Iron is stored in bone marrow as hemosiderin which makes nucleated erythrocytes to synthesize to hemoglobin. Iron stored outside of red blood cells is called extracellular iron. Iron granules are found in somepolychromatophilic normoblast, orthochromatic normoblast and mature red blood cells. Those cells are respectively called sideroblasts and siderocytes which belong tointracellular iron. This iron stain is based on the Prussian blue reaction. Acid potassium ferrocyanide reacts with intracellular iron and extracellular iron to form an insoluble blue ferric ferrocyanide
precipitate in iron depots of cells.