Nipple

The nipple is a small projection located at the center of the breast. It is where the milk ducts of the breast terminate, and thus where milk is released during breastfeeding. The nipple is surrounded by an area of different coloration called the areola.

The nipple is present in both men and women, but its function in milk production is active only in women after pregnancy and during breastfeeding. The size, color, and shape of the nipple vary considerably among individuals, depending on genetics, sex, age, and breastfeeding history.

Anatomy

Several milk ducts pass through the nipple, running from the breast tissue and opening at small pores on the nipple surface. The nipple contains smooth muscle that functions independently; when touched or exposed to cold, it contracts and becomes firm. The areola contains small glands (Montgomery's glands) that secrete an oily substance to protect the skin during breastfeeding.

Breastfeeding function

When an infant suckles the nipple, sensory signals are sent to the brain, triggering release of the hormone oxytocin. Oxytocin causes milk in the breast tissue to be pushed toward the nipple ducts, a process called the milk letdown reflex. The hormone prolactin, for its part, stimulates milk production. Thus, frequent breastfeeding maintains milk supply.

Variation and medical conditions

Some individuals have inverted nipples, which are usually a normal variation. Other conditions include soreness in breastfeeding women, mastitis (breast inflammation), and nipple discharge that is not milk. A sudden change in nipple shape, inversion, or bloody discharge warrants medical evaluation, as these can sometimes be signs of breast cancer.