The Sexual Cycle and Ovulation

At sexual maturity (puberty), the female begins a sexual cycle (Le.. a menstrual cycle in bumans) that is besed on rhythmic interaction between the hypothalamo/hypophyseal axis and the ovaries. A given cycle is initiated when the hypothalamus begins secreting gonadotropin-releasing hormone (GnRH) into the hypothalamo-hypophyseal portal system where it travels to the anterior hypophysis (pituitary) and stimulates gonadotropes to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LII).

Under the influence of these two gonadotropic hormones from the pituitary, primordial follicles begin growing in the ovary.

During this developmental process known as folliculogenesis, a large antral cavity forms in the center of the follicle and a thick layer of collagenous connective tissue forms around its perimeter. As the follicle grows, it begins secreting androgens and estrogens.

Ovarian b -estradiol promotes the expression of gonadotropin receptors on the plasma membranes of follicular cells. A follicle is said to be mature when it is endowed with an adequate population of ganadotropin receptors that are responsive to LII and/or FSH.

At this stage of the sexual cycle, the elevated level of circulating b - estradiol induces a sudden increase in GnRH secretion from the neurosecretoly cells of the hypothalamus, and this releasing hormone causes a surge in LII and FSH secretion from the pituitary gland. This surge in gonadotropins initiates the ovulatory process.

During the next several hours, androgen and estrogen secretion is replaced by a marked increase in ovarian progesterone synthesis. The rise in this progestin signals the onset of luteinization of the ovarian follicle. In addition, the elevation in circulating progesterone inhibits further secretion of GuRN, LH and FSH.

The hypothalamo/hypophyseal axis begins to secrete these hormones once again to initiate the next sexual cycle only after the corpus Interim begins to deteriorate (i.e., undergo luteolysis) and progesterone secretion is diminished.