Structure and Physiology

The mammary glands are the distinguishing feature of all mammals. Human breasts are conical in form and are often unequal in size. The breast extends from the level of the second or third rib to the level of the sixth or seventh rib, from the sternal edge to the anterior axillary line. The "tail" of the breast extends into the axilla and tends to be thicker than the other breast areas. This upper outer quadrant contains the greatest bulk of mammary tissue and is frequently the site of neoplasia. Figure 16-1 illustrates the normal breast.

The normal breast consists of glandular tissue, ducts, supporting muscular tissue, fat, blood vessels, nerves, and lymphatic vessels. The glandular tissue consists of 15 to 25 lobes, each of which drains into a separate excretory duct that terminates in the nipple. Each duct dilates as it

enters the base of the nipple to form a milk sinus. This serves as a reservoir for milk during lactation. Each lobe is subdivided into 50 to 75 lobules, which drain into a duct that empties into the excretory duct of the lobe.

Both the nipple and areola contain smooth muscle that serves to contract the areola and compress the nipple. Contraction of the smooth muscle makes the nipple erect and firm, thereby facilitating the emptying of the milk sinuses.

The skin of the nipple is deeply pigmented and hairless. The dermal papillae contain many sebaceous glands, which are grouped near the openings of the milk sinuses. The skin of the areola is also deeply pigmented but, unlike the skin of the nipple, contains occasional hair follicles. Its sebaceous glands are commonly seen as small nodules on the areolar surface and are termed Montgomery's tubercles.

Cooper's ligaments are projections of the breast tissue that fuse with the outer layers of the superficial fascia and serve as suspensory structures.

The blood supply to the breast is carried by the internal mammary artery. The breast has an extensive network of venous and lymphatic drainage. Most of the lymphatic drainage empties into the nodes in the axilla. Other nodes lie beneath the lateral margin of the pectoralis major muscle, along the medial side of the axilla, and in the subclavicular region. The main lymph node chains and lymphatic drainage of the breast are illustrated in Figure 16-2.

Several physiologic changes occur in the breast. These changes are a result of the following factors:

Growth and aging The menstrual cycle Pregnancy

At birth, the breasts contain a branching system of ducts emptying into a developed nipple. There is elevation of only the nipple at this stage. Shortly after birth, there is a slight secretion of milky material. After 5 to 7 days, this secretory activity stops. Before puberty, there is elevation of the breast and nipple, called the breast bud stage. The areola has increased in size. At the onset of puberty, the areola enlarges further and darkens. A distinct mass of



Figure 16-2 Lymphatic drainage of the breast.

Figure 16-2 Lymphatic drainage of the breast.

Figure 16-3 Stages of breast development.

glandular tissue begins to develop beneath the areola. By the onset of menstruation, the breasts are well developed, and there is forward projection of the areola and nipple at the apex of the breast. One to 2 years later, when the breast has reached maturity, only the nipple projects forward;the areola has receded to the general contour of the breast. The stages of breast development from birth to adulthood are illustrated in Figure 16-3. Figure 24-46 (Chapter 24, The Pediatric Patient) further illustrates and describes the breast developmental stages.

The nodularity, density, and fullness of the adult breast depend on several factors. Most important is the presence of excess adipose tissue. Because the mammary gland consists mainly of adipose tissue, women who are overweight have larger breasts. Pregnancy and nursing also alter the character of the breasts. Often, women who have nursed have softer, less nodular breasts. However, because the glandular tissue is approximately equal in all women, the size of the breast is unrelated to nursing. With menopause, the breasts decrease in size and become less dense. There is an associated decrease in elastic tissue as women age.

The major physiologic change related to the menstrual cycle is engorgement, occurring 3 to 5 days before menstruation. This is an increase in the size, density, and nodularity of the breasts. There is also an increased sensitivity of the breasts at this time. Because the nodularity of the breasts increases, the examiner should not attempt to diagnose a breast mass at this time. The patient should be reevaluated during the midperiod of the next cycle.

With pregnancy, the breasts become fuller and firmer. The areola darkens, and the nipples become erect as they enlarge. As the woman approaches the third trimester, a thin, yellowish secretion, called colostrum, may be noted. After the birth of the child, if the mother begins nursing within 24 hours, the secretion of colostrum stops, and the secretion of milk begins. During nursing, the breasts become markedly engorged. After the woman has stopped nursing, lactation continues for a short time.

The neuroendocrine control of the breasts can be outlined as follows. Suckling produces nerve impulses that travel to the hypothalamus. The hypothalamus stimulates the anterior pituitary to secrete prolactin, which acts on the glandular tissue of the breast to produce milk. The hypothalamus also stimulates the posterior pituitary to produce oxytocin, which stimulates the muscle cells surrounding the glandular tissue to contract and force the milk into the ductular system.

Many abnormalities of the breast are related to its embryology. An epithelial ridge, called the milk line, forms along each side of the body from the axilla to the inguinal region. Along this milk line are multiple rudiments for future breast development. In humans, only one rudimentary pair in the pectoral region persists and eventually develops

Accessory Nipples Milk Line

Figure 16-4 The milk line.

Figure 16-5 Accessory nipple.

Figure 16-4 The milk line.

into normal breasts. Accessory breasts or nipples are present in as many as 2% of white women. Accessory breasts may exist as glandular tissue, nipple, or only the areola. The axilla is the most common site for these anomalous structures, followed by a site just below the normal breast. In more than 50% of all patients with accessory breast tissue, the anomalies are bilateral. In general, accessory breast tissue is of little clinical significance. It usually has no physiologic function and is rarely associated with disease. Figure 16-4 illustrates the milk line. Figure 16-5 shows an accessory nipple.

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