BFU-E generates colony-forming unit erythroid (CFU-E). The earliest recognizable erythroid-specific progenitor is the burst-forming unit erythroid (BFU-E) that in semisolid media gives rise to large colonies of red blood cells (RBCs), identifiable between 7 and 10 days after plating of murine-derived cells. Identification relies on a selective enrichment via cell surface markers combined with culture and in vivo cellular assays. The first stage is the evolution of lineage-committed progenitors that are microscopically invisible ( Emerson et al., 1985 Rosse, 1976). The sites of definitive erythropoiesis are the fetal liver and postnatal bone marrow, and occur in three distinct stages. The very first site of hematopoiesis occurs in the yolk sac and provides primitive erythrocytes that are essential for the survival of the embryo until the next, definitive wave of hematopoiesis is established ( Tavassoli, 1991). As this process proceeds, there is progressive restriction in potential, first generating lineage-restricted progenitors, then morphologically identifiable precursors, and finally the mature blood cells. Hematopoiesis is the process by which a self-renewing population of stem cells provides a continuous replenishment of differentiated blood cells by generating progeny with sequentially altered gene expression patterns ( Kondo et al., 2003 Orkin, 2000). ![]() The focus of this chapter is to outline the mechanisms by which erythroblastic islands aid erythropoiesis, review the historical data surrounding their discovery, and highlight important unanswered questions. An additional control mechanism for cellular growth within the erythroblastic islands is through the modulation of apoptosis via feedback loops between mature and immature erythroblasts and between macrophages and immature erythroblasts. Such interactions enable regulatory feedback within islands via cross talk between cells and also trigger intracellular signaling pathways that regulate gene expression. Both macrophages and erythroblasts display adhesive interactions that maintain island integrity, and elucidating these details is an area of intense interest and investigation. Their pronounced adhesive properties, ability for avid endocytosis, lack of respiratory bursts, and consequent release of toxic oxidative species, make them perfectly adapted to function as nurse cells. The central macrophages are identified by their unique immunophenotypic signature. There is also accumulating evidence for the role of macrophages in promoting enucleation itself. It has been proposed that early in erythroid maturation the macrophages provide nutrients, proliferative and survival signals to the erythroblasts, and phagocytose extruded erythroblast nuclei at the conclusion of erythroid maturation. ![]() The interaction of cells within the erythroblastic island is essential for both early and late stages of erythroid maturation. These islands consist of a central macrophage that extends cytoplasmic protrusions to a ring of surrounding erythroblasts. Erythroblastic islands are specialized microenvironmental compartments within which definitive mammalian erythroblasts proliferate and differentiate.
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