A woman's breasts sit over the pectoralis major muscle and usually extend from the level of the 2nd rib to the level of the 6th rib anteriorly. The superior lateral quadrant of the breast extends diagonally upwards in an 'axillary tail'. A thin layer of mammary tissue extends from the clavicle above to the seventh or eight ribs below and from the midline to the edge of the latissimus dorsi posteriorly. Breasts begin developing in the embryo about 7 to 8 weeks after conception.
They are unrecognizable at this stage consisting only of a thickening or ridge of tissue. From weeks 12 to 16, the various sub-components become more defined. Tiny groupings of cells begin to branch out laying the foundation for future ducts and milk producing glands. Other tissues develop into muscle cells which will form the nipple (the protruding point of the breast) and areola (the darkened tissue surrounding the nipple). In the later stages of pregnancy, the mother's hormones, which cross the placenta into the fetus, cause breast cells to organize into branching tube-like structures thus forming the milk ducts. In the final 8 weeks, lobules, (milk producing glands), mature and actually begin to secrete a liquid substance called colostrum. In both female and male newborns, swellings underneath the nipples and areolae can easily be felt and a clear liquid discharge, colostrum or "witch's milk", can be seen. These represent the effect of the mother's hormones and subside in the first few weeks of life.
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.
The breast size and shape in different women varies a lot. Some women have more glandular tissue in their breasts, some have less. Some have more fatty tissue than others. Some have more connective tissue so their breasts are firmer, and yet some women are totally flat-chested. The size and shape also varies over time in the same woman because of the changes during menstrual cycle, pregnancy, after weaning, and during menopause. Most of the size differences between women are due to the amount of fatty tissue in the breast. But practically all breasts can make milk and help nurture the baby - and that is what makes breasts beautiful!
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.
There are several, well described congenital (present from birth) abnormalities of the breast, which are worth mentioning. The most common of these are accessory nipples and/or breast tissue. This occurs in 2-6% of the population and often goes unrecognized. Accessory nipples are seen anywhere along the milk line (a ridge of tissue, present only in the fetus, extending from the underarm to the groin from which breasts develop). They are frequently multiple. Accessory breast tissue can also be found in this distribution but most often occurs in the underarm area. There is no special clinical importance to these other than being aware of their presence and including them in physical examinations.
Severe underdevelopment or absence of one or both breasts is another congenital abnormality that can occur. Unlike accessory nipples and breast tissue, this defect is extremely rare. Usually a rudimentary nipple is present on the effected side in these individuals. Coexisting abnormalities of the underlying muscles and rib cage are common. Plastic surgical correction is possible and often provides significant improvement in quality of life.