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Although the nucleus has been extruded, the reticulocyte is still considered immature because it retains numerous organelles needed for hemoglobin production, such as ribosomes, mitochondria, and fragments of the Golgi apparatus. The reticulocyte is slightly larger (10 microns) than the mature erythrocyte. A reticulocyte normally remains in the bone marrow for one or two days before entering the circulation and its final 24 hours of maturation. The red cell is mature when hemoglobin production is complete and the organelles have disintegrated. Reticulocytes normally make up 0.5 - 1.5% of the peripheral blood red cells. They appear blue/gray on the Wright's stained smear. The residual RNA in the cytoplasm causes the blue/gray color. The terms, polychromasia or polychromatophilic, are used to describe these cells on a Wright's stained preparation. A supravital stain such as new methylene blue N or brilliant cresyl blue is used to stain reticulocytes for an actual count.

This slide shows a marrow aspiration smear with numerous ring sideroblasts. Normal red cell precursors have only one or at most two granules of iron in their cytoplasm. These abnormal red cell precursors have numerous iron containing granules in their cytoplasm indicating abnormal iron incorporation. This iron is actually incorporated into mitochondria. Ring sideroblasts can be seen in idiopathic sideroblastic anemia, and in sideroblastic anemia induced by drugs, lead poisoning, and alcohol abuse.