Iron mineralization in the radula teeth of the chiton <i>Acanthopleura hirtosa</i>

Abstract The structure, morphology and organization of the radula teeth of the chiton Acanthopleura hirtosa, together with their surrounding superior epithelium tissue, have been examined by using transmission electron microscopy and electron diffraction. Considerable amounts of iron are first seen in inclusions resembling ferritin aggregates and haemosiderin within the cells of the superior epithelium just before the onset of tooth-cusp mineralization. Iron appears to be delivered to the mineralizing surface of the cusp (presumably as FeII) via a series of microvilli, which extend out from the cells of the superior epithelium and which are contiguous with the surface of the tooth cusp. Before mineralization, the tooth cusp consists of a series of fine organic fibrils arranged perpendicular to the posterior edge but parallel to the anterior edge of the cusp. Mineralization within the cusp begins with the deposition of small spherules of ferrihydrite, which appear to have a close spatial relation to the organic matrix. Depending on their location within the cusp these spherules are rapidly replaced by crystals of either goethite, lepidocrocite and/or magnetite as mineralization proceeds. Mineralization is preferentially concentrated towards the posterior surface of the tooth cusp with a greater deposition of magnetite than is found on the anterior surface. We suggest that the initial control of mineralization in the tooth cusp is mediated by the organic matrix. However, the phase transformations among the several mineral phases found in the cusp may well be due to differential competition among reactions leading to these phases and this, in turn, may be controlled by the generation of differential redox and related potentials within the cusp.