Ultrastructural study on an appendiceal carcinoid tumor showing intra-nerve fiber growth.

Ultrastructural and immunohistochemical studies on an appendiceal carcinoid tumor in a 53-year-old man disclosed an intimate association of nerves and tumor cells. Electron microscopy revealed that the tumor nests were composed of neurosecretory cells containing neurosecretory granules and peripherally-located Schwann cells encasing axons. Some tumor nests were continuous with nerve fibers invested in the common basal lamina. These findings suggest that at least some carcinoid tumors of the appendix show intra-nerve fiber growth, and that the neurosecretory cells might be present in the nerve fibers from the beginning or infiltrated into the nerve fibers at a very early stage of neoplastic transformation.