Rate of Synthesis of Desoxyribonucleic Acid and Mitotic Rate in Liver and Intestine

The incorporation of radiophosphorus (P"") into desoxyribonucleic acid (DNA) has been shown to parallel the incidence of mitosis in various tissues (14).In tissues without mitotic activity, such as brain cortex, skeletal muscle, and cartilage, injected P32 was not found in DNA (4).Accordingly, it is generally thought that the presence of P32 in DNA occurs as a result of the synthesis of additional DNA which accompanies mitosis ( 14).In the present work, the rate of incorporation of P32 into DNA and the rate at which new cells are formed by mitosis were measured quantitatively and the results compared.This was done in rat liver, an organ with a low mitotic activity, and in rat intestinal mucosa, a tissue with a high mitotic activity.In both cases, twice as much phosphorus appeared in DNA as could be accounted for by the synthesis of the additional DNA required for the new cells.This finding may be explained only by assuming that all of the DNA phosphorus (DNA P) of the cells arising from mitosis is newly formed. MethodsEstimation of Rate of Cell Production-The rate of cell production in the liver was measured by estimating the daily increase in the total number of cells.Four groups of six rats, whose average body weights were 51, 150, 198, and 332 gm., respectively, were used.In each animal the liver was weighed and the number of nuclei per gm. of fresh tissue estimated by the method of Price and Laird (5), modified by replacing the nuclear stain, methyl green, by orcein (1 per cent in 45 per cent acetic acid).From the number of nuclei found per gm. of liver and the increase in the size of this organ per day in rats weighing 175 to 225 gm., the number of cells formed per day was calculated.The mucosa of the small intestine is composed of epithelium and lamina propria, both of which were included in cell counts as well as in chemical estimations.To estimate the daily rate of cell formation, six male rats weighing from 175 to 200 gm.were sacrificed 4 hours after a subcutaneous injection of 0.1 mg. of colchicine per 100 gm. of body weight, a dose which