Diagnostic immunoelectron microscopy in surgical pathology: assessment of various tissue fixation and processing protocols.

We have investigated various tissue fixation and embedding protocols in an effort to allow expanded use of immunoelectron microscopy in diagnostic surgical pathology. A sample of normal human small bowel mucosa was processed using seven different methods for subsequent postembedding localization of chromogranin A. In addition, several archival cases of neuroendocrine tumors previously fixed and routinely embedded for electron microscopy, stored in formalin, or snap-frozen were retrieved and variously processed for chromogranin A localization at the ultrastructural level. Precise localization of chromogranin A in dense core granules was achieved with protein A-gold on sections from all of the processing methods. The methods included retrieval into mild fixative of previously formalin-fixed or snap-frozen tissues followed by embedding in Lowicryl K4M (Polysciences Ltd., Eppelheim, Germany). Thus, tissue processed without foresight of the need for immunoelectron microscopic localization can be successfully used. Since embedding of tissues in Lowicryl K4M has been shown to preserve a variety of antigens, it may prove to be a superior resin for use in diagnostic immunoelectron microscopy.