Both males and females possess mammary glands. The mammary glands of males and immature females are similar. At puberty, estrogens and progesterone stimulate the development of female mammary glands. Estrogens start breast and mammary gland development, and progesterone stimulates the maturation of the mammary glands so that they are capable of secreting milk. Another hormone from the anterior lobe of the pituitary, prolactin, is required for milk production.

Mature mammary glands are female accessory reproductive structures that are specialized for milk production. They are located in the subcutaneous tissue of female breasts. The breasts are formed superficial to the pectoralis major. Breasts contain large amounts of areolar and adipose tissues that surround and cushion the mammary glands. Dense irregular connective tissue within the breasts is attached to the dermis and to the fascia of the pectoralis major to provide support for the breasts and mammary glands. Superficially, a circle of pigmented skin, the areola (ah-re^io-lah), is located near the anterior portion of each breast. A nipple containing erectile tissue is located in the center of each areola.

Each mammary gland consists of 15 to 25 lobes containing alveolar glands. Alveolar glands produce milk under stimulation by prolactin after the birth of an infant. Milk is carried from the alveolar glands by lactiferous ducts, which open into lactiferous sinuses that lead to the nipple and the external environment.