COMPARISON OF TEETH AND DERMAL DENTICLES (ODONTODES) IN THE TELEOST DENTICEPS CLUPEOIDES (CLUPEOMORPHA)

The present work is a contribution to an extensive comparative structu ral and developmental study we have undertaken to understand the evolu tion of the dermal skeleton in osteichthyans. We have investigated the structure of developing and functional tooth-like dermal denticles lo cated on the head of Denticeps clupeoides, a clupeomorph, and compared their features to those of oral teeth. Morphological (scanning electr on microscopy) and structural (light microscopy and transmission elect ron microscopy) observations clearly demonstrate that these small, sha rp, conical and slightly backward-oriented denticles are true odontode s, i.e., homologous to oral teeth. They are composed of a dentine cone surrounding a pulp cavity, the top being covered by a hypermineralize d cap. These odontodes are attached to a circular pedicel of attachmen t bone by a ligament that mineralizes, and the attachment bone matrix merges with that of the bony support. The pedicel of attachment bone s urrounds a vascular cavity that is connected to the pulp cavity which is devoid of blood vessels and of nerve endings. Once the odontode is functional, the deposition of collagen matrix (called circumpulpar den tine) continues against the dentine, ligament, and attachment bone sur faces, thereby provoking a narrowing of the pulp cavity. Odontodes are shed by resorption occurring at the base, but their pedicels of attac hment bone persist at the bone surface and become embedded in the bone matrix, within which they are clearly visible. The oral teeth are sim ilar in shape, size, and structure to the odontodes, and they show onl y small differences probably related to the different function of thes e elements: They are more firmly anchored to the attachment bone, and the amount of dentine is relatively smaller than in odontodes. Despite their different functions, this close structural agreement between te eth and odontodes in Denticeps suggests that 1) competent cells from t he same (ecto)mesenchymal population might be involved and 2) the gene tic control of the developmental processes could be identical. It is s uggested that the odontode expression in extra-oral positions is a rel atively late novelty in this lineage. (C) 1998 Wiley-Liss, Inc.