Breast Embryology, Anatomy, and Physiology

Fetal mammary gland development is not dependent on steroid hormones until beyond the 15th gestational week when breast structure development takes place. In the last weeks of gestation, the fetal breast is responsive to maternal steroid hormones, which are manifested in the neonatal period by the secretion of colostrum and palpable enlargement of the breast bud. This unilateral or bilateral breast enlargement and secretion of opaque fluid occurs in about 60% of normal newborns. However, both these findings resolve spontaneously during the first or second month after birth due to disappearance of maternal hormones from the infant’s bloodstream and do not require treatment. It can occur in both male and female infants. The development of the alveoli and their surrounding supportive framework remains quiescent until puberty once the perinatal influence of maternal hormones subsides. During childhood, breast tissue is dormant and mainly composed of ducts lined with epithelium and surrounded by connective tissue. Before breast development is completed, surgery around the breast bud in young girls must be avoided or exercised with great care to prevent injury and potential growth disturbance.