The breasts are composed of fatty tissue that contains the glands responsible for milk production in late pregnancy and after childbirth. Within each breast, there are about 15 to 25 lobes formed by groups of lobules, the milk glands. Each lobule is composed of grape-like clusters of acini (also called alveoli), the hollow sacs that make and hold breast milk.
The lobules are arranged around ducts that funnel milk to the nipples. About 15 to 20 ducts come together near the areola (dark, circular area around the nipple) to form ampullae - cavities that store the milk before it reaches the nipple surface. Montgomery's glands are small oil glands that are located around each areola. They release a lubricant that protects the nipples during nursing.
Breast Size And Shape
The breasts are not always exactly the same size or shape. They are incompletely developed at birth and - in men - remain small and undeveloped unless subjected to abnormal hormonal stimulation. In general, breast formation is complete within a year or two after the start of menstruation; however, the acini keep growing, and fibrous and fatty tissues are continually added during adolescence. Pregnancy andnursing cause further increases in breast size. As a woman ages, the fatty tissue of the breasts may become more prominent than the glandular tissue, and the breasts may feel softer. The breasts gradually atrophy (shrink) after menopause (the end of menstruation).
The breasts cover a large part of the chest wall. In front, the breast tissue may extend from the clavicle (collarbone) to the middle of the sternum (breastbone). On the side, breast tissue may continue into the axilla (armpit) and reach as far as the latissimus dorsi (muscle extending from the lower back to the humerus bone of the upper arm).
In fact, the anatomic relationship between the breasts and the underlying muscle is a very important consideration in surgical therapy. The breasts overlay vital chest wall muscles such as the pectoralis major (the 'pecs'), the pectoralis minor (thin, triangular muscle beneath the pecs), and the intercostals (muscles between the ribs). The breasts also may cover some of the serratus magnus (also called the serratus anterior; a slender muscle that is attached to the ribs/ rib muscles and connects with the shoulder blade) and the rectus abdominis (long, flat muscle that stretches up the torso from the pubic bone to the ribs).
Lymph is a clear, tan fluid that contains lymphocytes (white blood cells that fight disease). Lymph is drained from the breast tissues by a rich supply of vessels. Such lymphatic vessels connect with a network of lymph nodes that are located around the breasts' edges or in nearby tissues of the armpits and collarbone. The breasts' lymph nodes are not linked in a straight line. Instead, they are staggered and fixed within fat pads - an arrangement that complicates lymph node removal during breast cancer surgery.
Lymph nodes play a central role in the spread of breast cancer. The axillary (underarm) lymph nodes are particularly important, as they are among the first places that cancer is likely to be found if it metastasizes (spreads) from the breast. This lymph node cluster is often referred to as the 'tail,' or level I nodes. Level II nodes are located underneath the pectoralis minor muscle, and level III nodes are found near the center of the collarbone.