Specific binding sites for transferrin on ameloblasts of the enamel maturation zone in the rat incisor

During enamel maturation in rodents, an iron-containing pigment is deposited into the surface layer of the enamel. Maturation zone ameloblasts presumably are responsible for this deposition. The presence of large amounts of ferritin in the cytoplasm of these cells suggests that they receive iron, presumably from circulating transferrin. An in vivo radioautographic binding assay using iodinated transferrin was used to determine if indeed maturation ameloblasts possess transferrin receptors at their cell surfaces. Experimental rats received systemic injections of labeled transferrin while control rats received injections of labeled transferrin plus a large excess of unlabeled transferrin in order to compete with the labeled transferrin for available specific receptors. Light microscope radioautography showed that ruffle-ended ameloblasts (RAs) of the enamel maturation zone had a high density of specific receptors for transferrin relative to smooth-ended ameloblasts (SAs). Electron microscopy and energy-dispersive X-ray spectroscopy confirmed the presence of ferritin and iron, respectively, within these cells. It is postulated that the iron responsible for enamel pigmentation is transported by transferrin to maturation ameloblasts and is bound to specific transferrin receptors found mostly on RAs and that the modulation of these cells into SAs results in a loss of most of these receptors.