Evolution, development, and regeneration of tooth-like epithelial appendages in sharks.

Sharks and their relatives are typically covered in highly specialized epithelial appendages embedded in the skin called dermal denticles; ancient tooth-like units (odontodes) composed of dentine and enamel-like tissues. These 'skin teeth' are remarkably similar to oral teeth of vertebrates and share comparable morphological and genetic signatures. Here we review the histological and morphological data from embryonic sharks to uncover characters that unite all tooth-like elements (odontodes), including teeth and skin denticles in sharks. In addition, we review the differences between the skin and oral odontodes that reflect their varied capacity for renewal. Our observations have begun to decipher the developmental and genetic shifts that separate these seemingly similar dental units, including elements of the regenerative nature in both oral teeth and the emerging skin denticles from the small-spotted catshark (Scyliorhinus canicula) and other chondrichthyan models. Ultimately, we ask what defines a tooth at both the molecular and morphological level. These insights aim to help us understand how nature makes, replaces and evolves a vast array of odontodes. [Display omitted] • Odontodes represent all tooth-like structures including teeth, internal and external denticles; sharks and their relatives have all types in various combinations depending on the species. • Many studies have attempted to understand the homology of tooth-like structures, sharks offer the opportunity to study both denticles and teeth within the same animal. • We describe the current knowledge on the developmental and structural differences between denticles and teeth in sharks, where the defining feature of oral teeth is the presence of a dental lamina. • Some sharks e.g. , hammerheads (Sphyrnidae), possess both oral (internal) and skin (external) denticles, which could shed light on the origin of odontodes debate, whether they first appeared inside or out. [ABSTRACT FROM AUTHOR]