LYSOSOMAL DEGRADATION OF CELL ORGANELLES .2. ULTRASTRUCTURAL ANALYSIS OF UPTAKE AND DIGESTION OF INTRAVENOUSLY INJECTED MICROSOMES AND RIBOSOMES BY KUPFFER CELLS

Abstract

Rough and smooth microsomes, "mixed" or total microsomes, and ribosomes were isolated from one single rat liver and subsequently injected i.v. into a series of inbred rats. The uptake and the degradation of the injected organelles by Kupffer cells were followed by means of electron microscopic analysis. By 1 min after injection, microsomes were seen attached to the surface of Kupffer cells separated by a gap of 200-300 .ANG.. No attachment to hepatocytes, fat-storing cells, or endothelial cells was seen. By 5 and 10 min, most microsomes were phagocytosed and sequestered in large numbers within single membrane-enclosed vacuoles or phagosomes. The eugulfment proceeded by 2 mechanisms: most frequently, flaplike processes of cytoplasm embraced aggregates of microsomes, concomitant with the formation of indention of the cytoplasm; occasionally, single microsomal profiles were taken up by bristle-coated endocytic vacuoles. Ribosomes were also seen penetrating into the wormlike structures (micropinocytosis vermiformis) at the cell surface. At 30 min after injection, clear signs of alteration were noted starting with vesicle aggregation, clumping, and elongation of the microsomal profiles. The ribosomes were quickly stripped from their microsomal membranes and marginated to the inside of the vacuoles but separated from the limiting membrane by a distance of 200-300 .ANG.. By 1 and 2 h, disruption of the vesicles into membrane fragments and formation of dense material in and between the profiles occurred. By 8 h it was difficult to recognize the degradation products as membrane derivatives. The digestive vacuoles retained their size at this time interval. Typical pentalaminar structures were observed. By 14-24 h the digestive vacuoles became electron lucent and appeared to shrink, and in addition to containing various types of granular material, many were laden with lipid-like droplets presumed to be conglomerates of phospholipid remnants. Rough microsomes, when compared to smooth microsomes, gave rise to more granular material within the digestive vacuoles. Ribosomes were still identifiable 24 h after injection indicative of a somewhat slower rate of degradation. Accumulation of various types of lipid-like droplets in the residual bodies was typical after microsomal injections. Although microsomes appear to be phagocytosed at a quicker rate than mitochondria, they are apparently digested within the lysosomal apparatus of the Kupffer cells at a somewhat slower rate. This seems especially true for ribosomes. Heterophagy of microsomes is one cource of residual bodies.