The Role of the Visceral Mesoderm in the Development of the Gastrointestinal Tract

Early development of the lateral plate mesoderm. Diagram of a cross section through a Xenopus neurula. Dorsal is to the top. The visceral mesoderm (VM, orange) and somatic mesoderm (SM, brown) are 2 leaflets that are in continuity with the somitic (S) mesoderm at the lateral plate. They are situated between the endoderm (yellow) and the ectoderm (e). Endodermal Hh signals produced by the future gut endoderm induce BMP-4 expression in the VM. The ensuing molecular decisions in the VM are shown; BMP-4 activates expression of the transcription factor FoxF1. One of the early actions of FoxF1 is the inhibition of Iroquois (Iro) in the VM, thereby inhibiting the somatic mesoderm program in cells destined to become VM. FoxF1 also induces BMP4 expression. N, notochord; S, somitic mesoderm. Modified and reprinted with permission from El-Hodiri et al.¹⁶⁷

Schematic section through the adult GI tract. Each quadrant represents a different segment of the GI tract, from the stomach to the colon; mesenchymal structures are shown in detail. The following structures are conserved along the craniocaudal axis: the lamina propria mesenchyme, the muscularis mucosa, mesenchymal blood vessels, the ISEMFs, the Meissner's plexus, the longitudinal inner and circular outer layers of the muscularis propria, the Auerbach plexus, and the serosa. These structures are represented in all quadrants. The stomach (left upper quadrant) is characterized by an additional oblique inner layer of the muscularis propria. Its epithelium is characterized by deep, bifid glands (g) that fulfill the secretory and endocrine functions of the stomach. In the duodenum (left lower quadrant), the submucosa is recognizable by its abundance of Brunner's glands, which are epithelial in origin. Consistent with the rest of the small intestine, the duodenal epithelium is characterized by the presence of crypts and villi. The other 2 components of the small intestine are the jejunum and ileum (right lower quadrant). The ileum is characterized by the presence of Peyer's patches, which are large lymphoid aggregates in the submucosa. In the jejunum and the colon, the lymphoid aggregates are located in the mucosal tunic rather than in the submucosa. ISEMFs are adjacent to the epithelium lining crypts and villi. Finally, in the colon (right upper quadrant), the epithelium is characterized by straight glands (g) but does not include villi or crypts. The characteristic feature of the colon mesenchyme is the thinner, circular outer layer of the muscularis propria. Modified and reprinted with permission from T. Caceci, DVM (Virginia-Maryland Regional College of Veterinary Medicine).

Early specification of mesodermal tissues. Diagram representing the action of BMP-4 as a ventralizing morphogen. In most species, BMP signaling in combination with other pathways patterns the early mesoderm into different cell types. In Xenopus, zebrafish, and other species, BMP-4 acts as a mesodermal morphogen that specifies different mesodermal derivatives along the dorsoventral axis: blood, muscle, kidney, and notochord. According to its concentration at a given time and place, BMP-4 confers a distinct dorsoventral identity to different parts of the mesoderm. In Xenopus and zebrafish, the Vent homeobox-containing genes act downstream of BMP-4 to convert the signaling gradient into distinct cellular responses. The essential VM transcription factor FoxF1 is also downstream from BMP-4.³⁶ Although the exact “dose” of BMP-4 that regulates the specification and differentiation of lateral plate mesoderm has not been established, integration of combinatorial regulatory signals is an important paradigm that is repeated throughout development. The Vent pathway of ventral mesoderm specification has been confirmed in zebrafish and amphioxus, but mammalian studies are still pending.

The role of biniou/FoxF1 is conserved across species. Invertebrate example: Drosophila melanogaster. (A) Expression of VM marker fascilin III (Fas III) in stage 12 Drosophila embryos (arrow). Anterior is to the left. (B) The VM marker Fas III is absent in the biniou mutant, which lacks biniou, the Drosophila orthologue of FoxF1.³⁷ Vertebrate example: effect of FoxF1 loss of function in Xenopus laevis. (C) Ventral view of a normal gut coiling in a 5-day-old Xenopus embryo. (D) Both coiling and elongation are severely impaired when FoxF1 is knocked down using antisense morpholino-oligonucleotides.³⁶ The black cells in C are melanocytes; they are absent in the knockdown embryos. Cranial is to the top.

Transcription factors and signaling pathways during anterior-posterior and radial patterning. (A) Early mesoderm formation is under the control of the conserved homeobox genes. Studies in Drosophila have allowed the identification of some early steps in VM formation implicating homeobox genes. These include tinman (Nkx2.3-2.9), bagpipe (Nkx3.2), and other genes.¹⁸¹ Tinman is required for mesoderm formation, whereas its downstream target bagpipe promotes VM development. Vimar, a downstream factor from bagpipe, further specifies VM development.¹⁸⁴ In Xenopus, 2 bagpipe homologues have been identified and their expression has been analyzed, suggesting that these the role of these homeobox genes is conserved in development; Xbap (Nkx3.2) is expressed in the foregut musculature of Xenopus and mouse and zampogna in the musculature of the Xenopus midgut, with some overlap in the posterior foregut mesoderm. Expression of zampogna in the posterior indicates early anterior-posterior differentiation in Xenopus. Vimar is indicated in green in the figure because it has been identified in Drosophila but not in Xenopus. Foxf1, downstream of Nkx3.2 (bagpipe), is first expressed in the late gastrula and then throughout the VM of the embryo and the adult. Although the exact relationship between these genes and major signaling pathways is incompletely understood, in the early embryo, FGFs, Wnts, and retinoic acid (RA) promote posterior fates while inhibiting anterior fates. Figure of Xenopus embryo at stage 28: © 1994 Pieter D. Nieuwkoop and J. Faber.¹⁸⁷ (B) Spatial distribution of selected mesodermal forkhead and homeobox genes involved in vertebrate gastrointestinal differentiation, using mouse nomenclature. Not shown is Nkx2.5, which specifies the pylorus in the caudal segment of the foregut. Anterior is to the left, dorsal to the top. Diagram courtesy of Aaron M. Zorn. l, liver; p, pancreas.

Heterotopic recombinations of mesoderm and endoderm. Detailed dissection and recombination experiments in neurula-stage embryos (during regional specification of the primitive gut tube) of Xenopus laevis, the African clawed frog (equivalent to mice with 7–8 somites or a 22-day-old human embryo), have shown that early endodermal regionalization depends on mesodermal signals. Although the nature of these signals is incompletely understood, the major signaling pathways appear to be involved. For example, in the chick, the endoderm needs instructive BMP/activin family signals from the lateral plate mesoderm to differentiate⁶⁴; retinoic acid (RA) in the VM confers a posteriorizing gradient to the developing GI tract.¹⁹⁰ In mouse, Xenopus, and zebrafish, mesodermal Wnt signals confer temporally regulated anteroposterior information to the adjacent endoderm. Wnt antagonists in the anterior VM are necessary for stomach development (anterior)⁸³ and Wnts probably posteriorize the VM, but this has not been shown. Because this paradigm has been studied in detail in liver and pancreas development, we refer the reader directly to those studies. Modified and reprinted with permission from Horb and Slack.¹⁸⁹

Molecular cross talk between the VM and the endoderm during crypt-villus formation and in the adult. (A) Signaling pathways during crypt-villus formation. Hh ligands signal to the mesenchyme. In response, probably via ISEMFs, mesodermal Wnt ligands promote proliferation in the intervillus space and in the villus (starting at E16.25) and BMP signals regulate proliferation of the epithelium. Arrows indicate direction of signal. (B) In the adult with a fully developed crypt-villus axis. On the left, pathways expressed in the mesenchyme; on the right, pathways expressed in the epithelium. The signals in the adult likely occur simultaneously. Modified and reprinted with permission from Crosnier et al.¹⁹⁴