How effective is ‘high‐speed’ autoradiography?

SUMMARY A number of methods have recently been described for enhancing silver grain production in light microscope autoradiography by impregnating the emulsion with a 2,5‐diphenyloxazole‐based fluor system. After extensive testing of these methods it was found that none gave a significant increase in silver grain count. In the course of this work a number of observations were made which seem to account at least partially for these failures, and which question the feasibility of this type of fluorography.1 Tetrahymena nuclei, pulse labelled with 3 H‐thymidine, contain sufficient radioactivity to produce useful autoradiographs after 2 h exposure without fluorographic enhancement. 2 Although fluorographic autoradiography of this type requires that the fluor penetrate the photographic emulsion, direct evidence was obtained that such penetration does not occur. Thus it is unlikely that true fluorographic enhancement occurs with methods based on 2,5‐diphenyloxazole impregnation of the emulsion. (This is in contrast to the widely used gel fluorography techniques in which it is the gel rather than the photographic emulsion which is impregnated with fluor.) 3 In DNA fibre autoradiography, it is possible to obtain artefactual arrays of silver grains which closely resemble those expected of replicating DNA. This phenomenon alone, however, does not explain a recent report of successful DNA fibre autoradiography after an exposure of only 3 days. Calculations based on the maximum dpm per length of labelled DNA raise further questions concerning this result.