Electron microscopic diagnosis of medulloepithelioma.

A 20-month-old boy with a ciliary body tumor presented with two white flocculi floating in the anterior chamber of his left eye. This material was examined by electron microscopy. Both the clinical appearance of the tumor and the ultrastructural findings suggested the diagnosis of medulloepithelioma. The flocculi contained tumor cells forming lumina and displaying neuronal-type cilia, neurotubules, and a complex band of apical desmonosomal junctions. Since the last finding is not present in retinoblastoma rosettes in the absence of fleurette differentiation, it distinguishes medulloepithelioma from retinoblastoma. The electron microscopic diagnosis has permitted a trial period of conservative cryotherapy directed at the tumor and the associated glaucoma. The electron microscopic characteristics of the tumor favor the neuroepitheliomatous and neuroblastic differentiation of medulloepithelioma rather than ependymal differentiation.