Structure and Development of the Odontodes in an Armoured Catfish, Corydoras aeneus (Siluriformes, Callichthyidae)

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Abstract
In some living osteichthyans (e.g. the armoured catfishes) the postcranial dermal skeleton exhibits tooth‐like structures (odontodes) similar to those present in the dermal skeleton of the ancient craniates. We have undertaken this work to compare odontode with tooth development, structure, attachment to a bony support and replacement. We studied the odontodes fixed on the scutes (i.e. postcranial dermal plates) in a growth series of Corydoras aeneus using light, scanning and transmission electron microscopy. Odontodes are constituted of a pulp cavity surrounded by a cone of dentine itself capped with hypermineralized substance. The pulp cavity is devoid of nerves and blood vessels and there are no odontoblastic processes in the dentine. The dentine cone is firmly attached to a circular bony protuberance of the scute surface, the pedicel or attachment bone, by means of a ligament. An odontode anlage develops as a small invagination of a dermal papilla projecting into the epidermis, the basal cell layer of which constitutes a dental epithelium. First, dentine is deposited, next the hypermineralized substance, then the ligament and attachment bone. Odontodes develop in two positions with regard to the scute surface: a primary position when new odontodes form at the posterior border of the enlarging scute; a secondary position when new odontodes replace old odontodes that have been shed during thickening of the scute. In this case, the ligament and part of the base of the dentine cone are resorbed but not the pedicel of attachment bone, which is covered by deposition of scute matrix after the odontode has been shed. Within the scute matrix, the embedded pedicels of successive generations of odontodes are preserved, forming piles in the scutes of adult specimens.