The breast is one of two prominences located on the upper ventral region of the torso among humans and other primates. Both sexes develop breasts from the same embryological tissues. The relative size and development of the breasts is a major secondary sex distinction between males and females.
In females, it serves as the mammary gland, which produces and secretes milk to feed infants. Subcutaneous fat covers and envelops a network of ducts that converge on the nipple, and these tissues give the breast its distinct size and globular shape. At the ends of the ducts are lobules, or clusters of alveoli, where milk is produced and stored in response to hormonal signals. During pregnancy, the breast responds to a complex interaction of hormones, including estrogens, progesterone, and prolactin, that mediate the completion of its development, namely lobuloalveolar maturation, in preparation of lactation and breastfeeding.
Humans are the only animals with permanently developed breasts in females. At puberty, estrogens, in conjunction with growth hormone, cause permanent breast growth in female humans. This happens only to a much lesser extent in other primates—breast development in other primates generally only occurs with pregnancy. Along with their major function in providing nutrition for infants, a number of cultures ascribe social and sexual characteristics to female breasts and may regard bare breasts in public as immodest or indecent. Breasts have been featured in ancient and modern sculpture, art, and photography. They can figure prominently in the perception of a woman's body and sexual attractiveness. Breasts, especially the nipples, are an erogenous zone.
