7 Retinoic Acid Signaling and the Zebrafish Dentition …
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other tetrapod models (e.g. Simoes-Costa and Bronner 2016) and much has also
been learned from studies of zebrafish (Hernandez-Lagunas et al. 2005). Thus, it is
important to pay particular attention to the role of CNC-derived odontoblast cells
as the function and evolution of RA signaling in tooth development in zebrafish is
further considered.
Current State of the Field
Cell Signaling Control of Tooth Formation
Early Inhibition of RA Signaling (aldh1a2 Mutants)
The first investigation into specific roles of RA signaling during zebrafish tooth
development was to test whether RA is necessary for the initiation of tooth formation. However, because of known pleiotropic roles for RA in vertebrate development,
both at early stages and late, it was necessary to gradually home in on the exact timing
of when such a tooth-specific RA requirement might be present. Given the location
of the zebrafish dentition in the posterior pharynx, it is logical that the development of teeth might be particularly sensitive to the proper formation of the posterior
pharyngeal region in general, especially since pharyngeal patterning had been shown
previously to require RA signaling at very early developmental stages. Specifically, in
the absence of RA signaling via genetic inactivation of the RA synthesizing enzyme
aldh1a2 or by pharmacological inhibition of RA synthesis by DEAB or other methods of RA inhibition very early in development, the posterior pharyngeal region is
greatly disrupted in zebrafish embryos (Begemann et al. 2001, 2004; Grandel et al.
2002). Consequently, when the effect on tooth formation was directly examined in
aldh1a2 mutants or by early chemical RA inhibition, it perhaps wasn’t surprising
when it was found that teeth were completely absent (Gibert et al. 2010). The lack of
tooth formation in these experiments could merely be the result of the entire posterior
pharyngeal region being malformed or missing in these embryos.
Starting late in gastrulation, in the absence of RA signaling, CNC cells, which give
rise to odontoblasts, are misspecified in the posterior pharyngeal region (Kopinke
et al. 2006). Subsequently, zebrafish teeth are completely inhibited from forming
(Gibert et al. 2010). This result provides a link between RA signaling and zebrafish
odontoblast development that is reminiscent of the effects described previously in
mammalian studies.
Interestingly, even later RA inhibition was found to have catastrophic consequences for zebrafish tooth formation. Chemically blocking RA synthesis at around
40 h post fertilization (hpf), immediately prior to the time when the first teeth are
starting to undergo the earliest stages of morphological development, also completely prevented tooth formation (compare Figs. 7.3C and 7.4A) (Gibert et al. 2010).
Remarkably, at this time, RA inhibition has no noticeable effect on the patterning of
185
other tetrapod models (e.g. Simoes-Costa and Bronner 2016) and much has also
been learned from studies of zebrafish (Hernandez-Lagunas et al. 2005). Thus, it is
important to pay particular attention to the role of CNC-derived odontoblast cells
as the function and evolution of RA signaling in tooth development in zebrafish is
further considered.
Current State of the Field
Cell Signaling Control of Tooth Formation
Early Inhibition of RA Signaling (aldh1a2 Mutants)
The first investigation into specific roles of RA signaling during zebrafish tooth
development was to test whether RA is necessary for the initiation of tooth formation. However, because of known pleiotropic roles for RA in vertebrate development,
both at early stages and late, it was necessary to gradually home in on the exact timing
of when such a tooth-specific RA requirement might be present. Given the location
of the zebrafish dentition in the posterior pharynx, it is logical that the development of teeth might be particularly sensitive to the proper formation of the posterior
pharyngeal region in general, especially since pharyngeal patterning had been shown
previously to require RA signaling at very early developmental stages. Specifically, in
the absence of RA signaling via genetic inactivation of the RA synthesizing enzyme
aldh1a2 or by pharmacological inhibition of RA synthesis by DEAB or other methods of RA inhibition very early in development, the posterior pharyngeal region is
greatly disrupted in zebrafish embryos (Begemann et al. 2001, 2004; Grandel et al.
2002). Consequently, when the effect on tooth formation was directly examined in
aldh1a2 mutants or by early chemical RA inhibition, it perhaps wasn’t surprising
when it was found that teeth were completely absent (Gibert et al. 2010). The lack of
tooth formation in these experiments could merely be the result of the entire posterior
pharyngeal region being malformed or missing in these embryos.
Starting late in gastrulation, in the absence of RA signaling, CNC cells, which give
rise to odontoblasts, are misspecified in the posterior pharyngeal region (Kopinke
et al. 2006). Subsequently, zebrafish teeth are completely inhibited from forming
(Gibert et al. 2010). This result provides a link between RA signaling and zebrafish
odontoblast development that is reminiscent of the effects described previously in
mammalian studies.
Interestingly, even later RA inhibition was found to have catastrophic consequences for zebrafish tooth formation. Chemically blocking RA synthesis at around
40 h post fertilization (hpf), immediately prior to the time when the first teeth are
starting to undergo the earliest stages of morphological development, also completely prevented tooth formation (compare Figs. 7.3C and 7.4A) (Gibert et al. 2010).
Remarkably, at this time, RA inhibition has no noticeable effect on the patterning of