SUBPOPULATIONS OF STROMAL CELLS FROM LONG-TERM HUMAN BONE-MARROW CULTURES - ONTOGENY OF PROGENITOR CELLS AND EXPRESSION OF GROWTH-HORMONE RECEPTORS

Long-term culture of bone marrow derived stromal colony forming cells (S-CFC) in matrix and nutrient defined agar medium resulted in stromal cell colonies that pass sequentially through three distinct morpholog ical stages: firstly, aggregated loose syncytium of round to ovoid cel ls (stage I), a second developmental stage of large branching colonies in which the cells become enlarged, elongated with cytoplasmic projec tions forming a loosely anastomized network with adjacent cells (stage II), and finally cells become dissociated, loosing their long, thin c ytoplasmic filaments and breaking their contacts with one another, but remain large and retain a bi-polar nature (stage III). Cells were als o grown in Liquid medium in a culture microenvironment closely resembl ing conditions of haemopoiesis in vitro. Using a panel of well defined monoclonal antibodies reactive against the rat, rabbit and human grow th hormone receptors, this study found immunochemical evidence of the presence and localization of binding sites of growth hormone (GH) in t he cell membrane and extra-nuclear Golgi area of long-term bone marrow derived human stromal cells in liquid and semi-solid nutrient agar me diums. GH-receptor immunoreactivity was present in small proliferating progenitor cells, myofibroblast-like cells, large reticular fibroblas t cells, adipocytes and endothelial cells. Only MAb known to be reacti ve against human tissue resulted in strong immunoreactivity. The expre ssion of GH-receptors not only on small proliferating, but also on the well differentiated cells, indicates a role for growth hormone on non -progenitor cells. GH-receptor immunoreactivity on differentiating and /or differentiated cells suggests that GH is also necessary for, or ha s a trophic function in differentiation. We propose that direct GH act ion is necessary not only for differentiation of progenitor cells as i mplied by the dual effector hypothesis, but also their subsequent clon al expansion, differentiation and maintenance.