Now, let's get a closer look. The mammary glands of each breast consist of about 15 to 25 sections called lobes with each lobe containing many smaller structures called lobules.
Each lobule consists of a large number of alveolar glands, which are the small sac-like structures that produce milk.
The milk produced by the alveolar glands drain into the terminal ducts that join together to form the intralobular ducts.
These ducts converge further and form the lactiferous ducts, which drain the lobes and eventually lead to the nipple.
Before pregnancy, the mammary glands are considered to be in an inactive state, and they’re structurally different from mammary glands that are in the active state.
The active state occurs during pregnancy and during lactation after childbirth.
During pregnancy, the alveolar glands and the duct system will grow in preparation to produce milk for a newborn baby.
This low power image is an example of lactating mammary glands in the active state.
A portion of a lobe can be seen surrounded by a thick layer of connective tissue that contains a small number of fat cells or adipocytes within the connective tissue as well.
Within the lobe, thinner layers of connective tissue separate the lobe into lobules.
In a neighboring lobe, we can also see a couple of large lactiferous ducts.
These ducts are lined with a double layer of columnar or cuboidal cells with a surrounding layer of connective tissue.
If we take a closer look at one of the lobules, we can see some of the smaller intralobular ducts that are typically lined with 1 to 2 layers of cuboidal cells that also have a thin layer of connective tissue that surround the ducts. In this image of a different lobule, we can see a good example of a few alveoli and a longitudinal cross-section of the terminal duct that’s responsible for draining the milk produced by these alveoli.
Both the terminal ducts as well as the alveoli are lined with either cuboidal or columnar secretory cells.
The supportive tissue between the ducts and alveoli is called the intralobular stroma.