Periodic acid‐Schiff‐phosphotungstic acid as an electron histochemical method for ultrathin sections

Summary Structures which are regarded as ‘PAS positive’ may be strongly and selectively stained for electron microscopy in ultrathin sections by using the following procedure. Tissues were fixed by buffered osmium tetroxide or by glutaraldehyde followed by postfixation in osmium tetroxide. After dehydration the fixed tissues were embedded in butyl methacrylate-methyl methacrylate (10:1 v/v). Other embedding media do not give successful results. Ultrathin sections were cut and mounted on formvar-coated gold grids. The sections were treated with 1% w/v periodic acid in aqueous alcohol (water-ethanol 1:1 v/v) for 10 min, washed in water for 2–3 min, then treated with Schiff reagent for 20 min. The Schiff reagent was made according to the method of de Tomasi and allowed to age for 3–5 h. The sections were then washed in two changes of water for 3–5 min each, dried in air, stained in 1% w/v aqueous phosphotungstic acid for 1.5 h then rinsed in water for 2–3 min.