Collectively, the erythroid progenitors, terminally differentiating erythroblasts (precursors), and adult red cells are termed the erythron to reinforce the idea that they function as an organ. The widely dispersed cells comprising this organ arise from pluripotential hematopoietic stem cells. Following commitment to the erythroid lineage (unipotential progenitor), further maturation gives rise to the erythroid progenitors, burst-forming unit–erythroid (BFU-E) and, subsequently, colony-forming unit–erythroid (CFU-E), that can be identified by their development into representative clonal colonies of red cells in vitro. The CFU-E then undergoes terminal differentiation, progressing through four to five morphologic stages, each having characteristic light microscopic and ultrastructural features. During terminal erythroid differentiation, there is an increasing amount of hemoglobin synthesis accompanied by nuclear chromatin condensation, and at the final stage of differentiation, there is nuclear extrusion to generate an anucleate polychromatophilic macrocyte (reticulocyte with supravital staining). The human polychromatophilic macrocyte (reticulocyte) matures over 2 to 3 days, first in the marrow and then in circulation into the discoid erythrocyte. During reticulocyte maturation, cytoplasmic inclusions, including residual mitochondria and ribosomes, are degraded, and the reticulocyte loses surface area to achieve the mean cell volume and surface area of a discoidal erythrocyte. Mature erythrocytes are approximately 7 to 8 μm in diameter and undergo extensive deformation to pass through 3-μm-diameter capillaries and the 1-μm-wide and 0.5-μm-thick endothelial slits in the red pulp of the spleen. The ability of the red cell to undergo extensive reversible deformation is essential for both its function and its survival. Red cell deformability is a function of its geometry, the viscosity of the cytoplasm, largely determined by the concentration of hemoglobin. Decreased deformability is a feature of red cells in various pathologic states. The erythrocyte is unique among eukaryotic cells in that its principal physical structure is its cell membrane, which encloses a concentrated hemoglobin solution. Thus, all structural properties of this cell are in some way linked to the cell membrane. In contrast to other cells, the erythrocyte has no cytoplasmic structures or organelles. Among human cells, only red cells and platelets do not have a nucleus.
Acronyms and Abbreviations:
BFU-E, burst-forming unit–erythroid; CFU-E, colony-forming unit–erythroid; cP, centipoise; DIC, disseminated intravascular coagulation; EMP, erythroblast macrophage protein; ICAM-4, intercellular adhesion molecule-4; IL, interleukin; MCH, mean cell hemoglobin content; MCHC, mean corpuscular hemoglobin concentration; MCV, mean cell volume; MDS, myelodysplastic syndrome; SA:V, surface area-to-volume ratio; TTP, thrombotic thrombocytopenic purpura.
The mass of circulating erythrocytes constitutes an organ responsible for the transport of oxygen to tissues and the removal of carbon dioxide from tissues for exhalation*. Collectively, the progenitors, precursors, and adult red cells make up an organ termed the erythron, which arises from pluripotential hematopoietic stem cells. Following commitment to the erythroid lineage, unipotential progenitors mature into the erythroid progenitors, the burst-forming unit–erythroid (BFU-E) and, subsequently, the colony-forming unit–erythroid (CFU-E), which then undergoes further maturation to generate anucleate polychromatophilic macrocytes (reticulocytes ...