Abstract
An 18–month-old Standardbred filly had a large intraocular tumor involving the optic nerve. The tumor was a malignant medulloepithelioma, a rare intraocular neoplasm derived from the primitive medullary epithelium. By light microscopy the tumor had cords and lobules of primitive neuroepithelial cells that formed clefts and true rosettes. Electron microscopy of the rosettes showed a girdle of zonulae adherentes joining the apices of the cells as well as several basal bodies. This is the sixth report of equine intraocular medulloepithelioma, and, to the best of our knowledge, the first intraocular medulloepithelioma arising from the optic nerve head in a horse. Massive involvement of the optic nerve should be suspected when an eye containing an intraocular mass in the posterior segment lacks light perception. In such cases a long segment of optic nerve should be resected.