Milk sacs (the alveoli)

Where is the milk made?

Milk is made in the breasts by a special type of cell which lines the millions of little sacs present in a lactating breast. These sacs are called alveoli and develop at the ends of ducts. During the let-down the alveoli are squeezed by surrounding muscle fibers forcing the milk out of the alveoli and into the duct. The lower fat milk (foremilk) enters the duct first and the higher fat milk (hindmilk) follows later.

A) The structure of the milk sacs

In early pregnancy, the duct system of the breast grows and forms new branches and small milk sacs develop at the ends of the ducts. Each sac is about 0.1 mm (0.004 inches) wide and there are millions of them in each breast. The milk sacs are called alveoli. The organization of alveoli and milk ducts resembles a cluster of grapes.
The alveoli are lined with a special type of cell (lactocyte) that makes milk. When a mother is lactating, the milk sacs fill with milk. The milk fat tends to stick to the lactocytes.

B) How the milk sacs empty

Each of the millions of alveoli is surrounded by muscular cells that contract when the levels of the hormone oxytocin rises during the let-down. This squeezes the milk out of the sacs, into the milk ducts, and towards the nipple.
The center of the alveoli empties first and the milk fat is released later in the feed resulting in increasing levels of milk fat as the feeding progresses. Lower and higher milk fat are sometimes referred to as fore- and hindmilk respectively. Some mothers may unnecessarily worry about the baby getting the right amounts of fore- and hindmilk.