Cell-surface markers on haemopoietic precursors. Reagents for the isolation and analysis of progenitor cell subpopulations

The most intriguing aspects of developmental biology are the mechanisms controlling the generation of diverse cells types from common precursor cells . Haemopoiesis offers a unique opportunity to study such systems . Clinical and experimental observations suggest that all haemopoietic cells, including both myeloid and lymphoid cells, originate from a single type of stem cell .' ,' These multipotential stem cells have been defined by their ability for extensive 'self-replication' and proliferation and can generate progeny committed to one or more haemopoietic lineages.' However, it is still not known if the 'self-renewal' of stem cells under normal physiological conditions actually leads to the production of daughter cells that are exactly identical to the parental cells . Haemopoiesis can be arbitrarily subdivided into three compartments of increasing maturity : the stem cells, the lineage-restricted progenitors and the maturing cells. It is of some importance for developmental biology to determine the actual composition of these compartments, the interrelationships of the various lineages and the growth control mechanisms that govern their survival, 'self-renewal', commitment and maturation. Although the detailed composition of the progenitor cell compartments has not been resolved, regulation of haemopoiesis appears to depend on interactions among haemopoietic cells, stromal elements and a variety of growth factors .` It is evident that cell-surface molecules on haemopoietic cells