Retrospective demonstration of endogenous peroxidase activity in plastic-embedded tissues conventionally prepared for electron microscopy.

A procedure is described that permits retrospective demonstration of intracellular endogenous peroxidase activity in tissue conventionally prepared for electron microscopy, i.e., doubly fixed with aldehydes and osmium tetroxide, "stained" in block with uranyl acetate, and embedded in epoxy resins. Using sodium ethoxide, plastic was removed from 1 micrometer sections; subsequently, the sections were incubated for 20 min in diaminobenzidine solution (44 mg/100 ml) made in acetate-citric acid buffer, pH 5.6, with 0.01% hydrogen peroxide. After this treatment, the sections were rinsed, dehydrated, and mounted. Cell types known to have endogenous peroxidase activity (red blood cells, macrophages, and retinal pigment epithelium cells in our preparations) show positive granules in their cytoplasm--control sections were uniformly negative. This method, which could also be used prospectively, cytochemically demonstrates endogenous peroxidase activity upon optical microscopical examination of the treated tissues; correlative electron microscopic studies may be performed on the same tissue block, or even adjacent sections.