Old, new and new-old concepts about the evolution of teeth

Abstract

The evolutionary origin of teeth from dermal denticles (odontodes) that developed in the mouth cavity, designated as outside-in hypothesis, has long been undisputed. The outside-in hypothesis is based on the conclusion that dermal denticles and teeth fulfil the criteria of homology in an exemplary manner. Over the past 15 years, this hypothesis has been challenged. Proponents of the alternative inside-out hypothesis suggest that teeth did not evolve from dermal denticles, that they are of endodermal origin (forming in conjunction with neural crest-derived mesenchyme) and that they evolved several times independently in different lineages of vertebrates. Key arguments for the inside-out hypothesis are mineralized structures of conodonts that are accepted as teeth, the exclusive acceptance of placoderm pharyngeal denticles as teeth, together with the rejection of the presence of teeth in basal placoderms. We summarize the results of recent studies that have been triggered by the fruitful discussion between the two conflicting hypotheses. New findings support the traditional outside-in hypothesis: the mineralized elements of conodonts are not teeth, and the oral cusps in basal placoderms are true teeth. Furthermore, new developmental and molecular data clarify homology between teeth and dermal denticles. Today a new synthesis is emerging about the evolutionary origin of teeth from dermal denticles and about the unity of the elements of the dermal skeleton

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function

“…To understand how teeth arrived at their position at the margins of the jaws and the regulatory mechanisms that induce tooth formation at these specific locations, researchers have investigated whether endodermal or ectodermal epithelia contribute to the earliest stages of tooth development. There are two main hypotheses for the origin of oral teeth: “outside to inside,” in which odontodes (i.e., dermal scales or denticles) with ectodermal origins migrated into the oral cavity to form marginal teeth; or “inside to outside,” in which endodermally derived pharyngeal denticles migrate outward to the jaw margin to form teeth . Studies across multiple classes of jawed vertebrates suggest teeth can be generated from epithelia with endodermal, ectodermal, or mixed endo‐ and ectodermal origins .…”

Endoskeleton/Exo (dermal) skeleton - Mesoderm/Neural Crest: Two pair of problems and a shifting paradigm

“…() turned to another teleost, the Japanese medaka Oryzias latipes to investigate the cellular origin of the fin rays and scales. Scales, which come in an abundance of types, compositions, and responses to agents such as hormones, form an important body covering for teleost fishes (Harris, ; Witten et al., ). They provide protection for the body (especially for such superficial organs as the lateral lines), a barrier between organism and environment and against infection, and a source of Ca 2+ and PO 4 3− that can be mobilized by resorption and influenced by hormones (Levin and Levina, ).…”

“…They provide protection for the body (especially for such superficial organs as the lateral lines), a barrier between organism and environment and against infection, and a source of Ca 2+ and PO 4 3− that can be mobilized by resorption and influenced by hormones (Levin and Levina, ). Teleost scales, which are primarily collagenous, have been both distinguished from the dermal armour or exoskeleton (Smith and Hall, , ; Yang et al., ), and, classified as exoskeletal because of their development from odontodes, a feature they share with teeth, with which they are homologous (Sire and Akimenko, ; Hall, ; Sire et al., ; Harris, ; Witten et al., ).…”

Supernumerary teeth in the pharyngeal dentition of slow-developing zebrafish (Danio rerio , Hamilton, 1822)

“…Several features of the dentition are assumed to be taxon‐specific and are used as criteria by taxonomists for species identification and taxonomical classification (Cox & Hautier, ; Davalos, Velazco, Warsi, Smits, & Simmons, ; Huysseune, ). Despite the fact that basic processes in tooth formation are highly conserved among vertebrates (Huysseune & Sire, ; Underwood et al., ; Witten, Sire, & Huysseune, ), and that the development of the dentition is considered to be well canalized (Cox & Hautier, ), numerous studies have shown the occurrence of variations in dental patterns and in tooth shape (Roth, ; Golubtsov, Dzerjinskii, & Prokofiev, ; Mahler & Kearney, ; Shkil, Levin, Abdissa, & Smirnov, ; Cox & Hautier, ; among others). Alterations in the shape of the teeth, their number and the number of tooth rows are mostly assigned to one of the following categories: modifications as a symptom of pathological processes; acquired alterations, such as wear marks or traumatic tooth loss; and intraspecific variation (Cox & Hautier, ; Eastman & Underhill, ; Huysseune, ).…”