Erythroid maturation

The proerythroblast, the first identifiable erythroid precursor, has a diameter of 14 to 19 µM, a large, oval homogenously-staining nucleus, indistinct nucleoli, and darkly basophilic cytoplasm. The basophilic normoblast is 12 to 17 µM in diameter with basophilic cytoplasm, and coarsening and prominent clumping of the nuclear chromatin (spoked wheel or cartwheel appearance). Nucleoli are generally not seen. Accumulation of hemoglobin is seen in the polychromatic normoblast by the presence of less basophilic and muddy gray cytoplasm. The last nucleated RBC precursor, the orthochromatic normoblast approaches the diameter of a reticulocyte (8 to 12 µM); the eosinophilic staining cytoplasm contains nearly a full amount of hemoglobin, with a condensed pyknotic nucleus. Extrusion of the nucleus results in the reticulocyte (not shown), a cell slightly larger than a fully mature erythrocyte. The reticulocyte has a fine granular or reticular network of ribosomal RNA observed with supravital stains, such as cresyl blue or methylene blue. Such cells are present in small quantities in the peripheral blood of normal persons (1 to 2 percent), but are increased in response to stress on the erythroid lineage (eg, hemolysis, blood loss, hypoxia).