Reproductive

Overview

Oogenesis: process of forming female gametes that occurs in the ovaries 

Females are born with practically all the primary oocytes they will have 

There have been recent studies that predict it may be possible to develop more, however more validation is required (Telfer, et al., 2016). 

Women are born with one to two million, with a loss of 11,000 oocytes every month before puberty. Each month after puberty, about 1,000 oocytes are lost (at the start of puberty a woman will have 300,000 to 500,000). In the 20s, there are between 150,000 to 300,000, by the late 30s there are about 27,000 (fertility begins to decrease in early 30s and rate of oocyte loss increases at 35). By the early 40s, there are only 5,000 to 10,000 oocytes and there is less than a 5% chance of pregnancy per menstrual cycle. Only 300 to 500 oocytes actually reach maturity, most are lost to follicular atresia. (Note: this is in comparison to the more than 1 trillion mature spermatozoa that males produce and there is no age in which males stop producing sperm) 

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During oogenesis, a diploid germ cell undergoes meiosis to produce a haploid ovum. The process begins during fetal development, when oogonia (precursor cells) divide mitotically to produce primary oocytes. These primary oocytes are arrested in prophase I of meiosis until puberty. At puberty, one primary oocyte is selected each month to resume meiosis and complete the first meiotic division, producing a secondary oocyte and a polar body. The secondary oocyte then begins the second meiotic division but is arrested in metaphase II until fertilization occurs. If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature ovum and a second polar body.

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Follicles are clusters of cells that surround and nourish the primary oocyte

Primary follicle -> secondary follicle -> vesicular follicle 

The secondary oocyte is in the dominant follicle, and is the only one that actually resumes meiosis I.