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Chapter 2: Cell Structure and Function by John F. NeasOogenesis Oogenesis begins in fetal life when diploid (2N) oogonia develop from primordial germ cells and populate the ovarian cortex. Oogonia serve as stem cells during fetal life. Through repeated mitotic division, the oogonia produce daughter oogonia that incorporate into primordial follicles of the ovarian cortex. Mitotic divisions continue until about the fifth month of fetal life. At this time, each ovary contains more than 3 million oogonia. The oogonia enlarge and divide repeatedly. Therefore, unlike the male in which spermatogonia increase by mitosis throughout life, all oogonia develop prenatally and their numbers continuously decrease after birth. Beginning in the third fetal month, some oogonia differentiate into primary oocytes (2N) that enter the first meiotic prophase. In humans, this process concludes by the end of the seventh month of gestation at which time all primary oocytes are in the first meiotic division. The latent period during which they continue in prophase of the first meiotic division extends until ovulation at puberty. Then, the next maturation division begins, ultimately ending with production of haploid (1N) ova. During this time, the ovary loses many primary oocytes through a degenerative process called atresia. At 20 weeks gestation age, the ovaries contain more than two million primary oocytes or their precursors. Most of these regress during childhood. There are only about 300,000 primordial follicles by 7 years of age and fewer than 40,000 by puberty. Only about 450 oocytes ovulate during the approximately 30 years of reproductive life in the female. Spermatogenesis Spermatogenesis is the process of meiotic divisions that produces male germ cells (spermatozoa), a process that occurs in the recesses of the Sertoli cells in the seminiferous tubules of the testes. The first stage of spermatogenesis is spermatocytogenesis in which the spermatogonia develop into spermatocytes and then into spermatids. The second stage in the formation of spermatozoa is spermiogenesis (spermateliosis), in which the spermatids transform into spermatozoa. Spermatocytogenesis involves developmental changes of the nucleus, but spermiogenesis chiefly involves the cytoplasm. At any time during reproductive life in the male, the different seminiferous tubules exhibit all phases of spermatogenesis. There are waves of activity where particular areas show a particular stage of development while other areas may be temporarily quiescent. Spermatogenesis, like oogenesis, is a function of meiotic division, but now the enlargement of one spermatogonium produces a primary spermatocyte (46 chromosomes, 44 + XY, 4N). Primary spermatocytes enter Prophase I promptly after their formation. Meiosis I produces two secondary spermatocytes (23 chromosomes as dyads, or 2N). Secondary spermatocytes are short-lived; they quickly enter meiosis II to form four spermatids (two with 22 + X chromosomes and two with 22 + Y chromosomes). The spermatids mature into sperm.
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