The loss of "bound" S35 that occurs during various mounting procedures used in autoradiography was studied in healing surface wounds of rats treated with either methionine-S35 or Na2S35O4. Valid autoradiography of bound S35 in this tissue is not possible until 48 hours after radiosulfate and 24 hours after radiomethionine injection, when the S35 is almost entirely bound in large protein and polysaccharide molecules.

Autoradiograms of S35 given in both the organic and inorganic form reveal substantial over-all loss of the bound isotope from sections subjected to contact with solvents prior to autoradiography. A comparison of autoradiograms prepared by dry-mounting sections of frozen-dried tissue with autoradiograms of wet-mounted sections of the same tissue suggest that the loss is proportional to the extent of the contact with solvents. Evidence suggests that loss of the isotope occurs during contact of the ribbon or section itself with solutions after fixation and cutting and prior to radiation exposure. No appreciable loss of the bound isotope seems to occur during contact of the intact tissue specimen with a variety of fluid fixatives except for a marginal zone at the excision edges of the tissue. The potential hazard of displacement of the isotope during fixation, however, remains.

Technics which prevent loss of the isotope and fogging of the nuclear emulsion permit the use of thinner sections and emulsion films and the fine resolution of image rendered possible by the physical properties of S35.

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