Block staining with p-phenylenediamine for light microscope autoradiography.

A study was designed to test the suitability of p-phenylenediamine (pPd) as a block stain for light microscope autoradiography. This was done to obviate the conventional method of staining through the emulsion with histological stains after exposure and development. Ten rats were injected with 3H-thymidine and 1 to 3 days later were perfused with glutaraldehyde. Tissue samples were processed by three different schedules: 1) direct embedding in Araldite-Epon; 2) postfixation in OsO4 before embedding; 3) postfixation in OsO4 and staining in 1% pPd before embedding. Autoradiographs of these tissues showed that pPd-treated tissues were well stained with clearly defined cellular detail. The nuclear region was pale-staining, which highlighted the overlying silver grains. There was no evidence of chemography and the nuclear labeling intensity did not differ significantly from the fixed and post-fixed tissues. It was concluded the pPd is a most useful block prestain to use for light microscope autoradiography.