In Situ Hybridization

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Abstract
Twenty years ago Gall and Pardue1 reported a method to detect RNA-DNA hybrid molecules in cytologic specimens. Since then diagnostic in situ hybridization has become a standard method to detect DNA and RNA. The method has also been modified for use in electron microscopy2-4 but it is not easy to preserve both the hybridization properties and an acceptable morphology. Most postembedding techniques are useful for DNA hybridization but not for RNA analyses. Some of our problems may be solved with microwave procedures for EM fixation,5 but at present, ultrathin cryosectioning is the method of choice to preserve both DNA and RNA molecules. This sophisticated and time-consuming technique is not, however, in widespread use in EM laboratories, and preembedding nuclear acid hybridization will probably be preferred for quite some time. Colloidal gold rather than radioactive probes seems to be the marker to favor. Protein A-gold complexes have been used to demonstrate cellular RNA sequences,3 and preembedding in situ hybridization with colloidal gold has been used in order to demonstrate cytomegalovirus DNA in human fibroblasts.4 The in situ hybridization electron microscopy method demonstrates which genes are present, their rearrangements, viral genomes, and so on, and even shows their exact subcellular localization. Detection of viral DNA as well as oncogenes may once more focus the interest of the ultrastructural pathologist on the nucleus. In combination with immunoelectron microscopy, we have a powerful tool to follow the dynamic patterns of oncogenes, the resulting mRNA, and finally, the protein product within the tumor cells. Ultrastructural Pathology welcomes papers on the techniques and applications of in situ hybridization electron microscopy.