Oogonia divide by mitosis and enter meiosis during the first five months of fetal development. They do not complete meiosis, but are arrested in prophase of meiosis I. These are the primary oocytes that have a fixed number at birth (no more are produced after birth). These dormant primary oocytes are stored within primordial follicles in the ovaries. Some 40,000 remain at puberty. Of these about 400 ovulate over the lifetime of the female.
At the start of each menstrual cycle, some 12-20 primary oocytes begin to develop within primary follicles. A few of the primary follicles continue to develop into secondary follicles, which accumulate follicular fluid within the follicular antrum. Within the secondary follicle, one primary oocyte per month completes meiosis I and begins meiosis II, then halts at metaphase.
This secondary oocyte is surrounded by granulosa cells, forming a corona radiata. Between the granulosa cells and the secondary oocyte is a thin clear gel, the zona pellucida. The secondary oocyte with its surrounding granulosa cells is exposed to the follicular fluid of the antrum, forming a tertiatry (Graafian) follicle. The granulosa cells produce proteoglycans that draw fluid into the antrum, eventually causing it to rupture on day 14 of the menstrual cycle, releasing the oocyte to be captured by the fimbriae of the Fallopian tube.
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