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An 8-year-old white boy had a mass of the ciliary body in his left eye. Both the translucent characteristics and the magnetic resonance imaging findings suggested that this lesion was cystic. Fluid aspiration biopsy and iridocyclectomy were performed. By light microscopy the neoplasm showed the characteristic appearance of a neuroid tumor; however, on electron microscopic examination, the tumor exhibited characteristic features of a smooth muscle neoplasm and was diagnosed as a mesectodermal leiomyoma. Observation of melanin granules in scattered tumor cells further confirmed that the tumor had the same origin as uveal melanocytes, which also derive from the neural crest.

Two benign tumors of the ciliary body diagnosed as neurogenic tumors by light microscopy were shown by electron microscopy to be composed of smooth muscle cells with unusual morphologic features. The light-microscopic appearance of a background fibrillary matrix imparting a "neural" appearance was the result of the interweaving of myriad cell processes filled with thin cytoplasmic filaments possessing fusiform densities. The perikaryon of the tumor cells was relatively free of filaments and displayed mitochondria and stacks of rough-surfaced endoplasmic reticulum suggestive of Nissl substance. The basis for these remarkable "hybrid" tumor cells--combining myogenic and neurogenic characteristics--is believed to reside in the neural crest origin of the smooth muscle of the ciliary body (mesectoderm). The histogenetic implications of mesectodermal tissue and the tumors that it spawns are discussed.