Mesectodermal leiomyoma of the ciliary body. An ultrastructural study.

Electron microscopic observation of a mesectodermal leiomyoma of the ciliary body, in which clear perinuclear halos were present, revealed the ultrastructural characteristics of smooth muscle cells, which include myofilaments with fusiform densities, dense attachments, external lamina, and pinocytotic vesicles. In addition, fibrillary cytoplasmic processes containing large numbers of myofilaments with fusiform densities were notably arranged in a neurogenic pattern when observed under a light microscope. These are characteristic of and specific to mesectodermal leiomyomas. Masses or cords of large, polygonal cells with clear halos surrounding the large, round-to-oval nuclei were found to contain only a few myofilaments. Such features are atypical of mesectodermal leiomyomas. Melanocytes were scattered throughout the tumor, providing additional evidence for the neural crest theory of origination of mesectodermal leiomyomas, since melanocytes are well-recognized descendants of the neural crest. A small number of lipid droplets, abundant lysosomes, and, occasionally, phagosomes were also observed in the cells of this mesectodermal leiomyoma.