There are few if any alveoli in the non-lactating gland . Most of the epithelial elements within the lobule in the resting gland are intralobular ducts . During pregnancy, there is growth and branching from the terminal parts of the duct system, and many alveoli are formed. Colostrum is rich in IgA synthesized by plasma cells of the intralobular connective tissue. The lactating breast after parturition contains many alveoli with distended lumina.
Milk has many components, including proteins, lipids, and sugars.
In the case of the lipid , fat droplets arise in the cytoplasmic matrix and are moved toward the apical ends of the cells, where they are pinched off from the cell along with a small amount of cytoplasm. This is true apocrine type of secretion since some cytoplasm is lost. The endocrine dependence of the mammary gland is complicated. Estrogen promotes growth of the duct system. Estrogen and progesterone both have roles in development of alveoli.