Ultrastructural immunolabeling in diagnostic surgical pathology: illustrative cases

Ultrastructural immunolabeling techniques can be reproducibly applied to resolve diagnostic dilemmas in surgical pathology. The technique utilized for processing and labeling the cases presented in this article is simple and reproducible. The specimens were fixed in Carson-Millonig phosphate-buffered 4% formaldehyde fixative and embedded in LR White resin following a well-described protocol. Immunogold labeling allows precise localization of antigenic sites without compromise of the underlying ultrastructural morphologic findings; thus detailed immunomorphologic assessment is achieved. Immunoelectron microscopy techniques permit clarification of doubtful immunocytochemistry results by labeling antigenic epitopes in cases in which light microscopy immunocytochemistry techniques fail to show unequivocal positive results (probably as a consequence of small amounts of detectable antigen present in the tissues). Not only does ultrastructural labeling permit identification of focally positive reactions essential for accurate diagnosis in a given case, but it also localizes the antigenic determinants to specific cellular sites, providing immunomorphologic correlation. The specificity of the interpreted results can then be judged accordingly. Overall, ultrastructural immunolabeling is more sensitive than light microscopic immunocytochemistry. Ultrastructural immunolabeling can play a crucial role in the characterization of some tumors that cannot be accurately classified with other diagnostic techniques, even when combined.