gynecology and obstetrics
Female Reproductive Endocrinology
Ovarian Follicular Development
A female fetus has a finite number of egg precursors (germ cells). Germ cells begin as primordial oogonia, which migrate from the extraembryonic endoderm of the yolk sac into the undifferentiated genital ridge and become oogonia. Oogonia differentiate into primary oocytes and proliferate markedly by mitosis (single division into two identical diploid cells), resulting in about 7 million oocytes by the 5th month of gestation. During the 3rd month of gestation, some oogonia begin meiosis and arrest in the prophase of meiosis I until puberty. Meiosis occurs in two phases; it results in division into four haploid cells. By the 7th month of gestation, a layer of granulosa cells develops around all viable germ cells and forms a primordial follicle. After the 4th month of gestation, oogonia (and later oocytes) start to be lost spontaneously in a process called atresia; eventually, 99.9% are lost. Gametogenesis is the process of development from primordial germ cells to mature gametes: oogenesis in females and spermatogenesis in males. In both females and males, it begins with diploid germ cells that then undergo mitosis, meiosis, and cytodifferentiation into haploid gametes. At puberty, the oocyte completes meiosis I to produce one secondary oocyte and one polar body; these cells arrest in metaphase of meiosis II. FSH induces follicular growth in the ovaries. During each menstrual cycle, 3 to 30 follicles are recruited for accelerated growth. Usually in each cycle, only one follicle achieves ovulation. This dominant follicle releases its oocyte at ovulation and promotes atresia of the other recruited follicles. A single ovum is formed as a result of the two meiotic divisions—one just before ovulation and the second when the sperm penetrates. Polar bodies containing excess genetic material are extruded at each meiotic division. With increasing maternal age, the long time that surviving oocytes spend arrested in meiotic prophase may account for the increased incidence of genetically abnormal pregnancies ( Ovarian follicular development reference ). 1.Jones KT: Meiosis in oocytes: Predisposition to aneuploidy and its increased incidence with age.Hum Reprod Update14:143–158, 2008. 