Results of growth history studies on IMR-90 and WI-38 showed that the two cell strains were equivalent in population doublings achieved per life span. However, IMR-90 exhibited higher cell yields in phase II than did WI-38. In addition, entry of IMR-90 cells into phase III occurred more abruptly than in WI-38 cultures. Cell sizing analysis showed that phase II and phase III IMR-90 cell populations contained greater numbers of cells in the small volume categories. At senescence, both cell lines contained similar numbers of cells in all size categories. These data suggest that IMR-90 may not be equivalent in all respects to current stocks of WI-38.

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