Tooth Removal in the Leopard Gecko and the de novo Formation of Replacement Teeth.

Kirstin S Brink,Katherine Fu,Theresa M Grieco,Jesus Rodolfo Martin Del Campo,Joaquín Ignacio Henríquez,Joy M Richman

doi:10.3389/fphys.2021.576816

Abstract

Many reptiles are able to continuously replace their teeth through life, an ability attributed to the existence of epithelial stem cells. Tooth replacement occurs in a spatially and temporally regulated manner, suggesting the involvement of diffusible factors, potentially over long distances. Here, we locally disrupted tooth replacement in the leopard gecko (Eublepharis macularius) and followed the recovery of the dentition. We looked at the effects on local patterning and functionally tested whether putative epithelial stem cells can give rise to multiple cell types in the enamel organs of new teeth. Second generation teeth with enamel and dentine were removed from adult geckos. The dental lamina was either left intact or disrupted in order to interfere with local patterning cues. The dentition began to reform by 1 month and was nearly recovered by 2–3 months as shown in μCT scans and eruption of teeth labeled with fluorescent markers. Microscopic analysis showed that the dental lamina was fully healed by 1 month. The deepest parts of the dental lamina retained odontogenic identity as shown by PITX2 staining. A pulse-chase was carried out to label cells that were stimulated to enter the cell cycle and then would carry BrdU forward into subsequent tooth generations. Initially we labeled 70–78% of PCNA cells with BrdU. After a 1-month chase, the percentage of BrdU + PCNA labeled cells in the dental lamina had dropped to 10%, consistent with the dilution of the label. There was also a population of single, BrdU-labeled cells present up to 2 months post surgery. These BrdU-labeled cells were almost entirely located in the dental lamina and were the likely progenitor/stem cells because they had not entered the cell cycle. In contrast fragmented BrdU was seen in the PCNA-positive, proliferating enamel organs. Homeostasis and recovery of the gecko dentition was therefore mediated by a stable population of epithelial stem cells in the dental lamina.

Highlights

Polyphyodonty, or life-long tooth replacement, is a developmental process shared by most nonmammalian vertebrates
Other potential mechanisms proposed for reptiles include a reaction-diffusion model or ‘zone of inhibition’ model that creates spacing between teeth that change through growth, creating an emergent alternating replacement pattern (Osborn, 1970, 1971; Westergaard and Ferguson, 1987; Murray and Kulesa, 1996; Fraser et al, 2008)
Our results show that patterning is not greatly affected following removal of second generation teeth, and that the process of tooth regeneration is robust in the gecko since non-proliferative, dental epithelial progenitor cells are maintained in the dental lamina

Summary

Introduction

Polyphyodonty, or life-long tooth replacement, is a developmental process shared by most nonmammalian vertebrates. Other potential mechanisms proposed for reptiles include a reaction-diffusion model or ‘zone of inhibition’ model that creates spacing between teeth that change through growth, creating an emergent alternating replacement pattern (Osborn, 1970, 1971; Westergaard and Ferguson, 1987; Murray and Kulesa, 1996; Fraser et al, 2008). These models have all been proposed based on examination of tooth initiation in embryos, and so it is still not known how patterns of tooth cycling are maintained in adults

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Conclusion