Histology of breast development in early life.

Histological examination of the breasts of 26 infants and young children who died suddenly between the ages of 3 weeks and 2 years was performed. The glands were composed of well formed lobules surrounded by dense interlobular stroma, while within the lobules there was looser connective tissue. The lobules contained ducts, many of which were dilated and contained secretions. Foci of extramedullary haematopoiesis were found, and in the older infants, fat was prominent within the connective tissue of the breast. Myoepithelial cells were regularly present. No sex differences in breast development at this time were noted. Newborn breast development did not regress rapidly after birth and secretory activity continued for many months in both sexes. This study shows that the human mammary gland remains active for many months after birth and may continue to grow and secrete. The findings are not consistent with the current view that breast development in infancy results from stimulation from 'pregnancy hormones.' It is more likely that the infant's own gonadal secretions are responsible.