Development of the ovary and oogenesis

Abstract

This review of the literature deals with processes ranging from the migration of germ cells from the yolk sac to the germinal ridge, to the atresia of the Graafian follicle. The migration stage is followed by sex differentiation. Whereas oogonia disappear, oocytes develop as far as the diplotene (resting) stage of the meiotic prophase, and only during sexual maturity is meiosis resumed at the diakinesis stage. In the female, the second meiotic division is arrested in the metaphase until fertilization occurs. The factors controlling this are obscure, they may be connected with stimulating factors produced by sex differentiation, by the mesonephros or the rete ovarii; the resumption of meiosis after resting is controlled by gonadotrophins. The origin of the granulosa cells and of the zona pellucida is also discussed; the interdigitation of microvilli growing from the oocyte and from the granulosa cells into the zona pellucida may indicate the transfer of material from the mother to the embryo. For follicular growth, gonadotrophins are needed as a trigger, but then development proceeds wihout them during the preantral stage; in the later stages, however, they are needed permanently. The factors regulating th degeneration of germ cells (atresia) are poorly understood; they may be genetic (e.g., loss of one X chromosome), environmental (loss, or genetic incompatibility of granulosa cells, and gonadotrophins produce atresia, estrogens act in the opposite way), or metabolic. The purpose of atresia is to regulate litter size and to increase the supply of steroid hormones.