Tissue specificity for incorporation of [3H]thymidine by the 10- to 12-somite mouse embryo: alteration by acute exposure to hydroxyurea.

Radioautograms from 10- to 12-somite mouse embryos labeled for 30 min in vitro with [3H]thymidine were examined for frequency and intensity of incorporation. Results from ten tissues showed that values ranged from 82% of nuclei with a mean of 16.6 grains for visceral yolk sac to 17% of nuclei labeled with a mean of 4.4 grains for epithelium of the anterior gut tube. Labeling in the ten tissues indicated (1) a tissue-specific spectrum of incorporation of [3H]thymidine, (2) close correlation between frequency and intensity of labeling within a tissue and (3) asymmetrical quantities of incorporation between right and left somatopleure. Treatment with hydroxyurea in vitro reduced the frequency of labeled nuclei by 85% to 12% of control values. Mean numbers of grains over treated nuclei, 3.3-4.6 grains, were well above background but were clustered below the low end of the control range. Tissues exposed to hydroxyurea showed (1) labeling of significant numbers of nuclei, (2) inhibition of labeling in selected tissues and (3) equalization of bilateral asymmetry in quantity (frequency and intensity) of incorporation in somatopleure. The selective reduction of thymidine incorporation and equalization of asymmetrical rates of proliferation may constitute mechanisms by which hydroxyurea causes abnormal morphogenesis.