Ring sideroblasts and sideroblastic anemias.

Ring sideroblasts Ring sideroblasts are erythroblasts with iron-loaded mitochondria visualized by Prussian blue staining (Perls’ reaction) as a perinuclear ring of blue granules (Figures 1D and 2C). The International Working Group on Morphology of Myelodysplastic Syndrome (IWGM-MDS) recommended that ring sideroblasts be defined as erythroblasts in which there are a minimum of five siderotic granules covering at least one third of the circumference of the nucleus. Ring sideroblasts are found exclusively in pathological conditions, and should not be confused with ferritin sideroblasts, which are present in normal bone marrow. These latter are normal erythroblasts that, after Prussian blue staining, show a few blue granules scattered in the cytoplasm, representing endosomes filled with excess iron not utilized for heme synthesis (siderosomes). While the iron of ferritin sideroblasts is stored in cytosolic ferritin, whose subunits are encoded by the FTH1 and FTL genes, the iron of ring sideroblasts is stored in mitochondrial ferritin, encoded by the FTMT gene. Indeed, mitochondrial ferritin is specifically detected in ring sidero blasts, as illustrated in Figure 2D. Classification of sideroblastic anemias The sideroblastic anemias include both hereditary and acquired conditions, and the main disorders are reported in Table 1. Representative peripheral blood and bone marrow smears from a patient with X-linked sideroblastic anemia (XLSA) and a patient with refractory anemia with ring sidero blasts (RARS) are shown in Figures 1 and 2, respectively.