Changes in subpopulations of Li-7 cells during culture and effects on cell morphology and tumorigenicity. a) Analysis of CD13 and CD166 expression in Li-7 cells during culture by flow cytometry. The proportion of cells expressing CD13 decreased and that of cells expressing CD166 increased with the number of passages. b) Morphological changes in the bulk Li-7 cells after 2 months in culture. Upper panel, Li-7 cells after one week of culture; lower panel, Li-7 cells after 2 months of culture. c) Injection of Li-7 cells (1 × 106) after one week of culture into nude mice caused tumor formation in all mice after 2 months, whereas the cells injected after 2 months of culture were non-tumorigenic even at 4 months (right).