Role of the microenvironment on hematopoiesis. II. Regulation of cell kinetics in vitro during granulopoiesis and megakaryocytopoiesis.

A long-term liquid culture system derived from adult Syrian hamster spleen that supports hematopoiesis from a single inoculum without supplemental coritcosteroids was used. Clonal assays from progenitor cells in the supernatant and adherent phases of liquid cultures revealed an increased concentration of myeloid and megakaryocytic progenitor cells in the adherent layer. Cell kinetic studies with these stable unstimulated cultures indicate that the initially labelling cohort of cells were present in the adherent cell layer. However, incorporation of tritiated thymidine into DNA occurred in less than 5% of the total cells present in the adherent cell layer. The data indicate that regulation of hematopoiesis by the stromal microenvironment was achieved by maintaining hematopoietic cells in a quiescent G0 or prolonged G1 phase and was associated with the initially labeling cells. The assays demonstrate the phenomenon that megakaryocyte precursors were present almost exclusively on the adherent stromal cell layer, which also contained a storage pool of mature megakaryocytes. The mature cells did not undergo further DNA synthesis and did not show morphologic changes of senescence. Cell kinetic studies revealed that megakaryocytes were released into the supernatant phase from the adherent stromal layer after endomitosis had occurred and cytoplasmic maturation ensued, indicating a role of stromal cells in megakaryocyte differentiation, maintenance, and maturation. The results of these investigations provide additional data on the regulation of megakaryocytopoiesis by the microenvironment.