Female germ cells develop in the embryonic yolk sac and migrate to the genital ridge where they participate in the development of the ovary (Table 38.1). Without germ cells, the ovary does not develop. The germs cells, called oogonia, actively divide by mitosis. Oogonia undergo mitosis only during the prenatal period. By birth, the ovaries contain a finite number of oocytes, estimated to be about 1 million. Most of them will die by a process called atresia. By puberty, only 200,000 oocytes remain,- by age 30, only 26,000 remain,- and by the time of menopause, the ovaries are essentially devoid of oocytes.
When oogonia cease the process of mitosis, they are called oocytes. At that time they enter the meiotic cycle (or meio-sis, to prepare for the production of a haploid ovum), become arrested in prophase of the first meiotic division, and remain arrested in that phase until they either die or grow into mature oocytes at the time of ovulation. The primordial follicle (Fig. 38.4) is 20 |xm in diameter and contains an oocyte, which may or may not be surrounded by a single layer of flattened (squamous) pregranulosa cells. When pregranulosa cells surround the oocyte, a basement membrane develops, separating the granulosa from the ovarian stroma.
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