The breast is a mound of glandular, fatty and fibrous tissue located over the pectoralis muscles of the chest wall and attached to these muscles by fibrous strands (Cooper's ligaments). The breast itself has no muscle tissue, which is why exercises will not build up the breasts. A layer of fat surrounds the breast glands and extends throughout the breast. This fatty tissue gives the breast a soft consistency and gentle, flowing contour. The actual breast is composed of fat, glands with the capacity for milk production when stimulated by special hormones, blood vessels, milk ducts to transfer the milk from the glands to the nipples and sensory nerves that give feeling to the breast. These nerves extend upward from the muscle layer through the breast and are highly sensitive, especially in the regions of the nipple and areola, which accounts for the sexual responsiveness of some women's breasts.

Because the breast is made up of tissues with different textures, it may not have a smooth surface and often feels lumpy. This irregularity is especially noticeable when a woman is thin and has little breast fat to soften the contours; it becomes less obvious after menopause, when the cyclic changes and endocrine stimulation of the breast have ceased and the glandular tissue softens. Estrogen supplements after menopause can cause continued lumpiness. The breast glands drain into a collecting system of ducts that go to the base of the nipple. The ducts then extend through the nipple and open on its outer surface. In addition to serving as a channel for milk, these ducts are often the source of breast problems. Experts now believe that most breast cancer begins in the lining of the ducts and sometimes the milk glands. Benign fibrocystic changes also originate from these ducts.

The ducts end in the nipple, (which projects from the surface of the breast), and are a conduit for the milk secreted by the glands and suckled by a baby during breast-feeding. There is considerable variation in women's nipples. In some, the nipple is constantly erect; in others, it only becomes erect when stimulated by cold, physical contact or sexual activity. Still other women have inverted nipples.

Surrounding the nipple is a slightly raised circle of pigmented skin called the areola. The nipple and areola contain specialized muscle fibers that make the nipple erect and give the areola its firm texture. The areola also contains Montgomery's glands, which may appear as small, raised lumps on the surface of the areola. These glands lubricate the areola and are not symptoms of an abnormal condition.

Beneath the breast is a large muscle, the pectoralis major, which assists in arm movement; the breast rests on this muscle. (Portions of three other muscles are also found under the lower and outer portions of the breast.) Originating on the chest wall, the pectoralis major extends from deep under the breast to attach to the upper arm. It also helps form the axillary fold, created where the arm and chest wall meet. The axilla (armpit) is the depression behind this fold.

Normal anatomy on a mammogram will image differently depending on a woman's weight, age, presence of surgical scars and presence of superficial or sub-muscular implants, as well as the amount of fatty tissue in her breasts.

Each woman's breasts are shaped differently. Individual breast appearance is influenced by the volume of a woman's breast tissue and fat, her age, a history of previous pregnancies and lactation, her heredity, the quality and elasticity of her breast skin and the influence of hormones.  

The breast is responsive to a complex interplay of hormones that cause the breast tissue to develop, enlarge and produce milk. The three major hormones affecting the breast are estrogen, progesterone and prolactin, which cause glandular tissue in both the breast and uterus to change during a woman's menstrual cycle. Because of reduced hormonal levels, the breasts are less full for 1 to 2 weeks after menstrual flow; therefore, it may be easier to detect breast lumps during this time. Reduction of hormonal levels is also responsible for the breast's return to its pre-pregnant state after breast-feeding is concluded.

Breast shape and appearance change as a woman ages. In the young woman the breast skin is stretched and expanded by the developing breasts. The breast in the adolescent is usually hemispherical, rounded and equally full in all areas. As a woman gets older, the topside of the breast tissue settles to a lower position, the skin stretches and the shape of the breast changes. After menopause, with the decrease of hormonal activity, the composition of the breast changes; the amount of glandular tissue decreases and fat and ductal tissue become the predominant components of the breast. Reduction in glandular volume can result in further looseness of the breast skin.

Puberty

The breast grows rapidly, its ducts and lobules subdividing and maturing -- estrogen and progesterone are responsible for this.

The Menstrual Cycle

In preparation for pregnancy each month, the breast feels lumpy, increases in size, tenderness, firmness and blood supply (engorgement). Ducts enlarge with an increase in the number of cells lining the ducts and the breast retains more fluid. The breasts are least engorged 7-10 days after the beginning of menses, when it is best to perform breast self-examination (BSE).

Pregnancy and Lactation

The breast increases in size, tenderness and firmness caused by multiplying acini (lobules). By the end of pregnancy, the breast becomes an almost entirely glandular structure.

Lobules and acini begin to decrease in all women in their early twenties or following their first pregnancy, whichever comes first.

Menopause

The breast becomes less glandular. The breast initially increases in fatty tissue (perimenopausal), then decreases in fatty tissue (post-menopausal) and decreases in firmness. These changes can cause the breast to sag (ptosis) but can make it easier to detect a cancer.  

The Immune System * *

The immune system is an intricate network of specialized cells and organs that defends the body against attack by "foreign" invaders. When the immune system is functioning correctly, it fights off infections by agents such as bacteria and viruses. When it does not function properly, it can no longer protect the body against diseases such as allergy, arthritis, cancer and AIDS.

The immune system contains high concentrations of white blood cells called lymphocytes. These cells orchestrate the body's responses to foreign invaders such as bacteria, viruses and "foreign" tissue such as tumor cells. The organs of the immune system are located throughout the body and include the bone marrow, thymus, lymph nodes, spleen, tonsils and adenoids.

Stem cells, abundant in bone marrow, are cells from which all blood cells evolve. Bone marrow is the soft tissue in the hollow shafts of long bones.

The Lymphatic System and Blood Supply

The lymphatic system has two components:

1. A network of vessels (much like blood vessels) for transporting lymph fluid and lymphocytes.

2. Lymph nodes (small bean-shaped structures) that function as filters of foreign material in a particular area of the body.

Lymph nodes are scattered along the lymphatic system with clusters in the neck, armpits (axilla), abdomen and groin. Lymphocytes pass freely between lymphatic vessels and blood vessels (venous routes) patrolling for infection or malignant cells. Unfortunately, lymphocytes are not always successful in fighting off foreign material such as tumor cells, which are then capable of spreading through the lymphatic and venous routes.

Breast tissue is drained by lymphatic vessels that lead to axillary nodes (which lie in the armpit) and internal mammary nodes (which lie along each side of the breastbone). When breast cancer spreads, it is frequently to these nodes first.

Adapted from Breast Buddy Volunteer Curriculum © 1994

* * Source and Suggested Reading: "Understanding the Immune System," National Institute of Health, Publication No. 92-529.

* Source and Additional Resource: Excerpted with permission and adapted from Berger, Karen and Bostwick III, John: A Woman's Decision: Breast Care, Treatment, and Reconstruction, Quality Medical Publishing, St. Louis, 1994.

Last Updated: 06/01/2004

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