Modeling Digestive and
18 Respiratory System Development
and Disease in Xenopus
Scott A. Rankin and Aaron M. Zorn
CONTENTS
18.1. Introduction.............................................................................................................................................................. 259
18.2. Temporal Overview of Xenopus Endoderm Organogenesis .................................................................................... 260
18.3. The Molecular Basis of Endoderm Formation ......................................................................................................... 261
18.3.1. Maternal Pre-Pattern Phase ....................................................................................................................... 262
18.3.2. Endoderm Induction Phase ....................................................................................................................... 263
18.3.3. Endoderm Commitment Phase ................................................................................................................. 263
18.4. Endoderm Patterning ............................................................................................................................................... 263
18.4.1. Gastrula-Stage Endoderm Patterning ........................................................................................................ 263
18.4.2. Neurula-Stage Endoderm Patterning ........................................................................................................ 265
18.5. Induction of Endoderm Organ Fate ......................................................................................................................... 266
18.5.1. Pharyngeal Endoderm and Thyroid .......................................................................................................... 268
18.5.2. Lung, Trachea, and Esophagus ................................................................................................................. 268
18.5.3. Pancreas and Liver .................................................................................................................................... 268
18.5.4. Stomach..................................................................................................................................................... 270
18.5.5. Intestine..................................................................................................................................................... 270
18.6. Concluding Remarks ................................................................................................................................................ 271
Acknowledgments ................................................................................................................................................................ 271
References............................................................................................................................................................................ 271
18.1. INTRODUCTION
mechanisms of endoderm organogenesis and congenital disThe endoderm is the innermost germ layer of the vertebrate
embryo that gives rise to the epithelial lining of the respiratory and digestive tracts as well as associated organs such
as the stomach, liver, and pancreas. Research over the past
50 years has shown that the molecular basis of endoderm
organogenesis is largely conserved from animals to human
( Zorn and Wells 2009). Among vertebrate model organisms
used for biomedical research, the experimental advantages
of Xenopus embryos have proven particularly useful in elucidating key steps in endoderm formation, patterning, and
early organogenesis. Large, externally developing Xenopus
embryos allow microsurgery, explant culture, and targeted
microinjection for tissue-restricted analysis of gene or pathway function, and pharmacological manipulation of pathway/gene activity is easily performed at any desired stage
of development. In addition, transgenics, CRISPR-mediated
genome editing, and cutting-edge genomics are widely used
in Xenopus research. The combination of all these strategies
readily permits the determination of epistatic relationships
and temporal requirements of developmental events and has
accelerated the utility of Xenopus for interrogating molecular
ease in ways that are much more challenging in mammals.
Xenopus experiments in the 1990s were the frst to reveal
the growth factor signaling pathways that induce the endoderm and mesoderm lineages in the gastrula embryo (Smith
et al. 1990; Asashima et al. 1990); these fndings directly led
to the development of protocols enabling the directed differentiation of mouse and human endoderm tissue from pluripotent stem cells (PSCs) (Kubo et al. 2004; D’Amour et al.
2005; Loh et al. 2014). Experimental embryology in Xenopus
has also been instrumental in def ning conserved signaling events between the endoderm and adjacent splanchnic
mesoderm that govern induction of cardiac progenitor cells
(Nascone and Mercola 1995). Indeed, bi-directional signaling between the endoderm and mesoderm is a conserved feature of vertebrate gut tube patterning and organogenesis; this
knowledge, determined in large part from Xenopus embryos,
has helped facilitate the generation of human PSC-derived
organoids useful for disease modeling and hopefully one day
for regenerative medicine (McCauley and Wells 2017).
Xenopus is increasingly used to study how disruptions in
developmental pathways and genes can lead to birth defects
and pediatric disease of endodermal organs such as diabetes/
DOI: 10.1201/9781003050230-21
259
18 Respiratory System Development
and Disease in Xenopus
Scott A. Rankin and Aaron M. Zorn
CONTENTS
18.1. Introduction.............................................................................................................................................................. 259
18.2. Temporal Overview of Xenopus Endoderm Organogenesis .................................................................................... 260
18.3. The Molecular Basis of Endoderm Formation ......................................................................................................... 261
18.3.1. Maternal Pre-Pattern Phase ....................................................................................................................... 262
18.3.2. Endoderm Induction Phase ....................................................................................................................... 263
18.3.3. Endoderm Commitment Phase ................................................................................................................. 263
18.4. Endoderm Patterning ............................................................................................................................................... 263
18.4.1. Gastrula-Stage Endoderm Patterning ........................................................................................................ 263
18.4.2. Neurula-Stage Endoderm Patterning ........................................................................................................ 265
18.5. Induction of Endoderm Organ Fate ......................................................................................................................... 266
18.5.1. Pharyngeal Endoderm and Thyroid .......................................................................................................... 268
18.5.2. Lung, Trachea, and Esophagus ................................................................................................................. 268
18.5.3. Pancreas and Liver .................................................................................................................................... 268
18.5.4. Stomach..................................................................................................................................................... 270
18.5.5. Intestine..................................................................................................................................................... 270
18.6. Concluding Remarks ................................................................................................................................................ 271
Acknowledgments ................................................................................................................................................................ 271
References............................................................................................................................................................................ 271
18.1. INTRODUCTION
mechanisms of endoderm organogenesis and congenital disThe endoderm is the innermost germ layer of the vertebrate
embryo that gives rise to the epithelial lining of the respiratory and digestive tracts as well as associated organs such
as the stomach, liver, and pancreas. Research over the past
50 years has shown that the molecular basis of endoderm
organogenesis is largely conserved from animals to human
( Zorn and Wells 2009). Among vertebrate model organisms
used for biomedical research, the experimental advantages
of Xenopus embryos have proven particularly useful in elucidating key steps in endoderm formation, patterning, and
early organogenesis. Large, externally developing Xenopus
embryos allow microsurgery, explant culture, and targeted
microinjection for tissue-restricted analysis of gene or pathway function, and pharmacological manipulation of pathway/gene activity is easily performed at any desired stage
of development. In addition, transgenics, CRISPR-mediated
genome editing, and cutting-edge genomics are widely used
in Xenopus research. The combination of all these strategies
readily permits the determination of epistatic relationships
and temporal requirements of developmental events and has
accelerated the utility of Xenopus for interrogating molecular
ease in ways that are much more challenging in mammals.
Xenopus experiments in the 1990s were the frst to reveal
the growth factor signaling pathways that induce the endoderm and mesoderm lineages in the gastrula embryo (Smith
et al. 1990; Asashima et al. 1990); these fndings directly led
to the development of protocols enabling the directed differentiation of mouse and human endoderm tissue from pluripotent stem cells (PSCs) (Kubo et al. 2004; D’Amour et al.
2005; Loh et al. 2014). Experimental embryology in Xenopus
has also been instrumental in def ning conserved signaling events between the endoderm and adjacent splanchnic
mesoderm that govern induction of cardiac progenitor cells
(Nascone and Mercola 1995). Indeed, bi-directional signaling between the endoderm and mesoderm is a conserved feature of vertebrate gut tube patterning and organogenesis; this
knowledge, determined in large part from Xenopus embryos,
has helped facilitate the generation of human PSC-derived
organoids useful for disease modeling and hopefully one day
for regenerative medicine (McCauley and Wells 2017).
Xenopus is increasingly used to study how disruptions in
developmental pathways and genes can lead to birth defects
and pediatric disease of endodermal organs such as diabetes/
DOI: 10.1201/9781003050230-21
259