7 Retinoic Acid Signaling and the Zebrafish Dentition …
183
It wasn’t until the beginning of the genome sequencing era in the late 1980s and
early 1990s that it became apparent how many genes and gene functions are shared
in a recognizable state across all vertebrates, and thus how powerful a fish model of
embryonic development could be. For example, a zebrafish genetic mutation initially
named spadetail, based on its malformed embryonic phenotype, was isolated in an
early mutagenesis screen (Kimmel et al. 1989). In spadetail zebrafish embryos, it
was straightforward to observe how the gene defect caused abnormal mesoderm
formation in the developing trunk and tail. The gene responsible was later identified
as a member of the T-box class of transcription factors, a gene family shared across
vertebrates, that exhibits very similar control of early embryonic development in
divergent species including mammals (Griffin et al. 1998; Ahn et al. 2012).
With numerous similar studies soon focusing on different mutant phenotypes and
genes, zebrafish were quickly established as a species particularly well suited for
the discovery and validation of the function of essential, and usually evolutionarily
conserved, genes controlling vertebrate embryonic development. At the same time,
due to the phylogenetic position of zebrafish as distantly related to mammals but still
within the vertebrates, studies of zebrafish development allowed extensive knowledge
to be gathered about evolutionary changes in vertebrate development. A few of
these investigations included a focus on various aspects of RA signaling control of
embryonic development, such as in the central nervous system (Maves and Kimmel
2005), the limbs (Grandel et al. 2002; Gibert et al. 2006), and the heart (Waxman et al.
2008). Specific studies looking into RA regulation of zebrafish tooth development
were not long to follow.
Minnows, like zebrafish and other members of the order Cypriniformes such as
carp, have reduced dentition relative to most other fish lineages. No member of
this order is known to have teeth on their oral jaws (Britz et al. 2009), and for those
species that have teeth at all, the dentition is restricted to the pharyngeal region (Stock
2001). The zebrafish dentition is particularly reduced, as it only possesses teeth in
the posterior, ventral pharynx (Fig. 7.3). Given this relatively limited dentition and
its internal, pharyngeal location in the embryo and larva, it perhaps isn’t surprising
that teeth went largely unnoticed in early zebrafish developmental research. Tooth
germs and teeth were mentioned in a few studies (e.g. Hatta et al. 1991; Schilling
et al. 1996), but it wasn’t until almost the turn of the twenty-first century that they
were studied in detail (Huysseune et al. 1998; van der Heyden and Huysseune 2000).
Once the cellular and anatomical development of zebrafish teeth were well described,
gene expression and functional investigations were rapidly undertaken.
Gene Expression and Function in Zebrafish Tooth
Development
The first studies of gene expression and function in zebrafish tooth development
mostly focused on genes that had been already identified in mammals as important
183
It wasn’t until the beginning of the genome sequencing era in the late 1980s and
early 1990s that it became apparent how many genes and gene functions are shared
in a recognizable state across all vertebrates, and thus how powerful a fish model of
embryonic development could be. For example, a zebrafish genetic mutation initially
named spadetail, based on its malformed embryonic phenotype, was isolated in an
early mutagenesis screen (Kimmel et al. 1989). In spadetail zebrafish embryos, it
was straightforward to observe how the gene defect caused abnormal mesoderm
formation in the developing trunk and tail. The gene responsible was later identified
as a member of the T-box class of transcription factors, a gene family shared across
vertebrates, that exhibits very similar control of early embryonic development in
divergent species including mammals (Griffin et al. 1998; Ahn et al. 2012).
With numerous similar studies soon focusing on different mutant phenotypes and
genes, zebrafish were quickly established as a species particularly well suited for
the discovery and validation of the function of essential, and usually evolutionarily
conserved, genes controlling vertebrate embryonic development. At the same time,
due to the phylogenetic position of zebrafish as distantly related to mammals but still
within the vertebrates, studies of zebrafish development allowed extensive knowledge
to be gathered about evolutionary changes in vertebrate development. A few of
these investigations included a focus on various aspects of RA signaling control of
embryonic development, such as in the central nervous system (Maves and Kimmel
2005), the limbs (Grandel et al. 2002; Gibert et al. 2006), and the heart (Waxman et al.
2008). Specific studies looking into RA regulation of zebrafish tooth development
were not long to follow.
Minnows, like zebrafish and other members of the order Cypriniformes such as
carp, have reduced dentition relative to most other fish lineages. No member of
this order is known to have teeth on their oral jaws (Britz et al. 2009), and for those
species that have teeth at all, the dentition is restricted to the pharyngeal region (Stock
2001). The zebrafish dentition is particularly reduced, as it only possesses teeth in
the posterior, ventral pharynx (Fig. 7.3). Given this relatively limited dentition and
its internal, pharyngeal location in the embryo and larva, it perhaps isn’t surprising
that teeth went largely unnoticed in early zebrafish developmental research. Tooth
germs and teeth were mentioned in a few studies (e.g. Hatta et al. 1991; Schilling
et al. 1996), but it wasn’t until almost the turn of the twenty-first century that they
were studied in detail (Huysseune et al. 1998; van der Heyden and Huysseune 2000).
Once the cellular and anatomical development of zebrafish teeth were well described,
gene expression and functional investigations were rapidly undertaken.
Gene Expression and Function in Zebrafish Tooth
Development
The first studies of gene expression and function in zebrafish tooth development
mostly focused on genes that had been already identified in mammals as important