246
Xenopus
the extreme anterior domain (EAD) through facial transplants
(Jacox, Dickinson, and Sive 2014). GOF can be temporally
restricted using heat shock constructs in transient transgenic
embryos (Dickinson and Sive 2009). These features make
Xenopus an excellent model system.
One key question is whether the frog is a faithful model
for human face development. Overall, it appears that vertebrate craniofacial development is well conserved (Brugmann
et al. 2007; Blum, Schweickert et al. 2014; Blum, Feistel et al.
2014). The steps in Xenopus face formation appear similar
to those described in mouse, chicken, and zebraf sh systems
(Szabo-Rogers et al. 2010; Mork and Crump 2015; Chen
et al. 2017; Fish 2019). In frogs and mammals, craniofacial
development involves migratory neural crest in conjunction with endoderm and mesoderm. These tissues form the
pharyngeal or branchial arches, and a series of prominences
originate from these to form the different regions of the face
(Gilbert 2010). The mouth forms at the center of the prominence originating from the frst pharyngeal arch but does not
include tissue from the neural crest (Jacox, Sindelka et al.
2014). The early stages of face formation that can be readily
studied in frogs are diffcult to study in mammals, as the
developing face is obscured by the large forebrain. Although
it is true that every animal develops somewhat differently,
the frog system allows for greater insight than any other system and is one of the most accessible vertebrate models for
analysis of craniofacial development. Assays, ideas, and new
signaling connections can suggest productive approaches in
amniote models and contribute new information to the craniofacial community. As we will explore, Xenopus laevis
and Xenopus tropicalis are yielding insights into craniofacial anomalies, informing underlying mechanisms and diagnosis of affected people.
17.3. CELLS THAT MAKE UP THE CRANIOFACIAL
SKELETON: THE NEURAL CREST
We present here a concise view of neural crest cell contribution to craniofacial development.
17.3.1. NEURAL CREST CELLS—
CONTRIBUTIONS FROM XENOPUS
Much of the craniofacial skeleton arises from neural crest
cells (NCCs). These are multipotent cells that arise at the
border between the neural and the non-neural ectoderm
during neural tube formation. Formation of the NCCs is
initiated at gastrulation by induction of neural crest progenitors at the neural plate border (NPB) (LaBonne 1998;
Aybar and Mayor 2002). As the neural plate closes to form
the neural tube, neural crest progenitors delaminate, lose
their epithelial nature, and become migratory mesenchymal
cells. Migratory NCCs divide into four major populations
based on their position, the cranial, cardiac, vagal, and trunk
neural crests, with each contributing to distinct cell and tissue populations (Trainor 2014). Much understanding of
molecular mechanisms leading to NCC formation and differentiation has come from studies in Xenopus (reviewed in
(Meulemans and Bronner-Fraser 2004; Betancur, BronnerFraser, and Sauka-Spengler 2010; Klymkowsky, Rossi, and
Artinger 2010; Barriga et al. 2015)).
17.3.2. CRANIAL NEURAL CREST CELLS
The cranial neural crest (CNC) population gives rise to
craniofacial structures under the action of multiple signaling pathways and transcription factors (Figure 17.1A).
Subsequent to initial induction of CNC, migrating cranial
neural crest cells arising from the frst arch and frontonasal ectodermal zone give rise to the bones of the head and
face (Figure 17.1B). Signaling events between the neural
crest, ectoderm, and endoderm regulate cranial neural crest
migration (Olesnicky Killian, Birkholz, and Artinger 2009;
Theveneau and Mayor 2010; Theveneau et al. 2010; Kalcheim
2018; Duband 2006), proliferation, and differentiation into
cartilage and bone (Jandzik et al. 2014; Monsoro-Burq 2015;
Green, Simoes-Costa, and Bronner 2015; Shao et al. 2015; da
Costa, Trentin, and Calloni 2018). Defects in signaling pathways between the ectoderm, endoderm, and neural crest are
associated with craniofacial anomalies in humans (Trainor
2010; Huh and Ornitz 2010; Curtin et al. 2011). Cranial neural crest cells are further subdivided into forebrain, midbrain,
and hindbrain sub-populations. Gradients of FGF, BMP, and
WNT proteins specify these cells, and a hox gene expression gradient along the antero-posterior axis divides the CNC
into two different domains (Gavalas et al. 2001; Trainor and
Krumlauf 2001; Couly et al. 2002; Steventon and Mayor
2012; Raible and Ragland 2005; da Costa, Trentin, and
Calloni 2018). Interestingly, a hox-negative CNC population
at the rostral end produces the entire facial skeleton (Couly,
Creuzet et al. 2002; Creuzet, Couly, and Le Douarin 2005).
Although both hox-negative and hox -positive CNC domains
are able to generate cartilage, only the anterior region forms
the bones of the facial skeleton and palate (Vieux-Rochas
et al. 2013; Creuzet, Couly, and Le Douarin 2005; Dickinson
and Sive 2007). Defects in CNC development are associated
with human birth disorders such as cleft lip, cleft palate, craniosynostosis, craniofacial-microsomias, and ciliopathies, as
will be discussed in Section 7.
17.4. THE MOUTH IS AN ESSENTIAL
FACIAL STRUCTURE
17.4.1. MOUTH FORMATION IS CONSERVED
In vertebrates, the mouth is an integral part of the craniofacial system and is essential for eating and life. All multicellular animals have a mouth (Chen et al. 2017), and mouth
development appears to have arisen once during evolution
(Chen et al. 2017). Our group identifed a cohort of genes
during Xenopus mouth development (Dickinson and Sive
2009), many of which are expressed in similar regions in
other species (Chen et al. 2017). Xenopus is proving an
Xenopus
the extreme anterior domain (EAD) through facial transplants
(Jacox, Dickinson, and Sive 2014). GOF can be temporally
restricted using heat shock constructs in transient transgenic
embryos (Dickinson and Sive 2009). These features make
Xenopus an excellent model system.
One key question is whether the frog is a faithful model
for human face development. Overall, it appears that vertebrate craniofacial development is well conserved (Brugmann
et al. 2007; Blum, Schweickert et al. 2014; Blum, Feistel et al.
2014). The steps in Xenopus face formation appear similar
to those described in mouse, chicken, and zebraf sh systems
(Szabo-Rogers et al. 2010; Mork and Crump 2015; Chen
et al. 2017; Fish 2019). In frogs and mammals, craniofacial
development involves migratory neural crest in conjunction with endoderm and mesoderm. These tissues form the
pharyngeal or branchial arches, and a series of prominences
originate from these to form the different regions of the face
(Gilbert 2010). The mouth forms at the center of the prominence originating from the frst pharyngeal arch but does not
include tissue from the neural crest (Jacox, Sindelka et al.
2014). The early stages of face formation that can be readily
studied in frogs are diffcult to study in mammals, as the
developing face is obscured by the large forebrain. Although
it is true that every animal develops somewhat differently,
the frog system allows for greater insight than any other system and is one of the most accessible vertebrate models for
analysis of craniofacial development. Assays, ideas, and new
signaling connections can suggest productive approaches in
amniote models and contribute new information to the craniofacial community. As we will explore, Xenopus laevis
and Xenopus tropicalis are yielding insights into craniofacial anomalies, informing underlying mechanisms and diagnosis of affected people.
17.3. CELLS THAT MAKE UP THE CRANIOFACIAL
SKELETON: THE NEURAL CREST
We present here a concise view of neural crest cell contribution to craniofacial development.
17.3.1. NEURAL CREST CELLS—
CONTRIBUTIONS FROM XENOPUS
Much of the craniofacial skeleton arises from neural crest
cells (NCCs). These are multipotent cells that arise at the
border between the neural and the non-neural ectoderm
during neural tube formation. Formation of the NCCs is
initiated at gastrulation by induction of neural crest progenitors at the neural plate border (NPB) (LaBonne 1998;
Aybar and Mayor 2002). As the neural plate closes to form
the neural tube, neural crest progenitors delaminate, lose
their epithelial nature, and become migratory mesenchymal
cells. Migratory NCCs divide into four major populations
based on their position, the cranial, cardiac, vagal, and trunk
neural crests, with each contributing to distinct cell and tissue populations (Trainor 2014). Much understanding of
molecular mechanisms leading to NCC formation and differentiation has come from studies in Xenopus (reviewed in
(Meulemans and Bronner-Fraser 2004; Betancur, BronnerFraser, and Sauka-Spengler 2010; Klymkowsky, Rossi, and
Artinger 2010; Barriga et al. 2015)).
17.3.2. CRANIAL NEURAL CREST CELLS
The cranial neural crest (CNC) population gives rise to
craniofacial structures under the action of multiple signaling pathways and transcription factors (Figure 17.1A).
Subsequent to initial induction of CNC, migrating cranial
neural crest cells arising from the frst arch and frontonasal ectodermal zone give rise to the bones of the head and
face (Figure 17.1B). Signaling events between the neural
crest, ectoderm, and endoderm regulate cranial neural crest
migration (Olesnicky Killian, Birkholz, and Artinger 2009;
Theveneau and Mayor 2010; Theveneau et al. 2010; Kalcheim
2018; Duband 2006), proliferation, and differentiation into
cartilage and bone (Jandzik et al. 2014; Monsoro-Burq 2015;
Green, Simoes-Costa, and Bronner 2015; Shao et al. 2015; da
Costa, Trentin, and Calloni 2018). Defects in signaling pathways between the ectoderm, endoderm, and neural crest are
associated with craniofacial anomalies in humans (Trainor
2010; Huh and Ornitz 2010; Curtin et al. 2011). Cranial neural crest cells are further subdivided into forebrain, midbrain,
and hindbrain sub-populations. Gradients of FGF, BMP, and
WNT proteins specify these cells, and a hox gene expression gradient along the antero-posterior axis divides the CNC
into two different domains (Gavalas et al. 2001; Trainor and
Krumlauf 2001; Couly et al. 2002; Steventon and Mayor
2012; Raible and Ragland 2005; da Costa, Trentin, and
Calloni 2018). Interestingly, a hox-negative CNC population
at the rostral end produces the entire facial skeleton (Couly,
Creuzet et al. 2002; Creuzet, Couly, and Le Douarin 2005).
Although both hox-negative and hox -positive CNC domains
are able to generate cartilage, only the anterior region forms
the bones of the facial skeleton and palate (Vieux-Rochas
et al. 2013; Creuzet, Couly, and Le Douarin 2005; Dickinson
and Sive 2007). Defects in CNC development are associated
with human birth disorders such as cleft lip, cleft palate, craniosynostosis, craniofacial-microsomias, and ciliopathies, as
will be discussed in Section 7.
17.4. THE MOUTH IS AN ESSENTIAL
FACIAL STRUCTURE
17.4.1. MOUTH FORMATION IS CONSERVED
In vertebrates, the mouth is an integral part of the craniofacial system and is essential for eating and life. All multicellular animals have a mouth (Chen et al. 2017), and mouth
development appears to have arisen once during evolution
(Chen et al. 2017). Our group identifed a cohort of genes
during Xenopus mouth development (Dickinson and Sive
2009), many of which are expressed in similar regions in
other species (Chen et al. 2017). Xenopus is proving an