Autoradiographic Study of the Effect of Estradiol and Irradiation on Nucleic Acid Metabolism of the Thymus and Lymph Node of Mice

Autoradiographs of ${\rm UR}\text{-}5\text{-}{}^{3}{\rm H}$ and ${\rm TdR}\text{-}{\rm CH}_{3}\text{-}{}^{3}{\rm H}$ incorporation in the thymus and lymph node have been prepared at intervals from mice after 150 R or 0.5 mg estradiol. Epithelial reticular cells were the primary site of increased incorporation of ${\rm UR}\text{-}5\text{-}{}^{3}{\rm H}$ 8 hours postirradiation. At the time of maximum incorporation 72 hours after x-ray, however, transitional cells were heavily labelled. The sequential changes in ${\rm UR}\text{-}5\text{-}{}^{3}{\rm H}$ labelling in the estradiol-treated thymus were qualitatively similar though quantitatively less striking and no significant change occurred in the lymph node after either agent. The possible role of enhanced RNA synthesis in epithelial reticular cells is discussed in the relationship to the subsequent recovery of thymic lymphocytes. ${\rm TdR}\text{-}{\rm CH}_{3}\text{-}{}^{3}{\rm H}$ labelling in the thymus virtually ceased after x-ray and markedly decreased after estradiol. Reappearance first occurred in perivascular transitional cells and was followed by incorporation into the recovering lymphocyte lineage. The same general pattern occurred in the lymph node after irradiation but not after estradiol. The failure of ${\rm TdR}\text{-}{\rm CH}_{3}\text{-}{}^{3}{\rm H}$ to label epithelial reticular cells is considered to be an important point against the concept that they are the progenitors of thymic lymphocytes.