Development and fine structure of pharyngeal replacement teeth in juvenilezebrafish (Danio rerio) (Teleostei, Cyprinidae)

Teeth are commonly used model systems for the study of epithelial mesenchym al interactions during organogenesis. We describe here the ultrastructural characteristics of developing pharyngeal replacement teeth in juvenile zebr afish, an increasingly important model organism for vertebrate development. Replacement teeth develop in close association with the dental organ of a functional tooth. Morphogenesis is well advanced prior to the start of cyto differentiation. Fibrillar enameloid matrix is formed first, followed by th e deposition of predentine. Initial mineralization of the enameloid proceed s quickly; maturation involves the presence of ruffled-bordered ameloblasts . Dentine mineralization is inotropic and is mediated by matrix vesicles. W oven-fibred attachment bone matrix is deposited before completion of dentin e mineralization. Eruption of fully ankylosed teeth is a fast process and m ay involve degenerative changes in the pharyngeal epithelium. Mononucleated osteoclasts and elastic cells located in the pulp cavity intervene in toot h resorption prior to shedding. Structural differences with larval, first-g eneration zebrafish teeth include the presence of dentinal tubules and the absence of an electron-dense covering membrane. Part of these differences m ay relate to size differences of the teeth. Others, like the site of the re placement tooth bud, suggest that initiation may take place in already comm itted epithelium from the first initiation event in the larval stage.