A Simple Method for Obtaining Increased Contrast in Araldite Sections by Using Postfixation Staining of Tissues with Potassium Permanganate

Epoxy resin appears to be the best embedding material for preserving tissues for electron micros-copy. Its low linear coefficient of contraction I ensures that no appreciable distortion can occur owing to polymerization damage or excessive shrinkage (1). This plastic also is exceptionally stable in the electron beam, so that surface damage to the section is slight. Although Araldite blocks are often more difficult to cut than those of methacrylate, the introduction of the diamond knife (2) with its superior cutting abilities over The linear contraction is less than 1 per cent, an amount even lower than that of the polyester resins like Vestopal W, which contract about 2.5 per cent (author's measurements, unpublished data). the glass knife has considerably reduced this difficulty. The greatest disadvantage of Araldite is the low contrast of sections of osmium tetroxide-fixed material in the electron microscope. In the author's experience, satisfactory lead staining (3, 4) of Araldite sections is difficult to obtain. More contrast is achieved by staining Araldite sections with potassium permanganate (5, 6), but a fine precipitate on the stained sections is troublesome. Potassium permanganate-fixed tissues (7) show satisfactory contrast in Araldite, but permanganate fixation appears less perfect than osmium tetroxide fixation in several respects (8). Mitochondria are nearly always swollen, and some structures, e.g.