Effect of spleen cell conditioned medium on megakaryocytopoiesis in a short-term bone marrow culture system.

The effect of pokeweed mitogen-stimulated spleen cell conditioned medium (PWCM) on the proliferation of megakaryocytes and megakaryocyte progenitor cells (CFU-M) was studied with a short-term liquid culture (STLC) system. Adherent and nonadherent cells were sampled daily for acetylcholinesterase-positive cells and CFU-M. The proliferative capacity of CFU-M was determined by culturing cells from STLC in secondary methylcellulose cultures and counting the number of megakaryocytes per colony. Positive dose-related effects were observed between the number of megakaryocytes and CFU-M in liquid culture and the concentration of PWCM in the culture. In contrast, the proliferative capacity of CFU-M was lower in cultures containing high concentrations of PWCM compared with cultures containing low concentrations of PWCM. Furthermore, mean megakaryocyte diameter was significantly smaller in cultures containing high levels of PWCM compared with cultures with low concentrations. These data suggest that at low levels of conditioned medium, megakaryocytopoiesis is characterized by production of fewer CFU-M with a higher proliferative capacity and fewer large megakaryocytes. In turn, high concentrations of PWCM promote the production of a greater number of CFU-M with reduced proliferative capacity and an increased number of small megakaryocytes.