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The purpose of reproduction is to produce offspring. The physiological processes of reproduction include fertilization, pregnancy and delivery. Reproduction is modulated by the neuroendocrine system of hypothalamus-adenohypophysis-gonadal axis.
The testes must descend out of the abdomen to function properly. Spermatozoa are produced by mitosis, followed by meiosis, and then followed by spermatogenesis. The main hormones of spermatogenesis are LH, testosterone, and FSH, together with several auxillary hormones.
The first half of the ovarian cycle involves follicular development, which is estrogen-dominated and is separated by a short ovulatory phase from the progesterone-dominated second half, or luteal phase, which prepares the uterus for implantation. All of a woman's primary oocytes (2 million) are present at birth; no more are made afterwards. The ovarian cycle consists of one of the several follicles developing into a primary follicle, then Graafian follicle, then a corpus luteum under the influences of FSH and LH. Ovulation is triggered by a powerful surge of gonadotrophins. The ovarian cycle is "shadowed" by the menstrual cycle during which the uterine endometrium is firstly proliferative (due to estrogen) then secretory (due to estrogen and progesterone). Decline in progesterone and estrogen when the corpus luteum degenerates precipitates a menstruation.
Human pregnancy lasts 40 weeks, divided into three trimesters. Only 1 per 100,000 sperm of the 200~400 million contained in the 3~4 ml of the ejaculate deposited near the cervix, where capacitation takes place, reach the fallopian tubes where fertilization ideally occurs. Enzymes from the acrosomes of the many sperm digest the intercellular cement between the cumulus cells, allowing a single sperm to penetrate. On penetration by the first sperm, the ovum becomes impervious to other sperm. A second polar body is released. About 6 days after fertilization, the zygote (now a blastocyst) implants. The placenta takes over the production of estrogen and progesterone from the corpus luteum after the first trimester of the pregnancy. Placenta progesterone inhibits inappropriate contractions of the myometrium. Labor is divided into three phases: first－from onset of contractions to full dilatation of the cervix; second－to delivery of the fetus; third－the expulsion of the placenta.
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