Cell death by apoptosis during involution of the lactating breast in mice and rats

TLDR

Involution of mammary tissue is characterized by shrinkage while preserving basic architecture. The study aimed to determine the role of cell death during involution of the lactating breast in mice and rats. Light and electron microscopy were employed to examine cellular changes. Apoptosis was the principal mechanism of cell loss in both species, with mice showing rapid shedding of apoptotic bodies from alveolar epithelial cells that were later phagocytosed by macrophages, whereas rats exhibited a more gradual, less conspicuous shedding; additionally, apoptosis of myoepithelial cells occurred only in mice, capillary endothelial apoptosis led to rapid capillary bed regression in both species, and macrophage numbers increased and migrated during involution.

Abstract

Abstract The role of cell death in involution of lactating breast was investigated in mice and rats by light and electron microscopy. Apoptosis, recognized by sharply demarcated compaction of chromatin against the nuclear envelope and by shrinkage and budding of the whole cell to form membrane‐bounded apoptotic bodies, was responsible for major loss of cells in both species. In the mouse, rapid involution during the first 2 days was associated with shedding of large numbers of apoptotic bodies derived from alveolar epithelial cells into alveolar lumens. This was followed by more gradual regression, during which the bodies were mostly phagocytosed by macrophages within the epithelium. In the rat, glandular involution was a more gradual and uniform process, with shedding of apoptotic epithelial cells into alveolar lumens being much less conspicuous. Apoptosis of myoepithelial cells was observed in mice, the resulting apoptotic bodies being phagocytosed by intraepithelial macrophages, but was not detected in rats. Apoptosis of capillary endothelial cells caused rapid regression of the capillary beds in both mice and rats. Intraepithelial macrophages increased in number during involution, developed cytoplasmic lipofuscin pigment, and either remained within the epithelium or migrated to the interstitium and regional nodes. Cell loss by apoptosis has been demonstrated during involution and atrophy of a variety of other glands. It characteristically results in shrinkage of a tissue without disruption of its basic architecture.