Morphologyand Serum Dependenceof Cloned Cell Lines UndergoingSpontaneousMalignantTransformationin Culture

SUMMARY A number of morphological changes were found to come late with the occurrence of spontaneous neoplastic trans formation in sublines of five rigidly isolated clones of mouse embryo fibmoblasts.These morphological changes included increased cytoplasmic basophilia, reduced spreading of cells on the substrate, increased nuclear:cytoplasmic ratio, greaten heterogeneity in the size and shape of cells and nuclei, and more random orientation of cells. Because these changes were reproducible, occurring in some sub lines of all five clones, they have been described and illus trated to serve as a guide for identifying spontaneous trans formants 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 fibnoblasts derived from the same cell, showed similar saturation dens ities and serum dependence. Some clones had a higher transformation frequency than the parental line, which me mained nonneoplastic for years. Thus, the capacity for con tinuous 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.