Electron microscopic dry-mounting radioautography for diffusible compounds by means of ultracryotomy

Cultured HeLa cells or mouse liver and pancreas tissues were labeled with 3H-thymidine, -uridine or -glycine for varying periods in vitro, frozen in liquid nitrogen and cut on an LKB ultrotome equipped with LKB Cryokit. Dry ultrathin sections were mounted on grid meshes and were either air-dried, freeze-substituted or freeze-dried, and were covered with dry films of radioautographic emulsions, exposed, developed, stained and were observed in electron microscopes. After a number of trials, it was possible to obtain fairly good preservation of both cell structure and radioisotopes by means of freeze-dried and dry-mounted ultrathin frozen sections. However, the results are not completely satisfactory at the present time.