Histopathological and tissue culture studies of a melanizing cell line derived from a retinoblastoma.

Histological examination of the enucleated eye of a 7-mth-old child revealed a retinoblastoma with areas of rosette formation as well as focal areas of melanin pigmentation. Biopsy derived cells readily established a continuous cell line in liquid culture. The cells which have now been cultured continuously for over 3 yr, were shown to be malignant by being non-contact inhibitable, by readily forming colonies in semi-solid nutrient agar medium and by producing tumours in nude mice. When grown to the point of overcrowding in liquid culture the cells became heavily melanized. Electron microscopy of the melanized cultured cells showed the melanin to be contained in melanosomes. These findings suggest that retinoblastomas may be derived from bipotential primitive retinal cells which retain the capacity for both nuclear cell and pigment cell differentiation. Alternatively, separate malignant transformations may have occurred in each of 2 different progenitor cell types committed to a separate differentiation pathway. The clinical behaviour of this tumour has not differed from that expected of non-pigmented retinoblastomas.