Any tumor composed of cells with APUD (amine precursor uptake and decarboxylation) cytochemical properties. Ultrastructurally these tumors often display electron-dense granules and they produce multiple regulatory hormones and amines, neuron-specific enolase, synaptophysine and chromogranin A or C. These cells are thought to give rise, in addition to the islet cell tumors of the pancreas, to carcinoid tumors, medullary carcinomas of the thyroid, melanomas and pheochromocytomas. This explains the striking similarities in the histology of these tumors and pancreatic islet cell tumors. See also carcinoid gastrointestinal and islet cell tumor of the pancreas.

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