Each breast develops from the epidermal milk line, an embryonic ridge of tissue between the upper and lower limb buds. The two symmetrical ridges normally atrophy except in the thoracic region, where two thickenings develop into the nipples. Cords of cells grow downward from the nipple, developing lumens to form the ducts of the breast. This degree of development occurs in both sexes during fetal life.
At puberty, under the influence of female sex hormones, the female breast develops further. Outpouchings arise from the terminal ducts that branch extensively into the lobules of the breast. The adult female breast is composed of five to ten segments, each draining at the nipple by a separate lactiferous duct (Figure 56-1).
Structure of the breast and sites in which common pathologic lesions originate.
In the nonpregnant breast, the parenchyma represents only about 10% of the volume. Much of the breast enlargement that occurs at puberty is due to an increase in the amount of fibroadipose stroma, which is also directed by the female sex hormones. Histologically, the normal nonpregnant breast is composed of breast lobule units, comprising approximately 10–20 acini around a terminal ductule. Lobular units are separated from one another by abundant fibroadipose stroma.
The breast responds cyclically to menstruation. During the preovulatory phase, estrogen causes the glands and ducts to undergo mild dilation and hypertrophy. During the postovulatory phase, progesterone causes stromal proliferation and edema. These changes may result in mild enlargement of the breast toward the end of the cycle.
During pregnancy, there is marked hyperplasia of the glands that displace the fibroadipose stroma of the breast (Figure 56-2). Enlargement of the breast occurs in the third trimester and becomes prominent during lactation. Secretion of colostrum, the first milk, begins in the third trimester of pregnancy. The lactating breast is composed of closely packed dilated glands with little intervening stroma. After lactation, the glands atrophy to a level that approaches the prepregnant state.
Lactating breast, showing extreme hyperplasia of the acini (A), which have replaced the normal interlobular adipose tissue. Many acini show secretion into the lumen. D = ductule.
After menopause, glands, ducts, and adipose tissues atrophy further (see Chapter 16: Disorders of Cellular Growth, Differentiation, & Maturation), causing progressive shrinkage in breast size.
Manifestations of Breast Disease
A mass in the breast is the earliest manifestation of breast carcinoma and therefore the most important symptom of breast disease. Any mass in the breast must be evaluated for its cancer potential. This is particularly important (1) if the patient ...