Commitment to the Erythroid Lineage

Commitment to the erythroid lineage is marked by the appearance of CD71 (transferrin receptor), which increases gradually during maturation paralleled by a decrease in CD38.230 Although expressed on BFU-E, CD34 and the myeloid antigen CD33 are absent from immediate BFU-E descendants, which continue to be HLA-DR+CD71+ and express transiently the CD41a/CD61 platelet glycoprotein (GP) Ilb/IIIa complex.231 Expression of glycophorin by CD34+CD41+ precursor cells distinguishes erythroid from megakaryocytic precursor cells.232 Another erythroid progenitor cell antigen that is shared with the megakaryocytic lineage is CD36, the thrombospondin receptor or GPIV.231,233 CD36 is lost from erythroid cells at the reticulocyte stage concomitantly with the loss of fibronectin receptors, decreased attachment to bone marrow stromal cells, and migration of mature cells to the peripheral blood, whereas in megakaryocytic and monocytic cells, CD36 is a marker of maturation.233 HLA-DR is lost early, before the appearance of erythroid morphologic characteristics, whereas CD71 and CD36 slowly decrease with maturation. Differentiation from proerythroblasts to basophilic, polychromatophilic, and orthochromatic erythroblasts and finally to erythrocytes is characterized by acquisition of the red cell membrane components glycophorin A, band 3, and the blood group ABH antigen sialoglycoproteins and also of the structural cytoplasmic proteins, hemoglobin and spectrin.234

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