Human hematopoietic tissue represents a complex developmental system that is closely regulated to ensure the maintenance of appropriate levels of circulating blood cells. At least nine distinct lineages of cells at various stages of maturation have been described (1 ). Substantial progress in the study of hematopoietic stem cell ontogeny has resulted from the development of semisolid culture methods for the detection of committed progenitor cells. The assays define specific individual precursor cells indirectly by the ability of these cells to form colonies consisting of clonally expanded progeny at varying stages of differentiation. In turn, these methods have led to the identification, characterization, and synthesis by recombinant DNA techniques of some of the growth factors capable of stimulating colony formation and infer to an in vivo regulatory role for such factors in hematopoiesis.
