Week 6 Study Guide2

Week 6 Study Guide2 - Lecture#10 Tuesday Menstrual Cycle...

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Lecture #10: Tuesday, 2/9/10 Menstrual Cycle Female Gametogenesis Ovarian Cycle Follicular Phase Ovulatory Phase Luteal Phase Endometrial Cycle Menstrual Phase Proliferative Phase Secretory Phase Female Gametogenesis Oogenesis is the process of female gametogenesis, forming a mature ovum from immature precursors. Refer to Figure 31 below. In contrast to males, who continuously produce spermatocytes from the onset of puberty, females have a limited number of ova. By about 20 weeks of fetal age, the female fetus produces its maximum number of seven million oogonia, which individually consist of a single primordial follicle. By birth, the female baby bears only 1- 2 million primary oocytes, which are arrested at Prophase I; and finally, at puberty, her stores have dwindled to about 500,000 immature ova. Every month after puberty, when the pulsatile secretion of GnRH begins, about 10-15 of the primary oocytes are recruited to undergo one round of the menstrual cycle. By about Day 9 of the cycle, only one of the primary oocytes, called the dominant follicle, remains as all others have undergone atresia, which is hormonally regulated apoptosis. Immediately preceding mid-cycle, the LH surge causes the primary oocyte to complete Meiosis I and commence Meiosis II, transforming the primary oocyte to a secondary oocyte; however, the secondary oocyte remains arrested in Metaphase II. In addition to generating the secondary oocyte, the completion of Meiosis I yields the first polar body, which consists of the condensed set of haploid chromosomes resultant of the reductional division. The polar body degenerates soon after its formation. About 36 hours after the onset of the LH surge, which is a result of the positive feedback loop generated by increasing levels of estradiol, the secondary oocyte is ovulated, or ejected into the oviduct. After ovulation, the secondary oocyte is propelled forward toward the uterus by the ciliary action of the fallopian tube. The secondary oocyte remains viable about 24-36 hours after ovulation; furthermore, spermatozoa can remain viable in the oviduct up to 3-5 days after ejaculation. Sperm fusion and fertilization of the secondary oocytes triggers the completion of Meiosis II, yielding the second polar body, which disintegrates quickly thereafter, and the mature haploid ovum. Fertilization generally occurs in the ampulla, the proximal region of the oviduct, closest to the ovary. Subsequently, the blastula implants into the uterus about 8-10 days after fertilization.
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Figure 30. Oogenesis. Note that oogenesis generates only one viable haploid ovum, whereas spermatogenesis generates 4 viable haploid spermatozoa through a single round of Meiosis I and II. Figure 31. The Menstrual Cycle.
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Week 6 Study Guide2 - Lecture#10 Tuesday Menstrual Cycle...

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