The Heterogeneity of Macrophage-Granulocyte Precursor Cell Populations

The complex process of in vivo regulation of granulopoiesis still remains an enigma (24). Numerous studies utilizing the method of cloning of progenitor cells in semi-solid in vitro cultures, however, have revealed important developmental steps that permit some elucidation of the feedback mechanism(s) controlling granulopoiesis (5,14,18). Essentially, the committed granulocytic precursor cell is differentiated from the multipotential stem cells by intrinsic as well as extrinsic mechanisms (3). The early granulocyte precursor cell, known as the colony-forming unit culture (CFU-c) or granulocyte-macrophage colony-forming cell (GM-CFC), is also assumed to be a precursor for monocytes and macrophages (M-CFC) (14). The extreme heterogeneity in size and shape of the agar colonies suggests the presence of several subpopulations. Attempts have been made to describe some of them (11,12,14,15). It has not yet been determined whether all these different granulocyte-macrophage (GM) precursor cells are stimulated by the same colony-stimulating activity (CSA).