Immunostaining of DNA in electron microscopy: an amplification and staining procedure for thin sections as alternative to gold labeling.

We describe a new electron microscopic on-section staining technique with high specificity and sensitivity for DNA-containing structures. Lowicryl HM20 sections of specimens obtained by cryofixation and freeze-substitution are incubated in a first step with a primary IgM antibody specific for double-stranded DNA. The layer of bound antibodies at the section surface is amplified in a successive step by a secondary IgG antibody. Finally, electron scattering of the antibody layer produced is enhanced by staining with a mixture of uranyl acetate and potassium permanganate. The applicability of the method is exemplified by the detection of shape and distribution of various types of bacterial and eukaryotic chromatin.