The differential responsiveness of MPCs to endogenously produced and exogenously supplemented factors. IL-3 exerts its action (+) on both HCs and non-HCs throughout SF suspension culture, resulting in the expansion of cells in both these compartments. IL-3 has also been shown to promote the growth of CD45^–CD123⁺ cells, which are capable of giving rise to CFU-Fs and CFU-Os. However, on average, only about 24% and about 22% of the total attainable CFU-Fs and CFU-Os, respectively, are recovered from a CD45^–CD123⁺ cell population by day 21 of the study period, suggesting that the mesenchymal stem cell compartment is phenotypically heterogeneous. Furthermore, the growth of HCs is also directly influenced by IL-3 (and SCF). It has also been demonstrated from these studies that HCs potentiate the growth of MPCs via the release of endogenous factors (factor(s) X) that target MPCs, resulting in their growth.