Mammalian Chromosomes In Vitro. X. Heteroploid Transformation in Neoplastic cells2

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Abstract
Three tissue-culture strains were successfully established from Novikoff hepatoma cells originally carried intraperitoneally in Sprague-Dawley rats. The populations in vivo consisted roughly of 60 percent s (s = 2n-3 = 39) and 40 percent 2s (2s = 4n-6 = 78) elements. The populations in vitro at first showed a decrease in the number of s cells and an increase in 2s cells. Later, the proportion of the 2s group also decreased, and many cells with chromosome numbers in the 60's or low 70's appeared (the stage of spread of chromosome spectrum). After the stage of “spread,” the chromosome numbers narrowed and the populations may be represented by a modal chromosome number or by a small group of connecting numbers. Strain N-1 was characterized by possessing 67 or 68 chromosomes; strain N-3, 59 to 61 chromosomes; and strain N-4, 74 or 75 chromosomes. If we consider the s cells “diploid” and the 2s cells “tetraploid,” the route of “heteroploid transformation” in vitro taken by Novikoff hepatoma cells may serve as a model for the transformation of normal tissues. Prolonged residence in vitro seemed to cause the cells to lose their transplantability and perhaps also malignancy. The cell strains finally failed to produce tumors when reintroduced into hosts. These hosts developed a certain degree of immunity against subsequent challenges of tumor cells in vivo. Cultivating tumor cells in vitro thus appeared to attenuate the virulence of the cells without causing them to lose their antigenicity.