Hierarchical organization of hematopoietic microenvironments: role of proteoglycans.

The orderly process of hematopoietic cell differentiation depends on the organization of specific microenvironments which function by assembling progenitor cells and appropriate concentrations of necessary growth factors. We propose that this organization involves a spectrum of microenvironmental complexity concomitant with changing cellular requirements, that it is achieved by the extracellular matrix produced by marrow stromal cells, and that its specificity is conferred by the heterogeneity intrinsic in the conformation of proteoglycans and their constituent glycosaminoglycans.