An unusual form of differentiation was observed in 18 retinoblastomas. In these neoplasms, certain tumor cells differed morphologically from the undifferentiated retinoblastoma cells and also from those forming the Flexner-Wintersteiner rosettes. These cells exhibited more advanced differentiation and produced unequivocal photoreceptor elements. Cytologically, these highly differentiated tumor cells are considered less anaplastic. Such a cytologic impression is supported by the followup clinical data.
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