How the Body Produces Breastmilk

Did you know that women's breasts begin developing six weeks after conception, two weeks after the heart forms? They start with a "milk ridge," which forms as paired glands running side by side in parallel rows along the fetus' body from above the armpit to the groin. By nine weeks, most of the milk ridge disappears except for two main elevations, which will become the breasts.

When our breasts grow, it is the first sign of puberty. Our bodies change from girlish to womanly. The areola surrounding the nipple darken and our breasts take on their shapes - round, cone-like, or oval. Breasts become part of our identity and femininity. In many cultures, and for many artists, from Rubens to Picasso, their voluptuousness heralds beauty, womanhood, and sensuality.

For most of our lives, the breasts are hardly considered functional. But when we get pregnant, everything changes. At around seven months gestation, a hormone called prolactin (produced by the pituitary gland) increases and then surges. At that point, milk production (in the early form of colostrum) begins, though we often remain unaware of our newly developed function. The female hormones, estrogen and progesterone, high in pregnancy, keep the growing milk supply in check. After the baby is born, estrogen and progesterone fall, while prolactin reigns - time to make milk!

Our breasts do so in an elaborate process. Within the fat of breasts, there are around 15-20 lobes, which contain milk-producing glands called lobules (little lobes). The little lobes contain clusters of acini, which means grape in Latin. It seems that the Romans thought of breasts as vineyards. Maybe that explains why many Renaissance paintings depict bosomed maidens holding grapes in their hands. While grapes make Champagne, the acini make breast milk. They are connected to stems and larger vines that deposit the milk into ampullaes - jugs in Latin - that
can pour milk out into the nipple.

When the baby suckles, the breast becomes erect. People never talk about erections in women, but they happen. They happen as a result of a highly coordinated process known as "the milk-reflex." In other words, the breasts and the brain are connected. The breasts are mostly composed of fat, glands, and an intricate network of nerves. The nerves in the breasts send messages to the hypothalamus - a structure in the brain located in the cerebral cortex (the thinking part of the brain). The hypothalamus regulates body functions like sleeping, body temperature, and in the case of breasts, milk production. Then the pituitary gland - another structure in the brain - responds to signals from the hypothalamus by releasing two hormones: prolactin, that causes milk production, and oxytocin, that causes milk release. The oxytocin stimulates smooth muscle cells (myoepithelial cells) around the breast to contract. The milk is "let-down."

It is awe-inspiring to consider the exquisite coordination and interdependence between the brain and the breasts, and a mother and her newborn. Our emotions and thoughts also play a role. Later, oxytocin may be released during lovemaking, or when we hear our babies cry, or even when we think about them. That explains why the woman in the grocery aisle suddenly leaks milk (and when we become her, all we can do is laugh).

Notes:

Niels H. Lauersen, M.D., Ph.D. and Eileen Stukane, The Complete Book of Breast Care, pp. 13-16, 29-30 (Fawcett Columbine, 1998).

R. Michael Akers, Lactation and the Mammary Gland, Table - p. 78 (Wiley- Blackwell, 2002).