Intermediate filaments as histologic markers: an overview.

Determination of the type of intermediate filaments (IFs) present in a cell or tissue can yield information about its origin. Thus cells can be grouped into six different classes, i.e. epithelial cells characterized by cytokeratins, most but not all neurones characterized by neurofilaments (NFs), glial cells characterized by the presence of glial fibrillary acidic filaments, muscle cells characterized by the presence of desmin filaments, mesenchymal cells and certain other nonepithelial cell types characterized by the presence of vimentin, and other cells that appear not to contain IFs. The assignments made by immunologic techniques are supported by protein chemistry of the isolated proteins. Information derived from protein sequences as well as from DNA sequences establish that the major intermediate-filament proteins are different but related molecules and show also that the alpha-keratins of wool belong to this multigene family. Applications of IF typing to human pathologic material, and especially to the different major subgroups of human tumors, are reviewed. Thus, for instance, carcinomas continue to express cytokeratins, many tumors of neuronal origin express NFs, gliomas express glial fibrillary acidic protein (GFA), rhabdomyosarcomas express desmin, and nonmuscle sarcomas express vimentin. Further subclassification of epithelial cells and carcinomas is discussed. The subdivisions obtained by IF typing are striking because they follow well-known histologic principles; thus IF typing seems useful in certain instances where diagnosis is difficult by conventional techniques.