Resolution in electron microscope autoradiography. III. Iodine-125, the effect of heavy metal staining, and a reassessment of critical parameters.

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Abstract
Resolution for 125I-labeled specimens [plastic film and newt dorsal root ganglion] under EM autoradiographic conditions was assessed experimentally. With this isotope the size of the silver halide crystal was the most important resolution-limiting factor. Heavy metal staining such as is routinely used in preparing animal tissues for EM autoradiography produced an improvement in resolution of .apprx. 15-20%. For a 500-1000-.ANG. biological tissue section fixed with OsO4 and stained with uranyl acetate, resolution (half distance, HD) values of .apprx. 800 .+-. 120 .ANG. were obtained using Ilford L4 emulsion and 500 .+-. 70 .ANG. using a Kodak NTE-type emulsion. General aspects of resolution-limiting factors and comparison with 3H and 14C values are discussed.