Use of tannic acid and silver enhancer to improve staining for electron microscopy and immunogold labeling

Staining by uranyl acetate and lead citrate (UA-LC) of immunolabeled sections of unfixed glomerular basement membrane (GBM) digests embedded in low-acid glycol methacrylate (LA-GMA) is poor. The following were investigated for their ability to enhance contrast when applied to sections before UA-LC: potassium permanganate, phosphotungstic acid, gold chloride, osmium tetroxide, glutaraldehyde-osmium tetroxide, colloidal gold-silver enhancer, tannic acid, and glutaraldehyde-tannic acid. Silver enhancer (2 min incubation, no sodium thiosulfate step) gave dense GBM staining but not with immunogold labeling. Silver enhancer is recommended as a simple alternative to routine silver stains but not for contrast enhancement with immunogold labeling. Tannic acid (1% for 1 min or 0.005% for 10 sec) and glutaraldehyde (2.5% for 5 min) followed by tannic acid enhanced contrast and, when applied after immunolabeling, did not appear to affect probe levels. Tannic acid also enhanced the staining of fixed tissue in LR Gold acrylic resin and LA-GMA, but not of glutaraldehyde-osmium-fixed tissue in epoxy resin, even after sodium metaperiodate treatment. Treatment of sections with tannic acid is recommended for contrast enhancement in immunogold studies when osmium and tannic acid post-fixation must be avoided and the tissue is embedded in methacrylate or acrylic resin.