The breast contains 15-25 lactiferous ducts, which begin at the nipple, branch into smaller ducts and end in the terminal duct lobular unit (lobule), which is composed of a terminal duct and many small ductules (or acini). The ducts and ductules are lined by an inner layer of cuboidal to columnar epithelial cells and an outer layer of myoepithelial cells. The connective tissue within the lobule is composed of fibroblasts in a background of collagen and acid mucins, with histiocytes and occasional lymphocytes. The interlobular stroma is hypocellular and composed of fibroadipose tissue.
The epithelium and lobular stroma are hormonally responsive. During pregnancy there is a marked proliferation of ductules, resulting in very large lobules, and the epithelial cells have abundant cytoplasm filled with secretory vacuoles.