Carcinoid tumor of the vermiform appendix with Cushing's syndrome.Ultrastructural study of a case

A young woman with Cushing's syndrome recovered after removal of an appendiceal carcinoid tumor which contained ACTH in excess. The tumor was not associated with any features suggestive of serotonin production although the neoplastic cells were argyrophilic and demonstrated a positive argentaffin reaction. The cells contained pleomorphic dense granules and had other cytoplasmic characteristics of the enterochromaffin or argentaffin cells that are typically found in midgut derivatives. They displayed 4 ultrastructural features of polypeptide hormone-producing cells which are a high content of free ribosomes, low levels of rough endoplasmic reticulum, electron dense mitochondria, and membrane-bound secretion vesicles. However, they lacked one important ultrastructural feature of such cells, namely high levels of smooth endoplasmic reticulum in the form of vesicles. A sixth ultrastructural characteristic of polypeptide hormone-secreting cells, fixation lability of mitochondria, could not be assessed from the material available for study.