One of the most remarkable things about the breast is its life in three parts: pre-natal life, youth (teen years to 40s or so), and late life or post-fertile. Millions of cells make up the breast that support its primary function of feeding infants, but only some are used for that purpose — and usually only for a few years, if at all.

When you understand how the breast develops and how its tissue is structured and organized, you can better understand properties of normal breast tissue and how they relate to breast cancer risk and disease. What’s also remarkable is that this tissue remains present and functional through a woman’s entire life. It can be influenced by internal and external stimuli, including diet, medications, exercise, and environmental exposures.

The anatomy and development of the breast

The breast tissue isn’t usually the breast! Breast tissue starts in the armpit area and extends deep to the chest wall, and from the breastbone in the middle of the chest to the back of the armpit.

(Diagram with labels: Collarbone, Skin, Muscle, Breast Tissue)

Often there’s a ridge of fat at the bottom of the breast—the inframammary ridge. This ridge is perfectly normal, and it’s the result of the fact that breast tissue is contained in a special layer of fat behind the skin of the chest. The fat here serves to anchor the skin with tiny threads and hold it to the chest wall while remaining pliable. This enables the skin and deeper layers of breast tissue to move around as the arm and shoulder move, while still staying in place.

Breast development starts in utero and continues through puberty. In the earliest stages of fetal development, everyone has a similar group of cells that form the early breasts. With the influence of sex hormones (estrogen, testosterone), some of these cells mature into functional breast tissue or nipple structures.

Sometimes, “extra” milk lines or early breast structures will form in armpits and other areas (most commonly near the armpits). This is why some individuals may experience milk-leaking, swelling, and pain outside of the typical chest area during breastfeeding, post-pregnancy, or after weaning.

Why breasts bounce—because the breast tissue sits between layers of fat

Inside, the breast tissue is sandwiched between layers of fat; behind that is the chest muscle. The fat has some give to it, which is why breasts bounce. The breast tissue is firm and rubbery. The breast also has its share of the connective tissue that holds the entire body together. This material creates a solid structure—like gelatin—in which the other kinds of tissues are loosely set. This is sometimes called the stroma and is getting more attention as recent studies show its importance in breast cancer.

Like the rest of the body, the breast has arteries, veins, and nerves. There is another, almost parallel, network of vessels called the lymphatic system (or lymphatics) which works like recyclers or filters for helping the body fight infection. The job of this network is to collect the debris from the cells and strain it through the lymph nodes found scattered in nests throughout the body; they then send the filtered fluid back into the bloodstream to be reused.

Supporting Cast

Understanding the breast is critical to understanding cancer. But you also have to understand two supporting organs, the ovaries and adrenal glands, that help choreograph its behavior. These organs and their resulting hormones come into play in our current understanding of breast cancer and its treatment.