Primary hepatic carcinoid tumor. An electron microscopic and immunohistochemical study. A case of primary carcinoid tumor of the liver with striking morphologic and electron microscopic features is reported. Conventional histologic examination showed a prominent paranuclear clear zone in numerous tumor cells. By electron microscopic examination, this clear zone corresponded to a paranuclear mass of intermediate filaments admixed with neurosecretory granules and other cytoplasmic organelles. 