Hypotheses on the evolutionary origin of the vertebrate dentition have largely revolved around the developmental basis of pharyngeal
teeth (whether endoderm- or ectoderm-derived). As a corollary to our ‘modified outside in’ hypothesis (Huysseune et al., 2009), I propose
that the development of allegedly endoderm-derived pharyngeal teeth in teleost fish requires a contribution, or signalling, from the
ectoderm. Using the teleost fish Danio rerio (zebrafish) we have now shown that pharyngeal teeth derive from medial endoderm located
posterior to the sixth pharyngeal pouch. Yet, dental epithelial morphogenesis starts only after pouch endoderm has made contact with
the ectoderm, and invariably only after a layer of periderm-like cells have covered the odontogenic endodermal epithelium. Periderm is
an extra-embryonic layer that spreads over the embryo during gastrulation and covers the skin ectoderm. This research sabbatical is
intended to answer two questions, fundamental to the evolutionary developmental history of teeth. (1) Can we pinpoint the signal at the
pouch/ectoderm contact that allows tooth initiation in endoderm? This question will be addressed by studying knockout mutant zebrafish
in collaboration with Prof. Z. Gao (Wuhan, China). (2) Is the participation of periderm and/or periderm-like cells in the pharynx lining, and
subsequently in tooth formation, a derived feature proper to teleosts or common to all actinopterygians and perhaps rooted deeply in
gnathostome evolution? To answer this question I will examine series of embryonic stages of basal non-teleost actinopterygians, rare
material available only in the lab of Prof. R. Cerny (Prague, Czech Republic). Together, my observations will shed light on the potential role
of ectoderm and extra-embryonic tissues in building teeth and contribute to settling the evolutionary origin of teeth. The knowledge
gathered may well create paradigm-shifts at different levels.