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HUMAN REPRODUCTION BIOLOGY 269

HUMAN REPRODUCTION BIOLOGY 269. Assuming: You paired up with an appropriate partner You elicited normal sexual responses from each other A few milliliters of semen, containing 200- to 600-million sperm, were deposited high in the vagina, just below the cervix of the uterus. Scenario I:

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HUMAN REPRODUCTION BIOLOGY 269

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  1. HUMAN REPRODUCTIONBIOLOGY 269

  2. Assuming: You paired up with an appropriate partner You elicited normal sexual responses from each other A few milliliters of semen, containing 200- to 600-million sperm, were deposited high in the vagina, just below the cervix of the uterus

  3. Scenario I: The woman is not within a few days before or after ovulation • Her cervix will be • The few sperm which might make it into the Fallopian tubes • The sperm do not meet an oocyte and die within 48 hours • No fertilization occurs

  4. Scenario 2: The woman has just ovulated, or she will ovulate within 1 - 2 days. • The mucous plug in her cervix will be • Most sperm will die in her vagina, but a few million (1 - 2%) are pulled through the cervix, into the uterus, and into the Fallopian tubes where they are propelled toward the ampulla (+100,000 make it this far) • Sperm meet the oocyte, and fertilization can occur

  5. At ovulation, oocyte is surrounded by a thick membrane called the Fimbria

  6. Oocyte is viable for Fertilization occurs within If fertilization does not occur, oocyte will degenerate and die

  7. A few thousand sperm reach the ampulla of the Fallopian tube within 30 to 60 minutes after ejaculation But They are not capable of fertilizing oocyte Must undergo processes called and which will take 8 to 10 hours after ejaculation

  8. Capacitation: Prior to ejaculation, membrane around head of sperm is strong Semen contains chemicals to prevent weakening Tail is moderately active After ejaculation,

  9. Acrosome Reaction: Occurs as sperm make contact with corona radiata and zona pellucida Requires Acrosome swells,

  10. (Acrosome swells, becomes leaky, releases digestive enzymes) Enzymes digest path through corona radiata and zona pellucida Hundreds of sperm usually involved in this digestion

  11. Head of one sperm makes contact with outside of oocyte, binds onto it, and is quickly taken into it. Two systems prevent entry of more than one sperm: Fast block: Slow block:

  12. Once head of sperm enters oocyte, its nucleus is released into the cytoplasm. Nucleus of sperm (23 chromosomes) and nucleus of oocyte (23 chromosomes) meet and fuse together. Cell now called zygote, with 46 chromosomes

  13. Zygote undergoes repeated divisions (mitosis), called , as it is transported through Fallopian tube (oviduct) toward uterus Eventually forms solid mass of 20 – 30 cells called , same size as the zygote. This reaches uterus

  14. As oocyte was developing, follicle in ovary secreted , which caused At time of ovulation and fertilization, follicle develops into which begins to secrete progesterone, keeping When morula reaches uterus, endometrium is thick with lots of secretions.

  15. Morula does not embed in uterus immediately. Stays loosely attached for 4 to 5 days, nourished by secretions of endometrium. Continues dividing into +100 cells, forming hollow sphere called

  16. Blastocyst develops mass of cells at one end, called , which will develop. Cells surrounding cavity, called , will develop into the

  17. Cells of trophoblast secrete digestive enzymes, forming finger-like projections into endometrium. Blastocyst burrows into endometrium, which will heal over. This process is called.

  18. Trophoblast begins to secrete new hormone, HCG stimulates corpus luteum of ovary to remain active and to continue secreting This progesterone keeps . Thus, HCG = hormone detected by pregnancy tests HCG produced + 8 weeks, then placenta mature enough to produce progesterone. Corpus luteum in ovary slowly degenerates, but progesterone from placenta prevents new follicles from developing

  19. Back to the blastocyst As trophoblast is causing implantation and developing into placenta, Embryoblast is enlarging and developing into embryo Fluid-filled space called Embryoblast = flat disk between amniotic cavity and blastocyst cavity. Now called (12 – 14 days after fertilization)

  20. Pre-embryonicperiod now over, and embryonic period begins. During next six weeks of embryonic period, cells of embryoblast will differentiate into beginnings of all organs. After 8 weeks organs will grow, develop into their final forms, and begin to function during the fetal period. Fertilization – 2 wks = 2 wks – 8 wks = 8 wks – birth = x

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