HUYSSEUNE, A.*; WITTEN, P.E.; Ghent University, Belgium; University of Hamburg, Germany & Dalhousie University, Halifax, Canada: Patterning of development in continuously replacing osteichthyan dentitions

The dentition of osteichthyans presents an astonishing diversity in regard to distribution of the dentition in the oral cavity, tooth numbers, tooth arrangements, tooth shapes and sizes. Taking examples from different teleosts, this paper explores how the initial tooth pattern is set up, and how this relates to the establishment and maintenance (or modification) of the tooth replacement pattern. In teleosts, first-generation teeth (the very first teeth in ontogeny to develop in a particular locus in the dentition) are commonly initiated in adjacent or in alternate (odd and even) positions. The mechanism responsible for these divergent developmental patterns remains to be elucidated. However, patterns of adjacent or alternate tooth succession, set up by the first-generation teeth, can easily lead to replacement patterns where new teeth are initiated simultaneously every second, or even every third position. Recently, we have proposed that epithelial stem cells might underlie the process of continuous tooth replacement (Huysseune & Thesleff, 2004, BioEssays 26:665-671). To maintain a pattern that has been set up, stem cell activation − necessary for a new germ to develop − should be linked to the developmental stage of the predecessor. Our observations suggest that, once established, the replacement pattern is maintained (�default� state). Variations and modifications in the tooth replacement pattern that occur nevertheless are likely the result of local control at the level of the initiation of replacement teeth, i.e. by activation of putative stem cells. Further morphological and molecular data are needed to support the hypothesis of stem cell involvement and to reveal factors responsible for stem cell activation.