Bronchial carcinoid with fibrillary inclusions related to cytokeratins: an immunohistochemical and ultrastructural study with subsequent investigation of 12 foregut APUDomas.

A bronchial P cell carcinoid, which was negative for all hormones immunocyto chemically tested, showed a globular intracytoplasmic inclusion in almost every cell. The inclusions were not clearly distinguishable using the haematoxylin- eosin- safran procedure; they were best demonstrated with the Masson trichrome stain and the Grimelius technique and were easily detected in 1 micron thick Epon sections as target-like structures. On electron microscopy, they were found to be composed of filamentous aggregates entrapping a few endosecretory granules, which showed degenerative changes. The filaments, 8-10 nm in diameter, lacked any periodicity; they were randomly dispersed in the central area and arranged in broken concentric swirls at the periphery of the inclusions. The globules lacked the tinctorial properties of amyloid, but showed a strong immunostaining for keratin-like proteins. A systematic investigation of 12 APUDomas of bronchial or duodenopancreatic origin, using both light and electron microscopy, identified a few filamentous bodies in one case, a somatostatin cell tumour of ampulla of Vater. In both cases, the structures appeared similar to those previously reported in growth hormone cell pituitary adenomas as well as in a few bronchial or gut carcinoids. Whatever their nature, morphological data suggest that they are related to abnormalities in the secretory function, involving the Golgi apparatus, the endosecretory granules and the microtubular microfilamentous system.