A considerable decrease (24 to 40%) of DNA content per nucleus previously observed in the adrenal medulla of rats exposed intermittently to cold is followed by restoration to normal and supranormal values. This phenomenon has now been studied by use of H3-thymidine, which was given to normal rats, to rats exposed to cold, and to animals brought to room temperature after cold exposure. In the first two conditions, no significant labeling of nuclei was observed. In the third, labeling took place clearly in the 1st 3 days. The grain counts showed that the early labeled nuclei had more grains than those labeled later, indicating differences in the rate of DNA synthesis. A statistically significant correlation was found, on the same nuclei, between amount of Feulgen dye and number of grains. It is concluded that net synthesis of DNA takes place in the phase of recovery from cold. This fact is not related to cell division, as no mitoses could ever be detected, but rather to the cold-induced loss of DNA. Clear demonstration is thus given of a marked variation in the amount of DNA per nucleus in relation to the functional conditions of adrenal medulla cells.

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