Mitosis in the nontumorous parts of the retina with retinoblastoma.

Uninvolved retina in the posterior pole in an enucleated eye was examined by electron microscopy for retinoblastoma. A mitotic figure was occasionally found in the outer and inner nuclear layers. The dividing cells were clearly separated from the tumor mass by 450-600 micrometer. They were identified as immature Müller cells based upon the fine structural features. The existence of the dividing cells in the posterior retina after birth was abnormal and indicated immaturity of the retinal development.