oogonia

Oogonia are the female germ cells that give rise to oocytes. They arise from primordial germ cells that migrate to the developing ovaries during embryogenesis and proliferate by mitotic division, forming an extensive pool of oogonia.

During fetal life, oogonia differentiate into primary oocytes and enter meiosis I, becoming arrested in prophase I. Many die by atresia, and the surviving oocytes become enclosed in primordial follicles. By birth, the ovarian reserve consists of these primordial follicles containing primary oocytes; the total number is finite and declines with age.

From puberty onward, the pool declines further. In each menstrual cycle, a subset of primordial follicles is recruited; the selected primary oocyte completes meiosis I at ovulation to form a secondary oocyte and the first polar body. Meiosis II is completed only if fertilization occurs, yielding a haploid ovum and a second polar body.

Notes: In humans, oogonial division and formation of primary oocytes occur during fetal development; in other species timing may differ. The term “oogonia” refers to the proliferating germ cells before they enter meiosis, whereas “oocytes” denotes cells that have begun meiosis.