Ponceau 2R staining on semi-thin sections of tissues fixed in glutaraldehyde-osmium tetroxide and embedded in epoxy resins

SUMMARY Proteins were stained (brilliant red) by using 0·5% Ponceau 2R (C.I. 16150) in sulphuric acid (pH 2) at 40°C on sections 1–2 μm thick of tissues fixed in glutaraldehyde-osmium tetroxide and embedded in epoxy resins. The staining is free from precipitates and does not require the removal of the embedding media. Section bleaching with 1–2% periodic acid at 40°C for 30–90 min was also carried out in order to obtain a more brilliant colour of the proteins which may be photographed using black and white films.