The accumulated milk products cause the alveoli to rupture and damage the basal membrane, damage which allows the breakdown of milk components. The cellular components degenerate with the participation of the lysosomes of the cell. The myoepithelial cells apparently do not undergo degeneration and are important in bridging the gap to hold the surviving cells together after the necrotic cells are eliminated. Similar changes also occur in the involution process during normal lactation, but the changes are not as abrupt. In addition to the loss of epithelial cells, a decrease in the rate of activity per cell also occurs.
Biochemical alterations occur within hours after the cessation of milking or suckling.
These are a decrease in the respiratory quotient, a decrease in oxygen consumption and an accumulation of lactic acid in the mammary gland tissue. The oxidative phosphorylation of the mitochondria of the mammary gland is uncoupled within 12-24 hours after the young have been removed. The suckling stimulus plays an important role in the maintenance of lactation, for it has been shown that the regression process is retarded by the suckling of ligated mammary glands. Prolactin injections also retard the involution process. The suckling stimulus also maintains the nucleic acid content of the lactating mammary gland.