Anatomy and Physiology of the Breast

The breast is an organ which whose structure reflects its special function: the production of milk for lactation. The epithelial component of the tissue consists of lobules, where milk is made, which connect to ducts that lead out to the nipple. Most cancers of the breast arise from the cells which form the lobules and terminal ducts. These lobules and ducts are located spread throughout the background fibrous tissue and adipose tissue (fat) that make up the main mass of the breast. The structure of the male breast is nearly identical to that of the female breast, except that the male breast tissue lacks the specialized lobules, as there is no physiologic need for milk production by the male breast.

Anatomically, the adult breast sits atop the pectoralis muscle (the "pec" chest muscle), atop the ribcage. The breast tissue extends horizontally (side-to-side) from the edge of the sternum (the firm flat bone in the middle of the chest) out to the midaxillary line (the center of the axilla, or under arm). It is important to note that a tail of breast tissue called the "axillary tail of Spence” does extend into the axilla. This is important because a mass of breast cancer can develop in this axillary tail, even though it might not seem to be in the breast proper.

The breast tissue is encircled by a thin layer of connective tissue called fascia. The deep layer of this fascia sits immediately on top of the pectoralis muscle, and the superficial layer sits just under the skin. The skin covering the breast is similar to skin elsewhere on the torso and has similar sweat glands, hair follicles, and other characteristic features. A clinician will examine the skin in addition to the breast tissue itself when performing a breast exam.

The blood supply from the breast comes primarily from the internal mammary artery, which runs underneath the main breast tissue. The blood supply provides nutrients, such as oxygen, to the breast tissue. The lymphatic vessels of the breast flow in the opposite direction of the blood supply and drain into lymph nodes. It is through these lymphatic vessels that breast cancers metastasize to lymph nodes. Most lymphatic vessels flow to the axillary (under arm) lymph nodes, while a smaller number of lymphatic vessels flow to internal mammary lymph nodes located deep to the breast. Knowledge of this lymphatic drainage is important, because when a breast cancer metastasizes, it usually involves the first lymph node in the chain of lymph nodes. This is called the "sentinel lymph node,” and a surgeon may remove this lymph node to check for metastases in a patient with breast cancer.

Physiologically, the breast is an organ specialized for milk formation (lactation). Many additional changes are seen in the breast tissue during pregnancy and lactation due to the changes in hormones during those times.