The transition from primary culture to spontaneous immortalization in mouse fibroblast populations.

When compared with cells from other species, mouse fibroblasts cultivated in vitro have a high probability of originating immortal cell populations. The probability of immortalization is increased by varying the volume of the nutrient medium and by irradiation with low dose rate gamma-radiation. Immortalization of the fibroblast population is accompanied by the appearance of cells with new properties, favoring the hypothesis that the immortal cells are not present in the primary culture. The mouse genome seems to have different properties when compared with identical experiments performed with human fibroblasts, a cell system with a low probability of immortalization. The differences between the genomes of these two mammalian species could be the explanation for some aspects of their different behavior.