Caiman periodontium as an intermediate between basal vertebrate ankylosis‐type attachment and mammalian “true” periodontium

Abstract

The teeth of many fish, amphibia, and reptiles are attached to the alveolar bone via ankylosis. In contrast, mammalian periodontia are characterized by a gomphosis, an attachment of the tooth root in the alveolar bone socket via periodontal ligament fibers. Among the reptiles, the crocodilians are the only group featuring a gomphosis-type connection between tooth root and alveolar bone, while in other reptiles tooth-root and jawbone are connected via ankylosis. The purpose of the present study was to compare several key features of the crocodilian periodontium with those of the mammalian and noncrocodilian reptile periodontium. As experimental models for our study we chose the periodontium of newborn geckos (Hemidacylus turcicus), juvenile caimans (Caiman crocodilus crocodilus), and 10-day-postnatal Swiss-Webster mice (Mus musculus) as representative models for noncrocodilian reptiles, crocodilian reptiles, and mammals. The caiman periodontium emerged as an intermediary between the mineral-free mouse ligament and the mineralized gecko ankylosis-type attachment. Caiman ligament fibers were less organized than mouse ligament fibers but featured distinct fasciae surrounding ligament fiber bundles. Caiman Hertwig's epithelial root sheath (HERS) was similarly perforated as mouse HERS and distinctly different from the continuous gecko HERS. Both caiman and mouse HERS covered the entire tooth root length, while in the gecko HERS was limited to the coronal portion of the root, allowing for cementoid-mediated ankylosis at the apical tip of the root. We interpret our data to indicate distinct differences in mineral distribution, periodontal ligament fiber organization, and HERS distribution between noncrocodilian reptiles, crocodilian reptiles, and mammals. Mineral deposits in the caiman ligament may reflect an evolutionary position of the caiman periodontium between ankylosis and gomphosis.

Evolution and developmental diversity of tooth regeneration

“…This reduction in replacement appears correlated with the requirement of complex occlusion for efficient food processing [22,58]. The functional mammalian teeth sit in a socket in the jaw (thecodont attachment), a situation similar to Crocodilians but this appears to be a case of convergent evolution [59]. Two functional generations appears to be the primitive therian pattern, with replacement of teeth at the incisor, canine and premolar positions, while the adult molars are not replaced.…”

The tooth attachment mechanism defined by structure, chemical composition and mechanical properties of collagen fibers in the periodontium

“…The periodontium of a human tooth includes three mineralized tissues: alveolar bone, cementum and root dentin, of which cementum and alveolar bone interface with a soft fibrous tissue; the periodontal ligament (PDL), forming a fibrous joint or gomphosis [1,2]. Periodontal fibers and Sharpey's fibers in acellular cementum seemingly attach to root dentin [3][4][5].…”

Specialized stem cell niche enables repetitive renewal of alligator teeth

“…Squamates possess acrodont-pleurodont teeth (attached by connective filaments to the gum and maxillary/dental bones) (18), which differ from tooth attachment in mammals. Crocodilians have typical thecodont teeth, which show similar morphological characteristics as in mammalian teeth (they both are implanted in sockets of the maxillary/dental bones) (19). Crocodilians also have a secondary palate, like the palate found in mammals (20).…”