Interrelationships between TPO responsiveness and NbD undergone by megakaryocyte progenitors in vivo and in vitro. Relative responsiveness was defined as the absolute responsiveness (1/TPO₅₀) at each data point divided by the responsiveness of the least responsive MK progenitor. NbD have been calculated as detailed in “Materials and methods.” Inverse correlations were obtained between responsiveness and numbers of residual doublings undergone in vitro (NbD_r; left), and positive correlations were obtained between responsiveness and number of previous doublings undergone in vivo (NbD_p; middle). As a result, a reciprocal relationship was found between NbD_r and NbD_p, indicating that a high number of previous doublings were associated with a low number of residual doublings. The converse was also true (right). The P values for the correlation coefficients were less than .005.