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Replicative lifespan of cloned progenitor cells. After the initial 6 passages, estimated to require 22 CPD, 3 clonal progenitor lines were passaged at 7 day intervals by trypsinizing, counting, and replating at a fixed density of 1 × 104 cells/cm2. Points represent population doublings calculated from average cell counts of 2 cultures from each cell line.
