Fine structure of the pharyngeal teeth in the coelacanthid fish (Latimeria chalumnae)

Abstract

In this paper we describe the fine structure of erupted pharyngeal teeth in Latimeria chalumnae. Pharyngeal teeth, situated on the medial side of the left first branchial arch, were obtained from Latimeria chalumnae (B. L. 175cm, B. W. 85kg, Female) and were used in this study. Cross stripes consisting of longitudinal and transverse grooves which penetrated into the surface layer were observed. Filmy structures were observed in the longitudinal grooves. It is thought that these filmy structures consist of organic matter, because these structures seemed to continue the dentin and disappeared after removal treatment for organic matter. A number of stripes almost parallel to the surface were observed in the section of the surface layer. These stripes were considered to be incremental lines. The stripes obliquely ended at the boundary between the surface layer and the dentin. Because the stripes nearer the surface ended closer to the basal portion and the stripes nearer the dentin ended closer to the apical portion, the pattern formed by these stripes resembles the incremental lines of mammalian enamel. Though the intervals between the stripes were 0.2-0.7μm, in this specimen the tendency was that form the intervals of the surface side to be narrower than those of the dentin side. The crystal-like granules of the surface layer were columnar or spindle shaped, 60-80 A in width and 600-800 A in length. Their width and length were relatively uniform. However, the arrangement of the crystal-like granules was irregular under high magnification. The outline of the crystal-like granules was never straight, but irregular. A number of fine electron-lucent spot-like, oval-like and line-like structures were seen in the crystal-like granules. It is considered that organic matter was present around the crystals of the surface layer, because fine networks which made us think of a negative view of the undemineralized section were seen after phosphotungstic acid (PTA) stain for undemineralized section on the grid. The boundary between the surface layer and the dentin was clear in observation with scanning and transmisson electron microscopes under low magnification. On the other hand, the boundary between the surface layer and the dentin became unclear in observation with a transmission electron microscope under high magnification and a mixed layer between the two, 0.3-1 μm in width, which was recognized from the density and arrangement of the crystal-like granules, was seen. Concerning the organic matrix of the dentin, the outermost layer of the dentin, 1-1.2 μm in thickness, which consists of electron-dense amorphous substances and loose collagen fibers, was recognized in the specimen after chromium sulfate demineralization. From the results described above, it is considered that the surface layer of the pharyngeal teeth in Latimeria chalumnae corresponds to true enamel because of its optical property, resolvability to acid, mineralization, fine structure and feature of organic matrix. In particular, the feature of the surface layer resembled the thin aprismatic enamel in amphibia.