Retinoblastoma cell differentiation in culture

Human retinoblastoma cells were grown in tissue culture and their differentiation into Flexner-Wintersteiner rosettes was investigated. This process of photoreceptor cell differentiation was only observed in primary cultures and subsequent cell passages of tumors which showed these structures in vivo. Rosettes formed spontaneously within 5-9 d after plating of the tumor cells. Under optimal conditions in certain tumor cell strains, up to 80% of the cultured cell aggregates contained one or more differentiated Flexner-Wintersteiner rosettes. Exposure of retinoblastoma cells to RA or dcAMP did not alter the number of rosettes in culture. Retinoblastoma cells within rosettes continued to synthesize DNA, and mitotic figures were frequently observed in histological sections. Ultrastructural analysis of rosettes formed in vitro showed many of the characteristics described in those found in vivo including a polarized shape, established cell junctions (Zonula adherens), extensive accumulation of mitochondria and microtubules in the apical part of the cells, numerous basal bodies and centrioles as well as cilia typical of the mature photoreceptor cell. Lamellated stacks of membranes were also found and their nature is discussed.