Lysosomal degradation of cell organelles. I. Ultrastructural analysis of uptake and digestion of intravenously injected mitochondria by Kupffer cells.

An experimental model comparable to autophagocytosis is presented to evaluate lysosomal degradation of biologic membranes in vivo. This design takes advantage of the efficient phagocytic capacity of Kupffer cells. For this purpose, mitochondria were isolated from one rat liver (inbred strain) and subsequently injected intravenously into a series of rats, which were sacrificed at various time points. The uptake and intralysosomal digestion were examined by electron microscopy. Mitochondria were seen in the sinusoids and attached to the surface of Kupffer cells 1 to 3 minutes after injection. One or two mitochondrial profiles were trapped in cavelike structures often embraced by long hyaloplasmic pseudopods. By 5 and 10 minutes, the mitochondria were in single membrane-bound phagosomes within the Kupffer cells and early signs of mitochondrial alterations were present, e.g., loss of cristae and condensation. At 30 minutes and 1 hour the mitochondria were seen in large vacuoles which contained up to 20 profiles in a single section. The mitochondria showed evident signs of digestion such as membrane fragmentation and flocculent densities. By 4 hours and especially by 8 hours, most digestive vacuoles were laden with flocculent membrane debris. By 24 hours, the Kupffer cells showed multiple lysosomal structures which were irregular in shape and small. These often contained a number of lipid-like droplets of various electron densities and pentalaminar structures. By 2 and 5 days, the Kupffer cells had returned to their normal appearance although the lysosomal apparatus was still prominent. No uptake of mitochondria was seen in endothelial cells, fat-storing cells, or hepatocytes. It is concluded that lysosomes have an efficient capacity to digest mitochondria. Some lipid remnants are not completely or slowly degraded by Kupffer cell lysosomes but remain in "residual bodies" for up to 1 or 2 days.