Development and maturation of oocytes occur within ovarian follicles. Successful ovulation requires appropriate follicular development, during which the follicle has passed through a number of distinct developmental stages (Hirsheld, 1991). Throughout the life of a mammalian female preceding each ovarian cycle, some follicles are selected to develop to maturity for ovulation and potential fertilization. The most immature stage of ovarian follicular development is termed primordial. This is the stage at which follicles rst appear in the ovary of a developing female fetus. Development of a primordial (25 μm diameter) to an ovulatory follicle involves transitions through several stages as a pre-antral follicle (25-250 μm diameter; primordial, primary, small growing, large growing) and later as an antral follicle (>250 μm diameter; early antral and pre-ovulatory). The stages of follicular development toward ovulation involve a continuum of events, each providing further maturation of the follicular cells (Figure 5.1; Hoyer and Devine, 2001). Upon receipt of an as yet unknown signal for development, the primordial follicle is activated and becomes a primary follicle. As the follicle develops, there is proliferation of the granulosa cells surrounding the oocyte, and acquisition of a layer of theca interna cells surrounding the granulosa layer. Follicles progress from the primary stage to the growing stage when multiple layers of granulosa cells have formed around the FIGURE 5.1 Development of ovarian follicles. Primordial follicles are activated to grow and develop from primary through secondary and antral stages of follicular development until ovulation. Most follicles degenerate by the process of atresia before reaching the ovulatory stage. Those follicles which do ovulate luteinize to become corpora lutea, which support pregnancy if fertilization occurs. (From Hoyer, P.B. and Devine, P.J., Endocrinology and toxicology: The female reproductive system, In Handbook of Toxicology, 2nd edn., eds. M.J. Derelanko and M.A. Hollinger, CRC Press, Boca Raton, FL, 2001. With permission from CRC Press.) oocyte. When the follicle develops sufciently, an antrum (uid-lled space) develops within the granulosa cell layer. The antral follicle continues to grow, and at its most mature stage prior to ovulation is known as a Graafan (pre-ovulatory) follicle. Resumption of meiosis in the oocyte occurs only at the time of impending ovulation. The exact mechanism for selection of a follicle for ovulation is not well understood, but is believed to be under intra-ovarian control (Richards, 1980).
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