Normal Breast Development During Puberty

Puberty in girls begins at the age of 10 to 12 years as a result of the influence of hypothalamic gonadotropin-releasing hormones secreted into the hypothalamic–pituitary portal venous system. The basophilic cells of the anterior pituitary release follicle-stimulating hormone and luteinizing hormone. Follicle-stimulating hormone causes the primordial ovarian follicles to mature into graafian follicles, which secrete estrogens, primarily in the form of 17b-estradiol. These hormones induce the growth and maturation of the breasts and genital organs.

During the first 1 to 2 years after menarche, hypothalamic-adenohypophyseal function is unbalanced, because the maturation of the primordial ovarian follicles does not result in ovulation or a luteal phase. Therefore, ovarian estrogen synthesis predominates over luteal progesterone synthesis. The physiologic effect of estrogens on the maturing breast is to stimulate longitudinal ductal growth of ductal epithelium. Terminal ductules also form buds that precede further breast lobules. Simultaneously, periductal connective tissues increase in volume and elasticity, with enhanced vascularity and fat deposition.

These initial changes are induced by estrogens synthesized in immature ovarian follicles, which are anovulatory; subsequently, mature follicles ovulate, and the corpus luteum releases progesterone. The relative role of these hormones is not clear. In experimental studies, estrogens alone induce a pronounced ductular increase, whereas progesterone alone does not. The two hormones together produce full ductular-lobular-alveolar development of mammary tissues.

The marked individual variation in development of the breast makes it impossible to categorize histologic changes on the basis of age. Breast development by age has been described by external morphologic changes.

Morphology

Adult Breast
The adult breast lies between the second and sixth ribs in the vertical axis and between the sternal edge and the midaxillary line in the horizontal axis (Fig. 1-1). The average breast measures 10 to 12 cm in diameter, and its average thickness centrally is 5 to 7 cm. Breast tissue also projects into the axilla as the axillary tail of Spence. The contour of the breast varies but is usually domelike, with a conical configuration in the nulliparous woman and a pendulous contour in the parous woman. The breast comprises three major structures: skin, subcutaneous tissue, and breast tissue, with the last comprising both parenchyma and stroma. The parenchyma is divided into 15 to 20 segments that converge at the nipple in a radial arrangement. The collecting ducts that drain each segment are 2 mm in diameter, with subareolar lactiferous sinuses of 5 to 8 mm in diameter. Between five and ten major collecting milk ducts open at the nipple.