Staining sectioned biological specimens for transmission electron microscopy: conventional and en bloc stains

Post-staining of ultrathin sections and/or en bloc staining of specimens is necessary for differential contrast and improved resolution of cellular structures. Often specimens are fixed and stained with osmium tetroxide during fixation, but additional contrast is the result of additional heavy metal stains on the sections. The most common post-staining of sections is done on grids by aqueous uranyl acetate followed by lead citrate. When it is apparent that simple, aqueous uranium and lead post-staining is not adequate, other stains are invoked. These procedures can be as simple as en bloc staining with uranyl acetate after primary fixation and osmication. Over the years, several other treatments have been developed for use with the primary fixation or during dehydration. Tannic acid, paraphenylenediamine (PPD), and malachite green can all serve as en bloc stains and can contribute to overall improved visualization of ultrastructural details in biological specimens. Tannic acid and PPD improve membrane preservation, and malachite green is a phospholipid stain. All of these stains are compatible with aqueous fixatives and should be considered when the usual stains are not satisfactory. Marinozzi rings and microwave-assisted post-staining offer alternatives to traditional grid staining. In addition, stain precipitates on grids often can be removed by treatment with 10 % (v/v) acetic acid.