Studies on Radula Tooth Mineralization in the Polyplacophora

Abstract

This report is concerned with several aspects of the process of iron mineralization of the radula teeth of chitons (Polyplacophora. In Part I, the cusp cells of the radula sac, which are responsible for the deposition of iron into the major lateral teeth, are studied with the electron microscope. In Mopalia muscosa and Lepidochitona (Cyanoplax) hartwegi, it is found that these cells extend from a dorsal blood sinus to the surface of the teeth. The basal ends of the cusp cells, near the dorsal sinus, contain numerous rhopheocytotic vesicles filled with ferritin. Near the apical ends of the cells, there is a high concentration of iron-containing membrane-bound granules. Some granules contain ferritin, others are filled with ferruginous particles of electron-dense material, and others contain ferritin cores with an outer layer of ferruginous particles. In each cusp cell, the granule region is separated from the tooth surface by a bundle of microvilli that arises from a layer of mitochondria-rich cytoplasm and terminates on the tooth surface. No electron-dense material is found in the microvilli. Double-membrane structures are observed in regions where iron-containing granules occur near the microvilli. A model of the pathway of iron through the cusp cells is deduced from the observations on their ultrastructure. In Part II, it is shown that at least 90% of the iron contained in the blood of Mopalia muscosa (40 to 110 μg Fe per ml) occurs in the form of the protein-iron complex, ferritin. The blood ferritin is purified and compared with ferritin isolated from the superior epithelial cells and with vertebrate ferritins. In Part III, it is determined, by Fe59-labeling experiments, that the radula replacement rate of Mopalia muscosa is approximately 0.6 rows per day. The amount of iron contained in the major lateral teeth of Mopalia muscosa radulas is measured.