STUDIES ON MAMMARY GLAND INVOLUTION .2. ULTRASTRUCTURAL EVIDENCE FOR AUTO- AND HETEROPHAGOCYTOSIS

During enforced mammary gland involution in the rat, greatly increased numbers of cytosegresomes appear in the secretory epithelial cells 1-3 days after weaning. These cytosegresomes are interpreted as indicative of autophagocytosis. At 3 days and later, the cytosegresomes are few and appear to be replaced by large cytosomes, which are more numerous than in the lactating parenchyma. These findings support the theory that cytosegresomes are transformed to cytosomes. Increased numbers of macrophages were present between the epithelial cells in the alveoli during involution. Apparent heterophagocytic uptake of portions of epithelial cell cytoplasm and milk protein droplets in these macrophages appeared to be most pronounced at late intervals (3-10 days) after weaning. © 1968 Academic Press, Inc.