Breast

"The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants. Both men and women develop breasts from the same embryological tissues. However, at puberty, female sex hormones, mainly estrogen, promote breast development, which does not occur in men, due to the higher amount of testosterone. As a result, women's breasts become far more prominent than those of men.
During pregnancy, the breast is responsive to a complex interplay of hormones that cause tissue development and enlargement in order to produce milk. Three such hormones are estrogen, progesterone and prolactin, which cause glandular tissue in the breast and the uterus to change during the menstrual cycle. Each breast contains 15–20 lobes. The subcutaneous adipose tissue covering the lobes gives the breast its size and shape. Each lobe is composed of many lobules, at the end of which are sacs where milk is produced in response to hormonal signals."

Lactation

Breast milk is thought to be the best form of nutrition for neonates and infants. The properties of human milk facilitate the transition of life from in utero to ex utero. This dynamic fluid provides a diverse array of bioactive substances to the developing infant during critical periods of brain, immune, and gut development. The clinician must be familiar with how the mammary gland produces human milk and how its properties nourish and protect the breastfeeding infant.