Tooth replacement without a dental lamina: The search for epithelial stem cells in Polypterus senegalus

Abstract

Most actinopterygians replace their teeth continuously throughout life. To address the question of where and how replacement teeth form in actinopterygians, it is advisable to investigate well-chosen representatives within the lineage. The African bichir, Polypterus senegalus, belongs to the earliest diverged group of the actinopterygian lineage with currently living representatives. Its well characterized dentition, together with its phylogenetic position, make this species an attractive model to answer following questions: (1) when and where does the replacement tooth form and how is it connected with the dental organ of the predecessor, and (2) is there any evidence for the presence of epithelial stem cells, hypothesized to play a role in replacement? Serial sections show that one tooth family can contain up to three members, which are all interconnected by dental epithelium. Replacement teeth develop without the presence of a successional dental lamina. We propose that this is the plesiomorphic condition for tooth replacement in actinopterygians. BrdU pulse-chase experiments reveal cells in the outer and middle dental epithelium, proliferating at the time of initiation of a new replacement tooth. It is tempting to assume that these cell layers provide a stem cell niche. The observed absence of label-retaining cells after long chase times (up to 8 weeks) is held against the light of divergent views on cell cycling properties of stem cells. At present, our data do not support, neither reject, the hypothesis on involvement of epithelial stem cells within the process of continuous tooth replacement.

A comparison of the larval and juvenile dentition in Polypterus senegalus

“…In the present study we concentrated on the teeth of the lower jaw, as this allows us to compare our data to previous studies (Clemen et al., ; Wacker et al., ; Vandenplas et al., ). In the juvenile specimens of P. senegalus , the dentary, coronoid 1 and coronoid 2 are well‐developed.…”

“…Formation of a replacement tooth in juvenile P. senegalus has been described previously (Vandenplas et al, 2014). Importantly, initiation of a replacement tooth in juveniles starts off as a local thickening in the outer dental epithelium (ODE) of its predecessor at the postero-lingual side.…”

“…While the functional dentary teeth in juvenile P. senegalus tend to show a similar stage of maturation in alternate tooth positions, this alternate pattern, strikingly, is not reflected in the level of differentiation of their replacement teeth. Instead, many of these replacement teeth are in the same stage of cytodifferentiation (Vandenplas et al, 2014;and Fig. 1a).…”

“…The clearing was monitored on a daily basis and the heads were eventually stored in 100% glycerine. Details of the staining protocol adapted for P. senegalus can be found in Vandenplas et al (2014).…”

Craniofacial skeleton of MEXICAN tetra (Astyanax mexicanus): As a bone disease model

“…Several teleost fish species have already been identified as animal models for craniofacial research. This includes zebrafish (Yelick and Schilling, ; Stock, ; Mork and Crump, ), medaka (Kimura et al, ; Atukorala et al, ; Witten et al, ), cichlids (Fraser et al, ; Powder and Albertson, ), stickleback (Kimmel et al, ; Ellis et al, ), carp (Gidmark et al, , ), Polypterus (Vandenplas et al, ; Noda et al, ) and many others (Meredith and Butler, ; Ferry et al, ; Staab et al, ; Lyon et al, ; Edds‐Walton et al, ; Fritsch et al, ; Thiery et al, ).…”