Tooth be told, location matters
By Mizuki Tojo
Imagine waking up one morning, realizing that you know how to grow fingers out of your head with complete volition. Sure, this may not be advantageous unless you’re living in a world where evolution has taken a very strange turn. However, being able to know when and where a transplant organ should be introduced in order to ensure optimal success can be incredibly valuable. In a recent study, Chen and colleagues explore and compare spatial and temporal competency for tooth formation in the zebrafish and stickleback (Chen et al., 2025).
While most internal organs in vertebrates exist in tightly conserved partnerships with other organs and tissues, epithelial appendages (e.g., teeth, hair, feathers and scales) vary widely in both location and number across species (Ellis et al., 2015; Miller et al., 2014; Oeschger et al., 2022). Interestingly, for teeth, the number and organization can be highly varied even within species, yet the cell types that make up the organ are shared across vertebrates. As such, studying the tooth organ provides a unique opportunity to explore how organs that develop through a conserved genetic program can be highly plastic in their spatial organization across evolution.
Researchers have already succeeded in inducing the differentiation of ectopic and redundant teeth through overexpressing the gene Ectodysplasin (Eda). Mutating Eda also conversely leads to prominent teeth loss (Harris et al., 2008; Wucherpfennig et al., 2019). Previous studies to ubiquitously overexpress the zebrafish Eda gene in both zebrafish and Mexican Cavefish resulted in new teeth forming in sites beyond their native, or endogenous locations (Aigler et al., 2014; Jandzik and Stock, 2021). The new tooth fields were not merely an extension of the existing field but rather utilized a tooth differentiation program at entirely independent locations. Notably, these ‘ectopic’ tooth fields were found to naturally occur in other wild fish species, indicating that these findings replicate naturally occurring variation in tooth patterning. However, such studies did not have the tools to address temporal aspects of tooth competency. For example, is there a developmental competency window, and if so, when does that begin and end? Can ectopic teeth persist beyond the cessation of exogenous Eda?
In the present study, Chen and colleagues took a comparative approach to explore spatial and temporal competency for tooth formation in the zebrafish and stickleback, two fish species separated by 250 million years of evolution, with highly contrasting dental structures. Specifically, they developed a system in which both fish species could overexpress a species-specific version of the tooth-inducing gene, Eda, in a heat shock-inducible manner, enabling timed control of Eda expression. It turns out that ectopic teeth are most receptive to forming during the window when the first endogenous teeth are specified, which is around 2 dpf for zebrafish. Moreover, tooth remodeling continues for up to several months after the initial heat shock activation of Eda early in development in both zebrafish and stickleback.
Yet, the level of tooth induction competency, both in space and time, was strikingly higher in sticklebacks than zebrafish. Tyler Square, the senior author on the work, described being “floored” when he first saw sticklebacks with specified tooth organs on their face! Indeed, when he and colleagues assessed the formation of ectopic teeth following twice-daily heat shocks during larval stages (17-25 dpf), they noticed tooth germ initiation around the lower and upper jaws, a region spatially distinct from typical pharyngeal teeth. In contrast, zebrafish were not able to form these ‘face teeth’ upon Eda overexpression across a comparable developmental timeline.
To assess the long-term maintenance of the Eda-induced teeth, the authors conducted a pulse-chase live bone staining two months following Eda induction and only observed active tooth turnover in pharyngeal tooth fields. This was corroborated by the molecular differences between the tooth types (see the summary schematic). Several genes previously implicated in epithelial organ formation were expressed in the stickleback pharynx, whereas only one of them was expressed in facial regions where face teeth were specified. The authors speculated that face teeth may involve a separate downstream pathway from that in the pharynx, beyond initial Eda activation.
What might explain the lower competency for tooth differentiation in zebrafish? Square explains that zebrafish have highly organized dentition similar to mammals, and that perhaps “losing competency for tooth organization could be a possible tradeoff”. The findings in the paper open many avenues for exploration, including what differentiates an ectopic organ unit that can establish the next generation of stem cells to self-sustain, from a unit that cannot—such as the face teeth. Knowing that there is a prime window for ectopic teeth induction also highlights the importance of precise timing in optimizing regenerative and therapeutic outcomes.
Summary schematic from Chen et al., 2025.
About the authors:
Zoe Chen, the first author on the work, was an undergraduate in Craig Miller’s lab at UC Berkeley and is now a M.D. Candidate at UMass Chan Medical School, studying anesthesiology using mouse models. Her motivation in science comes from watching abstract textbook ideas come to life at the bench, as she saw how studying tooth development could inspire broader questions in evolution and human health. In her spare time, she enjoys rock climbing with her friends.
Tyler Square is an Assistant Professor in the Department of Microbiology and Cell Science, University of Florida, Gainesville, FL 32611, USA. He started working on stickleback teeth as a post-doc with Craig Miller at UC Berkeley. He did research on lampreys as an undergraduate student at the University of Colorado Boulder. There, he observed fingernail-like spikes in their mouths, which over time began looking like teeth, at which point he became interested in epithelial appendage evolution. He enjoys spending time with his sticklebacks, since “Florida is hot, but the stickleback facility is set to 18–19 degrees Celsius.”
About the Science Writer:
Mizuki Tojo is currently a third year PhD student in the Walker Lab at Dartmouth College, comparing the mechanisms underlying peripheral nerve development and regeneration in zebrafish. Stemming from her own academic background in psychology, she enjoys thinking about science from different perspectives. One of her goals this year is to spend less time negotiating experiments with zebrafish and more time embracing the harsh weather in the Upper Valley by trying out winter sports.
References:
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