Abstract
The radula of Mollusca is an excellent material to investigate the biomineralization process of iron and silica in living organism, because there is crystalline goethite and amorphous opal in the limpet’s radula. Because of the wearing of teeth by scraping on the rock, the limpet secretes organic matrix from the sac and deposits iron and silica precisely on the organic matrix framework through different stages. Thus, the process of radula biomineralization is in a linear and dynamic balance. The limpet Cellana toreuma was collected from the coast of Shi-man. We used tissue section, scanning electron microscope (SEM) and transmission electron microscope (TEM) to investigate the biomineralization in stage I, stage II and radular sac of the limpet. A new tissue containing iron was found. This tissue extends from the sac and covers the superior epithelial cells of the radula. Cells with sidersomes constructed by ferritin particles distribute sparsely in the tissue. In stage II, the cellular matrix of the tissue is filled with iron crystals that are about 30-50 nm in diameters. The existence of this tissue is an observation which has not been reported previously. In the radular sac, we observed newly formed teeth, and a clear boundary between the cusp and the base. There are two types of cells near the newly formed teeth. The cells adjacent to the cusp have microvilli and may function in transferring materials into the cusp. Some of these cells have tight junctions and differ from the cells with microvilli in stage II. The other type of cells adjacent to the base contains mulberry-like cysts which show high electron density. These newly observed cysts are proposed to be related to the forming of the tooth. Furthermore, we analyzed the proteins in the radula, and found one protein expressed not only in the tooth, but also in the superior epithelial cells and in the peripheral tissue. By amino acid composition analysis, its pI value is about 5.01. There was a difference of molecular weight using different analytical approaches. Its molecular weight in SDS-PAGE is about 16 kDa, in mass spectrum is 11.558 kDa and we propose the difference may due to glycosylation. Antibodies were generated from a rabbit. Using TEM and immunogold staining techniques we found that the ratio of immunogold density in cusp /base /superior epithelial cell /peripheral tissue is about 16/ 10/ 4/ 5, showing that the protein is predominantly expressed in the cusp and the base. Thus, we propose it should be an acid glycoprotein associated with the biomineralization process in the limpet’s radula.