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Collectively, the progenitor 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 undifferentiated, pluripotential stem cells. Following commitment, erythroid progenitors progress through several replicative stages, each having a characteristic ultrastructural morphology. As the cells mature, hemoglobin is synthesized with increasing intensity. The nucleus becomes more pyknotic and eventually is extruded. The mature erythrocyte adopts a variety of forms within two sequences—from discocyte to echinocyte and from discocyte to stomatocyte. These series are stages through which a single erythrocyte can pass reversibly as a result of changes in pH, plasma protein levels, and presence of amphipathic drugs. Other shapes, once acquired, are irreversible and frequently represent a particular pathophysiologic process. Names derived from Greek terms have been attached to these erythrocyte shapes because there is significant benefit to standardized terminology.

Acronyms and Abbreviations

Acronyms and abbreviations that appear in this chapter include: BFU-E, burst-forming unit—erythroid; CFU-E, colony-forming unit—erythroid; DIC, disseminated intravascular coagulation; DMT1, divalent metal transporter 1; ICAM-4, intercellular adhesion molecule-4; ISC, irreversibly sickled cell; MCHC, mean corpuscular hemoglobin concentration; MCV, mean corpuscular volume; TTP, thrombotic thrombocytopenic purpura.

The mass of circulating erythrocytes constitutes an organ responsible for the transport of oxygen. Because the oxygen concentration per unit volume of blood is indistinguishable from the volume percent of oxygen in air, each cell of the multicellular organism that is the human body has gas (O2, CO2) exchange equivalent to that which would be available to a unicellular organism. Collectively, the progenitor and adult red cells making up this organ are termed the erythron, which arises from undifferentiated, pluripotential stem cells. Following commitment, erythroid progenitors progress through several replicative stages, becoming more functionally specialized with maturation. In the process, they acquire many of the human blood group antigens.1 Eventually the reticulocyte and finally the mature circulating erythrocyte are produced.

In the adult stage of development, the total number of circulating erythrocytes is in steady state unless perturbed by pathologic or environmental insult. This is not so during growth in utero, particularly in the early stages of embryonic development. Consequently, erythrocyte production in the adult differs markedly from that in the embryo/fetus.

The Earliest Erythron

In the very early stages of human growth and development, there are two forms of red cell maturation: primitive and definitive (see Chap. 6).2–6 The primitive stage originates from mesothelial cells that migrate through the primitive streak in the yolk sac. The primitive erythron supplies the embryo with oxygen during the phase of rapid growth before the definitive form of maturation has had a chance to develop and seed an appropriate niche. The hallmark of this primitive erythron is a semisynchronous release of nucleated erythroid precursors containing primitive hemoglobin. Although primitive in the sense that the cells contain nuclei ...

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