Bronchial carcinoid cells contain neural-type intermediate filaments.

Monospecific antibodies and indirect immunofluorescent microscopic examination, combined with immunochemical analysis, were used to examine intermediate filaments in four cases of bronchial carcinoid tumors. The results show that carcinoid cells express intermediate filaments of neural type (neurofilaments) but are negative for intermediate filaments of mesenchymal type (vimentin), epithelial type (keratin), muscle type (desmin), and glial type (glial fibrillary acidic protein). Since the expression of intermediate filaments shows a high degree of tissue specificity, the results suggest either derivation of bronchial carcinoid cells from maternal cells displaying neural characteristics or from cells with the capacity to acquire neural properties on neoplastic growth. It is also suggested that antineurofilament antibodies can be used as a useful aid in differential diagnosis of bronchial carcinoids from other pulmonary tumors.