I

Primary oocyte

Primordial follicle

Granulosa cells

Granulosa cells

Theca Interna

Primary follicle

Zona pellucida Theca externa

Theca interna interna

Antrum fcL.

Primordial follicle y " y

Primary follicle

Zona pellucida Theca externa

Theca interna interna

Theca Externa
Secondary follicle

Figure 2.1 From the pool of primordial follicles, every day some begin to grow and develop into secondary (preantral) follicles, and this growth is independent of FSH. Then, as the cycle progresses, FSH secretion recruits primary follicles to begin development into secondary (antral, Graafian) follicles. During the last few days of maturation of secondary follicles, estrogens, produced by follicular and thecal cells, stimulate increased production of LH by the pituitary (Fig. 2.13), and this hormone causes the follicle to enter the preovulatory stage, to complete meiosis I, and to enter meiosis II where it arrests in metaphase approximately 3 hours before ovulation.

factor-9 (GDF-9), a member of the transforming growth factor-^ (TGF-^) family In cooperation, granulosa and thecal cells produce estrogens that (a) cause the uterine endometrium to enter the follicular or proliferative phase; (b) cause thinning of the cervical mucus to allow passage of sperm; and (c) stimulate the pituitary gland to secrete LH. At mid-cycle, there is an LH surge that (a) elevates concentrations of maturation-promoting factor, causing oocytes to complete meiosis I and initiate meiosis II; (b) stimulates production of progesterone by follicular stromal cells (luteinization); and (c) causes follicular rupture and ovulation.

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