Fibrous histiocytoma of the corneoscleral limbus: The ultrastructure of a distinctive inclusion.

A case of epibulbar fibrous histiocytoma is presented, with emphasis on the electron microscopic aspects due to the paucity of ultrastructural descriptions of this rare tumor. A tan-yellow conjunctival tumor developed at the corneoscleral limbus in a 31-year-old male. The clinical diagnosis was confirmed by routine histopathologic evaluation, which demonstrated a typical matted or storiform cellular pattern. Electron microscopy revealed a mixed cell population, composed primarily of histiocytic and fibroblastic cells. The histiocytic cells contained lysosomes and phagosomes and had interdigitating cell processes. The fibroblastic cells were characterized by prominent rough-surfaced endoplasmic reticulum and variable amounts of lipid within vacuoles, but did not possess lysosomes or phagosomes. Both cells occasionally contained an unusual curvilinear structure, suggesting a shared histogenesis. This distinctive inclusion appeared to be a modification of the rough-surfaced endoplasmic reticulum. The nature and origin of the tumor cells are discussed on the basis of their ultrastructure.