Mammary Gland Relaxed
Alveoli are round balloon-type structures surrounded by lactocytes (milk-producing cells). Bands of contractile (smooth-muscle) cells surround the alveoli. The infant's sucking stimulates nerves in the nipple which carry a message to the brain. The brain releases hormones that regulate milk production including oxytocin. When the alveoli are relaxed (see image) they can fill with milk.

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