Ultrastructural studies of a thymic carcinoid tumor

A case of a thymic carcinoid tumor is presented. The tumor was a mass measuring 7 by 6 by 5 cm in size. Microscopically, the tumor was composed of a mixture of round, cytoplasm-rich cells in addition to the predominantly spindle-shaped cells. The ultrastructural observation showed many secretory granules, about 120 mmu in size, in almost every cell of the tumor. In the cytoplasm many microfilaments were seen, in places forming large nodules and bundles. Centrioles were often observed in the tumor cells. Sometimes three or even more centrioles were seen in a single cell.