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Ovary is a part of the female reproductive system. The ovary has two main functions: production of steroid hormones and generation of mature oocytes (female gametocytes). The ovarian follicle is the functional unit of the ovary, in which folliculogenesis takes place. In this process, the oocytes mature, surrounded by granulosa cells and thecal cells, which produce the androgen substrate required for ovarian estrogen biosynthesis.
Clusters of primordial germ cells (PGCs) with somatic cells come closer to form ovigerous cords, which is first discernible in the fetal ovary upon establishment of initial contact between germ cells and somatic cells near the surface of the ovarian epithelium.
The first mechanistic difference between an XX and an XY germ cell in the genital ridge is reactivation of the inactive X in the female PGCs. The XX germ cells continue to divide and then enter meiosis at around E12.5. Subsequently, the female germ cells arrest at the diplotene stage of meiosis I and do not resume meiosis until postnatal ovarian folliculogenesis.
Folliculogenesis: Before formation of an ovarian follicle, oocytes are present within germ cell clusters (cysts or nests). The first stage of ovarian folliculogenesis involves the formation of the primordial follicle, which occurs when oocytes that survive the process of germ cell cluster breakdown are individually surrounded with squamous pre-granulosa cells. This takes place during the 20th week of gestation, in humans, and in the first postnatal days, in mice. In mammals, the population of primordial follicles serves as a resting and finite pool of oocytes available during the female reproductive life span. In the transition from the primordial follicle into the primary follicle, the flattened granulosa cells transform into cubic cells. The secondary follicle is characterized by multiplication of the granulosa cells to form a second layer around the oocyte, while the antral (tertiary) follicle is characterized by the formation of a fluid-filled cavity, adjacent to the oocyte, called the antrum. Granulosa and theca cells continue to undergo mitosis concomitant with an increase in antrum volume. Antral follicle growth is gonadotropin-dependent, in contrast to preantral follicular development, which does not require stimulation by the pituitary gonadotropins. At the end of its growth, the mature follicle reacts to a discharge of gonadotropic hormones by transformations which lead to follicular rupture (ovulation) and subsequent formation of a corpus luteum (CL).