Each breast has 15-25 secretory lobes in the adipose tissue. The mammary gland resembles a sweat gland with a few modifications. A compound tubular acinar gland resides in each of these lobes. The acini discharge into ducts lined by cuboidal or low columnar epithelial cells and encircled by myoepithelial cells. Each lobule’s duct empties into a lactiferous duct on the nipple’s surface. In the nipple’s region, these ducts are encircled by smooth muscle, which contracts to raise the nipple.

Reference: https://www.histology.leeds.ac.uk/female/FRS_breast.php