The incorporation of tritiated thymidine into the DNA of preneoplastic liver parenchyma (hypobasophilic, basophilic, and hyperbasophilic populations), as well as in normal liver and hepatomas, was investigated by radioautographic methods in rats fed the azo dye 4-dimethylaminoazobenzene (DAB).

Quantitative determinations of the incidence of radioactive nuclei revealed that the number of radioactive nuclei in the basophilic population was higher than in normal liver. The formation of hyperbasophilic regions was accompanied by a significant increase in the number of labeled nuclei and a still higher value was obtained for the incidence of radioactive nuclei in hepatomas. The increase in DNA synthesis occurring in preneoplastic liver was found to follow closely the phenomenon of hyperbasophilia which appears as a turning point in the carcinogenic process.

Extranuclear chromosome fragments and pyknotic granules resulting from abnormal mitoses and nuclear degeneration were found in the different populations of preneoplastic liver cells and hepatomas. The chromosome fragments and pyknotic granules present in basophilic cells were not radioactive, while those found in hyperbasophilic regions and hepatomas gave positive reactions indicating that such particles can be the site of DNA synthesis. It appears from the present study that the neoplastic transformation is associated with an increase in the number of nuclei synthesizing DNA and also with an uptake of tritiated thymidine in extranuclear chromosome fragments and pyknotic granules.

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This investigation was supported by a grant from the National Cancer Institute of Canada to Dr. A. Cantero, Director of the Research Laboratories. Preliminary reports of this work were presented at the 55th and 56th Annual Meetings of the American Association for Cancer Research held in Chicago, April 9–11, 1964, and in Philadelphia, April 7–10, 1965 (8, 32).

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