Sideroblasts are normoblasts of bone marrow which contain fine iron loaded granules (siderosomes) demonstrated by the iron stain. They constitute at most 4% of the normoblasts.
Sideroblasts (left image) must be differentiated from ringed sideroblasts (right image). Sideroblasts are normal in the bone marrow. They contain diffusely scattered over the whole cytoplasm one to few iron granules.
Ringed sideroblasts are always pathological and occur in sideroachrestic disorders. In ringed sideroblasts, the iron is in the mitochondria which are arranged around the nucleus. By definition, a ringed sideroblast contains 5 or more granules. The granules are more coarse than in sideroblasts.