11.4 Sexual Reproduction
Essential idea: Sexual reproduction involves the development and fusion of haploid gametes.
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11.4 U 1 Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation
11.4 U 1 Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation
Stages of spermatogenesis in the wall of the seminiferous tubule.
Spermatogenisis
Spermatogenisis
Definition
Gametogenesis is the process by which cells of the germinal epithelium undergo cell division and differentiation to form haploid gametes.
Spermatogenesis is the production of sperm which takes place in the testes. Testes are composed of seminiferous tubules with interstitial cells (sometimes called Leydig cells) filling up the gaps in between. The figure to the left shows a cross section of seminiferous tubules in the testes.
Stages of spermatogenesis.
Important
Leydig cells in the interstitial space of the testis produce the hormone testosterone, which allows spermatocytes to complete the meiotic divisions and mature into spermatozoa.
Spermatogenesis and male fertility are both dependent upon the presence of testosterone in the testis.
OOGENESIS
Oogenesis involves the production of ova (egg cells), which takes place in the ovaries of a female. A female is born with around 400 000 primary follicles. The primary follicle consists of a primary oocyte, surrounded by a single layer of follicle cells. However, the development of the primary oocyte has been halted at the first meiotic division.
11.4 U 2 Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm
11.4 U 2 Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm
Similarities and differences between spermatogenesis and oogenesis
Similarities and differences between spermatogenesis and oogenesis
11.4 U 3 Fertilization in animals can be internal or external.
11.4 U 3 Fertilization in animals can be internal or external.
Internal Fertilization
Internal Fertilization
The fertilization process occurs within the body of a female is called internal fertilization. It is a specialization for the protection of egg, but it depends on the birth method . Reptiles and birds have thick shell, covering the egg to protect it from dehydration and destruction. However, fertilization occurs inside the body otherwise, sperm has to enter through a thick wall.
Animals such as mammals also have internal fertilization, where embryo develops inside mother, which enhances the protection to the embryo1.
Internal fertilization facilitates the survival of the embryo, selective fertilization, and longer protection and minimizes the wastage of gametes.
External Fertilization
External Fertilization
In external fertilization fusion of sperm and egg occurs externally of the female body. External fertilization needs water to facilitate their fertilization, so it occurs in wet environments. Female and male gametes are released to the water, and male gamete is mostly mobile. This type of fertilization can be seen in lower plants. The advantage of external fertilization is that it produces a large number of offspring due to the external hazards. So survival of the embryo is comparatively lower. Amphibians and fish are examples for these types of animals.
Comparing and contrasting internal and external fertilization
Comparing and contrasting internal and external fertilization
The internal fertilization process occurs within the body of female whereas, in external fertilization, fusion of sperm and egg occurs externally of the female body.
After the internal fertilization, egg will come out of the body having a thick shell whereas, in external fertilization, eggs are produced with thin tertiary membrane or without membrane .
External fertilization needs water, whereas internal fertilization does not need water to fertilize.
Organisms involved in external fertilization have mobile male gametes with flagella, whereas organisms involved in internal fertilization has immobile male gametes.
In internal fertilization, wastage of gametes is lower, whereas wastage of gametes is higher in external fertilization.
Organisms that involved in internal fertilization produce lower number of gametes, whereas organisms involved in external fertilization produce a large number of gametes.
Survival of organisms that involved in internal fertilization is higher than the survival of organisms involved in external fertilization.
11.4 U 4 Fertilization involves mechanisms that prevent polyspermy
11.4 U 4 Fertilization involves mechanisms that prevent polyspermy
Fertilization involves the acrosome reaction, fusion of the plasma membrane of the egg and sperm, and the cortical reaction.
Acrosome Reaction
sperm migrates through follicle cells (corona radiata)
sperm binds to zona pellucida glycoproteins
exocytosis of acrosomal hydrolytic enzymes
enzymatic digestion through zona pellucida (proteins + polysaccharides)
Egg and sperm membrane fusion
acrosomal reaction exposes sperm membrane
binding proteins of egg and sperm match
binding triggers egg membrane depolarization as Na+ gates open
fast block to polyspermy
Cortical reaction
depolarization triggers release of egg G proteins
G proteins trigger release of Ca++ from ER
Ca++ triggers fusion of cortical granules with egg membrane
enzymes released by exocytosis catalyze hardening of zona pellucida
slow block to polyspermy
Fertilization
egg microvilli endocytotically take in sperm nucleus
envelopes of both nuclei dissolve and share in common spindle apparatus
first mitosis of embryo = cleavage division
11.4 U 5 Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy.
11.4 U 5 Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy.
Zygote development summary
Zygote development summary
The zygote (fertilized egg cell) continues dividing into 4 then 8 cells as it travels down the fallopian tube.
A hollow ball of cells called a ‘blastocyst’ is formed and cells begin to differentiate.
The zygote has become an embryo and next implants into the uterus lining (endometrium)
If the embryo successfully implants it will keep growing and produce hCG hormone.
hCG hormone causes the ovary to continue producing progesterone – pausing the menstrual cycle.
11.4 U 6 HCG stimulates the ovary to secrete progesterone during early pregnancy.
11.4 U 6 HCG stimulates the ovary to secrete progesterone during early pregnancy.
presence of HCG
can be detected in mother’s urine by ELISA (enzyme-linked immunosorbent assay)
as a positive test of pregnancy commonly used in pregnancy test kits
When a human embryo is implanted into the endometrium or the uterine lining, it starts to produce the hormone, human chorionic gonadotrophin (HCG).
embryo secretes HCG
which acts like pituitary LH,maintaining corpus luteum
which produces estrogen and progesterone, maintaining endometrium and cervical mucus/plug
stimulates growth of placenta and uterine enlargement
inhibits menstruation
by second trimester, HCG declines
corpus luteum disintegrates
placenta produces progesterone/estrogen
11.4 U 7 The placenta facilitates the exchange of materials between the mother and fetus.
11.4 U 7 The placenta facilitates the exchange of materials between the mother and fetus.
The placenta develops from the trophoblast layer of the blastocyst. When developed three blood vessels contained within umbilical cord connect the placenta to the growing fetus.
Two umbilical arteries carry deoxygenated blood and waste away from the fetus to the placenta. As maternal blood enters the placenta it leaves the arteries and enters the inter-villous space, where it pools and surrounds the placental villi. The placental villi are finger-like fetal tissues that have a large surface area for the exchange of materials such as gases, nutrients and wastes.
Fetal blood that circulates in capillaries within the villi and microvilli is very close to the surface, allowing for efficient exchange of materials between the fetal and maternal blood. Materials such as oxygen, nutrients and vitamins diffuse into the fetal capillaries from the maternal blood in the inter-villous space, while carbon dioxide and wastes diffuse out of the fetal capillaries into the inter-villous space. One umbilical vein carries oxygenated and nutrient rich blood back to the fetus from the placenta.
The cells that separate the fetal and maternal blood form a semi-permeable placental barrier. Materials are exchanged between the maternal and the fetal blood in the placenta.
Note: maternal and fetal blood is never mixed together.
11.4 U 8 Estrogen and progesterone are secreted by the placenta once it has formed.
11.4 U 8 Estrogen and progesterone are secreted by the placenta once it has formed.
When the pregnancy is at term, the fetus secretes hormones that signal the placenta to stop producing progesterone (progesterone inhibits the secretion of oxytocin by the pituitary gland). Oxytocin secreted by the anterior pituitary gland stimulates the muscle fibers in the uterus to begin to contract. As the muscles in uterus contract, mechanoreceptors in the uterine wall signal the pituitary to produce more oxytocin. More oxytocin increases the frequency and intensity of the contractions, thus stimulating the production of even more oxytocin. This is an example of positive feedback.
Contractions of the muscles of the uterus will cause the amniotic sac to break, releasing the amniotic fluid (This is when the “water breaks” in childbirth). Relaxation of the muscles in the cervix causes it to dilate, eventually allowing the increasing contractions to push the baby out through the vagina and the cervix. he placenta is expelled “afterbirth” about 15 minutes after the baby is born.
labor:
first stage of birth
progesterone decrease/estrogen increase: triggers formation of oxytocin receptors in uterus
prostaglandins: produced by fetus/placenta initiate uterine contractions
positive feedback: uterine contractions, via nervous system, stimulate production of oxytocin, a hormone, by posterior pituitary into blood
oxytocin: binds to uterine receptors, stimulating:
uterine contractions
prostaglandin production by placenta
delivery:
second stage of birth
uterine contractions force fetal head against cervix, which dilates
fetal head move through cervix, into vagina, exits
afterbirth:
third stage of birth: placenta expelled
11.4 A 1 The average 38-week pregnancy in humans can be positioned on a graph showing the correlation between animal size and the development of the young at birth for other mammals.
11.4 A 1 The average 38-week pregnancy in humans can be positioned on a graph showing the correlation between animal size and the development of the young at birth for other mammals.
There is a correlation with animal size (mass) and the development of their young (viewed as length of gestation period). In many cases, the longer the gestation period, the greater the mass size and development at birth. Species of mammals that give birth to smaller, immature and somewhat helpless offspring are called altricial species. Species of mammals that give birth to more mature offspring that are generally larger, have their eyes open at birth and are immediately mobile. These offspring are precocial.
11.4 S 1 Annotation of diagrams of seminiferous tubule and ovary to show the stages of gametogenesis.
11.4 S 1 Annotation of diagrams of seminiferous tubule and ovary to show the stages of gametogenesis.
The process of gametogenesis occurs in the reproductive organs (gonads) of the male and female
In males, the gametes are produced within the seminiferous tubules of the testes
In females, the gametes are produced by the ovaries
11.4 S 2 Annotation of diagrams of mature sperm and egg to indicate functions.
11.4 S 2 Annotation of diagrams of mature sperm and egg to indicate functions.
The male and female reproductive gametes (sperm and egg) have specialised structures which reflect their functions
The male gamete (sperm) is small and motile and only contributes the male’s haploid nucleus to the zygote
The female gamete (egg) is large and non-motile and contributes all the organelles and cytoplasm to the zygote
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