Craniofacial Development and
17 Disorders—Contributions of Xenopus
Ashwin Lokapally and Hazel Sive
CONTENTS
17.1. What Is Craniofacial Development? ........................................................................................................................ 245
17.2. Xenopus as a Model for Craniofacial Development ................................................................................................ 245
17.3. Cells That Make Up the Craniofacial Skeleton: The Neural Crest .......................................................................... 246
17.3.1. Neural Crest Cells—Contributions from Xenopus.................................................................................... 246
17.3.2. Cranial Neural Crest Cells ........................................................................................................................ 246
17.4. The Mouth Is an Essential Facial Structure ............................................................................................................. 246
17.4.1. Mouth Formation Is Conserved ................................................................................................................ 246
17.4.2. Cellular Organization of the Mouth .......................................................................................................... 247
17.4.3. Mouth Formation—Xenopus as a Model.................................................................................................. 247
17.5. The Extreme Anterior Domain Is a Craniofacial Signaling Center ......................................................................... 249
17.5.1. Identif cation of the Extreme Anterior Domain ........................................................................................ 249
17.5.2. Craniofacial Organizer Function of the Extreme Anterior Domain .......................................................... 249
17.6. Landscape of Craniofacial Anomalies ..................................................................................................................... 249
17.6.1. Craniofacial Anomalies Are Associated with Multiple Factors ................................................................ 249
17.6.2. Classes of Craniofacial Anomalies ........................................................................................................... 250
17.7. Treatment of Craniofacial Anomalies ...................................................................................................................... 251
17.7.1. Surgical Approaches ................................................................................................................................. 251
17.7.2. Tissue Engineering Approaches ................................................................................................................ 251
17.7.3. Cell-Based Approaches ............................................................................................................................. 252
17.8. Contribution of Xenopus to Understanding Craniofacial Anomalies ....................................................................... 252
17.9. Conclusion ............................................................................................................................................................... 252
References............................................................................................................................................................................ 253
17.1. WHAT IS CRANIOFACIAL DEVELOPMENT? the head and face (Twigg and Wilkie 2015), and more than
700 distinct craniofacial anomalies have been identif ed
The term craniofacial is used to defne the bones of the thus far (Terrazas et al. 2017).
cranium (head) and the face, and the abnormalities associated with it are known as craniofacial anomalies (CFAs).
The craniofacial complex is a three-dimensional structure 17.2. XENOPUS AS A MODEL FOR
consisting of the cranium, sense organs (i.e. eyes, ears,
CRANIOFACIAL DEVELOPMENT
nose, tongue), mouth, facial bones, connective tissue, and
the peripheral nerves (Wilkie and Morriss-Kay 2001; Chai In Xenopus, embryos develop externally to the mother, and
and Maxson 2006). In humans, initial facial development the facial primordium is readily visible, facilitating live morextends from week 3 to week 12 of gestation (Som and phological observations and imaging. Since frog genes are
Naidich 2013). The formation of the head and face involves conserved with those of mammals, application of molecular
a series of coordinated events that include the primary gain and loss-of-function assays and the rapid development of
embryonic tissues, ectoderm, endoderm, and mesoderm, embryos make assays rapid and accessible to imaging. Large
using multiple signaling cues to regulate proliferation, clutches of at least 500 eggs make for signifcant sample sizes.
migration, and differentiation. A population of cells termed Loss-of-Function (LOF) can be elicited by injecting antisense,
neural crest gives rise to the skull, face, jaw, and cartilages morpholino-oligonucleotides (MOs) into the one- to two-cell
of ears and nose. Craniofacial development is susceptible embryos (Nutt et al. 2001), using RNAi (Nakano et al. 2000),
to genetic and environmental perturbations, with orofacial and inserting mutations obtained through CRISPR-mediated
anomalies appearing in 1 of 700 live births (Yoon, Pham, gene editing, including analysis of F0 animals (Nakayama et
and Dipple 2016), while craniofacial anomalies result in al. 2013; Guo et al. 2014; Willsey et al. 2018). We have develapproximately one-third of all congenital birth defects of oped a technique to limit LOF or gain-of-function (GOF) to
DOI: 10.1201/9781003050230-20
245
17 Disorders—Contributions of Xenopus
Ashwin Lokapally and Hazel Sive
CONTENTS
17.1. What Is Craniofacial Development? ........................................................................................................................ 245
17.2. Xenopus as a Model for Craniofacial Development ................................................................................................ 245
17.3. Cells That Make Up the Craniofacial Skeleton: The Neural Crest .......................................................................... 246
17.3.1. Neural Crest Cells—Contributions from Xenopus.................................................................................... 246
17.3.2. Cranial Neural Crest Cells ........................................................................................................................ 246
17.4. The Mouth Is an Essential Facial Structure ............................................................................................................. 246
17.4.1. Mouth Formation Is Conserved ................................................................................................................ 246
17.4.2. Cellular Organization of the Mouth .......................................................................................................... 247
17.4.3. Mouth Formation—Xenopus as a Model.................................................................................................. 247
17.5. The Extreme Anterior Domain Is a Craniofacial Signaling Center ......................................................................... 249
17.5.1. Identif cation of the Extreme Anterior Domain ........................................................................................ 249
17.5.2. Craniofacial Organizer Function of the Extreme Anterior Domain .......................................................... 249
17.6. Landscape of Craniofacial Anomalies ..................................................................................................................... 249
17.6.1. Craniofacial Anomalies Are Associated with Multiple Factors ................................................................ 249
17.6.2. Classes of Craniofacial Anomalies ........................................................................................................... 250
17.7. Treatment of Craniofacial Anomalies ...................................................................................................................... 251
17.7.1. Surgical Approaches ................................................................................................................................. 251
17.7.2. Tissue Engineering Approaches ................................................................................................................ 251
17.7.3. Cell-Based Approaches ............................................................................................................................. 252
17.8. Contribution of Xenopus to Understanding Craniofacial Anomalies ....................................................................... 252
17.9. Conclusion ............................................................................................................................................................... 252
References............................................................................................................................................................................ 253
17.1. WHAT IS CRANIOFACIAL DEVELOPMENT? the head and face (Twigg and Wilkie 2015), and more than
700 distinct craniofacial anomalies have been identif ed
The term craniofacial is used to defne the bones of the thus far (Terrazas et al. 2017).
cranium (head) and the face, and the abnormalities associated with it are known as craniofacial anomalies (CFAs).
The craniofacial complex is a three-dimensional structure 17.2. XENOPUS AS A MODEL FOR
consisting of the cranium, sense organs (i.e. eyes, ears,
CRANIOFACIAL DEVELOPMENT
nose, tongue), mouth, facial bones, connective tissue, and
the peripheral nerves (Wilkie and Morriss-Kay 2001; Chai In Xenopus, embryos develop externally to the mother, and
and Maxson 2006). In humans, initial facial development the facial primordium is readily visible, facilitating live morextends from week 3 to week 12 of gestation (Som and phological observations and imaging. Since frog genes are
Naidich 2013). The formation of the head and face involves conserved with those of mammals, application of molecular
a series of coordinated events that include the primary gain and loss-of-function assays and the rapid development of
embryonic tissues, ectoderm, endoderm, and mesoderm, embryos make assays rapid and accessible to imaging. Large
using multiple signaling cues to regulate proliferation, clutches of at least 500 eggs make for signifcant sample sizes.
migration, and differentiation. A population of cells termed Loss-of-Function (LOF) can be elicited by injecting antisense,
neural crest gives rise to the skull, face, jaw, and cartilages morpholino-oligonucleotides (MOs) into the one- to two-cell
of ears and nose. Craniofacial development is susceptible embryos (Nutt et al. 2001), using RNAi (Nakano et al. 2000),
to genetic and environmental perturbations, with orofacial and inserting mutations obtained through CRISPR-mediated
anomalies appearing in 1 of 700 live births (Yoon, Pham, gene editing, including analysis of F0 animals (Nakayama et
and Dipple 2016), while craniofacial anomalies result in al. 2013; Guo et al. 2014; Willsey et al. 2018). We have develapproximately one-third of all congenital birth defects of oped a technique to limit LOF or gain-of-function (GOF) to
DOI: 10.1201/9781003050230-20
245