The cyclic series of events that lead to the release of mature egg cells (oocytes) from the ovary, i.e. ovulation, occurring in most female primates, including human females. It is controlled primarily by the level in the bloodstream of the gonadotrophin hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH) released by the anterior pituitary, which is stimulated by the hypothalamus. The ovarian cycle starts at the time of menstruation and is synchronized with the menstrual cycle, which prepares the uterus to receive a fertilized egg. During the follicular phase, several primary oocytes begin to mature in the ovary, each surrounded by a cluster of cells forming a Graafian or ovarian follicle. Initially the follicles release the sex hormone oestradiol at low levels; then the level rises to a peak, which triggers a surge in secretion of LH and FSH. This prompts the maturing follicle to move to the surface of the ovary and rupture, releasing the ovum into the fallopian tube. In humans usually only a single follicle matures; the others disintegrate. The second half of the cycle is the luteal phase, during which the residual follicle forms the glandular corpus luteum. This secretes progesterone and oestradiol, which promote thickening of the endometrium of the uterus, inhibit the release of gonadotrophin-releasing hormone (GnRH) by the hypothalamus, and hence reduce blood levels of FSH and LH (see illustration). If fertilization of the egg does not occur, continuing low levels of gonadotrophins cause the corpus luteum to degenerate and oestradiol levels to fall, thereby disinhibiting the hypothalamus and enabling another ovarian cycle to begin. In humans the ovarian cycle lasts about 28 days, like the menstrual cycle, with ovulation taking place around day 14.

Ovarian cycle: positive and negative feedback in hormonal control