The breasts are modified apocrine sweat glands that develop to the same extent in both males and females during embryonic life and are similar structures in both sexes until thelarche when they develop in the female under the influence of the sex hormones to become the organs of lactation providing milk and nourishment for the newborn.
In the female, the breasts continue to change in structure and function throughout life. In the male they have no known function but are still subject to change under the conditions of obesity, and the abnormal influence of estrogen or progesterone. In the female their mst important function is the nutrition and nourishment of the newborn and infant. They are also secondary sex organs.