Breast Anatomy and Development

Introduction
The breasts, or mammary glands, of mammals are important for the survival of the newborn and thus of the species. Nursing of the young in the animal kingdom has many physiologic advantages for the mother, such as aiding postpartum uterine involution, and for the neonate, in terms of the transfer of immunity and bonding. In humans, social influences have reduced the prevalence of breast-feeding of neonates and may have interfered with its physiologic role. It has become increasingly apparent that the advantages of nursing are substantial for both mother and child.

An understanding of the morphology and physiology of the breast and the many endocrine interrelationships of both is essential to the study of the pathophysiology of the breast and the management of benign, preneoplastic, and neoplastic disorders.

Embryology
During the fifth week of human fetal development, the ectodermal primitive milk streak, or "galactic band," develops from axilla to groin on the embryonic trunk. In the region of the thorax, the band develops to form a mammary ridge, whereas the remaining galactic band regresses. Incomplete regression or dispersion of the primitive galactic band leads to accessory mammary tissues, found in 2% to 6% of women.

At 7 to 8 weeks' gestation, a thickening occurs in the mammary anlage (milk hill stage), followed by invagination into the chest wall mesenchyme (disc stage) and tridimensional growth (globular stage). Further invasion of the chest wall mesenchyme results in a flattening of the ridge (cone stage) at 10 to 14 weeks' gestation. Between 12 and 16 weeks' gestation, mesenchymal cells differentiate into the smooth muscle of the nipple and areola.

{embed="ref/7breast"}Epithelial buds develop (budding stage) and then branch to form 15 to 25 strips of epithelium (branching stage) at 16 weeks' gestation; these strips represent the future secretory alveoli. The secondary mammary anlage then develops, with differentiation of the hair follicle, sebaceous gland, and sweat gland elements, but only the sweat glands develop fully at this time. Phylogenetically, the breast parenchyma is believed to develop from sweat gland tissue. In addition, special apocrine glands develop to form the Montgomery glands around the nipple. The developments described thus far are independent of hormonal influences.

During the third trimester of pregnancy, placental sex hormones enter the fetal circulation and induce canalization of the branched epithelial tissues (canalization stage). This process continues from the twentieth to the thirty-second week of gestation. At approximately term, 15 to 25 mammary ducts are formed, with coalescence of duct and sebaceous glands near the epidermis. Parenchymal differentiation occurs at 32 to 40 weeks with the development of lobuloalveolar structures that contain colostrum (end-vesicle stage). A fourfold increase in mammary gland mass occurs at this time, and the nipple-areolar complex develops and becomes pigmented. In the neonate, the stimulated mammary tissue secretes colostral milk (sometimes called witch's milk), which can be expressed from the nipple for 4 to 7 days postpartum in most neonates of either sex. In the newborn, colostral secretion declines over a 3- to 4-week period, owing to involution of the breast after withdrawal of placental hormones. During early childhood, the end vesicles become further canalized and develop into ductal structures by additional growth and branching.