Colony-forming ability of wild-type, c-Mpl^-/-, and STAT5ab^-/- BM cells stimulated with hematopoietic cytokines in vitro. BM cells were harvested from both hind limbs of wild-type and mutant mouse strains. The total BM cellularity was determined to allow for comparison of the absolute number versus the frequency of colony-forming units in culture (CFU-Cs). The frequency of CFU-Cs assayed in methylcellulose was multiplied by the total BM cellularity to derive the absolute number of BM CFU-Cs per both hind limbs. (A) BM cells from wild-type (n = 3), c-Mpl^-/- (n = 3), and STAT5ab^-/- (n = 3) mice were stimulated with GM-CSF. The number of CFU-Cs per 1 × 10⁵ cells plated is shown. (B) Shown is the absolute number of BM myeloid progenitors counted for the cells in panel A and normalized for the total BM cellularity. (C) BM cells from wild-type (n = 5), c-Mpl^-/- (n = 6), and STAT5ab^-/- (n = 5) mice were stimulated with IL-3, IL-6, SCF, and EPO. The number of CFU-Cs per 1 × 10⁵ cells plated is shown. (D) Shown is the absolute number of BM myeloid progenitors counted for the cells in panel C and normalized for the total BM cellularity. ^*P ≤ .05 in comparison with wild-type BM cells. CFU-C counts from individual mice were derived from the average of duplicate plates, and the error bars represent the standard deviation.