Importance of cell-cell interactions in regulation of erythropoiesis.

In the bone marrow, erythropoiesis occurs in distinct anatomic units called erythroblastic islands that consist of a central macrophage surrounded by a ring of developing erythroblasts. The formation of erythroblastic islands involves adhesive interactions between the central macrophage and the ring of developing erythroblasts, between adjacent erythroblasts in the ring, and between macrophages/erythroblasts and the components of extracellular matrix. These adhesive interactions are mediated by specific pairs of cell surface receptors and counterreceptors (ligands) including integrins, the immunoglobulin superfamily, and cadherins. Several lines of evidence have suggested that cell-cell interactions among various cell types in the bone marrow play an important role in supporting and regulating hematopoietic differentiation. Cell surface adhesion molecules expressed during erythroid development and the likely counterreceptors for these molecules are discussed in this review.