Novel Insights into Tooth Row Development: From Old Ideas to New Concepts

Field: Biochemistry, Genetics and Molecular Biology (Life Sciences)

Teeth are highly mineralized structures that develop in specific morphogenetic zones along the jaws within a distinct superficial epithelial unit, the dental lamina.[1] Both represent shared derived characters of all living vertebrates with jaws,[2] but form, number, and arrangement of teeth vary greatly so that they produce either homodont (all teeth are similar) or heterodont (teeth differ morphologically) dentitions. Additionally, teeth might be distributed all over the inner surface of the mouth and branchial cavities to varying degrees or restricted to definite locations. The early evolution of teeth and mechanisms resulting in the various patterns, however, are a subject of continuous debate. The recent paper by Sadier, Jackman, Laudet, and Gibert is important because it proposes a generalized model in which tooth rows develop sequentially from a single first-formed tooth starting the inhibitor cascade for subsequent tooth development.[3] This hypothesis is not novel, but the new study integrates recent evidence obtained from zebrafish and mouse into a unifying hypothesis. Both are model organisms, but they differ significantly in arrangement and location of teeth. The Zebrafish has teeth in the branchial chamber, which are continuously replaced, but lack teeth in the mouth cavity, a phenomenon that represents a secondary loss, evolutionarily speaking. The mouse, conversely, displays the very specific mammal dental heterodonty and arrangement of teeth into a single tooth row along the jaws. The mouse differs from other mammals in that it shows highly derived and adapted functional dentitions with reduced tooth types consisting only of continuously growing incisors and non-replaced molars. Both tooth types are separated by a diastema, which is the result of a discontinuous dental lamina characterizing the specialized dentition of rodents. The mouse, as all mammals also lacks pharyngeal teeth. Molar tooth row development in the mouse is initiated by an inhibitor tooth, as is pharyngeal tooth development in zebrafish. Anterior and posterior teeth in mammals, however, develop from different germ layers during embryogenesis with anteriors arising from the ectoderm while the molars are formed by the endoderm, as are the pharyngeal teeth in the zebrafish. The developmental boundary between anterior and posterior dentitions could indicate different evolutionary pathways for anterior and posterior dental row partitions representing distinct tooth families and might be the reason why not all mammals follow the inhibitory cascade. Changes in activation/inhibition signals during jaw development are assumed to result in variations of tooth development between species and clades.[3] So far, the inhibitor cascade only has been ascertained for molar tooth rows in mammals and pharyngeal teeth in fishes, which are both of endodermal origin. And yet, the dental lamina only extends into the molar, but not into the branchial region. Whether distinct parts of a discontinuous dental lamina initiate unconnected inhibitor cascades in the mouth cavity or a single inhibitor cascade creates a complete oral tooth row even across developmental boundaries remains ambiguous. The lack of a dental lamina in the pharyngeal zebrafish dentition, however, might imply that tooth row development is independent of such a tissue: It possibly represents an ancient conserved feature such as the development of oral tooth replacement in the bichir, which plesiomorphically is without a dental lamina.[4] Fishes, such as medaka, with both oral and pharyngeal teeth, or basal mammals having the complete set of heterodonty, could represent expedient model organisms for future research directions focusing on embryonic tooth family development and correlated inhibitor cascades and genetic patterns. The author declares no conflict of interest.