Research advances in regulating mechanisms of mammalian ovarian folliculogenesis

Ovary, the female gonad in mammals, is a heterogeneous organ consisting of oocytes and various types of somatic cells. The functions of ovary is not only governing the health of individual female by regulating endocrine status, but also determining the production of mature oocytes which allow the continuation of species. As the fundamental unit of female reproduction, ovarian follicle consists of germline oocyte and follicle somatic cells, and the folliculogenesis is an accurate and orderly process of internal coordination and external regulation in mammals. The gonadotropin-dependent stage of follicle development, from early antral follicle to ovulation, directly regulates the reproductive cycles of the female, has been extensively investigated. Recently, increased lines of evidence show that the fine tuned early folliculogenesis plays a pivotal role in the maintenance of female reproductive lifespan. Further exploration of the mechanism of follicular development could lead to a more comprehensive understanding about how females maintain their proper reproductive lifespan in mammals, which may provide the possibility to design new therapeutic approaches against female reproductive ageing in future. With the advances of technologies and methods, especially the widespread application of genetically modified animals and novel microscopic technology, the research on regulating mechanisms of in vivo follicular development, especially the early stage development of follicles, has made great progress. In this review, we summarized the regulating mechanisms of in vivo folliculogenesis around the key developmental events under physiological conditions, with a focus on the research progress of the early development of follicles in recent years.