FURTHER OBSERVATIONS ON THE LATE LABELLING ASSOCIATED WITH STIMULUS‐RESPONSIVE CELLS IN SKIN

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Abstract
The presence of a long‐lived DNA precursor pool which may show some specificity for stimulus‐responsive cells has been demonstrated. Autoradiography, biochemical analysis, hydroxyurea sensitivity, and the temporal response all suggest that the late incorporation is into DNA in cells in the basal layer of the skin that respond to stimulation. The effect is observed with various doses of tritiated thymidine and both methyl and 6 labelled material. 125Iododeoxyuridine also shows late labelling in skin. It is believed that the late labelling is readily distinguishable from reutilization and from possible slow utilization of catabolites into molecules other than DNA. Skin of the ear and dorsum shows similar degrees of late incorporation, while tail and foot skin apparently have smaller long‐lived pools. This may indicate smaller stimulus‐responsive cell populations. If tritiated thymidine is given to cells after stimulation there is still some slow or delayed uptake. The distribution of labelled cells in the autoradiographs suggests that the late labelling cells may be associated somehow with pre‐existing labelled cells (cells in S at the time of thymidine administration).

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