The tubuloalveolar mammary glands, derived from modified sweat glands in the epidermis, lie in the subcutaneous tissue. The inactive adult mammary gland is composed of 15 to 20 irregular lobes separated by fibrous bands of connective tissue. They radiate from the mammary papilla, or nipple, and are further subdivided into numerous lobules known as terminal duct lobular units (TDLUs) (Fig. 23.30). Some of the fibrous bands, called suspensory or Cooper’s ligaments, connect with the dermis. Abundant adipose tissue is present in the dense connective tissue of the interlobular spaces.
Each gland ends in a lactiferous duct that opens through a constricted orifice into the nipple. Beneath the areola, the pigmented area surrounding the nipple, each duct has a dilated portion, the lactiferous sinus. Near their openings, the lactiferous ducts are lined with stratified squamous keratinized epithelium. The epithelial lining of the duct shows a gradual transition from stratified squamous to two layers of cuboidal cells in the lactiferous sinus and finally to a single layer of columnar or cuboidal cells through the remainder of the duct system.
The epidermis of the adult nipple and areola is highly pigmented and somewhat wrinkled and has long dermal papillae invading into its deep surface (Fig. 23.31). It is covered by keratinized stratified squamous epithelium. The pigmentation of the nipple increases at puberty, and the nipple becomes more prominent. During pregnancy, the areola becomes larger and the degree of pigmentation increases further. Deep to the areola and nipple, bundles of smooth muscle fibers are arranged radially and circumferentially in the dense connective tissue and longitudinally along the lactiferous ducts. These muscle fibers allow the nipple to become erect in response to various stimuli.
The areola contains sebaceous glands, sweat glands, and modified mammary glands (glands of Montgomery). These glands have a structure intermediate between sweat glands and true mammary glands, and they produce small elevations on the surface of the areola. It is believed that Montgomery glands produce a lubricating and protective secretion that changes the skin’s pH and discourages  microbial growth. Numerous sensory nerve endings are present in the nipple; the areola contains fewer sensory nerve endings.
Terminal duct lobular unit (TDLU) of the mammary gland represents a cluster of small secretory alveoli (in lactating gland) or terminal ductules (in inactive gland) surrounded by intralobular stroma.
Successive branching of lactiferous ducts leads to the terminal duct lobular unit (TDLU). Each TDLU represents a grapelike cluster of small alveoli that forms a lobule (Fig. 23.32) and consists of the following.  Terminal ductules are present in the inactive gland. Dur ing pregnancy and after birth, epithelium of the terminal ductules, which is lined by secretory cells, differentiates into fully functional secretory alveoli producing milk.  The intralobular collecting duct carries alveolar secre tions into the lactiferous duct.  The intralobular stroma is specialized hormonally sensi tive loose connective tissue that surrounds the terminal ductules and alveoli. The intralobular connective tissue contains little adipose cells.