The prepubertal breast in both boys and girls consists primarily of lactiferous ducts that exhibit some branching without evidence of progressive alveolar differentiation, although some rudimentary lobular structures may be seen. The breast is maintained in this state until the time of puberty. Extramedullary hematopoiesis in the stroma, a feature seen in the fetal breast, may persist until 4 months of age (Fig. 17.1, e-Fig. 17.1).1

At puberty in females, there is duct elongation, branching, and development of a thickened epithelium, which is primarily due to the influence of estrogen.2 This is accompanied by an increase in the density of periductal connective tissue, which is also the result of relative estrogen dominance. Deposition of stromal adipose tissue also occurs, and it is this adipose tissue that is largely responsible for the enlargement and protrusion of the breast disc at this time. Cyclical exposure to progesterone following exposure to estrogen during ovulatory cycles promotes lobulo-acinar growth as well as connective tissue growth. Although the majority of lobulo-acinar development occurs during puberty, this process continues into the third decade and terminal differentiation of the breast is induced only by pregnancy.

The adolescent male breast is composed of fibroadipose tissue and ducts lined by low cuboidal epithelial cells.

A number of breast lesions that occur in adults may be seen in children and adolescents, including fibroadenomas, phyllodes tumors, and intraductal papillomas. Gynecomastia may be seen in adolescent boys, as discussed in Chapter 16. This chapter will focus on several lesions that are primarily seen in children and adolescents.