Epithelial Label-Retaining Cells Are Absent during Tooth Cycling in Salmo salar and Polypterus senegalus.

Sam Vandenplas,Tom Hansen,P Eckhard Witten,Per Gunnar Fjelldal,Maxime Willems,Ann Huysseune,Laurent Viriot

doi:10.1371/journal.pone.0152870

Sam Vandenplas, Tom Hansen + Show 5 more

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https://doi.org/10.1371/journal.pone.0152870

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Abstract

The Atlantic salmon (Salmo salar) and African bichir (Polypterus senegalus) are both actinopterygian fish species that continuously replace their teeth without the involvement of a successional dental lamina. Instead, they share the presence of a middle dental epithelium: an epithelial tier enclosed by inner and outer dental epithelium. It has been hypothesized that this tier could functionally substitute for a successional dental lamina and might be a potential niche to house epithelial stem cells involved in tooth cycling. Therefore, in this study we performed a BrdU pulse chase experiment on both species to (1) determine the localization and extent of proliferating cells in the dental epithelial layers, (2) describe cell dynamics and (3) investigate if label-retaining cells are present, suggestive for the putative presence of stem cells. Cells proliferate in the middle dental epithelium, outer dental epithelium and cervical loop at the lingual side of the dental organ to form a new tooth germ. Using long chase times, both in S. salar (eight weeks) and P. senegalus (eight weeks and twelve weeks), we could not reveal the presence of label-retaining cells in the dental organ. Immunostaining of P. senegalus dental organs for the transcription factor Sox2, often used as a stem cell marker, labelled cells in the zone of outer dental epithelium which grades into the oral epithelium (ODE transition zone) and the inner dental epithelium of a successor only. The location of Sox2 distribution does not provide evidence for epithelial stem cells in the dental organ and, more specifically, in the middle dental epithelium. Comparison of S. salar and P. senegalus reveals shared traits in tooth cycling and thus advances our understanding of the developmental mechanism that ensures lifelong replacement.

Highlights

The enthralling ability to continuously replace teeth throughout life has fascinated scientists for decades
The dental organ is composed of the inner dental epithelium (IDE) of the functional tooth, the IDE of its successor, a middle dental epithelium (MDE) and the outer dental epithelium (ODE)
The 3D reconstruction of the dental organ of a S. salar tooth family confirms features described earlier in Huysseune and Witten [8] based on histological serial sections, i.e. new tooth germs arising at the lingual and posterior side of the dental organ, the absence of a dental lamina, and the presence of an MDE positioned between the functional tooth and its successor

Summary

Introduction

The enthralling ability to continuously replace teeth throughout life has fascinated scientists for decades. Label-Retaining Cells and Tooth Replacement continuous tooth replacement in a wide range of species: lesser spotted catshark (Scyliorhinus canicula) [1,2,3]; African bichir (Polypterus senegalus) [4]; rainbow trout (Oncorhynchus mykiss) [5,6,7]; Atlantic salmon (Salmo salar) [8,9]; zebrafish (Danio rerio) [10,11,12]; medaka (Oryzias latipes) [13]; leopard gecko (Eublepharis macularius) [14,15] and corn snake (Pantherophis guttatus) [16] These studies reveal considerable interspecific variation in morphological features, organization and patterning of the dentition; almost all species have teeth that are organized in tooth families, i.e. one functional tooth and all of its successors [17]. The latter authors coined the term middle dental epithelium (MDE) for this tier, and hypothesized that it could functionally substitute for a dental lamina by supplying the outer dental epithelium with cells before its differentiation into a placode

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