Haploid unit-ploidy transition of tetraploid and octaploid H1 (ES) cells in long-term culturing

Haploid unit-ploidy transition in tetraploid and octaploid mouse H1 (ES) cells (4H1 and 8H1 cells, respectively) during long-term culturing was observed using flow cytometry. The DNA content of 4H1 cells was elevated from 3.5C to 4.5C, and that of 8H1 cells was degraded from 6.5C to 5.5C, in addition to gradual DNA loss (C: complement). The timing of the transition was not predetermined. Cell cycle parameters, doubling time and phase durations, were essentially the same before and after the transition, suggesting that most cells in a cell population were induced to undergo the ploidy transition at the same time. Cellular morphology was altered before and after the transition, suggesting that the ploidy shift changed cellular characteristics; however, pluripotency was maintained irrespective of DNA content. Cell volume correlated with DNA content during the final stage of culturing. Diploid and hexaploid H1 (ES) cells--2H1 and 6H1 cells, respectively--were used as control cells in which the ploidy was maintained for about 300 days of culturing. The haploid unit-ploidy transition was explained using a hypothesis concerning the DNA structure of polyploid cells: closing homologous chromosomes causes inhomogeneous cell division accompanying a haploid DNA set, suggesting the existence of a coupling apparatus connecting DNA fibers with a single haploid DNA set.