Pharyngula

Human milk is potent stuff. The Greek for milk was gala, and as you might be able to see if I hadn't had to reduce this Tintoretto so much, the galaxies were created from the spray of milk from Hera's breasts. Modern astronomers might quibble with that explanation of the origins of the extrasolar universe, but what do I know...I'm a biologist. I'll stick to biology now, and with that, here's a short summary of the biology behind mother's milk.

A recipe:

Milk is a rich (but not too rich) food source for suckling infants. It contains sugars and fats enough to keep a baby alive; cow's milk is roughly similar in fat content at about 5%. That fat content is not particularly high (marine mammals hold the record: some seal milk has a fat content of 50-60%), though, so infants need to nurse at frequent intervals during the day and night, as all parents know.

A little-appreciated fact is that we mammals are generally fairly slimy creatures. Our skin is a glandular organ that is constantly secreting fluids, oils, fats, and proteins, many with a defensive function. Our sweat and tears contain a protein called lysozyme, an antibiotic agent that digests bacteria. We also dribble out a particular kind of antibody called immunoglobulin-A, which can bind to and coagulate bacterial invaders. Note that human milk contains the same agents, in higher concentration; α-lactalbumin is a protein related to lysozyme. Milk also contains lactoferrin, a protein that carries iron and also has antibiotic and fungicidal properties. During the first few days of lactation, the breast secretes an immunological cocktail called colostrum exclusively, and only later adds the fats to the mix. Milk is thought to have evolved from simple, sweaty secretions from the skin that were lapped up by the infant, and we see something similar in monotremes that just secrete milky fluids from nippleless glands on their chest.

Several hormones regulate milk production. The glands of the breast are normally quiescent, until triggered by estrogen to mature and begin actual milk production. The elevated levels of estrogen in the later months of pregnancy prepare the breast for secretion; the breast doesn't actually produce anything because a second hormone that is maintained at high levels during pregnancy, progesteron, suppresses secretion. After birth, when progesterone levels decline, the breast is then ready and free to begin making milk.

Another signal is needed to actually trigger secretion by the cells of the gland: prolactin. The production of prolactin by the anterior pituitary stimulates milk synthesis and production by the glands that have been prepared by estrogen. What triggers prolactin? Another hormone, oxytocin. Stimulation of the nipple triggers the release of oxytocin from the posterior pituitary, which has multiple effects. Oxytocin directly causes the contraction of smooth muscle cells surrounding the ducts, forcing milk from the nipple (it is also the hormone that stimulates contractions of the smooth muscle of the uterus, initiating labor). Oxytocin inhibits the release of hypothalamic factors that suppress prolactin, and so causes a prolactin surge to ten times the resting level; that prolactin increase maintains the production of milk. Not nursing for a few days allows prolactin levels to decline, and milk secretion will end.

Males have the capacity to produce milk as well. They don't, usually, because their estrogen levels aren't high enough, so the glandular tissue of their breasts are not fully differentiated. Men who have been treated with estrogen as part of cancer therapy will develop functional breasts, and even without estrogen they possess the circuitry to produce prolactin surges in response to nipple stimulation. Diamond, in his book Why is Sex Fun, has a nice story about that:

My favorite case is the chauvinist husband who kept complaining about his wife's "miserable little breasts", until he was shocked to find his own breasts growing. It turned out that his wife had been lavishly applying estrogen cream to her breasts to stimulate the growth craved by her husband, and the cream had been rubbing off on him.

One last hormonal effect of nursing is that it suppresses the production of yet another hormone, gonadotropin releasing hormone (GnRH), by the hypothalamus. The reduction in GnRH reduces levels of two other hormones, luteinizing hormone and follicle stimulating hormone, that are essential in inducing ovulation. Reduced likelihood of ovulation reduces fertility while lactating.