Siderocytes are anucleate erythrocytes with iron-containing (siderotic) cytoplasmic inclusions. The inclusions can be due to aggregates of iron in the cytoplasm or within mitochondria (the latter are called Pappenheimer bodies). A nucleated erythoid cell with the same inclusions (usually within mitochondria) is termed a sideroblast. Siderocytes should be distinguished from basophilic stippling (aggregates of RNA) because they mean different things. Generally, siderocytes have small focal accumulation of light blue to grey material, whereas basophilic stippling is more chunky and diffusely distributed throughout the cytoplasm. Siderocytes will also stain blue with Prussian blue reaction (detects ferric iron), if the iron is in sufficient amounts to readily visualize (small amounts do not stain well with Prussian blue).
Increased numbers of siderocytes may also be seen in connection with:
Hemolytic anemia: Increased iron turnover can result in siderocyte formation, e.g. immune mediated hemolytic anemia. Siderocytes are seen most frequently in dogs with hemolytic anemia.
Abnormal heme synthesis. Iron may accumulate in RBC if hemoglobin production is inhibited. This can occur with drugs (e.g. chloramphenicol), lead poisoning, and vitamin B6 and copper deficiency in pigs (B6 is required for heme synthesis and copper is also required for release of iron from macrophages and enterocytes). Another drug associated with siderocytes is hydroxyzine, presumably due to alterations in heme synthesis. Siderocytes have also been seen in some dogs with portosystemic shunts (may be due to abnormal iron metabolism with iron accumulation occurring in RBC). Siderocytes can be a feature of dyserythropoiesis due to myelodysplastic syndrome and acute myeloid leukemia in cats (but generally are not seen in dogs with these disorders). Acquired dyserythropoietic syndromes with siderocytes have been described in dogs (Canfield et al., 1987, Weiss, 2005).