Acid phosphatase was localized at the fine structural level in rat endometrial phagocytes during the period of postpartum involution. These cells showed intense phagocytotic and pinocytotic activities, which were accompanied by the development of abundant lysosomes. Phagosomes acquired their enzymatic complement by fusion with lysosomes; the same appeared to be true in the case of pinocytotic vesicles, but, because of the small size of these vesicles, this point could not be established with certainty. Digestion within some phagolysosomes led to the formation of electron-lucent vacuoles containing solubilized products. Other phagolysosomes showed accumulation of lipid residues in the form of droplets and myelin figures, and the structures acquired the appearance of residual bodies. In many macrophages, overfeeding led to the formation of unusually large numbers of phagolysosomes, which occupied almost the entire cytoplasm with exclusion of other cell organelles. In these cells the presence of abundant lead deposits, apparently free in the cytoplasm suggested an intracytoplasmic release of hydrolases.

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