Breast Anatomy: Understanding the Structure, Function, and Role in Breast Health

Here, find essential aspects of breast anatomy to help you maintain breast health

Everyone’s breasts are unique, but they are all made of the same basic components. Understanding the elements that make up breast anatomy can help people keep track of their breast health. It also makes it easier to understand screening and development of breast cancer.

What is breast anatomy?

Breast anatomy is the group of structures that make up the breast and how they all work together. The main structures that make up the breast include:

• Lobes and lobules (the “glandular” part)
• Milk ducts
• Fibrous connective tissue (together with the lobes, lobules and ducts, the fibroglandular tissue)
• Fat
• Lymph nodes and lymphatic vessels
• Nerves and blood vessels
• Nipple and areola

The Main Parts of Female Breast Anatomy

The major structures of the breast each have important roles, as explained below.

Nipple and Areola

Each of the 12 or so ducts has an opening in the nipple where milk can be secreted. The darker skin surrounding the nipple is called the areola. The tiny bumps on the areola are called Montgomery’s glands, and they secrete an oil to protect the skin from chafing during breastfeeding. Changes to the nipple and areola—such as flattening or inverting—can be signs of breast cancer.

Milk Ducts

Working back from the nipple, milk ducts are thin tubes that transfer breast milk from the lobules to the nipples, where it is secreted. Each breast has about 12. When the sacs at the ends of the ducts become blocked, they can form fluid-filled cysts. Cysts are very common: “We see them in about two-thirds of women before menopause,” says BCRF researcher and radiologist Dr. Wendie Berg.

Sometimes while breastfeeding, ducts can get clogged, become narrow, or grow swollen and produce a lump. This can cause a painful bacterial infection called mastitis that often presents as flu-like symptoms. It is important to get any new or worsening breast lump evaluated by your healthcare provider.

Lobes and Lobules

These are the glandular tissues that make up the female breast. Lobes are sections of tissue that spread out around the nipple like petals in a flower. Each breast has about 15-20 lobes. Inside each lobe is a milk duct and its branches that terminate in 20-40 smaller structures called lobules, which are groups of tiny sacs called acini.  Each little sac has an inner layer of epithelial cells (which can become breast cancer). Surrounding the epithelial cells is an outer layer of myoepithelial cells that act like tiny muscle cells to help express the milk.  Outside these two layers of cells is a “basement membrane” that helps support the sacs and ducts.

“Multiple little sacs in a collection make a lobule, and then multiple lobules make a lobe,” says Dr. Wendie Berg. The lobule is connected to tiny branches of ducts, and together these make up the “terminal duct lobular unit” where most breast cancers and also most benign breast lesions develop.  The same types of cells comprise both the sacs and the ducts. Surrounding the sacs and the ducts is fibrous connective supportive tissue.

In lactating women, acini pull nutrients and water from the bloodstream to produce and store milk. Changes in normal epithelial cells can lead to breast cancer. The majority of breast lumps are benign (not cancer), but most invasive breast cancers (70-80%) begin in the terminal duct lobular unit.

Lobular breast cancer cells are distinguished by loss of cell-cell adhesion. Invasive lobular cancer makes up about 10-15% of all breast cancers.

When cancer is confined to the acini and milk ducts and has not disrupted the myoepithelial cell layer nor broken through the basement membrane, it is known as ductal carcinoma in situ (DCIS) and referred to as pre-cancer or Stage 0 breast cancer because it is non-invasive. About 20-25% of new breast cancer cases are DCIS.

Fatty and Connective Tissue

Breasts contain a mix of connective and fatty tissue. Fibrous connective tissue holds the breast structures in place, while fatty tissue helps define breast size and shape. Glandular tissue is where the milk is produced, in small sacs called acini within lobules. Fibrous and glandular tissue together are called fibroglandular tissue. On mammography, fibroglandular tissue appears white because it absorbs X-rays. Some breasts have relatively more fat and little fibroglandular tissue, and other breasts have relatively more fibroglandular tissue. Radiologists describe these variations using one of four categories of breast density. The denser the breast, the harder it can be to see cancer on a mammogram. Dense breasts also increase the risk of breast cancer. In September 2024, the U.S. Food and Drug Administration (FDA) mandated that all mammography facilities notify women if they have dense breasts.

A mammogram, Dr. Berg says—not the look or feel of breasts—can help indicate if you have dense breasts. Women with dense breasts should ask their providers about supplemental screening options such as an ultrasound or breast MRI.

Lymph Nodes and Lymphatic Vessels

Lymph nodes are part of the lymphatic system, which filters out foreign matter and waste and maintains fluid balance in the body. Lymph nodes are the small organs that trap bacteria, viruses, and other bad cells. They contain white blood cells, which fight infection. There aren’t many lymph nodes within the breast itself, but there are about 20-40 lymph nodes located in the armpit, or axilla.

Lymph vessels connect to lymph nodes and transport lymph, a fluid that helps your body fight infection. But they can also “take up anything foreign, and that includes cancer cells,” Dr. Berg says. If breast cancer spreads, it’s usually to the lymph nodes first.

Lymph nodes and lymphatic vessels are vitally important in breast cancer diagnosis and staging; finding out if or how many lymph nodes contain cancer is part of determining the extent of the disease.

How Breast Anatomy Changes Throughout Life

Breasts naturally change throughout a woman’s life due to shifting hormones (estrogen and progesterone). Breasts begin to develop during puberty, when the ovaries start making estrogen. During this stage, the milk duct system develops and glandular and connective tissue build up, reaching their peak development usually in the late teen years.

The changing hormone levels in the body can cause the breasts to feel tender, swollen, or lumpy before a woman gets her period each month. These symptoms usually go away once her period starts.

In pregnancy, hormones ramp up—and the breasts grow rapidly—to get ready for lactation. A woman may grow one or two full cup sizes. These changes can also make the breasts feel swollen and tender. Breastfeeding, too, causes breasts to swell, as milk glands get bigger and fill up. Once a woman stops breastfeeding, her breasts usually return to their pre-pregnancy size.

During menopause, estrogen levels drop and glandular tissue decreases. It can be replaced by fatty tissue, which can make the breasts softer and can cause them to lose firmness and droop. Though not always. As estrogen decreases, breast density may too, but only a little bit. Skin also loses elasticity, which can also cause the breasts to sag.

While these are normal changes in the breast, they may partially explain why so many cancers originate there.

“One of the reasons that breast cancer is more common is that cells are dividing more throughout our lifetime,” says Dr. Berg. “They go up and down throughout the menstrual cycle, and even after menopause, there’s still estrogen around. And these cells are sensitive to estrogen, so they tend to be dividing more. And when cells divide, they can make mistakes. It is that accumulation of mistakes that eventually can result in cancer.”

Why Breast Anatomy Matters for Breast Health

Understanding breast anatomy helps women stay informed about their breast health and aware of any changes. It is also important for understanding breast exams, mammograms, and other types of screenings, as well as whether or not you have dense breasts. Knowing your breasts allows you to better communicate any changes with your healthcare providers. If it turns out that any of those changes are due to breast cancer, it can also help you understand the diagnosis.

Breast Anatomy and Breast Cancer

Most cancers begin in the cells that make up the terminal duct lobular unit (the milk-producing structure of the breast). Breast cancer can be classified as invasive ductal carcinoma (IDC, about 85% of breast cancers) or invasive lobular carcinoma (ILC, 10-15% of breast cancers).

If the cancer is confined to the ducts, it is called ductal carcinoma in situ. Lobular breast cancer doesn’t always present with a lump; instead, it can appear as a thickening, swelling, or area of increased firmness. Rarely, cancer can begin in another area of the breast, such as the nipple, connective tissue, or blood vessels.

Understanding Breast Anatomy Drives Better Breast Health

As researchers continue to uncover new insights into how breast tissue develops, functions, and changes over time, that knowledge is helping improve breast cancer prevention, diagnosis, and treatment for people everywhere.

Support the research behind these discoveries. Donate, start a fundraiser, or explore other ways to give and help fund the science that’s moving breast cancer research forward.

Frequently Asked Questions

Does breast size affect cancer risk?

No. Breast size does not determine your risk of developing breast cancer. Factors such as age, genetics, family history, hormone exposure, and certain lifestyle factors have a much greater influence on breast cancer risk than breast size. However, weight gain and having more fatty tissue may play a role.

Do men have breast tissue?

Yes. Men and women are both born with breast tissue, including nipples, ducts, and fatty tissue. Because male breast tissue does not fully develop after puberty, it contains much less glandular tissue. Although rare, men can also develop breast cancer, making breast awareness important for everyone.

Why are lymph nodes important in breast health?

Lymph nodes are small glands that help filter harmful substances and support the immune system. In breast cancer, nearby lymph nodes are often checked for cancer cells since their presence there indicates that the cancer has spread beyond the breast. Knowing if the lymph nodes are involved helps guide diagnosis, staging, and treatment decisions.

What are dense breasts?

Dense breasts contain relatively more glandular and connective tissue than fatty tissue, compared to women with non-dense breasts. Having dense breasts is common and isn’t something you can feel, but it can make mammograms more difficult to interpret and is associated with a higher risk of breast cancer. Your healthcare provider can explain what your breast density means for you.  Additional screening tests may be recommended if you have dense breasts, especially if you have other risk factors.