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An Electron Microscope Study of Sarcoma I in a Homologous Host

II. Changes in the Fine Structure of the Tumor Cell during the Homograft Reaction^*

( Department of Microbiology, University of Washington, School of Medicine, Seattle, Washington)

Changes in the ultrastructure of Sarcoma I ascites tumor cells during homograft rejection by the C57BL/6K mouse are described. Several contacts between the plasma membranes of host macrophages and tumor cells and between the plasma membranes of adjacent tumor cells are illustrated.

In degenerating tumor cells the mitochondria were swollen, and the mitochondrial matrix showed reduced electron density. The cristae were shortened. Many of the narrow profiles of rough-surfaced endoplasmic reticulum, which are characteristic of cells before the onset of degeneration, were apparently replaced by distended circular profiles during degeneration. Other profiles of endoplasmic reticulum became greatly elongated and assumed a coiled arrangement. Large, smooth-surfaced vesicles were also prominent in degenerating cells, and the contents of such vesicles were usually lacking in electron density.

The cluster arrangement of free ribosomes which was evident in nondegenerating cells was lost. The ribosomes became distributed in a diffuse, disorganized fashion throughout the cytoplasm of the peripheral regions of the degenerating cell. The significance of this change is discussed in the light of the recent reports on polysomes.

Cytoplasmic blebbing was a prominent feature of degenerating cells of regressing tumors. The blebs were packed with free ribosomes, which were diffusely distributed. The blebs contained a few strands of rough-surfaced endoplasmic reticulum. The retention of ribosomes and their diffuse arrangement in the degenerating Sarcoma I cell during the homograft reaction may differentiate the degeneration produced by transplantation immunity from the immunological tissue reactions effected by heterologous humoral antibody and complement.

^* Supported in part by United States Public Health Service Grant Ca-05698.

Received 3/ 2/64.