Electron microscopic observations were made serially of E.L.4 and C1498 ascites tumor cells following the transplantation of the tumors in C57BL/10J mice, a susceptible strain, and in B10.D2 mice, a coisogenic resistant subline which differs from the former mice only in a single allele at the histocompatibility-2 locus. As early as 2 days after tumor transplantation, proliferation of lymphoid tissue including plasma cells took place in the mesenteric and retroperitoneal lymph nodes and spleens of both sublines of mice, but only in the resistant line was this process progressive and extensive. At 5 days after inoculation, destructive effects were observed in both the cytoplasmic and nuclear structures of the tumors in the B10.D2 mice. These effects were observed in many tumor cells in which there was no contact with histiocytes. No evidence was found in the present study of cytoplasmic exchange between tumor cells and histiocytes prior to the phagocytosis of the degenerating tumor cells. Phagocytosis of the tumor cells by histiocytes in the ascites was followed by digestion of the degenerated tumor cells, presumably by the enzymes of the histiocytic lysosomes which entered the phagocytotic vesicles. Myelin-like residues seen within the histiocytes were possibly end-stages of tumor cell membrane destruction by the enzymes of the phospholipase-poor lysosomes. From the results it is clear that the degenerative consequences of the homograft reaction can be observed earlier by electron microscopic than by light microscopic methods.

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This study was supported in part by grants C-6503, C-6019, and C-2568 from the National Cancer Institute, National Institutes of Health, USPHS, and by the Hector Escobosa Fund.

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