Progression of spontaneous malignant transformation of epithelial rat liver cell lines.

Cultured epithelial cell lines from normal rat livers were shown to undergo gradual transformation and malignancy which increased with time. Morphological changes appeared both before and after cells had attained a malignant state, as detected by agar tests. The progression of the degree of malignancy was determined by the morphological appearance of the cells, the increase in the number and size of cell colonies in soft agar, the expression of gamma glutamyl transferase (gamma GT) and the shortening of the latency period necessary for tumor formation after transplantation to syngeneic rats of cells from sequential passages.