Differentiation markers as an aid in the histological diagnosis of small-cell carcinoma of the lung: synopsis of intermediate filament protein and synaptophysin expression.

Small-cell carcinomas of the lung (SCLCs) are frequent tumors of the bronchopulmonary tract which are distinguished by their neuroendocrine (NE) features, indicative of a derivation from the sparse neuroendocrine cells present in the normal epithelium. Because of the lack of marked morphological details, the differential diagnosis of this tumor is very difficult. We show that the epithelial nature and origin of SCLCs can be demonstrated, biochemically and immunocytochemically, by the expression of cytokeratins, notably cytokeratins 8 and 18, often together with desmosomal proteins as another independent differentiation marker. The NE character of SCLCs, on the other hand, is revealed by antibodies to certain NE-specific components, notably the broad range NE marker, the membrane protein synaptophysin. Both kinds of differentiation markers are also expressed in a number of SCLC-derived cultured cell lines which therefore may serve as valuable biological model systems to study the biology of SCLC.