What's Inside a Woman's Breasts?

We spend most of our lives consuming food; it is bizarre to consider how we suddenly start producing food. The process is called lactogenesis (meaning, birth of milk). To understand it, we must have a basic knowledge of genetics.

In each of the cells in our body, we inherit forty-six chromosomes (twenty-three from the mother and twenty-three from the father), which provide a map of our hereditary characteristics. On the chromosomes, there are around twenty thousand genes, which encode the formation of specific functions and proteins in our body. The puffer fish has almost as many genes as we do, but clearly, the genes in every species code for a slightly different product. We now know that genes can be turned on and off in a dynamic process that allows for complex biological responses throughout life. Pregnancy turns the certain genes on, which in turn promote synthesis of alactalbumin, the predominant sugar in breast milk.

Under a microscope, cells in our breasts before pregnancy appear mainly filled with large fat droplets, and at this point they lack machinery to secrete milk. During pregnancy and immediately after delivery, a combination of maternal hormones stimulates the cells to mature. Breast cells gain an advanced protein-making structure, called the endoplasmic reticulum (ER), and a distribution and shipping department, called the Golgi apparatus (GA). The ER and GA are found in the majority of cells in our body, but they work specifically depending on the needed function: in the case of breast milk production, proteins and lipids that are built in the ER bud off into tiny bubble-like vesicles and float through the cell until they reach the GA, where they are further adapted for the infant. These GA secretions are combined with other nutrients that have floated in through the blood stream. After some additional and highly complex processing steps, breast milk is ready.

It is postulated that maternal hormones promote the dilation of vessels, which leads to increased blood flow towards our breasts. Since our heart pumps blood to the body, we can think of breast milk as coming straight from the heart, both literally and figuratively speaking. Less romantically, we can think of it as coming from our stomachs. The exact supply of nutrients changes slightly depending on what we eat. This is the reason why breast milk might taste differently after a meal of Pasta and Pesto versus a desert of chocolate cake. While milk is a complex fluid with many hundreds of minor components (and immunoglobulins, which protect against disease), the major nutrients include fat, proteins, carbohydrates, ions, and water. It is a species specific combination (if not person specific). For example, milk in seals and whales has little carbohydrate but a higher concentration of ions. In contrast, milk of primates is low in ions but high in carbohydrates. The exact combination of water, specific proteins (the most common proteins are casein and whey), and lactose differ in humans, cows, blue whales, and every other mammal.

The biggest difference between human and cow's milk is that the latter has much more casein protein, which can upset the infant's belly. Thus, formula manufacturers have to modify cow's milk and try to get the casein out, one formidable task among many. That factory process is never perfect. It cannot compete with the intricate manufacturing capabilities of our own cells.

Bibliography:

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).