Anatomy of the Breast

If you asked most women what kinds of tissue are in their breasts, they probably could not tell you, except that they make milk for nursing a child. The composition of breasts is complex and the specific ratio of each component varies between individuals. Some breasts are denser than others and have more fibrous connective tissue, making mammograms more difficult to interpret.1 This is not to say that mammograms are ineffective in women with dense breasts, but other tests may be needed to detect cancer in lesions that look suspicious as a result of the breast anatomy.

The majority of the breast is fatty tissue and connective tissue that hold the other components in place. When a lot of connective tissue is present, the breast appears to be a faint white color with scattered densities on mammogram. Because this spongy tissue is difficult to compress, masses are more difficult to see. Fatty tissue is less dense and appears darker on mammogram. As you age, your tissues lose their elasticity. This is true even in the breast, which makes them easier to compress and provides for a better mammogram image. Breast cancer often contains deposits of calcium that appear bright white on mammogram. A cluster of “micro-calcifications” is more concerning for cancer than scattered “macro-calcifications”, which can be caused by injury and the healing process at that site.

A Sophisticated Structure

The breast lays directly on top of your pectoralis major and minor muscles, which make up your chest wall and separate the breast from the ribcage. The nipple and surrounding areola have tiny openings that branch into a system of ducts inside the breast. These ducts continue to branch into smaller ducts and terminate into a lobule. A lobule produces milk toward the end of a pregnancy, and the milk is carried within this series of ducts to the nipple, where suckling from a baby allows its release. The anatomy of the breast can be compared to a bunch of grapes, where the main stem branches into a series of smaller stems that terminate with a grape. The stems are similar to the system of ducts in the breast and the grape resembles a lobule.

The lymphatics are another complex system of channels that route to specific areas of our bodies, providing a way for our immune system to recognize infection and cancer, and supply the necessary cellular components to fight disease. These channels have lymph nodes along their path that all ultimately drain into the blood stream. If infection or cancer is present, the lymph nodes that drain that site may be enlarged and firm. The breast has scattered lymph nodes in the tissue itself, but cancer typically drains to the clusters of lymph nodes that can be found in the axilla, or armpit. If breast cancer has spread to the lymph nodes, a non-tender lump may be felt in the armpit area and can sometimes be noticed before a breast mass is found.

The breast is also composed of veins, arteries and nerves that tend to run together and are surrounded by fatty tissue that holds them in place. The arteries and veins supply and drain blood, respectively, while the nerves provide sensory and motor functions.

How Breast Cancer Begins and Spreads

Breast cancer can be found in any part of the breast. It may be contained within a duct (DCIS), invade through the walls of the duct and into surrounding tissue, or begin in a lobule and invade into surrounding tissue. Invasive breast cancer begins as a contained group of cells that continue to divide uncontrollably, disregarding the normal signal to stop dividing. As they divide, they form masses of cancer cells called tumors that grow into and destroy normal tissue. The body recognizes this and provides necessary components of the immune system to fight it. The lymphatic channels drain the site of the cancer and carry cancer cells into lymph nodes. The cancer then has a route of entry into the bloodstream, and if it goes undetected for too long, can spread throughout the body.

Genetics and Breast Cancer

Genetics do have a role in the risk of developing breast cancer. Certain genes (which are made of DNA) can be inherited from your family that alter cell growth and allow cells that are growing uncontrollably to bypass normal check points. Sometimes, genes can mutate spontaneously leading to cancer. There are two types of genes that are known to be involved in cancer development.2

  • Tumor suppressor genes – genes that signal a cell to die when it begins to divide uncontrollably and are unable to do so when mutated, which leads to cancer; BRCA1 and BRCA2 are tumor suppressor genes that predispose people to a number of cancers, including breast cancer
  • Oncogenes – genes that have the potential to cause cancer, that when mutated lead directly to cancer

Breast cancers can be placed into different subtypes based on their molecular gene expression profiles. Two major subtypes are luminal and basilar, which can be further classified as luminal A, luminal B or basilar. Of these, the basilar subtype carries the worst prognosis, as it fails to express receptors that can be used to target the cancer for treatment. These molecular subtypes are not being used as part of standard medical practice at this time, but are for research purposes only. They do, however, help us understand breast cancer on the molecular level and with time could enhance the diagnosis and treatment of breast cancer


1 Birdwell, Robyn L.; Debra M. Ikeda; Kathryn F. O’Shaughnessy; and Edward A Sickles. (April 2001) Mammographic Characteristics of 115 Missed Cancers Later Detected with Screening Mammography and the Potential Utility of computer-aided Detection. Radiology. v219; 198-202.

2 Hartmann, Lynn C and Charles L. Loprinzi (2005) Mayo Clinic Guide to Women’s Cancers. New York: Mayo Clinic Health Information.

This article was originally published on July 27,2012 and last revision and update of it was 9/2/2015.