Morphology and serum dependence of cloned cell lines undergoing spontaneous malignant transformation in culture

A number of morphological changes were found to correlate with the occurrence of spontaneous neoplastic transformation in sublines of five rigidly isolated clones of mouse embryo fibroblasts. These morphological changes included increased cytoplasmic basophilia, reduced spreading of cells on the substrate, increased nuclear:cytoplasmic ratio, greater heterogeneity in the size and shape of cells and nuclei, and more random orientation of cells. Because these changes were reproducible, occurring in some sublines of all five clones, they have been described and illustrated to serve as a guide for identifying spontaneous transformants among rodent fibroblasts in culture. Neoplastic transformation was determined by the growth of the cells as malignant neoplasms in syngeneic hosts. The spontaneous transformants, as compared with nonneoplastic fibroblasts derived from the same cell, showed similar saturation densities and serum dependence. Some clones had a higher transformation frequency than the parental line, which remained nonneoplastic for years. Thus, the capacity for continuous growth in vitro can be independent of malignant potential. The use of horse serum as supplement to the medium did not accelerate or increase the frequency of neoplastic transformation.