Breasts are complex, specialized organs whose primary function is to produce milk for the infant and baby. They are made up mainly of fat and breast tissue, intermingled with nerves, blood vessels, lymph vessels and lymph nodes, muscle tissue, and connective tissue.
Breast tissue extends from the breastbone to the underarm, and from below the collarbone to the level of the sixth or seventh rib. Breast tissue is composed of lobules, lobes, and mammary ducts. Fifteen to twenty lobes are arranged beneath the nipple and areola (the shaded area around the nipple), like petals on a flower. Each lobe contains thousands of tiny round sacs called lobules, and lobules hold the bulbs that produce breast milk. A single breast contains up to a million lobules. The lobules are connected to the mammary ducts like bunches of grapes on stems. Mammary ducts are the canals that carry breast milk to the nipple openings. Fat covers the breast tissue and gives the breast its shape.
The breast contains lymph vessels that drain into a network of lymph nodes located around the edges of the breast, in the underarm (axilla), and near the collarbone. These lymph nodes are embedded in fat, which makes it difficult to remove them. Underarm lymph nodes are often the first place breast cancer spreads.
Several arteries supply the breasts with blood: the internal mammary, axillary, and intercostal (between the ribs) arteries. The internal mammary, axillary, and intercostal veins carry blood away from the breasts. The breast's surface veins carry blood to these veins and to the lungs. Cancerous cells may travel through the surface veins to the lungs, where they may form tumors. The intercostal veins run to the spine, allowing cancerous cells in the breast to spread to bone tissue.
The chest muscles are overlaid by a fibrous covering. They are located under the breasts and include the following:
- Major and minor pectorals. The main chest muscles, which lie directly behind the breast, in front of the ribs. They attach to the collarbone, the breastbone, the humerous (upper arm) bone, the shoulder joint, and the third, fourth, and fifth ribs and the muscles between those ribs.
- Serratus magnus. Attaches to the first eight or nine ribs and rib muscles and connects with the shoulder blades in the back.
- Rectus abdominus. Extends from the pubic bone and attaches to the cartilage of the fifth, sixth, and seventh ribs.
The skin of the upper breast is innervated (supplied with nerves) that branch off from nerves in the neck. The skin of the lower breast is innervated with nerves that extend from the spine and between the ribs. The thoracic nerve crosses over the underarm lymph nodes and helps to move the upper arm. Surgeons have to be careful not to injure this nerve when operating near the underarm.
Fascia (fibrous connective tissue) separates the breast tissue from the skin and also from the chest muscles. Ligaments run from the deep fascia throughout the breast tissue and attach to the dermis (the second layer of skin). Connective tissue helps hold all the components of the breast in place.
Changes During Pregnancy and Birth
The function of the breast is to produce milk and colostrum for the infant. The breast undergoes many important changes when a woman becomes pregnant, in preparation for producing milk. Estrogen and progesterone stimulate cell growth and development of the breast. Special hormones called prolactin and somatomammotrophin, produced during pregnancy, also play a role. The cells in the glands of the breast tissue change shape, become bigger, and increase in number. Later on in the pregnancy, the lobules start to produce milk, which begins to accumulate in the ducts. Muscle tissue around the lobules helps to squeeze the milk into the ducts.
During breastfeeding, glands on the areola secrete a fluid to lubricate the nipple, and muscles in the nipple allow it to become erect. A protein and antibody-rich fluid called colostrum is produced during the last weeks of pregnancy and during the first few episodes of nursing.
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