetic cause of developmental disabilities and mental retardation.”

– The embryo on the other hand faces a life and death situation and tries to stay alive at any price. So it’s not asking politely but deploying thousands of infiltration units: Bubbles filled with genetic material, kind of like a human virus! They sort of invade the uterine cells and try to brainwash them, so they help it attach itself, rather than rejecting it.

Another tool to mediate the chemical communication between the mother and the embryo is the extracellular vesicles. These can have different functions and can be filled with hormones, bits of genetic information or other molecules. They are secreted bidirectionally both by the endometrium and the embryo. The following review paper discusses the different types and functions.

#Poh et al. Omics insights into extracellular vesicles in embryo implantation and their therapeutic utility. Proteomics. 2023.

https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/pmic.202200107
Quote: “Pre-implantation embryos release EVs both in vitro [33, 34, 53, 125, 149] (from spent media collected from incubated embryos) and in vivo [150], and act in both autocrine (on trophoblast) [151] and paracrine (on endometrium) [53] fashion. The abundance and composition of EVs are modulated during pregnancy [152], highlighting their contribution in conceptus-endometrial interactions during this process [57, 81, 153, 154]. Indeed, a proportion of EVs identified in the uterine lumen during early pregnancy [55] have been shown to be uniquely contributed by the foetus [34, 56, 57, 150, 154], and levels of placental EVs increase in human maternal circulation across gestation [155]. Embryo-derived EVs can interact with various cell types associated with the uterus (e.g., endometrium, immune [156] cells) to modulate processes implicated in receptivity [84, 157, 158] in addition to remodelling the extracellular matrix [158, 159]—a critical network in supporting the development and implantation competence of human embryos [160]. Because of the very small quantity of EVs released by an embryo, cells of trophectodermal [84, 157-159] or embryonic [33, 34, 53, 129] origin remain the most widely used approaches to understand their composition and function (Table 2).”

Figure Caption: "Extracellular vesicles (EVs) and their contribution to functional hallmarks of embryo implantation. The endometrium/uterus and embryo releases EVs containing bioactive molecules that drive biological processes determining implantation success. These include promotion of embryo hatching, determining the site of implantation by inducing localized responses in the endometrium, and embryo development, attachment, and invasion. EVs also modulate the uterine environment during embryo implantation, and have demonstrated immune regulation, antioxidant activity, and arterial remodelling capabilities. Created with BioRender. HSPG2: Heparan sulfate proteoglycan 2, CD55: Complement decay-accelerating factor, CD47: Cluster of Differentiation 47, DPP3: Dipeptidyl peptidase 3, FAK: Focal adhesion kinase, FN1: Fibronectin, ITGβ: Integrin subunit beta 3, ITGα1: Integrin subunit alpha 1, CDH5: Cadherin 5, EMMPRIN: Extracellular matrix metalloproteinase inducer, LGALS1: Galectin 1, LGALS3: Galectin 3, S100A4: S100 calcium binding protein A4, S100A11: S100 calcium binding protein A11, SERPINC1: Antithrombin 3, PIBF: Progesterone induced blocking factor, HSPE1: Heat shock protein family E (Hsp10) member 1, ROS: Reactive oxygen species, SOD1: Superoxide dismutase 1, CAT: Catalase, PRDX2: Peroxiredoxin 2"

Quote: “In addition to transiently available markers such as maternal antibodies—which wane 4–6 months after birth—a new layer of complexity within the feto-maternal “communicatome” arose from the discovery of pregnancy-associated chimerism (Owen, 1945; Billingham et al, 1953). Chimerism is defined as the presence of genetically distinct cells or DNA originating from another individual. During pregnancy, chimerism occurs naturally, opposed to e.g., chimerism induced by blood transfusion or transplantation (Mathe et al, 1963). The currently available detection methods of chimeric cells in mother and fetus indicate a frequency of less than one percent of all cells (Hall et al, 1995; Nelson, 2012; Stelzer et al, 2021). Hence, the term microchimeric cells (MC) was coined. Intriguingly, not only cells but also extracellular vesicles (EVs) can be vertically transferred between mother and fetus and add to pregnancy-associated chimerism. EVs are lipid-membrane-bound particles in the size range of nano- to micrometer and continuously secreted by a wealth of cell types into the extracellular space (Smith et al, 1974; Knight et al, 1998; Théry et al, 2018; Sheller-Miller et al, 2019).”

Quote#2: “As mentioned above, EVs can be vertically transferred between mother and fetus, hereby contributing to pregnancy-associated chimerism. EVs are spherical nano-scaled particles, enclosed by a lipid bilayer harboring biomolecules, and can be continuously secreted by almost every cell type into the extracellular space (Smith et al, 1974; Théry et al, 2018). Fetal tissues—such as the placenta—release EVs into the maternal circulation (Knight et al, 1998; Redman and Sargent, 2000; Sarker et al, 2014; Tong and Chamley, 2018). Here, EVs—together with FMc—contribute to the microchiome. Conversely, EVs derived from maternal tissues can also reach the fetus, similar to MMc (Sheller-Miller et al, 2019; Kaisanlahti et al, 2023).”

– During this time one of the weirdest features of pregnancy occurs: the uterus provides uterine milk, not real milk but a clear fluid filled with nutrients and hormones that the young embryo sucks up hungrily to gain additional energy until it gets inside.

In the early stage of pregnancy, the intervillous space is filled with a clear fluid containing uterine gland secretions, which are phagocytosed by the syncytiotrophoblast and serve as a nutrient source for the developing fetus. Among their varied secretions are growth factors that regulate placental development, including epidermal growth factor, vascular endothelial growth factor (VEGF), transforming growth factor beta (TGF-β), and leukemia inhibitory factor. So until the placenta, therefore the full blood connection, is fully established, most nutrition does not happen through blood. This might be happening for a good reason though.

There are villous structures initially but there are mostly secretions in the intervillous space instead of blood. Also in the first trimester the environment is hypoxic, and actually oxygen at this stage can be damaging to the embryo. If they are exposed to oxygen at this point, they won't be able to grow a villus tree properly.

The invasive activity of the extravillous trophoblast cells is at a maximum during the first trimester of gestation, peaking at around 10-12 weeks and declining thereafter. It might be that the low oxygen environment keeps trophoblasts to differentiate into their invasive form. Before the 10th week the oxygen level in the uterus is similar to nonpregnant uterus.

#Burton GJ, Watson AL, Hempstock J, Skepper JN, Jauniaux E. Uterine glands provide histiotrophic nutrition for the human fetus during the first trimester of pregnancy. J Clin Endocrinol Metab. 2002

https://pubmed.ncbi.nlm.nih.gov/12050279/

Quote: “AMONG EUTHERIAN MAMMALS, two principal pathways have evolved to transfer nutrients from the mother to her fetus. These are termed histiotrophic and hemotrophic, respectively (1). Histiotroph is an extracellular material derived from the endometrium and the uterine glands that accumulates in the space between the maternal and fetal tissues. It is phagocytosed initially by the trophectoderm of the blastocyst, and later by the trophoblast of the placenta or the endoderm of the yolk sac. By contrast, hemotrophic nutrition is the exchange of blood-borne materials between the maternal and fetal circulations. This is facilitated by the extensive and intimate apposition of the maternal and fetal tissues that occurs within the placenta. Before implantation, nutrition of the mammalian conceptus is therefore essentially histiotrophic. Once the placenta is established, hemotrophic nutrition becomes predominant, although the two pathways may coexist for much of gestation in certain species (1).”

#Gauster M, Moser G, Wernitznig S, Kupper N, Huppertz B. Early human trophoblast development: from morphology to function. Cell Mol Life Sci. 2022 https://pmc.ncbi.nlm.nih.gov/articles/PMC9167809/#Sec1
Quote: ”Implantation and initial syncytium formation. A The blastocyst is attached to the uterine epithelium. This epithelium is covered by a massive glycocalyx. At the site of blastocyst attachment, the pinopodes of the uterine epithelial cells allow direct interaction between these cells and the trophectoderm cells of the embryonic pole of the blastocyst, bypassing the glycocalyx. B At the embryonic pole of the blastocyst, the trophectoderm cells start to proliferate and form multiple cell layers of daughter cells, the primitive cytotrophoblasts. Some of the primitive cytotrophoblasts further differentiate and fuse with each other to generate the primitive syncytiotrophoblast, a multinucleated trophoblast structure. Only this primitive syncytiotrophoblast seems to be able to penetrate the uterine epithelium, allowing invasion into the uterine stroma. C By means of the invading primitive syncytiotrophoblast, the embryo has passed the uterine epithelium and is now fully embedded in the uterine stroma. The primitive syncytiotrophoblast fully surrounds the underlying mono-nucleated cytotrophoblasts as well as the embryo and thus is the only embryonic layer in contact with maternal tissues. Already at this stage of placental development, the primitive syncytiotrophoblast invades into a uterine gland to allow nutritive support of the embryo using glandular secretion products, called uterine milk”

#Caniggia I, Winter J, Lye SJ, Post M. Oxygen and placental development during the first trimester: implications for the pathophysiology of pre-eclampsia. Placenta. 2000

https://pubmed.ncbi.nlm.nih.gov/10831118/

Quote: “During early pregnancy, placentation occurs in a relatively hypoxic environment which is essential for appropriate embryonic development. Intervillous blood flow increases at around 10-12 weeks of gestation and results in exposure of the trophoblast to increased oxygen tension (PO2). Prior to this time, low oxygen appears to prevent trophoblast differentiation towards an invasive phenotype. In other mammalian systems, oxygen tension effects are mediated by hypoxia inducible factor-1 (HIF-1). We found that the ontogeny of HIF-1alpha subunit expression during the first trimester of gestation parallels that of transforming growth factor-beta3 (TGFbeta3), an inhibitor of early trophoblast differentiation. Expression of both molecules is high in early pregnancy and falls at around 10 weeks of gestation when placental PO2 levels are believed to increase.”

#Massimiani M, Lacconi V, La Civita F, Ticconi C, Rago R, Campagnolo L. Molecular Signaling Regulating Endometrium-Blastocyst Crosstalk. Int J Mol Sci. 2019

https://pmc.ncbi.nlm.nih.gov/articles/PMC6981505/

Quote: “Endometrial glands produce and secrete a cocktail of molecules, the histotroph, including amino acids, glucose and growth factors, which appear to be involved in embryo survival, trophectoderm activation, endometrial invasion and nourishment of the implanted embryo [48,49,50,51,52,53,54,55,56]. Leukemia inhibitory factor (LIF) and vascular endothelial growth factor (VEGF) are produced by uterine glands [57].”

– If the embryo is allowed in, the implantation was successful, the first critical hurdle. The next step is to reach the blood vessels of its mother, the only way to survive if it doesn't want to starve. The trophoblasts begin to clone themselves on a massive scale and turn into different specialists. One of them turns into a violent invader that starts drilling into the uterine tissue like a tiny parasitic octopus with many arms, spreading and growing. This is a brutal process, ordering loads of it’s mom’s uterus cells to destroy themselves, killing others directly or even devouring some cells whole.

There are different types of trophoblast that differentiate from the initial trophoblast stem cell population. A single layer of syncytiotrophoblasts (SYN) outlines the outermost surface of the villous trees and acts as the main cellular barrier between the fetal compartment and maternal blood. Underlying the SYN layer are the undifferentiated, mononucleated cytotrophoblasts (CTBs). CTBs are progenitor cells and can fuse to replenish the SYN layer or differentiate into mononucleated extravillous trophoblasts (EVTs), which are located at the tips of the anchoring villi. EVTs are the invasive type that migrate out of the placenta and invade into the decidua.

#Galán A, O'Connor JE, Valbuena D, Herrer R, Remohí J, Pampfer S, Pellicer A, Simón C. The human blastocyst regulates endometrial epithelial apoptosis in embryonic adhesion. Biol Reprod. 2000

https://pubmed.ncbi.nlm.nih.gov/10906047/

Quote: “Cytotrophoblasts at the tips of anchoring villi primed for EVT differentiation (EVT progenitors) undergo a carefully orchestrated programme of differentiation, whereby they progressively gain migratory and invasive capacities [34]. EVT progenitors first differentiate into proximal cell column trophoblasts (pCCTs), identified by expression of NOTCH-1, EGFR and CCR1, that retain their proliferative capacity and adhere to neighbouring pCCTs as they migrate away from the villus in columns [17, 35–37].
[...]
Within the decidua, EVT can be further distinguished into distinct subtypes. The two most studied of these are interstitial trophoblasts that are found in the within the decidual stroma, or endoarterial trophoblasts that colonise the uterine spiral arteries and facilitate their remodelling into wide bore conduits that do not respond to maternal vasoconstrictive stimuli. This process of spiral artery remodelling is an important physiological adaptation to reduce the velocity of blood entering the intervillous space and ensure optimal placental perfusion [41–43]. Endoarterial trophoblasts also form loose ‘plugs’ in the spiral arteries during the frst trimester, which aid in limiting the flow of oxygenated maternal blood to the intervillous space and create a physiologically normal low oxygen environment favourable for early placental development [43]. In the absence of significant blood flow, histotrophic nutrition derived from

glandular secretions provides an important source of nourishment for the developing embryo in early pregnancy [44].

Finally, EVTs are also found in the uterine lymphatic vessels (endolymphatic trophoblast), as well as the uterine veins (endovenous trophoblast) where they are thought to play key roles in eroding these vessels such that they open to the intervillous space to complete this circulation [45].”

#Gauster M, Moser G, Wernitznig S, Kupper N, Huppertz B. Early human trophoblast development: from morphology to function. Cell Mol Life Sci. 2022 https://pmc.ncbi.nlm.nih.gov/articles/PMC9167809/#Sec6
Quote: “As outlined above, primary villi start to emerge from day 13 p.c. and consequently result in the formation of primitive villous trees. Some stems of those villous trees remain attached to the maternal side, where they are called anchoring villi. This villous type does not only facilitate mechanical stability but is also the source for the extravillous trophoblast populations [54, 55]. The connection between an anchoring villus and the uterine decidua basalis is characterized by a cell column of mononuclear extravillous trophoblasts (EVTs). At the very tip of anchoring villi, there is no covering syncytiotrophoblast. Hence, proliferation of cytotrophoblasts results in the formation of multiple layers of daughter cells, called trophoblast cell column. During the first trimester of pregnancy, some of the cell columns still seem to have a surrounding syncytiotrophoblast layer [55].”

https://personalpages.manchester.ac.uk/staff/j.gough/lectures/the_cell/diffdev1/page5.html

#Greenbaum S et al. A spatially resolved timeline of the human maternal-fetal interface. Nature. 2023
https://pmc.ncbi.nlm.nih.gov/articles/PMC10356615/#Sec7

Quote: “Decidualization is a fascinating process with no other normative precedent in human biology. In this process, the structure and function of the maternal endometrium transforms to promote the regulated invasion of genetically dissimilar fetal cells. The decidua plays a dual role by permitting EVT invasion in the first trimester and later limiting it by inducing EVT apoptosis48. EVT invasion can also be limited by morphological changes such as EVT fusion, which leads to polyploidization that limits invasion owing to nuclear size49. Given the lack of tractable and relevant animal models and the inability to study decidualization prospectively, our understanding of it is immature relative to other areas of human physiology. Therefore, our study aimed to understand how global, temporally dependent changes in decidual composition are coupled to local regulation of vascular remodelling in pregnancy. Initial invasion of placental EVTs is prompted by a shift towards a permissive milieu, whereas progression of SAR depends on the subsequent migration and perivascular accumulation of EVTs, where they are thought to participate in cooperative cell–cell interactions with maternal fibroblasts, NK cells and macrophages2. Thus, the formation of the maternal–fetal interface is mediated by global, temporally dependent cues that serve as a gating function for remodelling processes that are regulated in the local tissue microenvironment.”

#von Rango U, Krusche CA, Kertschanska S, Alfer J, Kaufmann P, Beier HM. Apoptosis of extravillous trophoblast cells limits the trophoblast invasion in uterine but not in tubal pregnancy during first trimester. Placenta. 2003

https://pubmed.ncbi.nlm.nih.gov/14580375/

Quote: “During the first trimester of uterine pregnancy extravillous trophoblast cells (EVT) invade the maternal decidua beyond the endometrial-myometrial border (Benirschke and Kaufmann, 2000), normally until the inner third of the myometrium. The extent of this invasion is tightly regulated and declines from first trimester of pregnancy to term (Kemp et al., 2002). The control mechanisms regulating the invasion depth are still unknown. Either a basic genetic program of EVT stem cells, or the maternal decidua should influence the extent of invasion (Morrish, Dakour and Li, 1998; Bischof, Meisser and Campana, 2000).”

#Boeddeker SJ, Hess AP. The role of apoptosis in human embryo implantation. J Reprod Immunol. 2015

https://pubmed.ncbi.nlm.nih.gov/25779030/

Quote: “Human endometrium undergoes regular cyclic changes throughout each menstrual cycle scheduling for embryo implantation, and in its absence, for menstruation. Both processes are accompanied by cellular mitosis, differentiation, and apoptosis. Embryo implantation into a receptive endometrium is the pivotal process in early pregnancy influencing the course of pregnancy tremendously. The embryo–maternal dialog, which is pivotal for successful implantation and subsequent pregnancy, is conducted via a broad spectrum of factors including secreted cytokines and chemokines in addition to the expression of their corresponding receptors and co-receptors. After leaving its protective zona pellucida, the embryonic trophoblast facilitates contact with the endometrium and starts penetration of the endometrial epithelium leading to complete apoptosis of the epithelial layer in a locally determined fashion, followed by invasion into the endometrial stroma and the inner third of the myometrium, which seems to be rather distinctly regulated in space (Cohen et al., 2010). Disturbance of this tightly regulated procedure can lead to inadequate implantation, which in turn enables the occurrence of several diseases such as intrauterine growth restriction (IUGR), HELLP syndrome, and/or preeclampsia (Pijnenborg et al., 2008).”

#Li Y, Sun X, Dey SK. Entosis allows timely elimination of the luminal epithelial barrier for embryo implantation. Cell Rep. 2015

https://pubmed.ncbi.nlm.nih.gov/25865893/

Quote: “During implantation, uterine luminal epithelial (LE) cells first interact with the blastocyst trophectoderm. Within 30 hr after the initiation of attachment, LE cells surrounding the blastocyst in the implantation chamber (crypt) disappear, allowing trophoblast cells to make direct physical contact with the underneath stroma for successful implantation. The mechanism for the extraction of LE cells was thought to be mediated by apoptosis. Here, we show that LE cells in direct contact with the blastocyst are endocytosed by trophoblast cells by adopting the nonapoptotic cell-in-cell invasion process (entosis) in the absence of caspase 3 activation. Our in vivo observations were reinforced by the results of co-culture experiments with primary uterine epithelial cells with trophoblast stem cells or blastocysts showing internalization of epithelial cells by trophoblasts. We have identified entosis as a mechanism to remove LE cells by trophoblast cells in implantation, conferring a role for entosis in an important physiological process.”

#Scheliga I et al. Dead or Alive: Exploratory Analysis of Selected Apoptosis- and Autophagy-Related Proteins in Human Endometrial Stromal Cells of Fertile Females and Their Potential Role During Embryo Implantation. Int J Mol Sci. 2024

https://pmc.ncbi.nlm.nih.gov/articles/PMC11720002/pdf/ijms-26-00175.pdf

Quote: “The data suggest that apoptosis and autophagy are involved in the decidua’s response to decidualization, potentially impacting early pregnancy success. The upregulation of apoptosis receptors and increased autophagy proteins in primary ESCs during decidualization indicate their essential roles in preparing the endometrium for embryo reception. Furthermore, there appears to be an interaction between apoptosis and autophagy, which is likely influenced by Sdc1.”

– While this sounds brutal, and it is, this process is highly regulated and doesn’t hurt the mom. It is another test. The mom’s body monitors carefully how the embryo is doing – is it growing quickly or if it is more chill. If an embryo has genetic damage or chromosomal abnormalities it will spend way more energy on repairing itself and maybe grow more erratically. This makes it metabolically noisy, releasing loads of chemicals the mom’s immune cells pick up. Which provokes them and makes it more likely that they will destroy it. On the other hand, if the embryo is weak it is metabolically too quiet and the mom will stop talking to it – which also ends the pregnancy.

#Macklon NS, Brosens JJ. The human endometrium as a sensor of embryo quality. Biol Reprod. 2014

https://pubmed.ncbi.nlm.nih.gov/25187529/

Quote: “The mechanism underlying this novel concept of human embryo selectivity was unclear and difficult to study directly. To address these challenges, a study was designed in which decidualizing cells were exposed for 12 h to pooled conditioned medium from human embryos deemed insufficient for uterine transfer and high-quality embryos that resulted in an

on-going pregnancy after single embryo transfer, thus providing unequivocal proof of their developmental competence. A third pool of unconditioned embryo culture media was applied

as a control. Genomewide expression profiling uncovered only 15 decidual genes responsive to soluble signals from competent human embryos. In contrast, and consistent with the previously observed cytokine response [23], 449 maternal genes were deregulated in response to medium conditioned by poor-quality embryos. Gene ontology studies showed half of these genes to be associated with the broad biological process of transport, translation, and cell cycle regulation [14].
This concept of the decidualized endometrium responding more profoundly to the developmentally incompetent than the competent embryo is consistent with previous studies asserting that the healthy embryo, needing only to invest energy in growth and development, is metabolically quiet in comparison to less viable embryos that must additionally engage in repair and apoptosis [24–26]. This concept introduces the possibility that an embryo may be so quiet that it fails to trigger any supportive maternal response, a hypothesis that would be

consistent with the conjecture that some degree of aneuploidy may confer certain advantages to the implanting embryo. The converse notion to that of the high-quality, quiet embryo is that of the struggling, metabolically noisy embryo producing a source of putative signals to which the decidualized stroma responds by deregulating a repertoire of genes implicated in implantation.”

#Moffett A, Shreeve N. Local immune recognition of trophoblast in early human pregnancy: controversies and questions. Nat Rev Immunol. 2023
https://pmc.ncbi.nlm.nih.gov/articles/PMC9527719/#Sec6

Quote: “The leukocyte population in endometrium and decidua in the first few weeks of human pregnancy is dominated by distinctive NK cells specific to this location. In addition, despite the intrusive nature of human placentation deep into the uterine mucosa, an inflammatory response is not seen.
[...]
Breaching a mucosal barrier, as occurs during implantation and trophoblast invasion of the decidua, would be predicted to activate an inflammatory response. The endometrium is receptive to implantation during a window of only 3–6 days in the mid-secretory phase, and a common view is that the receptive endometrium is an inflammatory environment28,29. However, mast cells, which are a key trigger of inflammatory responses, are not present in the functional layer of the endometrium or decidua, although they do populate the myometrium30. Despite some reports to the contrary, neutrophils, which would be the hallmark of an acute inflammatory response and are easily and definitively identified by histology alone31,32, are present only when the endometrium breaks down at menstruation and miscarriage33,34. In decidua, neutrophils are confined to the narrow zone of necrosis —

Nitabuch’s layer — at the boundary between the anchoring villi and superficial decidua basalis. Macrophages are constantly present in the endometrium and decidua, and a recent report using imaging mass cytometry has defined several subsets of macrophages (including an HLA-DRnegative subset present in first-trimester decidua basalis) and their distribution throughout gestation35”

– This embryo is just right, so the mom’s cells release a flood of different chemicals to help it out, support it growing and most importantly, they activate her immune system. Usually this would be very bad – the immune system kills everything that is not part of the mom’s body, and this embryo is clearly not part of her. But the immune cells of the uterus surround the embryo and start helping it, guiding the trophoblast to grow further. They create a physical and chemical safe zone that tells dangerous immune cells like T Cells to stay away.

The uterus has a special population of Natural Killer Cells that are not cytotoxic unlike the peripheral ones. They regulate trophoblast invasion and spiral artery remodelling in the first trimester, produce factors that stimulate blood vessel formation and attract fetal derived placental cells called extravillous trophoblast and are typically most active around the time of implantation and the first trimester of pregnancy.

#Moffett A, Shreeve N. Local immune recognition of trophoblast in early human pregnancy: controversies and questions. Nat Rev Immunol. 2023
https://pmc.ncbi.nlm.nih.gov/articles/PMC9527719/#Sec6

Quote: “The leukocyte population in endometrium and decidua in the first few weeks of human pregnancy is dominated by distinctive NK cells specific to this location.
The composition of leukocytes changes in the endometrium throughout the menstrual cycle and in the decidua throughout gestation19–25. Following ovulation in the secretory phase of the menstrual cycle, ~20% of CD45+ cells in the non-pregnant endometrium are macrophages and ~10% are T cells. Only 1% of CD45+ cells are dendritic cells, and sparse B cells are present in basal lymphoid aggregates. The main population of CD45+ cells in endometrium and early decidua are specialized CD56bright uNK cells, which account for ~70% of leukocytes21. By contrast, at term, uNK cells account for less than 50% of leukocytes in the endometrium, with the number of T cells having increased proportionally26,27. Thus, during early pregnancy when the boundary between the decidua and the placenta forms, uNK cells and macrophages, rather than adaptive immune cells, dominate the decidual immune landscape, although their relative importance seems to decline as gestation proceeds. A recent detailed study by mass cytometry described the decidual leukocytes that are present throughout gestation, excluding blood leukocytes26, which is particularly important when one is interpreting studies of cell isolates from term decidua, as contaminating maternal and fetal blood leukocytes will inevitably be present. In addition, samples taken after delivery are affected by the inflammatory events characteristic of labour and birth. By contrast, there is little evidence that a classic inflammatory response occurs early in pregnancy during placentation despite the large numbers of innate immune cells.”

#Koch CA, Platt JL. T cell recognition and immunity in the fetus and mother. Cell Immunol. 2007.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2703468/

Quote: "The Fas/FasL pathway is also thought to be important for the control of maternal immune responses at the fetal-maternal interface. Apoptosis can be detected at the fetal-maternal interface throughout gestation [27–29]. This apoptosis might reflect the normal turnover of cells in an area of high cellular proliferation and growth; however, it could also represent a mechanism to prevent the maternal immune system from attacking the fetus. The expression of FasL mRNA can be detected at the fetal-maternal interface as early as 6 days post-conception in mice and as early as 12 days post-conception in humans [30, 31]. Coumans et al. [32] found that human CD3+ T cells undergo apoptosis when cultured with human trophoblast due to the expression of FasL. Vacchio and Hodes [33] studied the role of Fas/FasL in the maternal response to the H-Y antigen using TCR transgenic mice. The authors found that 50% of the H-Y specific T cells were deleted in mothers carrying male fetuses; however, when the mothers were deficient in Fas the deletion of H-Y reactive T cells did not occur. These results suggest that Fas/FasL interactions are necessary for the deletion of maternal T cells reactive to the fetus; however, no increase in fetal abortion was reported. The results of Vacchio and Hodes [33] as well as the fact that gld mice, deficient in FasL, and lpr mice, deficient in Fas, are still able to reproduce suggests that Fas/FasL is not critical to the survival of allogeneic fetuses."

#Moffett A, Colucci F. Uterine NK cells: active regulators at the maternal-fetal interface. J Clin Invest. 2014

https://pubmed.ncbi.nlm.nih.gov/24789879/

Quote: “Pregnancy presents an immunological conundrum because two genetically different individuals coexist. The maternal lymphocytes at the uterine maternal-fetal interface that can recognize mismatched placental cells are T cells and abundant distinctive uterine NK (uNK) cells. Multiple mechanisms exist that avoid damaging T cell responses to the fetus, whereas activation of uNK cells is probably physiological. Indeed, genetic epidemiological data suggest that the variability of NK cell receptors and their MHC ligands define pregnancy success; however, exactly how uNK cells function in normal and pathological pregnancy is still unclear, and any therapies aimed at suppressing NK cells must be viewed with caution. Allorecognition of fetal placental cells by uNK cells is emerging as the key maternal-fetal immune mechanism that regulates placentation.”

There are several mechanisms that induce tolerance in the maternal immune system.

#Ander SE, Diamond MS, Coyne CB. Immune responses at the maternal-fetal interface. Sci Immunol. 2019

https://pubmed.ncbi.nlm.nih.gov/30635356/

Quote: “Maternal tolerance also may occur through species-specific mechanisms. In humans, placental EVTs express HLA-G, a nonclassical major histocompatibility complex (MHC) molecule, for which there is no homolog in the mouse genome. Unlike the canonical class I MHC molecules, of which exist thousands of allelic variations that serve to distinguish self from nonself, there are only 16 protein variants of HLA-G (66). Solely expressed by EVTs, HLA-G binds to dNK inhibitory receptors KIR2DL4 (67) and LILRB (68) to protect the trophoblasts from NK-mediated cytolysis (69).”

#Joo JS, Lee D, Hong JY. Multi-Layered Mechanisms of Immunological Tolerance at the Maternal-Fetal Interface. Immune Netw. 2024

https://pubmed.ncbi.nlm.nih.gov/39246621/

Quote: “Among MHC class I molecules, non-classical HLA-G Ag is specifically expressed by trophoblasts during gestation (34,35). In fact, trophoblasts are devoid of most of MHC Ags but EVTs which invade maternal decidual tissue and encounter maternal immune cells express high levels of HLA-G (35). HLA-G in trophoblasts is present as a homodimer that preferentially binds Ig-like transcripts (ILTs) on decidual APCs (36). This interaction induces a regulatory phenotype in decidual myelomonocytic cells, suppressing T cell responses and impairing Ag presentation via MHC class II molecules (37). NK cells also express other inhibitory receptors for HLA-G Ag such as killer cell Ig-like receptor (KIR) 2DL4 (38). Thus, the specific expression of HLA-G with the absence of classical MHC class I molecules, HLA-A and HLA-B in EVTs promotes tolerogenic microenvironment in maternal-fetal interface. Alongside HLA-G expression, EVTs express other MHC class I molecules HLA-C and HLA-E. Unlike HLA-A and HLA-B, which are major TCR ligands, HLA-C in EVTs does not activate maternal T cells but binds KIRs expressed on uterine NK cells, thereby promoting EVT function (39,40). Furthermore, while expression of MHC class I proteins is regulated by NOD-like receptor family CARD domain containing 5 (NLRC5), the master regulator of MHC class I molecules, expression of HLA-C is regulated by transcription factor E74 like ETS transcription factor 3 (ELF3) independently of NLRC5 (41). Expanding on these findings to clinical relevance, the expression of HLA-G in placenta and its release into maternal circulation is impaired in PE patients while expression of HLA-B is significantly elevated in these patients (42,43).”

– The embryo has its own motives and doesn't want to rely on its mothers goodwill alone – so defensive trophoblasts send out signals that kill her immune cells if they get too close and could start attacking it.

#Ayala-Ramírez P et al. Assessment of Placental Extracellular Vesicles-Associated Fas Ligand and TNF-Related Apoptosis-Inducing Ligand in Pregnancies Complicated by Early and Late Onset Preeclampsia. Front Physiol. 2021

https://pmc.ncbi.nlm.nih.gov/articles/PMC8342945/

Quote: “Preeclampsia (PE) is a hypertensive disorder that affects 2–8% of pregnancies and is one of the main causes of fetal, neonatal, and maternal mortality and morbidity worldwide. Although PE etiology and pathophysiology remain unknown, there is evidence that the hyperactivation of maternal immunity cells against placental cells triggers trophoblast cell apoptosis and death. It has also been reported that placenta-derived extracellular vesicles (EV) carry Fas ligand (FasL) and Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and trigger apoptosis in Jurkat T cells. This study aimed to quantify and compare FasL and TRAIL expression in EV derived from cultures of placenta explants from women with PE (early versus late) and women with uncomplicated pregnancies. Also, the study assessed EV capacity to induce apoptosis in Jurkat T cells. The authors isolated EV from placenta explant cultures, quantified FasL and TRAIL using ELISA, and analyzed EV apoptosis-inducing capability by flow cytometry. Results showed increased FasL and TRAIL in EV derived from placenta of women with PE, and increased EV apoptosis-inducing capability in Jurkat T cells. These results offer supporting evidence that EV FasL and TRAIL play a role in the pathophysiology of PE.”

– Hundreds of cells go even further and leave the embryo behind. They spread all across the mom’s body, entering organs, even the brain. We don’t know what all of these cells are doing but we think that they are probably telling her immune system that the embryo is not to be attacked. That it should be left alone or even protected. These cells may stay inside the mother for years or even decades. It is likely that parts of you are still in your mother.

#Graf I, Urbschat C, Arck PC. The 'communicatome' of pregnancy: spotlight on cellular and extravesicular chimerism. EMBO Mol Med. 2024 https://pmc.ncbi.nlm.nih.gov/articles/PMC11018796/pdf/44321_2024_Article_45.pdf
Quote:”The presence of fetal cells in the maternal organism is referred to as fetal microchimerism. Vice versa, the presence of maternal cells in the fetus is termed maternal microchimerism. Both, fetal microchimeric cells (FMc) and maternal microchimeric cells (MMc) can not only be detected during pregnancy, but also long after birth in the respective host (Maloney et al, 1999; O’Donoghue, 2008). Intriguingly, pregnancy-acquired microchimerism can create a pool of genetically foreign cells within an organism. For example, the pool of FMc increases with each subsequent pregnancy in the maternal body, hereby expanding the diversity of FMc in the mother. In consequence, younger siblings

are considered to receive a more diverse composition of MMc, which not only includes maternal cells, but can also contain cells from older siblings, or their grandmother and possibly even elder siblings of their mother (uncles, aunts) (Kinder et al, 2017; Shao et al, 2023). This pool of trans- and intergenerational microchimeric cells within an organism has been termed microchiome.
[...]

Hematopoietic and mesenchymal stem cells contribute to the majority of the FMc pool during pregnancy in the maternal bloodstream (Bianchi, 1999; Osada et al, 2001). Hence, fully differentiated fetal cells that can be detected in maternal tissues years or even decades after pregnancy may have differentiated from fetal-derived stem cells (Johnson et al,2002; O’Donoghue, 2008). However, their functional role is intensely debated and potential functions cover a broad repertoire of functions as depicted in Fig. 4, ranging from advantageous health effects such as tissues repair, to disadvantageous health consequences, e.g., autoimmune reactions and the progression of cancer (Lambert and Lee Nelson, 2003; Gilmore et al, 2008; Boddy et al, 2015; Sedov et al, 2022). The latter is supported by the detection of FMc in various malignant tumors, e.g., in the breast, colon, brain, lung, thyroid, cervix and skin (Cirello et al, 2008; Dubernard et al, 2008; Broestl et al, 2018).”

#Kinder JM, Stelzer IA, Arck PC, Way SS. Immunological implications of pregnancy-induced microchimerism. Nat Rev Immunol. 2017

https://pubmed.ncbi.nlm.nih.gov/28480895/

Quote: “Early studies that used various methods (TABLE 1) to identify fetal microchimeric cells that express unique paternally derived markers showed that fetal cells are present among maternal peripheral blood mononuclear cells (PBMCs) at a frequency of 1 in 103 cells by 14–15 weeks of gestation20 and that fetal cells can be detected in maternal blood as early as 7 weeks gestation, which is before the complete establishment of the fetal placental vasculature21. The number of fetal cells in maternal blood progressively increases throughout pregnancy, reaching peak levels of more than 100 fetal cells per millilitre of maternal blood at parturition 21,22. Similarly, cells containing fetal DNA have been found in the cadaveric lung, spleen, liver, kidney and heart tissues of women during pregnancy 9 . Importantly, fetal microchimeric cells are intact cells, and they are a completely distinct source of fetal DNA from the placenta derived, cell-free fetal DNA that is present in the serum of mothers during pregnancy and that is increasingly being used for prenatal analysis.”

– Around 8 weeks after the egg was fertilized, the transition from the embryo to fetus begins – the size of an olive, its organs begin to form and it turns from a blob into something vaguely human-like.

#Foundations Practical - Week 1 to 8.

https://embryology.med.unsw.edu.au/embryology/index.php?title=Foundations_Practical_-_Week_1_to_8
Quote: “In the first 8 weeks the embryo grows from 0.1 mm to about 3 cm in length.”

#John Hopkins Medicine. The First Trimester. Retrieved March 2025.

https://www.hopkinsmedicine.org/health/wellness-and-prevention/the-first-trimester

Quote: “The most dramatic changes and development happen during the first trimester. During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester, it becomes a fetus that is fully formed, weighing approximately 0.5 to 1 ounce and measuring, on average, 3 to 4 inches in length.”

– Meanwhile construction trophoblasts are busy building a spongy fingerlike structure that expands further into the uterus, a completely new organ: The placenta – an organ that you once grew inside your mother, that was ejected after your birth and died a silent death while everybody was busy welcoming you into the world.

The formation of the human placenta from trophoblasts is a complex process involving many steps and cell types. The following paper gives a good overview.

#Cindrova-Davies T, Sferruzzi-Perri AN. Human placental development and function. Semin Cell Dev Biol. 2022

https://pubmed.ncbi.nlm.nih.gov/35393235/

Quote: “The placenta is a transient fetal organ that plays a critical role in the health and wellbeing of both the fetus and its mother. Functionally, the placenta sustains the growth of the fetus as it facilitates delivery of oxygen and nutrients and removal of waste products. Not surprisingly, defective early placental development is the primary cause of common disorders of pregnancy, including recurrent miscarriage, fetal growth restriction, pre-eclampsia and stillbirth. Adverse pregnancy conditions will also affect the life-long health of the fetus via developmental programming[1]. Despite its critical importance in reproductive success and life-long health, our understanding of placental development is not extensive, largely due to ethical limitations to studying early or chronological placental development, lack of long-term in vitro models, or comparative animal models. In this review, we examine current knowledge of early human placental development, discuss the critical role of the maternal endometrium and of the fetal-maternal dialogue in pregnancy success, and we explore the latest models of trophoblast and endometrial stem cells. In addition, we discuss the role of oxygen in placental formation and function, how nutrient delivery is mediated during the periods of histotrophic nutrition (uptake of uterine secretions) and haemotrophic nutrition (exchange between the maternal and fetal circulations), and how placental endocrine function facilitates fetal growth and development.”

– In our story the placenta is now the new home of the fetus. An enormous fortress, protecting it from microbes that could infect and kill it in its still pretty fragile state. It even has its own mini immune system, placental immune cells that gobble up anything that poses a threat.

#Ding J, Maxwell A, Adzibolosu N, Hu A, You Y, Liao A, Mor G. Mechanisms of immune regulation by the placenta: Role of type I interferon and interferon-stimulated genes signaling during pregnancy. Immunol Rev. 2022

https://pmc.ncbi.nlm.nih.gov/articles/PMC9189063/

Quote: “Pregnancy is a unique condition where the maternal immune system is continuously adapting in response to the stages of fetal development and signals from the environment. The placenta is a key mediator of the fetal/maternal interaction by providing signals that regulate the function of the maternal immune system as well as provides protective mechanisms to prevent the exposure of the fetus to dangerous signals. Bacterial and/or viral infection during pregnancy induce a unique immunological response by the placenta, and type I interferon is one of the crucial signaling pathways in the trophoblast cells. Basal expression of type I interferon-β and downstream ISGs harbors physiological functions to maintain the homeostasis of pregnancy, more importantly, provides the placenta with the adequate awareness to respond to infections. The disruption of type I interferon signaling in the placenta will lead to pregnancy complications and can compromise fetal development. In this review, we focus the important role of placenta derived type I interferon and its downstream ISGs in the regulation of maternal immune homeostasis and protection against viral infection. These studies are helping us to better understand placental immunological functions and provide a new perspective for developing better approaches to protect mother and fetus during infections.”

– Other placental cells creep along the inside of the mom's blood vessels, stretching them and connecting the fetus through the umbilical cord on the other side.

#Hunkapiller NM, Fisher SJ. Chapter 12. Placental remodeling of the uterine vasculature. Methods Enzymol. 2008

https://pmc.ncbi.nlm.nih.gov/articles/PMC2857511/

Quote: “Specialized epithelial cells that arise from the placenta, termed cytotrophoblasts (CTBs), are responsible for redirecting maternal blood to the developing conceptus, which occurs as a result of the cells’ aggressive invasion through the maternal endometrial stroma (interstitial invasion) and resident blood vessels (endovascular invasion). The latter process involves displacement of maternal endothelium and induction of apoptosis in the surrounding smooth muscle. Together, these events result in a reduction of blood vessel elasticity and increased blood flow (Burton et al., 1999).

[...]

Uterine attachment requires CTBs to differentiate and acquire the ability to aggressively invade maternal tissues (Fisher et al., 1989; Librach et al., 1991). This process also initiates the remarkable cell-cell interactions that occur as CTBs remodel the uterine vasculature into chimeric vessels, comprised of fetal and maternal cells, that supply blood to the placenta. Key morphological aspects of the invasion process are diagrammed in figure 1B. CTB progenitors form a polarized epithelium that is attached to the basement membrane surrounding the mesenchymal cores of chorionic villi. During differentiation along the invasive pathway, CTBs leave this basement membrane to form columns of unpolarized cells that attach to and then penetrate the uterine wall. The ends of the columns terminate within the superficial endometrium, where they give rise to invasive (extravillous) CTBs. During interstitial invasion, a subset of these cells, either individually or in small clusters, commingles with resident decidual, myometrial and immune cells. During endovascular invasion, masses of CTBs breach and plug the vessels (Ramsey et al., 1976). Subsequently, these fetal cells replace the resident maternal endothelium and portions of the smooth muscle wall. Normally, this process encompasses the portions of uterine arterioles that span the decidua and the inner third of the myometrium. In contrast, only the termini of uterine veins are breached (Zhou et al., 1997). Together these two components of CTB invasion anchor the placenta to the uterus and permit a steady increase in the supply of maternal blood that is delivered to the growing embryo/fetus. Endovascular invasion, which is a major determinant of pregnancy outcome, incorporates the placenta into the maternal circulation.”

“Figure 1. Anatomy of the maternal-fetal interface. Anchoring chorionic villi (B) attach the placenta to the uterus and give rise to invasive CTBs. (A) Placental chorionic villi stem from the chorionic plate and lie within the intervillous space. The point of attachment between anchoring villi and the underlying tissue is referred to as the basal plate (box B). (B) Enlargement of the area in box B. Undifferentiated CTB progenitors in the anchoring villi give rise to invasive CTBs that invade the uterine interstitium (interstitial invasion) and the maternal endothelium (endovascular invasion).”

#Hanbo Liu, Fen Ning, Gendie E. Lash. Contribution of vascular smooth muscle cell apoptosis to spiral artery remodeling in early human pregnancy. 2022.

https://www.sciencedirect.com/science/article/abs/pii/S0143400422000406

Quote: “Uterine spiral arteries, the smallest terminal branches of the uterine artery, are the conduits extending from the myometrium to the decidua for delivery of oxygen and nutrients to the placenta, and ultimately the fetus, in pregnancy. During early pregnancy, in order to maintain healthy placental function and meet the high nutritional demands of the growing fetus, as well as removal of metabolic waste, it is necessary to adapt to the sharp increase in maternal blood volume and flow through remodeling of the uterine spiral arteries [4]. The remodeled spiral arteries are highly dilated and are converted from high-resistance/low-flow to low-resistance/high-flow vessels [9], allowing at least 3–4 fold increase in the maternal blood delivered to the intervillous space of the placenta [19,20]. The histological features of this vital physiological process are well described with vascular smooth muscle cell (VSMC) separation, endothelial cell swelling, vessel dilatation, extravillous trophoblast cell (EVT) invasion, fibrinoid deposition and eventual complete loss of the VSMCs [4,21,22], but little is known about its cellular and molecular regulation.

It has been reported in numerous articles that extravillous trophoblast cells play a vital role in facilitating spiral artery remodeling. EVT invade the decidua and inner third of the myometrium via two routes, interstitial and endovascular. Interstitial EVT invade the decidua and myometrium, cluster around the spiral arteries and after the initial separation of the VSMCs initiated by uterine natural killer (uNK) cells invade the wall of the spiral arteries. The endovascular EVT migrate along the lumen of spiral artery in a retrograde manner, temporally plugging the vessels causing transient loss of the endothelial cells. Both interstitial and endovascular EVT then invade the wall of the spiral artery where they replace the smooth muscle layer by depositing fibrinoid material within which they become embedded, and are then termed intramural EVT [4,23–27].”

– With the blood flow secured, it’s time to load it with as much food as possible. The placenta releases hormones that funnel glucose directly to the fetus, stealing energy from its mom. If the fetus goes too far and asks too much, this can sometimes lead to gestational diabetes for the mom during the pregnancy, starving her body of energy.

#Yu Y, He JH et al. Placensin is a glucogenic hormone secreted by human placenta. EMBO Rep. 2020

https://pmc.ncbi.nlm.nih.gov/articles/PMC7271319/

Quote: “In addition to mediating nutrient and waste exchange between maternal and fetal compartments, the placenta is also a hormone‐producing organ, capable of regulating maternal and fetal metabolism and hormonal balance 1. With the mother maintaining metabolic homeostasis for herself and the fetus, pregnancy is characterized by stage‐dependent increases in insulin resistance with minimal changes in serum glucose levels 4. During normal pregnancy, there is a progressive increase in the insulin/glucose ratio based on the oral glucose tolerance test and hyperinsulinemic–euglycemic clamp 6. In addition, 6–9% of pregnant women developed gestational diabetes mellitus (GDM) 7, showing increased risk for high blood pressure and preeclampsia 8 as well as for future development of type II diabetes in mothers and children 9.

[...]

During pregnancy, major changes in metabolic homeostasis took place due to increasing demand of the fetus and placenta. Maternal liver metabolism contributes to the increased energy demand of the placenta and the embryo by enhancing the production of glucose 27, 28. Since gluconeogenesis is minimal in the fetus, maternal glucose is the main source for the growing fetus 29. Commensurate with increasing demand during late gestation, there is an increase in maternal glucose production and gluconeogenesis 27. As pregnancy progresses, stage‐dependent increases in insulin resistance were found 6, 30, 31. Because diabetes‐like changes in glucagon secretion were not found in normal pregnant women or those with gestational diabetes mellitus in present and earlier studies 32, observed increases in insulin resistance during pregnancy have been ascribed to increased production of placental hormones. However, longitudinal measurement of diverse placental hormones (hCG, estradiol, progesterone, human placental lactogen, and prolactin), together with leptin and cortisol, during different stages of pregnancy showed no correlation between levels for these hormones and pregnancy‐associated increases in insulin resistance 33. Our findings of a glucogenic placental hormone and stage‐dependent increases in serum placensin levels during pregnancy progression provide an endocrine basis for pregnancy‐associated increases in maternal glucose production.”

#Castillo-Castrejon M, Powell TL. Placental Nutrient Transport in Gestational Diabetic Pregnancies. Front Endocrinol (Lausanne). 2017

https://pmc.ncbi.nlm.nih.gov/articles/PMC5682011/

Quote: “Gestational Diabetes Mellitus [GDM]

Pregnancy is a state characterized by profound metabolic and physiological changes to support fetal development and growth (14). As an adaptation in healthy pregnancy, maternal insulin sensitivity and insulin-mediated glucose consumption in peripheral tissues, including skeletal muscle and adipose tissue declines (15). The placenta secretes hormones such as estrogens, progesterone, growth hormone, and human placental lactogen which may contribute to the establishment and maintenance of pregnancy by regulating the maternal physiological adaptations to pregnancy (16). In pregnancy, insulin secretion increases due to hyperplasia of beta cells in the pancreas (17). Despite the increase in insulin secretion, pregnancy is characterized by a relative insulin resistance state, particularly in the third trimester, which favors the metabolic needs of the developing fetus (18). Pregnancies complicated by GDM are characterized by glucose intolerance first recognized in pregnancy, where maternal pancreatic β-cells are not able to secrete sufficient insulin to maintain normal glycemia in the mother (19).”

– The mothers body is trying to support the new life, but not at the cost of her own survival. In a sense the mothers genes inside the fetus still have a stronger allegiance to her than to the new being – but it’s father’s genes don’t. They want the fetus to survive at all costs. So while there is a sort of fragile peace between both parties, it is just that – their interests are not perfectly aligned and both sides have to deal with that.

#Reik W, Constância M, Fowden A, Anderson N, Dean W, Ferguson-Smith A, Tycko B, Sibley C. Regulation of supply and demand for maternal nutrients in mammals by imprinted genes. J Physiol. 2003

https://pubmed.ncbi.nlm.nih.gov/12562908/

Quote: “Not many more than 100 imprinted genes are expected in mammalian genomes. A substantial proportion of imprinted genes are involved in the control of fetal growth, and in general paternally expressed imprinted genes enhance fetal growth, whereas maternally expressed ones suppress it (Reik & Walter, 2001; Tycko & Morison, 2002). The popular genetic conflict hypothesis explains the evolution of imprinted genes by paternally derived genes that influence nutrient acquisition being selected to extract more resources from the mother (thus being more selfish) whereas maternally derived genes have to balance the nutrient provision to the current fetus with that to future fetuses of the same mother (but potentially different fathers). Thus maternally derived genes are more conservative with regard to resource provision to the fetus (Moore & Haig, 1991; Haig & Graham, 1991).”

#National Human Genome Research Institute. Genetic Imprinting. 2025.

https://www.genome.gov/genetics-glossary/Genetic-Imprinting
Quote: “Genetic imprinting is a rather mysterious phenomenon which has become somewhat better understood in the last few years. Essentially, what it refers to is the chemical modification of a DNA sequence. Keep in mind here that the DNA sequence itself is not changing. These are modifications to the DNA sequence itself that occur in a cell--usually refers to a germ cell, either an egg cell or a sperm cell--and that change is passed on from one generation to another. The reason it confused scientists for many years is that it is a non-sequenced-based mechanism of inheritance. Initially, it was thought that all inheritance is based on changes in sequence; this turns out not to be true. In one of those mechanisms, which is not involved in change of sequence, but rather an inherited chemical change to a DNA sequence, is referred to as imprinting. And that imprinting, the reason it's important is that chemical modification, which is passed on from the mother or the father to the offspring, changes the function of the gene or the gene product, whether it's expression or actually the function of the gene product itself.”

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