Overview
The nipple is the external projection of the mammary gland through which milk is delivered from a lactating mammal to its offspring. In humans it sits at the center of the areola and serves both as the exit for milk and as a sensory organ that can trigger neuroendocrine responses. Nipples occur in both sexes because they form during early embryonic development before the main processes of sexual differentiation.
Anatomy and structure
Externally the nipple and surrounding areola vary markedly in size, shape and pigmentation. Internally the nipple contains the openings of multiple lactiferous ducts, connective tissue, smooth muscle fibers that enable erection, and a dense supply of sensory nerve endings. The duct architecture and subareolar glandular tissue connect the lobules of the breast to the nipple tip. For a focused summary of the specialized parts and histology, see nipple structure.
Function and lactation
The primary biological function of the nipple is to enable suckling and efficient transfer of milk. Milk synthesized in the alveoli of the breast is transported through ducts and released at the nipple openings when the infant suckles. Suckling stimulates afferent nerves in the nipple and areola; this neural input supports oxytocin release for milk ejection and prolactin-mediated milk synthesis. Practical guidance on breastfeeding technique and positioning is often provided in resources about breastfeeding.
Development and hormonal control
Nipples develop early in the embryo and are present in both sexes. Lactation is regulated by a hormonal network: prolactin primarily drives milk production while oxytocin causes milk let-down. Other hormones and local factors modulate glandular activity. More general information about breast organization and function is available at breast anatomy, and about the physiology of milk production and hormonal regulation.
Variations, clinical notes and cultural aspects
Nipples show natural variation—some are inverted, flat, or more protruding. New changes such as unexplained discharge, pain, or sudden inversion should be evaluated clinically. Although male nipples do not usually lactate, rare hormonal imbalances, certain medications or stimulation can produce milk in people of any sex. Beyond biology, nipples carry cultural and social meanings that influence norms about modesty, public breastfeeding and healthcare messaging.
Comparative and practical considerations
Across mammals nipple number, position and form reflect reproductive strategy: some species have multiple nipples along mammary lines, while primates typically have a single pair. For parents and healthcare providers, attention to latch, skin care and signs of infection supports successful breastfeeding and comfort for both parent and infant.