Local & Imported Ingredients
The building blocks of milk arrive in the breast via two main pathways. In addition to normal digestion (discussed previously), which results in nutrients carried through the bloodstream to the breasts, other special components in milk, such as immune and growth factors, arrive through the lymphatic system. Immune tissue in the gut, for example, is the source of specialized cells that are then transported to breast tissue via this route. Once the components have arrived in the breasts, many undergo further processing and packaging. And in fact, some of breast milk’s most unique qualities are the result of this ingenious packaging.
The components leave the blood or lymphatic fluid and are taken up by the epithelial cells called lactocytes. Once inside these cells, there are several different routes by which they enter milk, though a few components are transported directly into the alveoli through the tiny spaces between the epithelial cells.
Different nutrients undergo different processing and are delivered into the alveoli via different specific mechanisms. The names of the sites in the cells and the pathways the nutrients follow sound very technical (and they are!), but think of them as various stops along a factory assembly line: here’s where they attach the doors to the car, or here’s where they wrap the package of cookies. Some nutrients arrive basically intact, while others are assembled pretty much from scratch.
Here is a quick rundown of the pathways:
- Exocytosis (“exo” meaning “exit”) is the name for the pathway by which milk components such as proteins and lactose, which are made and assembled within the lactocyte, are then transported out of the cell and into the alveoli. Amino acids are absorbed into the lactocytes from the bloodstream. Once inside, they are synthesized into proteins by ribosomes located along another special structure in the cell called the rough endoplasmic reticulum (RER) to form various milk proteins, such as casein. From the RER, the proteins are transferred to another structure, called the Golgi apparatus, where they are processed for delivery into the empty cavity, or lumen, of the alveoli. This processing can involve further packaging. Casein, for example, is attached to calcium and phosphorous to form a tiny unit called a micelle, which forms soft curds in the stomach. The simple sugar glucose is also absorbed into the lactocytes, where it is transformed by the Golgi apparatus into milk sugar, or lactose, the primary carbohydrate in breast milk.
- Lipid synthesis and secretion is the pathway by which milk fat-globules are assembled and transported. Milk fats also undergo transformation in the lactocytes. Building-block molecules, as well as some premade nutrients such as fatty acids, are taken up by the lactocytes. These components are then synthesized within the cells into fats called triglycerides. Next, tiny fat droplets fuse together into a larger drop of milk fat. The fat drop is forced out of the cell surrounded by a membrane, which is actually a tiny piece of the lactocyte cell membrane. It is now called a milk-fat globule. The packaging of fats in this way is critical because without the membrane surrounding the globules, fat droplets would coalesce into ever larger blobs of fat. This would make milk secretion very difficult.
- Transmembrane secretion. This is the pathway by which ions and water are transported directly across the lactocyte membrane into the alveolus. The small molecules transported by this route include sodium, potassium, chloride, some monosaccharides, and water.
- Transcytosis (“trans” meaning “across”) is the pathway by which intact proteins move through the interior of a lactocyte. The cell creates a tiny temporary bubble, called a vesicle, that engulfs the milk component on one side of the cell and then carries it and releases it on the other side of the cell into the empty cavity of the alveoli. This pathway is used for premade immunoglobulins, hormones, and growth factors. These components do not undergo transformation or packaging in the lactocyte.
- The paracellular pathway involves transport of substances through the tiny spaces between adjacent lactocytes. For the most part, these tight junctions are intended to prevent substances from leaking either into the milk or out of the milk back to the mother. At certain times, however, this route is important for the movement of specific substances, particularly those involved in fighting infections. In cases when the breast tissue becomes infected, anti-inflammatory substances and infectious waste products move into and out of the mammary tissue via this route.
Hormonal Preparation for Lactation
A cascade of hormones that begins in the second month of pregnancy triggers a series of changes, preparing the mother’s body to produce milk:
The hormones involved in lactation are produced in several places in the body, including the pituitary gland, ovaries, breast tissue, and placenta. Among the hormones that play roles in preparing for milk production are human placental lactogen (HPL), which stimulates breast, nipple, and areola growth before birth; estrogen, which stimulates the milk duct system to grow; progesterone, which influences the growth of alveoli and lobes; prolactin, which also contributes to the growth of the alveoli; and oxytocin, which contracts the smooth muscle of the uterus during labor. Prolactin and oxytocin work together to control milk production after birth.
By the fifth or sixth month of pregnancy, the breasts are basically ready to produce milk and are waiting for the hormonal changes triggered by birth. The initiation of milk production is called lactogenesis. It can be divided into three stages, each marked by changes in milk composition. Lactogenesis I is the initial stage, which begins in midpregnancy and ends 2-4 days after birth. Lactogenesis II can last for 2 or more weeks and is followed by lactogenesis III, in which mature milk is established and milk supply is maintained through supply-and-demand in response to the infant’s sucking.
If a woman does not breastfeed her infant, levels of prolactin and oxytocin fall and milk production ceases. Mammary tissue gradually returns to a resting state, but because new alveoli formed during pregnancy never totally disappear, the breasts do not completely return to their prepregnant state, although studies have found that the changes in the breast tissue that occur during lactation seem to protect the mother against breast cancer.
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Related Health Centers:
Infant Nutrition Health Center, Mother-Baby Bond Health Center, Mother’s Milk Health Center, Monthly Infant Development Calendar Health Center,Weekly Pregnancy Calendar Health Center