Breasts.
It's impossible to be neutral about them. Are they decorative? Functional? Recreational? Obscene? Totems of momism? Natural? Too big? Too small? Too saggy? Too artificial?
They might be one of the best marketing tools in Twentieth Century America and beyond. Seen a Daily Kos T-shirt draped on a pneumatic model, lately?
One thing is for certain: breasts are mammalian exocrine glands, and I'll take you on a short tour of their evolution, their forms, their function, their physiology and some of their quirks.
Evolution
From an evolutionary standpoint, breasts are modified sweat glands, closely related to the
apocrine sweat glands, and belong to the integumentary system. In the egg-laying mammals such as the platypus and echidna, the mammary glands open along the belly, lacking nipples, and the young just lap milk up, like so many beads of glorified sweat (right, platypus hatchlings lick lunch).
Link to kangaroo joey on nipple.
Marsupials and placental mammals (e.g. humans) have specialized nipples. While marsupials don't lay eggs, they nourish their embryos inside with a sort of yolk sac, then give birth at 4 to 5 weeks. What looks like a barely formed embryo crawls up into the mother's pouch or marsupium, attaches to a nipple and remains that way for a long time as development proceeds. In the link above, you see a newborn kangaroo joey attached to one of the nipples in the pouch.
Placental mammals (which would be all the rest, from mice to whales) all have nipples placed according to the needs of their young, most of whom are pretty independent immediately following birth, except, perhaps for the human, who has an enormous head and very little muscle tone. No one really knows why the human breast has filled out with fat the way it has; other primates remain flat-chested unless lactation is occurring. My horseback theory is that protuberant, mobile breasts are better suited to an uncoordinated infant who can't walk, lift his head or cling to fur, as the baby bonobo at right is doing.
We all have echoes of the milk-sweating platypus in our development. All lactating tissue forms along the "milk ridge," or “milk line,” which are two bands that in the human originate on the anterior thigh and run cephalad up the ventral skin, the front of the chest, and terminate in the axillae (See the figure of an 8-week embryo at left). In humans, two mammary glands are placed near the armpits, while in other animals the glands may be toward the hind limbs, or multiple glands may run along the entire length (e.g. carnivores like cats, with eight nipples).
Extra nipples and/or glandular tissue along the milk line is very common in humans(up to 2% occurrence, favoring females). The usual case is an extra nipple someplace along the embryonic milk line. Glandular tissue associated with the nipple is much more rare. In your authoress' case, she was born with a raised mole to the right of her bellybutton. It turned out to be a nipple, in keeping with her atavistic uterus. The proper term is polythelia.
Structure
Glandular tissue containing milk-producing alveoli is ensconced in a fat-padded breast, the shape of which is retained by Cooper’s ligaments in the skin and a ridge of connective tissue under each breast mass at the level of the fifth rib. Each breast contains 15-25 lobes of glandular tissue arranged radially and draining into a lactiferous sinus near the nipple. Tiny ducts (numerous and varying in number) drain from the nipple. In a nonpregnant or nonlactating state, the glandular tissue of the breasts is fairly undeveloped. Males also have this glandular tissue and theoretically lactate, although this does not happen under normal hormonal conditions.
Normal structure excludes silicone gel packs or mastopexy scars, nipple rings, tattoos, pasties or glued-on coconuts.
Function
Breasts are for feeding babies; human milk is the sole source of nutrition for a a baby from birth to 6-12 months, and it plays a decreasing role in the years thereafter, although it is not uncommon for very young children to nurse for several years. For an older child, breast milk supplements the regular diet and still provides some passive immunity. It is a vastly superior food when compared to formula.
A secondary function of breasts is to make enormous loads of money for various industries. I realized this when I was 15, and spied a cardboard box of potatoes decorated with a line drawing of a well stacked woman in a bikini kneeling by a pile of their product.
Lactation hormones: pregnancy through weaning
Breasts are cyclical critters, as almost any woman of childbearing age will affirm. Estrogen and progesterone cause breasts to swell and develop greater vasculature, and otherwise change during the luteal or postovulatory phase of the menstrual cycle, and if a pregnancy takes hold, the growing placenta will eventually secrete human placental lactogen (hPL) and human chorionic somatomammotropin (hCS). All these hormone work together together to mature the breasts for lactation. A friend of mine referred to her growing breasts during pregnancy as the "
cones of pain," with a wink at Get Smart.
Also during this time, the high and rising levels of estrogens, progesterone and hPL trigger the hypothalamus to release prolactin-releasing hormone (PRH), which stimulates prolactin, another hormone (found in many nonmammalian species) that promotes milk formation in pregnancy and especially throughout the early months of lactation. A yellowish, high-protein, high-/IGa antibody protomilk called colostrum comes in during the second trimester and in the earliest days following birth.
Nature is nothing if not wildly optimistic. At about any time following 16 weeks, miscarriage or premature birth will trigger lactation, and the milk will be very different in composition from the milk at term. “Premie” milk is ideal for feeding a premature infant because of the high protein content. I once lost a nonviable premie at 19 weeks and the milk-engorged breasts that ensued seemed like a particularly bad joke at the time.
At parturition, the sudden loss of the placental hormones, especially the estrogens, triggers full-blown lactation. After about two weeks, the milk is fully mature, and the quantity gradually increases, in a positive feedback loop driven by the mechanical stimulation of sucking. The pituitary hormone oxytocin stimulates a milk ejection reflex, and in the first few months, prolactin ensures a copious, high-fat supply for the young infant. The fat content drops off, with prolactin, around 6 months post-partum.
If a mother chooses not to feed her child human milk, the breasts will undergo involution over about 2 weeks, returning to their prepregnant state.
If, for some reason, a mother or a never-pregnant woman would like to relactate, the milk will always be the "garden variety" thinner milk of the older baby, never the rich stuff of the first six months.
For a varying length of time during lactation, the new mother’s ovulation will be suppressed by prolactin and her estrogen levels will remain at near-menopausal levels. Her cycles may return at any time, but a rough rule of thumb is that her cycles will return later the longer her child nurses exclusively, with no added formula or food. When cycles do return, they may be irregular, or may have a short luteal phase. As lactation trails off, the luteal phase will lengthen to the woman’s baseline (usually 14 days).
After 6 months or so, lactation is driven almost entirely by the mechanical stimulation of the infant’s sucking. If the breasts are not drained frequently, and there are fewer let-down reflexes (stimulating oxytocin & prolactin), the breasts will make progressively less milk. During the gradual process of weaning, which begins as soon as the child begins to eat and drink other things, the breasts accommodate the slower flow, vasculature becomes less prominent, and breast size begins to decrease.
Failure to lactate: very uncommon, but very real
Most women who quit breastfeeding claiming that they did not have enough milk usually have some other problem; most breasts make all the milk a baby needs. But a small percentage of women (5% or fewer) do have an identifiable problem with milk production, marked frequently by “
tubular hypoplasia” of the breasts. The breasts are small, widely spaced and lack fullness. They may not change much during pregnancy. Much of the tissue containing progesterone and prolactin receptors is missing, and these breasts are hypothesized not to respond well to the normal hormonal signals of pregnancy. There seems to be a link with endocrine disorders such as insulin resistance,
PCOS, hypothyroidism and excess testosterone. One hypothesis goes that the breasts may not have had the right mix of sex hormones at puberty, possibly because of PCOS and other problems. Follow the link above for more about tubular hypoplasia.
I chose to talk about lactation malfunction in the breasts because it seems to be a big issue with nurses who work in OB. The mother with ordinary but small breasts may be reassured by the encouragement that she can make more than enough milk, while the woman with an endocrine disorder or tubular hypoplastic breasts can be reassured that she did the best she could and that it is not her fault.
What does this all mean?
Boobies, of course.
Updated by rhubarb at Mon Mar 14, 2011, 01:27:26 PM
My diaries seem to meander in the wilds of Daily Kos, but sometimes, the Rescue Rangers touch them with their magic wand. Thanks!
As a psych nurse who deals daily with patients suffering from PTSD, I want to highlight this terrific effort by sricki.
It's an excellent teaching diary.