Oogenesis
Oogenesis The production of female gametes begins before birth. Primordial germ cells migrate from the yolk sac of the embryo to the gonads (reproductive organs), where they differentiate into oogonia. Oogonia undego multiple mitotic divisions, and are arranged in clusters surrounded by a layer of epithelial/follicular cells by the end of the 3 rd month of development. The oogonia continues to divide, however some of them start meiosis, arresting at the diplotene stage of meiosis 1(stage when crossing over occurs). These are now called primary oocytes. The number of germ cells in the ovaries reaches its maximum at about 5 months of gestation, at around 7 million. After 5 months, a portion of oogonia and primary oocytes begin to degenerate becoming atretic. By the 7 th month, most cells hav degenerated except for those near the surface, these have all entered meiosis 1 and most are surrounded by a layer of epithelial cells. A primary oocyte together with the epithelial cells is c