Epithelial cells remove apoptotic epithelial cells during post-lactation involution of the mouse mammary gland

Following the cessation of lactation, the mammary gland undergoes a physiologic process of tissue remodeling called involution in which glandular structures are lost, leaving an adipose tissue compartment that takes up a much larger proportion of the tissue. A quantitative morphometric analysis was undertaken to determine the mechanisms for clearance of the epithelial cells during this process. The involution process was set in motion by removal of pups from 14-day lactating C57BL/6 mice. Within hours, milk-secreting epithelial cells were shed into the glandular lumen. These cells became apoptotic, exhibiting exposure of phosphatidylserine residues on their surfaces, activation of effector caspase-3, staining for caspase-cleaved keratin 18, loss of internal organellar structure, and nuclear breakdown, but minimal blebbing or generation of apoptotic bodies. Clearance of residual milk and the shed epithelial cells was rapid, with most of the removal occurring in the first 72 h. Intact apoptotic epithelial cells were engulfed in large numbers by residual viable epithelial cells into spacious efferosomes. This process led to essentially complete involution within 4 days, at which point estrous cycling recommenced. Macrophages and other inflammatory cells did not contribute to the clearance of either residual milk or apoptotic cells, which appeared to be due entirely to the epithelium itself.