In females, it serves as the mammary gland , which produces and secretes milk to feed infants. At puberty , estrogens , in conjunction with growth hormone , cause breast development in female humans and to a much lesser extent in other primates. Breast development in other primate females generally only occurs with pregnancy. Subcutaneous fat covers and envelops a network of ducts that converge on the nipple , and these tissues give the breast its size and shape. At the ends of the ducts are lobules , or clusters of alveoli , where milk is produced and stored in response to hormonal signals.