Rebooting the Zahnreihen: Analysis of the Jaw‐wide Patterning cues in the Adult Gecko Dentition

Abstract

Unlike mammals, it is common for reptiles to replace their teeth throughout life. Through exploring the mechanisms of tooth replacement, particularly in adult reptiles we will learn about the cells and molecules that are required for the regeneration of the dentition. Wholemount preparations, x-rays and μCT scans of most lizards show that each functional tooth appears to line up with a series of unerupted teeth that are in progressively less mature stages of development going posteriorly (Zahnreihen). The position of the Zahnreihen is at approximately 147° to the jaw margin. The next teeth that are fated to replace a functional tooth do not appear in the mouth until they have reached a threshold size and have moved into position. It has been hypothesized that the unerupted teeth secrete factors that regulate the pace of tooth formation and spacing. All unerupted teeth are connected to each other via a dental lamina that extends around the jaw, extending deep into the soft tissue lingual to the functional teeth. Using an unbiased, surgical approach we asked whether there were signals emanating from the unerupted teeth and dental lamina. We carried out 4 types of surgeries in defined regions: enucleation of unerupted teeth with minimal disruption of the dental lamina, enucleation plus scrambling of the dental lamina, enucleation plus chemical cauterization of remaining epithelium or a sham control. Animals were analyzed at 7d, 14d, or 1–8 months. The macroscopic and microscopic phenotypes confirmed that teeth were removed successfully and that recovery was taking place except when cauterization was carried out. There was an initial wave of apoptosis that tapered off by 5 days. Next a spike in proliferation occurred in both torn epithelium and surrounding mesenchyme that lasted for at least 1 week. In long term followup we found that simple tooth removal delays but does not impact overall patterning. In contrast, cauterization of the dental lamina causes a block in tooth formation that does not recover from the cut edges (up to 8 months follow-up). The mesenchyme on its own is completely unable to induce new teeth from other types of epithelia such as oral mucosa or glandular epithelium. The dental epithelium is therefore instructive and is the key tissue that must be present for new teeth to form. Molecular readouts that will be presented include the location of putative stem cells (BrdU label retention, SOX2 antibody staining), changes in gene expression (in situs and RNAseq) and evidence of polarity of the epithelium being re-established (in situs). This data will be used to map out the sequence of recovery in dental epithelium leading to reestablishment of the Zahnreihen. Support or Funding Information Studies were supported by an NSERC Discovery Grant (RGPIN-2016-05477) to JMR. TMG was supported by an NIH F32 Postdoctoral Fellowship (F32DE024948). KB is supported by Killam and Michael Smith Foundation for Health Research postdoctoral fellowships. Palatal view of an adult gecko with approximately 40 functional teeth in each quadrant. Each marginal tooth defines the start of a Zahnreihen (diagonal row of unerupted teeth). Not shown is the continuous dental lamina to which all teeth are attached. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.