Pregnancy

Movement of Spermatozoa through female tract

In cows, the males deposit semen into the cranial vagina. Retrograde loss, otherwise known as spermatozoa backflow, and phagocytosis will occur, but from there, semen will enter the cervix where two different types of mucus are introduced. One type called sialomucin, is a low viscous mucus that allows healthy and motile sperm to swim through the crypts of the cervix. However, the other mucus sulfomucin, is responsible for trapping the unhealthy and nonmotile sperm so that phagocytosis can occur. This dynamic allows for privileged pathways to occur where the cervix can eliminate the non-motile sperm. From the cervix, spermatozoa enter the uterus where capacitation and more phagocytosis occurs. Capacitation is the process that occurs over several hours, where the carbohydrate portion of spermatozoa introduced by seminal plasma and part of the protein located on the spermatozoa head are removed in order to expose the acrosomal area. This process is necessary because the removal of these surface molecules will allow the spermatozoa to bind to the zona pellucida. After the uterus, spermatozoa travel into the oviduct where hyperactive motility is achieved. Hyperactive motility is necessary for contact to occur between the sperm and the oocyte. Finally, fertilization can occur along with the acrosomal reaction where spermatozoa penetrates the zona pellucida and fusion can occur with the plasma membrane of the oocyte. During the acrosomal reactions, sperm binds to the receptor ZP3 and a complex is formed between ZBR and ZP3. This causes a calcium influx through the membrane and the outer plasma membrane of sperm melts away, leaving only the inner acrosomal membrane. Then the cortical reaction has to occur in order for their to be a zona block to prevent other sperm from penetrating the zona pellucida, stopping polyspermy. Then in the perivitelline space, the sperm head is engulfed and decondensation of the sperm nucleus occurs in order for the male pronucleus to form. Finally, fusion between the male and female pronuclei can occur, a process called syngamy. Regarding artificial insemination, spermatozoa is deposited into only one uterine horn, a process called intercornual transport. When this occurs, spermatozoa are redistributed so that both horns contain an adequate amount of spermatozoa. Along with that, fertility is enhanced when spermatozoa is deposited within the uterine body or in a uterine horn (4).

Figure 11. An image from Pathways to Pregnancy and Parturition which depicts the pathway spermatozoa follow in the female tract along with the major steps (4)

Process of Maternal Recognition of Pregnancy

In the cow, the blastocyst is responsible for activation of maternal recognition of pregnancy. The free floating blastocyst produces a protein interferon that is referred to as bovine Interferon τ or bIFN-τ. Produced by the trophoblastic cells, bIFN-τ remains present in the uterus from day 13 to 21 and binds to the endometrium to prevent oxytocin receptor synthesis by the endometrial cells. Along with that, this interferon binds to the uterine glands in order to promote protein synthesis that is necessary for preimplantation embryonic survival. Furthermore, bIFN-τ is responsible for the production of blood proteins which are involved in immune response. Heifers without elevated levels of these blood proteins from day 17 to 20 would not be considered pregnant (4).

Placental type

Heifers have a cotyledonary syndesmochorial placental type. Cotyledonary placenta is defined as numerous button-like structures that range between 70 to 120 in the heifer. The placentome consists of a fetal cotyledon contributed by the chorion and maternal cotyledon from the caruncular region of the uterus. At day 25, the chorion starts attaching to the caruncles of the uterus and the placentomes form a convex structure. Near the end of gestation, the cotyledons will measure between 5 to 6 centimeters in diameter in order to provide an increased surface area for the transfer of nutrients and metabolic waste. The syndesmochorial type is the least intimate compared to the other placental types. While it also has six layers like the epitheliochorial type, the syndesmochorial differs in that the endometrial epithelium transiently erodes and regrows causing the maternal capillaries to be exposed to the chorionic epithelium. Along with that, a specific cell called the binucleate giant cells are found in ruminant placenta. Consisting of two nuclei, these cells appear between days 18 to 20 and originate from the trophoblast cells. Binucleate giant cells travel from the chorionic epithelium to the endometrial epithelium where they are responsible for transferring complex molecules from fetal to maternal placenta. They also secrete placental lactogen and pregnancy specific protein B, sometimes called pregnancy associated glycoproteins. Placental lactogen is responsible for growth of the fetus and mammary gland stimulation (4).

Figure 12. An image from the textbook Pathways to Pregnancy and Parturition which illustrates the cotyledonary placental type (4)

What is pregnancy like for dairy cows?

Since the production of milk relies on repeated pregnancies of the cow, it is important to know how she is affected by gestation. The optimal time of gestation for the Holstein cow is found to be between 275-277 days, or around nine months, in order to maximize calving ease and minimize still births (1). This length is about the same for most other cattle as well. Cattle typically will only have one offspring at a time, but the incidence of twins if found to be between 5.5% and 13.6% (2). Twins can lead to loss of early pregnancy as well as increase the risk of dystocia, retained placenta, and stillborn calves (7). The negative impacts of twins affect the profitability and possible future reproductive performance of the cow, so it is important to monitor when they might happen and know how to handle and select cows with lower incidences within the herd.

Figure 13. Twin bull calves born at the Kansas State fair in 2016 (6)

During a pregnancy, the cow will go through changes that will affect how her body functions. These changes will affect her nutritional needs, her reproductive tract, and her own vital signs as her pregnancy progresses. Her nutritional needs will change throughout her pregnancy to support the growing fetus. Early on, it is important to increase her condition to build up the maternal nutrition reserves that will support the needs of the fetus and lactation later on when she will not be able to eat as much. The cow will want to eat less as the fetus takes up more space and will likely go into ketosis, so building up these reserves is important to prevent too much loss in body condition when lactating after parturition. The cow's reproductive tract will protect the fetus by keeping the cervix relatively closed and later on will aide in parturition by increasing the vaginal and cervical mucus in the tract. Her uterine horn will begin to grow and stretch with the fetus until the maximal stretch is reached, which will then trigger parturition. The cow will undergo some changes to her vital signs as the fetus grows as well. There will be in increased blood volume, and therefore increased blood pressure and heart rate and the pregnancy progresses. Her respiration rate will increase as the fetal demands for oxygen increase and her lung capacity decreases as the fetus takes us space. Her blood will gain hyper-coagulability and she will have an increased GFR, or glomerular filtration rate, as her body responds to the increase in fluids and waste from the fetus.

The formation of all of the bodily systems including the reproductive tract take place during early embryo development in gestation. The embryo starts with three layers of tissue that go on to become the rest of the bodily systems. The endoderm, or the innermost cellular layer, will form the digestive system, lungs, and endocrine system in the fetus. The mesoderm, or the middle layer of cells, will be used to form muscles, bones, the cardiovascular system, and the reproductive system. The ectoderm, or the outermost cell layer, forms the nervous system, skin, and hair of the developing fetus. The sex of the fetus is determined at fertilization when the sperm from the sire delivers an X or Y chromosome to the oocyte. The Y chromosome carries the SRY gene, which will cause the production of testis determining factor (TDF) and the development of the testis in the male fetus. This will lead to further development of the male tract through the production of the sertoli cells, which will produce anti-mullerian hormone (AMH) and make the leydig cells differentiate. The testis will also descend during mid-gestation in bull calves. The absence of TDF in the female conceptus with two X chromosomes will result in the development of the ovaries and the female tract within the fetus. During this time the hypothalamus will develop sexual differences as well. Testosterone in the male fetus can cross the blood brain barrier where it will be converted to estradiol. That estradiol will cause a decrease in the development of the surge center. Estradiol in the female fetus cannot cross the blood brain barrier due to binding with α-fetal protein. This results in a developed tonic and surge center within the female hypothalamus.

How do the hormones work during pregnancy and parturition?

Early Pregnancy

After maternal recognition of pregnancy, the CL is maintained to keep progesterone levels high throughout pregnancy. The maintenance of the CL is critical during pregnancy in cows because the placenta doesn't take over for progesterone production until almost 8 months of gestation in the cow. A process called "progesterone block" prevents myometrial contractions in the uterus to keep the environment suitable for the fetus. Estrogen during this time is low due to high progesterone levels.

Mid Pregnancy

Progesterone remains high during mid pregnancy to maintain the quiescent environment for the developing fetus. Placental lactogen begins to rise during this period of development. It works similar to growth hormone to promote the growth of the fetus and early development of the mammary gland.

Late Pregnancy and Parturition

Lots of endocrine changes begin to occur in late pregnancy for parturition to take place. Fetal cortisol will promote the conversion of progesterone to estradiol. Estradiol is important for immune function during parturition to prevent infection from contamination during birth as well as promoting an increase in secretory activity within the tract to aide in lubrication for the expulsion of the fetus. Prolactin will begin to be produced by the anterior pituitary gland to finish mammary gland development and start milk synthesis.

What does birth look like?

Stage 1

Once the fetus has reached a size where the uterus cannot stretch anymore, it undergoes stress that causes the production of cortisol from the fetus. The cortisol produced promotes the change of progesterone to estradiol. The rise in estradiol signals the placenta to begin producing PGF2α, which has similar effects as it does during the estrous cycle. It will cause regression of the CL and therefore abolish the progesterone block in the uterus, which will lead to the beginning of contractions of the uterus. The fetus will turn to face the posterior of the dam and pressure will increase on the cervix. This pressure will cause an increase in systemic oxytocin and further facilitate the contractions that will expel the fetus. Once the fetus enters the cervical canal, this stage of parturition is complete.

Figure 14. A video from Oklahoma State extension showing the three stages of calving (3)

Stage 2

The release of PGF2α also triggers the synthesis of relaxin from the ovaries of the cow. Relaxin helps the connective tissue of the reproductive tract soften and increases the elasticity of the ligaments in the pelvic region. The rise in estradiol discussed earlier also increases the secretory activity of the cervix and the vagina, allowing them to lubricate the reproductive tract to help the fetus move through it during parturition. As the fetus moves through the cervix, the pressure on its membranes will cause the membranes to rupture and release the allentoic fluid into the tract. This fluid will also serve to lubricate the tract. The fetus becomes hypoxic, or lacking oxygen, as it moves into the birth canal and begins to move. This movement leads to stronger myometrial contractions and a shorter time for parturition to take place. If this stage is prolonged, dystocia may be occurring and it may be necessary to help the cow by pulling the calf. If this is needed you should contact your vet immediately for emergency services. Dystocia that goes on for too long could lead in the death of the calf as well as the cow.

Stage 3

The next stage is the expulsion of the placenta and membranes from the reproductive tract. In the cow, this should take 8-12 hours and if it doesn't happen within 12 hours it is referred to as a retained placenta (5). If the placenta does not come out on its own it should not be manually removed. The vet should be contacted for possible treatment plans. The chorionic villi have to remove themselves from the crypts of the maternal side. This process is believed to happen through vasoconstriction of the arteries in the villi (4).

References

Nogalski, Z., & Piwczyński, D. (2012). Association of length of pregnancy with other reproductive traits in dairy cattle. Asian-Australasian journal of animal sciences, 25(1), 22–27. https://doi.org/10.5713/ajas.2011.11084
López-Gatius, F., Garcia-Ispierto, I., & Hunter, R. H. F. (2020). Twin Pregnancies in Dairy Cattle: Observations in a Large Herd of Holstein-Friesian Dairy Cows. Animals : an open access journal from MDPI, 10(11), 2165. https://doi.org/10.3390/ani10112165
SUNUPTV. (2020, January 13). Cow-calf corner - stages of Calving (1/11/20). YouTube. Retrieved November 23, 2022, from https://www.youtube.com/watch?v=Pccgi8FozKU
Senger, P. L. (2012). Pathways to pregnancy and parturition. Current Conceptions.
Stuttgen, S. (n.d.). Three stages of bovine parturition. University of Wisconsin-Madison Extension. Retrieved November 26, 2022, from https://livestock.extension.wisc.edu/articles/three-stages-of-bovine-parturition/
Booker, A. (2016, September 12). It's girls ... and boys! Two sets of twin calves born at Fair - A Record. The Hutchinson News. Retrieved November 26, 2022, from https://www.hutchnews.com/story/news/state-fair/2016/09/12/it-s-girls-boys-two/20986661007/
López-Gatius F. (2020). Twins in Dairy Herds. Is It Better to Maintain or Reduce a Pregnancy?. Animals : an open access journal from MDPI, 10(11), 2006. https://doi.org/10.3390/ani10112006

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