Cellular interrelationships during in vitro granulopoiesis

Long-term production of fully differentiated granulocytes can be maintained in vitro in a liquid system of cultured bone marrow. Marrow is cultured in medical flasks and allowed to form an adherent layer over a three-week period, and then recharged with fresh marrow resulting in continued mature granulocyte production for several months. During the initial establishment of the adherent layer, three attached populations become apparent: phagocytic monocytes, an attached epithelial cell type, and aggregations of epithelial cells swollen to enormous proportions by the presence of numerous lipid-containing vacuoles. Without the formation of these aggregations, granulocyte production is not maintained beyond an initial period and the culture converts to phagocytic mononuclear cell production alone. Thus not only is the presence of the fat-containing aggregations necessary to continued granulopoiesis, but cultures in full granulocyte production show a characteristic clumping of granulocytes around these aggregates. Electron microscopy has shown that the epithelial cells from the adherent layer form a layer covering some of the attached cells in these areas and thus may provide the necessary in vitro microenvironment for granulopoiesis to occur. Pinocytotic vesicles and gap junctions have been observed between the adjacent membranes of the undifferentiated granulocytes (possibly stem cells) and the epithelial cells themselves.