Understanding Human Reproduction: Anatomy, Ovulation, and Fertilization

1.  Human reproduction involves intricate anatomy{see text} and complex behavior{previous lecture}

2. reflex ovulation results from an acute induction of LH secretion by mating-associated stimuli … most Insectivora (shrews), Lagomorpha (rabbits, hares), and Carnivora (cats, minks, raccoons) are reflex ovulators. {The male cat has spines on his penis that assist with stimulating the vagina during mating, it also contains an os penis or bone} Reflex ovulators are also found in Orders Rodentia (ground squirrels, voles, lemmings) and Artiodactyla (llamas, alpacas, camels). Fertilization of egg initiates development; what initiates ovulation? humans and most laboratory species are spontaneous ovulators … Spontaneous ovulations are primarily the result of an LH surge from anterior pituitary induced by ovarian steroid hormones … {regular cycling – next slide}

3. http://www.campuslife.utoronto.ca/services/SEC/frepro.html Day 1 of menstrual flow: low estrogen causes the hypothalamus to send a message to the pituitary gland to secrete FSH. The ovaries react to the FSH by beginning the maturation of an egg inside the follicles of the ovaries. The follicle produces estrogen. Estrogen causes the cells in the endometrium of the uterus to multiply and the lining gradually becomes thicker. The estrogen level continues to rise for around 10 days, until it is high enough to stimulate the pituitary gland to release LH (leutenizing hormone). This sudden rise in LH (called the LH Surge) acts on the ovaries and triggers the release of the egg from the follicle. Egg release (ovulation) occurs about halfway between the start of one menstrual flow and the next, and means the egg is available for fertilization if sperm are present in the reproductive tract. After an egg is released, the follicle becomes a corpus luteum (yellow body). The corpus luteum produces both estrogen and progesterone. … with the addition of progesterone from the corpus luteum, the lining changes to form distinct layers with tiny blood vessels … Progesterone also causes LH and FSH to drop gradually. When the level of LH is low enough, the corpus luteum stops producing progesterone and estrogen. This causes the uterus to shed its lining as the mesntrual flow - which is Day 1 of the next cycle. The low level of estrogen also signals the hypothalamus and the pituitary gand to start secreting more FSH again, thereby beginning the next reproductive cycle. Oral contraceptive pills consist of estrogen and progesterone (progestin), or progesterone only. Estrogen prevents the release of an ovum. Progesterone makes the the cervical mucus very thick {which} makes it more difficult for sperm to reach the uterus. Progesterone also makes the endometrium of the uterus unfavourable for implantation.

4. Concealed ovulation makes the rhythm method of contraception ‘probabilistic.’ http://www.abc.net.au/science/descent/trans1b.htm David Buss: Humans have what’s called concealed or cryptic ovulation, and this poses an especially important adaptive problem for men, and that is, the problem of how to know when to mate with the woman, These tests measure the increase in LH in urine, at the time of ovulation. … the leading hypothesis is that it has selected for the emergence of long term pair bonds, where the man had to stick around, form a relationship with the woman, have sex with her on a regular basis, simply because he didn’t know when she would be ovulating. And I think it’s not coincidence that these three things have co-evolved together. Concealed ovulation, long-term pair bonds, and heavy male parental investment.

5. Primordial germ cells differentiate into spermatogonia, the stem cells that give rise to sperm. As spermatogonia differentiate into spermatocytes and then to spermatids, meiosis reduces the chromosome number from diploid (2n = 46 in humans) to haploid ( n = 23). Cells move from the outer seminiferous tubule toward the lumen (central opening) and make their way to the epididymis, where they become motile. From spermatogonia to motile sperm, takes 65 to 75 days in the human male. In mature males, about 3 million spermatogonia start the process each day. {note temperature sensitivity & tight underwear} The head w/ the haploid nucleus is tipped with the acrosome: w/ enzymes that help the sperm penetrate the egg. Behind the head: large numbers of mitochondria w/ ATP for movement of the tail, which is a flagellum. Spermatogenesis is continuous and prolific in the adult human males. Each ejaculation of a human male contains 100 to 650 million sperm cells, & males can ejaculate daily w/o loss of fertility {sperm is NOT limiting}.

6. In the developing female embryo, oogonia, the stem cells that give rise to ova, multiply and then begin meiosis, but the process stops at prophase I. The cells at this stage, called primary oocytes, remain quiescent within small follicles until puberty. Beginning at puberty, FSH (follicle-stimulating hormone) periodically stimulates a follicle to grow and induces primary oocyte to complete meiosis I & start meiosis II. Meiosis then stops again {secondary oocyte}; Penetration of the egg cell by the sperm ‘Activates’ the secondary oocyte & triggers the completion of meiosis. Birth - puberty In humans, oogenesis differs from spermatogenesis in three major ways: 1. In meiotic divisions of oogenesis, almost all the cytoplasm is monopolized by a single daughter cell, the secondary oocyte, which forms the ova; the other products of meiosis, smaller cells called polar bodies, degenerate. w/ spermatogenesis all four products of meiosis develop into mature sperm. 2. Spermatogonia (stem cells) continue mitosis throughout the male’s life, this is not the case for oogenesis: At birth, an ovary already contains all the primary oocytes it will ever have. 3. Oogenesis is cyclic, spermatogenesis is not.

7. Figuring out how to ‘capacitate’ sperm In a glass dish was critical for in vitro fertilization { Secretions in the female reproductive tract enhance sperm motility & prepare sperm plasma membrane: this capacitation, requires about 6 hrs. } http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/fert.html The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the Zona Pellucida to triggerthe Acrosomalreaction allowing the zona to be pierced. {sperm surface molecule binds like peptide hormone to ZP surface receptor, triggers Na+ influx&cellulardepolarization (like in neuron) - ‘fast block to polyspermy:’ the zona pellucida hardens trapping runner-up sperm; sperm receptors in the zona pellucida are destroyed.} Penetration of the zona pellucida takes about twenty minutes. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html {In humans}The fertilization process takes about 24 hours. A sperm can survive for up to 48 hours. It takes about 10 hours to navigate the female productive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the the vagina, only 1%, 3 million, enter the uterus.

8. After penetrating the zona, incoming sperm enter the perivitelline space surrounding the egg and land on the egg plasma membrane, where the … sperm head initiates sperm-egg fusion. http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/fert.html Prior to fertilization, the egg is in a quiescent state, arrested in metaphase of the second meiotic division. Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes that are collectively called egg Activation. Prominent effects include completion of the second meiotic division and the so-called cortical reaction. The Cortical Reaction: a signal transduction cascade causes the endoplasmic reticulum to release Ca2+ , causing the cortical granules to fuse w/ the plasma membrane and release enzymes … viltelline layer becomes the fertilization envelope - ‘slow block to polyspermy.’ Sperm cells do not contribute any materials required for activation. Unfertilized eggs can be artificially activated by the injection of Ca2+ or by a variety of mildly injurious treatments, such as temperature shock. It is possible to artificially activate an egg that has had its own nucleus removed: mRNA coding for new proteins are stockpiled in an inactive form in the cytoplasm of the unfertilized egg.

9. Within 11 hours following fertilization, the oocyte has extruded a haploid polar body. {from 2nd meiotic division C&R Fig 46.13} 1.5 - 3 days post-ovulation The zygote now begins to cleave, with each division occurring into two cells called blastomeres. {In mammals, sperm tail taken in to become centrioles & spindles for mitosis} The zygote's first cell division begins a series of divisions, with each division occurring approximately every twenty hours. {In mammals, haploid sperm & oocyte nuclei don’t fuse until after 1st division} Each blastomere within the zona pellucida becomes smaller and smaller with each division.{& molecules in cytoplasm partitioned to establish polarity} When cell division has generated about sixteen cells, the zygote becomes a morula (mulberry shaped). It leaves the fallopian tube and enters the uterine cavity three to four days after fertilization. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html

10. 4 days post-ovulation … the morula enters the uterine cavity. Cell division continues, and a cavity known as a blastocel forms in the center of the morula. Cells flatten and compact on the inside of the cavity while the zona pellucida remains the same size. With the appearance of the cavity in the center, the entire structure is now called a blastocyst. The presence of the blastocyst indicates that two cell types are forming: the embryoblast (inner cell mass on the inside of the blastocele), and the trophoblast (the cells on the outside of the blastocele). http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html

11. 5 - 6 days post-ovulation The trophoblast cells secretes an enzyme which erodes the epithelial uterine lining and creates an implantation site for the blastocyst. In a cyclical process of hormonal stimulation, the ovary is induced to continue producing progesterone byhuman chorionic gonadotropin (hCG) released by the trophoblast cells of the implanting blastocyst. {detected in pregnancy tests} {later: hCG may protect the fetus from rejection by the maternal immune system by increasing the expression of Indoleamine 2,3-dioxygenase (IDO), which promotes degradation of tryptophan } Endometrial glands in the uterus enlarge in response to the blastocyst and the implantation site becomes swollen with new capillaries. Circulation begins, a process needed for the continuation of pregnancy. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html

12. Placenta C&R Fig 46.17 7 - 12 days post-ovulation Trophoblast cells engulf and destroycells of the uterine lining creating blood pools, both stimulating new capillaries to grow and foretelling the growth of the placenta. Ectopic pregnancies can occur at this time … The inner cell mass divides, rapidly forming a two-layered disc. The top layer of cells {epiblast} will become the embryo and amniotic cavity, while the lower cells {hypoblast} will become the yolk sac. Formation of the placenta. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html

13. To continue this narrative for 9 months, go to … http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html … 16 days post-ovulation … gastrulation continues … changing the two-layered disc into a three-layereddisc. The ectoderm grows rapidly over the next few days … The three layers {of epiblast} will eventually give rise to {from ‘fate mapping’} Endoderm that will form the lining of lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract. Mesoderm that will form the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive and excretory systems. Ectoderm that will form the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/db_tutorial.html

14. Images at UNSW are from the Kyoto Collection, reproduced with the permission of Prof. Kohei Shiota Fetus by Leonardo da Vinci

15. The uneven distribution of maternal RNAs & proteins in the egg cytoplasm (cytoplasmic determinants) sets the stage for pattern formation in development Then cell-to-cell signal gradients establish relative position for differentiation & organogenesis, {Fig 21.10b} ex: C&R Fig 21.10a induction of neural plate by Sonic Hedgehog ‘morphogen’ from notocord {see Fig 47.11} Sonic Hedgehog expression in the Notocord and Neural Tube floorplate. - Lance Davidson morphogens from neighboring cells induce & repress transcription of genes like the ‘homeobox’ genes {Fig 21.15}, important in segmentation across animals … and morphogens from vertebrate limb-bud organizer regions (Zone of Polarizing Activity) (including Sonic Hedgehog) shape differentiation of limbs. Note importance of cell death (apoptosis) in shaping limbs, fingers etc.

16. Sonic Hedgehog Shapes The Brain The size and shape of brain structures can be controlled by a signaling molecule known as Sonic Hedgehog, University of Chicago researchers show in a paper the March 16, 2001, issue of Science. During development, the brain becomes organized into highly specialized groups of neurons, called brain nuclei, each expressing its own set of genes … … this process can be coordinated by the secretion of a single molecule, Sonic Hedgehog, that operates as a 'positional signal'. "A positional signal is a neat mechanism for creating patterns of different types of cells," said Cliff Ragsdale, principal investigator in the study. "Target cells respond differently to a signaling molecule according to their distance from the source of the signal." http://www.chembio.uoguelph.ca/educmat/chm736/differtx.htm Sonic hedgehog, Shh is the human homolog of Drosophila hedgehog, Hh. Other Hh related genes include Indian hedgehog, Ihh, which acts in cartilage, and Desert hedgehog, Dhh, which acts in testicular tissue. Hh homologs have been reported in zebrafish (Mrs. Tiggywinkle) and nematodes (warthog).

17. Cancer is something like loss of control of development. Recovery from injury is something like ‘re-development’ The morphogen Sonic hedgehog is an indirect angiogenic agent upregulating two families of angiogenic growth factors. Pola et al. NATURE MEDICINE 7 (6): 706-711 JUN 2001Abstract:Sonic hedgehog (Shh) is … known to regulate epithelial/mesenchymal interactions during embryonic development. … the hedgehog-signaling pathway is present in adult cardiovascular tissues and can be activated in vivo. Shh was able to induce robust angiogenesis, … Shh also augmented blood-flow recoveryand limb salvage following operatively induced hind-limb ischemia {blood deprivation} in aged mice. … Shh might have potential therapeutic use for ischemic disorders. The role of hedgehog signalling in tumorigenesis.Wicking C, McGlinn E. CANCER LETTERS 173 (1): 1-7 NOV 8 2001Abstract:… members of this pathway are crucial to the processes of tumorigenesis. … mutations in the gene encoding the hedgehog receptor moleculepatched are responsible for both familial and sporadic forms of basal cell carcinoma (BCC), as well as a number of other tumour types. It is now known that a number of key members of the hedgehog cascade are involved in tumorigenesis, and dysregulation of this pathway appears to be a key element in the aetiology of a range of tumours.

18. Hox gene topology key to limb development. At some point in evolution the Hox genes, responsible for organizing structures along the trunk axis, were co-opted to regulate limb development. Intriguingly, Hox genes for digits are found at one extremity of the HoxD cluster, suggesting that limb skeletal organization is somehow controlled by the Hox genes' genomic topology. In this issue, the mechanism by which this collinearity produces digit morphology is unravelled. The cover (courtesy J. Zakany) shows digital morphology revealed in a clay impression of a mouse's 'foot'. Serial deletions and duplications suggest a mechanism for the collinearity of Hoxd genes in limbsMARIE KMITA et al.Nature420, 145–150 (2002) First come, first served. … Hox genes are arranged and expressed in the same order as the body parts they help to produce. New work … ROLF ZELLER & JACQUELINE DESCHAMPS Nature420, 138–139 (2002)

19. The NIH developed this primer to help readers understand … A. What are stem cells and why are they important? … they are unspecialized cells that renew themselves through cell division. … can be induced to become cells with special functions such as heart muscle or the insulin-producing cells of the pancreas. B. How are embryonic stem cells grown in the laboratory? … transferring the inner cell mass {of 4-5 day blastocyst} into a plastic laboratory culture dish … coated with mouse embryonic skin cells …called a feeder layer … If scientists can direct the differentiation of embryonic stem cells into specific cell types, they may be able to use the resulting, differentiated cells to treat certain diseases … Parkinson’s Disease Type I Diabetes

20. Immunology at the maternal-fetal interface: lessons for T cell tolerance and suppression.Mellor AL, Munn DHAnnual Review Of Immunology 18: 367-391 2000 … fetal alloantigensencoded by polymorphic genes inherited from the father oughtto provoke maternal immune responses leading to fetal rejection soon after blastocyst implantation in the uterine wall. No other tissue, when surgically transplanted between geneticallydifferent individuals, enjoys the impunity from lethal host (maternal) immune responses that characterize the maternal-fetalrelationship. This unique relationship resembles a parasiticcondition in which the fetus is nurturedand given immunological protection … Originally, Medawar proposed three broad hypotheses to explainthe paradox of maternal immunological tolerance to her fetus: (a) physical separation of mother and fetus; (b) antigenic immaturityof the fetus; and (c) immunologic inertness of the mother. { reject } { reject } { yes - but how?} The ‘Holy Grail’ of reproductive immunology hasbeen to elucidate the fundamental processes that explain fetalsurvival in all mammalian species.

21. Indoleamine 2,3-dioxygenase contributes to tumor cell evasion of T cell-mediated rejection.Friberg M et al. 2002. INTERNATIONAL JOURNAL OF CANCER 101:151-155. Prevention of T cell-driven complement activation and inflammation by tryptophan catabolism during pregnancy.Mellor et al. NATURE IMMUNOLOGY 2 (1): 64-68 JAN 2001Indoleamine 2,3 dioxygenase (IDO) {produced by fetal trophoblast} protects developing fetuses from maternal immune responses in mice. IDO inhibitor treatment triggered extensive inflammation at the maternal-fetal interface in susceptible mating combinations … These data show that IDO activity protects the fetus by suppressing T cell-driven inflammatory responses to fetal alloantigens. Tryptophan degradation by human placental indoleamine 2,3-dioxygenase regulates lymphocyte proliferation.Kudo et al. JOURNAL OF PHYSIOLOGY-LONDON 535 (1): 207-215 AUG 15 2001 CD4 T {helper} lymphocyte division was specifically suppressed by indoleamine 2,3 -dioxygenase-mediated tryptophan depletion. Our results show that mechanisms are present in the human placenta which are able to regulate cellular proliferation of the maternal immune system. This mechanism is dependent both on placental indoleamine, 2,3-dioxygenase-mediated tryptophan degradation and on tryptophan sensing systems within lymphocytes.

22. Genetic ConflictsIn Human-pregnancy.Haig D. Quarterly Review Of Biology 68:495-532 DEC 1993. Abstract:Fetal genes will be selected to increase the transfer of nutrients to their fetus, andmaternal genes will be selected to limit transfers in excess of some maternal optimum. During implantation, fetally derived cells (trophoblast) invade the maternal endometrium and remodel the endometrial spiral arteries into low-resistance vessels that are unable to constrict. … the fetus gains direct access to its mother's arterial blood. Placental hormones, including … human placental lactogen (hPL) … increase maternal resistance to insulin. … This action, however, is countered by increased maternal production of insulin. Gestational diabetes develops if the mother is unable to mount an adequate response to fetal manipulation. Similarly, fetal genes are predicted to enhance the flow of maternal blood through the placenta by increasing maternal blood pressure. Preeclampsia{a complex disease w/ hypertension} can be interpreted as an attempt by a … fetus to increase its supply of nutrients by increasing the resistance of its mother's peripheral circulation.

23. Genomic imprinting might have something to do with genetic conflicts & immune tolerance. Basics of gametic imprinting.Ruvinsky A. JOURNAL OF ANIMAL SCIENCE 77: 228-237, Suppl. 2 1999 Imprinted genes in mammals are expressed from only one chromosome: either the gene inherited from the mother or that from the father is silenced. Such genes include Igf2{insulin-like growth factor, maternal suppressed} and other genes involved in growth control; here, imprinting might work to balance maternal and paternal demands on the size of an embryo. The differential labelling of cytosine– guanine base pairs with methyl groups distinguishes the two copies of an imprinted gene. This differential methylation usually originates in the parental germ cells … … methylation of a gene ensures that it cannot be transcribed. Inheritable mechanisms that control gene expression without affecting gene sequence are said to be epigenetic.

24. Frequent loss of imprinting of IGF2 and MEST in lung adenocarcinoma.Kohda et al. 2001. Molecular Carcinogenesis 31:184-191. Abstract:… Accumulating evidence suggests that deregulation of imprinted genes, including loss of imprinting (LOI), plays a role in oncogenesis. … we investigated allelic expression of six imprinted genes in human lung adenocarcinomas as well as in matched normal lung tissue. LOI of the insulin-like growth factor 2 gene (IGF2) and mesoderm-specific transcript (MEST, also known as paternally expressed gene 1) was noted in 47% (7 of 15) and 85% (11 of 13) of informative cases, respectively.Monoallelic expression was maintained in all the matched normal tissues examined. … These findings indicated that independent deregulation took place in imprinted genes andsuggested that aberrant imprinting of IGF2 and MEST was involved in the development of lung adenocarcinoma.

25. Reproductive immunosuppression and diet - An evolutionary perspective on pregnancy sickness and meat consumption.Fessler DMT CURRENT ANTHROPOLOGY 43 (1): 19-61 FEB 2002Maternal immunosuppression, necessary for tolerance of the fetus, results in gestational vulnerability to pathogens. Throughout the period of maximal vulnerability, dietary behavior is significantly altered via changes in nausea susceptibility and olfaction and the development of marked aversions and cravings. Of food types, meat is both the most likely to carry pathogens and the principal target of gestational aversions and pregnancy taboos. Nausea and vomiting in early pregnancy: Its role in placental development.Huxley RR Obstetrics and Gynecology 95: (5) 779-782 MAY 2000 Nausea and emesis in early pregnancy is a common phenomenon affecting between 50% and 70% of pregnant women, but little is known about the etiology and possible function … Morning sickness … a positive effect on pregnancy outcome and is associated with a decreased risk of miscarriage, preterm birth, low birth weight (LBW), and perinatal death. Meaning of morning sickness still unsettled.Pirisi A. LANCET 357 (9264): 1272-1272 APR 21 2001

26. Ascription of resemblance of newborns by parents and nonrelatives.McLain et al. 2000.Evolution and Human Behavior 21:11-23. It has been hypothesized that human females ascribe the resemblance of their infants to the father or his relatives to promote assurance of paternity… renders fathers more likely to invest in and less likely to harm mothers and their children. … mothers were significantly more likely to ascribe resemblance to the domestic father than to themselves. This bias was exaggerated in the presence of domestic fathers. Yet, judges matched photographs of these mothers to their newborns significantly more frequently than they matched domestic fathers to newborns. The bias in how mothers remark resemblance does not reflect actual resemblance and may …. assure domestic fathers of their paternity. The low rate with which newborns are matched to fathers may be biologically significant. Concealment of paternity may be favored when suspicion of cuckoldry leads fathers to abandon or harm newborns. Genomic imprinting is one mechanism by which resemblance of newborns could be biased toward mothers. … { men selected to hide paternity ?} Reactions to children's faces - Resemblance affects males more than females.Platek et al. EVOLUTION AND HUMAN BEHAVIOR 23 (3): 159-166 MAY 2002