An unusual bronchial carcinoid tumor: light and electron microscopy.

An unusual bronchial carcinoid tumor was studied by light and electron microscopy. The tumor cells, which appeared to be monotonously uniform in hematoxylin and eosin stained sections, were found to be morphologically heterogeneous at the ultrastructural level with regard to the size, number, and morphology of the endocrine granules. Presumptive endocrine granules were seen in all tumor cells, but some cells contained only small round granules (2000 A largest diameter), other cells contained large round granules (some with as large a diameter as 1.0 mu), and some cells contained large polymorphic granules. Many of the cells stained positively at the light microscopic level when selective stains for endocrine cells were applied. All types of granules showed argyrophilia at the ultrastructural level. Numerous clusters of endocrine cells were observed in the otherwise normal bronchial and bronchial glandular epithelium. The spectrum of granule morphologies, as seen in the tumor cells, was displayed in cells of the intraepithelial clusters. Some mucous cells and sparsely ciliated cells within these clusters contained argyrophilic granules. Multiple continuities existed between the epithelial endocrine cell clusters and the underlying tumor mass. The intraepithelial clusters represent foci of carcinoma in situ, the genesis of which is discussed.