Spontaneous maturation and differentiation of B16 melanoma cells in culture.

B16 melanoma tumors and cultures are composed of cells with different melanin contents and replicative activities. The hypothesis was tested in vitro that these various cells constituted a population in the process of differentiation and maturation. Early cultures were predominantly composed of small, amelanotic cells with high replicative activity. Older cultures contained mostly larger and heavily melanotic cells with little or no replicative activity. Replicative activity, as measured by the uptake of tritiated thymidine, was inversely proportional to cell size and melanin content. Colony-forming ability was impaired if the original cells were melanotic. Tumorigenicity was unaffected except in very old (9-day) cultures. Our results support the concept that malignant melanocytes undergo a sequence of developmental changes which eventuates in the production of mature cells characterized by enlargement, elevated melanin content, and reduced replicative and colony-forming capacity.