The endocrinology of the menstrual cycle.

The ovulatory menstrual cycle is the result of the integrated action of the hypothalamus, pituitary, ovary, and endometrium. Like a metronome, the hypothalamus sets the beat for the menstrual cycle by the pulsatile release of gonadotropin-releasing hormone (GnRH). GnRH pulses occur every 1-1.5 h in the follicular phase of the cycle and every 2-4 h in the luteal phase of the cycle. Pulsatile GnRH secretion stimulates the pituitary gland to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH). The pituitary gland translates the tempo set by the hypothalamus into a signal, LH and FSH secretion, that can be understood by the ovarian follicle. The ovarian follicle is composed of three key cells: theca cells, granulosa cells, and the oocyte. In the ovarian follicle, LH stimulates theca cells to produce androstenedione. In granulosa cells from small antral follicles, FSH stimulates the synthesis of aromatase (Cyp19) which catalyzes the conversion of theca-derived androstenedione to estradiol. A critical concentration of estradiol, produced from a large dominant antral follicle, causes positive feedback in the hypothalamus, likely through the kisspeptin system, resulting in an increase in GnRH secretion and an LH surge. The LH surge causes the initiation of the process of ovulation. After ovulation, the follicle is transformed into the corpus luteum, which is stimulated by LH or chorionic gonadotropin (hCG) should pregnancy occur to secrete progesterone. Progesterone prepares the endometrium for implantation of the conceptus. Estradiol stimulates the endometrium to proliferate. Estradiol and progesterone cause the endometrium to become differentiated to a secretory epithelium. During the mid-luteal phase of the cycle, when progesterone production is at its peak, the secretory endometrium is optimally prepared for the implantation of an embryo. A diagrammatic representation of the intricate interactions involved in coordinating the menstrual cycle is provided in Fig. 1.