The ultrastructural localization of iron in rat incisor enamel

Previous studies have shown that the pigmented surface layer of rat incisor enamel contains up to 30% iron but have left the question open as to whether iron is bound to the hydroxyapatite crystals or is present as a separate crystalline phase. Maxillary incisors from adult rats were demineralized for an extended period of time in EDTA. An insoluble membranous structure representing the remaining superficial portion of the enamel was collected and analyzed in the electron microprobe. The analyses showed that the demineralization procedure had removed calcium and phosphorus nearly completely, whereas the iron content of the surface layer had not been appreciably reduced. Electron microscopic examination of these specimens showed that the residual material contained electron-dense particles whose size, form and orientation indicated that they were remnants of the original crystal population. Several other demineralized and undemineralized specimens were also examined electron microscopically. The results indicate that iron is not present as a separate particulate phase but is, probably, bound to the surface of hydroxyapatite crystals.