The nipple is a raised area of tissue on the breast, surrounded by the darker, pigmented skin known as the areola. It serves as the exit point for milk during lactation. Contrary to the idea of a single, large hole, the nipple surface contains multiple tiny openings, or pores. These pores are the final destinations of the breast’s internal milk transport system, and their presence is fundamental to the physiological process of feeding.
The Duct System and Nipple Pores
The internal structure of the breast is organized around a network of milk ducts that converge at the nipple. Milk is produced in tiny, grape-like sacs called alveoli, deep within the breast tissue. It travels from the alveoli into progressively larger tubes known as lactiferous ducts. Each major duct corresponds to a separate lobe of glandular tissue.
The number of distinct lactiferous ducts reaching the nipple surface typically ranges from 4 to 20. Each duct narrows significantly as it approaches the tip, terminating in an individual, microscopic opening, or nipple pore. This arrangement of multiple, separate channels ensures milk is released from several points simultaneously, rather than through a singular outlet.
Modern ultrasound studies show that the ducts remain narrow and milk is not stored in large volumes near the surface. Older anatomical models incorrectly suggested the ducts widened beneath the areola to form large storage reservoirs called lactiferous sinuses. Instead, the ducts branch closer to the nipple, and milk flow depends directly on the active process of release.
How Milk Exits the Nipple
Milk is expelled through the nipple pores via an active physiological event known as the Milk Ejection Reflex, or “let-down.” This reflex is initiated when an infant’s suckling stimulates specialized nerve endings in the nipple and areola. The stimulation sends a signal to the brain, prompting the release of the hormone oxytocin from the posterior pituitary gland.
Oxytocin travels through the bloodstream to the breast tissue, acting on myoepithelial cells surrounding the alveoli and ducts. The hormone causes these muscle-like cells to contract rhythmically, squeezing the milk out of the alveoli. This muscular contraction propels the milk through the lactiferous ducts and forces it out through the multiple, tiny nipple pores.
The let-down reflex is highly efficient, often occurring less than a minute after stimulation begins. Since the hormonal signal affects both breasts simultaneously, milk commonly drips or sprays from the non-feeding breast during the reflex. This coordination of hormonal release and muscular contraction ensures milk is readily available to the infant, flowing out of the multiple pores in jets or streams.
Visibility and Appearance of Nipple Openings
For most non-lactating individuals, the nipple pores are microscopic and practically invisible to the unaided eye. Their small size is the primary reason for the common misconception that the nipple is a solid structure with no openings. The skin of the nipple is covered by smooth muscle, which aids in erection upon stimulation, but does not make the tiny pores visually apparent.
The visibility of the nipple openings may occasionally increase during lactation, but they still remain very small. During this time, a person might notice a tiny white spot, often a blocked pore or “milk bleb,” caused by a clog of hardened milk. These occurrences temporarily highlight the exact location and microscopic size of the individual duct openings.
The areola contains small bumps called Montgomery glands, which are often mistaken for nipple pores. These glands secrete an oily substance that lubricates and protects the skin. Montgomery glands may become more noticeable during pregnancy and lactation due to hormonal changes, but they are separate from the milk-exiting pores.