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. The microtubules assist in the movement of secretory vesicles to the apex of the cell. The cytoplasm is a fluid matrix which constitutes a large portion of the mammary cell. The cytoplasm contains the enzymes, nutrients, etc. which are essential for cell function.