Effect of the inoculum size of cells on the maintenance of diploidy in cultured liver cells of the rat.

In culture, a cloned rat liver cell line, J-5-2, exhibited a high diploidy for 200 days after the last cloning (diploid line) but, thereafter, pseudodiploid cells gradually increased in number (pseudodiploid line). These diploid and pseudodiploid lines were inoculated at various sizes, and chromosome analysis was performed for 75 days of serial passages. Diploid line showed an increase of pseudodiploid cells at a large inoculum size. On the other hand, more than 80% of the cells examined maintained the diploid karyotype at a small inoculum size. Pseudodiploid line showed an increase of pseudodiploid cells at a large inoculum size and of diploid cells at a small inoculum size. These pseudodiploid cells contained three types of marker chromosomes, in which number 1 chromosome was found to be involved by the G-banding methods. The saturation density of the pseudodiploid line was much higher than that of the diploid line, and plating efficiency of the diploid line was also significantly higher than that of the pseudodiploid line, although the populating doubling time of these two lines was almost the same. Possible mechanisms for the effect of different inoculum sizes on the incidence of diploid cells and pseudodiploid cells are discussed.