A wide range of nonspecific and reversible ultrastructural responses occur in liver cells following acute and chronic exposure to hepatocarcinogens. Although most changes do not occur to a uniform extent in all cells in acute and chronic experiments, nor do they persist indefinitely after withdrawal of the carcinogen, nucleolar abnormalities and disturbance in the ribosome-ergastoplasm relationship are the most consistent alterations in acute and chronic intervals and in the tumor cells themselves. None of the carcinogens used produced tumors with ultrastructural features sufficiently characteristic to distinguish them from those produced by other carcinogens. Despite the limitations of sampling and methods of electron microscopy, it would appear from a morphologic point of view that, following administration of hepatocarcinogens, nucleolar abnormalities at the level of synthesis and assembly of ribosomal precursors probably are equal in importance to changes in the ribosome-ergastoplasm complex in the cytoplasm.

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Supported in part by USPHS Grants CA-5680 and CA-8055, and by the Kansas Division of The American Cancer Society.

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