The distribution of granulocyte macrophage colony- stimulating factor (GM-CSF) in human long-term bone marrow cultures (HLTBMC) was examined using two monoclonal antibodies raised using purified recombi-

The role of growth factors in promoting the survival, proliferation and development of multipotent haemopoietic stem cells and their more developmentally restricted progeny has been unequivocally established using short-term in vitro clonogenic culture systems (Fauser and Messner, 1979; Johnson, 1984; Toogood et al., 1980). In other culture systems, however, haemopoiesis occurs in the absence of added growth factors, provided that the haemopoietic cells are cultured in association with a bone marrow stroma (Dexter et al., 1984). The stroma of these ‘long-term bone marrow cultures’ consists of a complex network of fibroblasts, macrophages, adipocytes, endothelial cells, adventitial reticular cells and extracellular matrix molecules, which are cells that are representative of the bone marrow stroma in vivo. The adherent stromal cells exert their effect by direct cell contact with the haemopoietic cells, and if cell contact is prevented the haemopoietic cells die (Dexter et al., 1978; Dexter and Allen, 1984). This suggests that appreciable levels of diffusible growth factors are not normally produced by the stromal cells in these cultures. What then is the role of growth factors in stromal cell-mediated haemopoiesis? With the exception of macrophage colony stimulating factor (M-CSF), it has been difficult to detect biologically active haemopoietic cell growth factors in the supernatant media from long-term bone marrow cultures (Lanotte et al., 1982; Heard et al., 1982; Shadduck et al., 1983). However, radioimmunoassay techniques and molecular probes for mRNA of haemopoietic colony stimulating factors (CSF) such as macrophage CSF, granulocyte-macrophage CSF and granulocyte CSF has shown that stromal cells are capable of producing at least some of these growth factors. Furthermore, stimulation of long-term bone marrow cultures with phytohaemagglutinin (PHA), lipopolysaccharide (LPS) or irradiation, results in the accumulation of appreciable levels of growth factors into the supernatant 761 Journal of Cell Science 106, 761-769 (1993) Printed in Great Britain © The Company of Biologists Limited 1993