The secretory contents of the Golgi Apparatus are discharged into the lumen of the alveoli by the process of reverse pinocytosis. Pinocytosis is the process whereby cells can take in fluid and molecules too large to be carried across the plasma membrane by active transport mechanisms. During pinocytosis, material becomes adsorbed to the plasma membrane causing changes in surface tension and electrical property in the membrane. These changes cause an invagination in the membrane to form a pocket. Eventually the pocket is pinched off to float freely within the cell.
Lysosomes later digest the vesicle to release the contents.
For milk protein secretion, the Golgi Apparatus forms vacuoles which migrate and become contiguous with the plasma membrane. Through the reverse of the procedure just described the vacuole discharge it's contents without rupture of the cell membrane. Lysosomes are membrane bound particles which contain degradative enzymes. Lysosomes are especially active during involution of the mammary gland. The microtubules are essential for cell division and serve to maintain shape of the mammary cells.