The breeding cycle and the menstrual cycle

• In placental mammals the ovary becomes active at definite intervals (the breeding/sexual cycle) that coincide with mating.
• The length differs: once a year (lion, tiger), twice a year (cat, dog) or several times a month (rabbit, rat).
• In women it is the menstrual cycle, usually 28 days, in which the two ovaries alternate in producing a mature ovum.

Phases of the cycle (counting day 1 as the first day of bleeding):

(a) Menstruation (days 1–5): the endometrium breaks down and is shed with blood.

(b) Proliferation (follicular) phase (days ~5–13):

• the pituitary releases FSH, which stimulates the ovary to develop several follicles, of which one becomes the Graafian follicle (the ovum matures in about 10 days).
• The follicle secretes oestrogen, which stimulates the endometrium to grow (proliferate) and become thick and vascular.
• High oestrogen then stimulates a surge of LH.

(c) Ovulation (day 14):

• the pituitary releases a surge of LH, which causes the Graafian follicle to rupture and release the secondary oocyte (the ovum with the first polar body).
• The remains of the follicle turn into the corpus luteum (yellow body), which secretes progesterone and oestrogen.

(d) Secretory (luteal) phase (days 15–28):

• progesterone and oestrogen from the corpus luteum further thicken the endometrium, increase its blood supply and glands so that it is ready for the embryo.
• These hormones also inhibit FSH and LH (negative feedback), so no new follicle develops.

(e) If the ovum is not fertilised: the corpus luteum degenerates gradually; progesterone and oestrogen fall; the endometrium breaks down; the blood vessels rupture from uterine contractions; menstrual bleeding follows (about 3–5 days) and a new cycle (with the other ovary) begins.

(f) If the ovum is fertilised:

• the corpus luteum persists and continues secreting oestrogen and progesterone, which maintain the endometrium and inhibit ovulation, so the cycle stops until after birth.
• The corpus luteum is at its maximum at the end of the third month and starts to degenerate in the fourth month, when the placenta takes over secreting progesterone and oestrogen and preserves the endometrium.
• Progesterone also gradually develops the mammary glands.
• If the corpus luteum is removed before the fourth month, miscarriage (abortion) occurs; after that, it does not, because the placenta supplies the hormones.

Worked example 11 (calendar). A woman's cycle is regularly 28 days and her last bleeding began on 3 March. Ovulation is on day 14 = 16 March. The ovum lives 1–2 days, sperms 2–3 days: the fertile period is roughly 13–17 March. Next bleeding: 31 March (day 29 = day 1 of the next cycle).