“Hybrid” Type Cells in Combined Cultures of Two Different Mammalian Cell Strains

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Abstract
Cells of a high-cancer N1 line derived from NCTC 2472 clone of mouse fibroblasts and cells of N2 or N2T′ lines derived from low-cancer NCTC 2555 clone of the same drigin were cultivated together in vitro. The evolution of mixed-cell populations was compared with the behavior of component cells in separate cultures. Separately cultivated, the cell lines kept their essential characteristics fairly stable. N1 cells maintained a karyotype with a modal number of nearly 55 telocentric chromosomes—one extra long. N2 lines had modal chromosome numbers of 62 and 58 chromosomes, 13 biarmed. Higher polyploids in pure cultures were recognized as doubles or quadruples of stemline cells. In mixed cultures the proportion of N1 cells progressively decreased until they almost completely disappeared. At the 3d month, a new type of cell designated “M” appeared, which later reached the proportion of one third to one fourth of the whole cell population. The M cells had many “hybrid” chromosomal and biological characteristics. They showed a modal number of 115 to 116 chromosomes with 9 to 15 metacentrics, frequently with one extra long telocentric. M cells from mixed populations and secondarily isolated M-type cell clones produced, in isologous mice, malignant tumors in which M cells could be identified. The possible origin of the “hybrid” type cells is discussed.