Medaka and Zebrafish, small sized fresh water fishes, have numerous teeth in the pharyngeal region, which undergo cyclical replacement throughout life. With advancement of molecular analyses of these animals, their pharyngeal teeth are gaining attention as a potential model for the studies of tooth evolution and regeneration. Evolutionally, periodontal ligament first appears in Reptilian and is common in the dentition of higher vertebrates. In medaka, upper pharyngeal teeth are connected to the pedicle by attachment fibers, which resemble, at least functionally, to periodontal ligament of higher vertebrates, despite absence of cementum or alveolar sockets in this species. Objectives: and Methods: In order to evaluate compositional resemblance of attachment apparatus of medaka teeth to mammalian periodontal ligament, we sought to examine distribution of periostin, known to be enriched in mammalian periodontal ligament, in the developing pharyngeal teeth of medaka by immunohistochemistry using specific antibodies raised against zebrafish periostin that share high homology with that of medaka. Pharyngeal teeth of zebrafish, ankylosed to supporting bones, served as control. Results: and Discussion: In pharyngeal tooth germs of medaka, periostin-like immunoreactions were located exclusively along the cell membranes of inner and outer enamel epithelia, but not in the attachment fibers or other surrounding tissues. In the process of tooth eruption, periostin–like immunoreactivity was confined to the junctional epithelium and, thereafter, shifted to superficial epithelial cells around the tooth crown that showed enhanced reactions. In zebrafish, immunoreactions were broadly expressed in pharyngeal epithelium, most intensely in the junctional epithelium. No immunoreactivity was found in dental papilla or other dental mesenchymal elements in zebrafish. Conclusion: In medaka, periostin is not involved in fibrous tooth attachment to supporting bones but appears to play roles in dento-epithelial attachment throughout development. In ankylosed teeth of zebrafish, periostin may also play similar roles as in medaka if not identical.

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