A mammary alveolus is a small cavity or sac found in the mammary gland. Mammary alveoli are the site of milk production and storage in the mammary gland. They are formed by secretory cells surrounded by myoepithelial cells. In response to oxytocin stimulation, myoepithelium contracts, resulting in release of milk. Mammary alveoli cluster into groups called mammary lobules, and each breast may contain 15 to 20 of these lobules. The lobules drain milk through the lactiferous ducts out of the nipples.