Role of the stromal cells in the regulation of granulopoiesis in long-term bone marrow culture: effects of conditioning medium on granulopoiesis "in vitro".

Mouse abdominal wall-conditioned medium contains a factor which stimulated granulocyte-monocyte colony formation. To elucidate the role of colony-stimulating factor and the stromal microenvironment in long-term bone marrow cultures, effects of the conditioned medium on granulopoiesis were studied. Addition of the conditioned medium to the culture stimulated the maturation of stem cells (CFUs and GM-CFC) in suspension into granulocytes, leading to a rapid decline in the number of the stem cells. In contrast, stem cells trapped within the stromal microenvironment were protected from the direct effect of added conditioned medium. These results suggest that granulopoiesis is controlled by intercellular interactions with stromal cells and that exogenous factors act only on the stem cells outside the stromal microenvironment.