Development of the submucous plexus in the large intestine of the mouse

Enteric Nervous System Striped Patterning and Disease: Unexplored Pathophysiology

The enteric nervous system (ENS) controls gastrointestinal (GI) motility, and defects in ENS development underlie pediatric GI motility disorders. In disorders such as Hirschsprung's disease (HSCR), pediatric intestinal pseudo-obstruction (PIPO), and intestinal neuronal dysplasia type B (INDB), ENS structure is altered with noted decreased neuronal density in HSCR and reports of increased neuronal density in PIPO and INDB. The developmental origin of these structural deficits is not fully understood. Here, we review the current understanding of ENS development and pediatric GI motility disorders incorporating new data on ENS structure. In particular, emerging evidence demonstrates that enteric neurons are patterned into circumferential stripes along the longitudinal axis of the intestine during mouse and human development. This novel understanding of ENS structure proposes new questions about the pathophysiology of pediatric GI motility disorders. If the ENS is organized into stripes, could the observed changes in enteric neuron density in HSCR, PIPO, and INDB represent differences in the distribution of enteric neuronal stripes? We review mechanisms of striped patterning from other biological systems and propose how defects in striped ENS patterning could explain structural deficits observed in pediatric GI motility disorders.

Spatial gene expression profile of Wnt-signaling components in the murine enteric nervous system

Introduction

Wnt-signaling is a key regulator of stem cell homeostasis, extensively studied in the intestinal crypt and other metazoan tissues. Yet, there is hardly any data available on the presence of Wnt-signaling components in the adult enteric nervous system (ENS) in vivo.

Methods

Therefore, we employed RNAscope HiPlex-assay, a novel and more sensitive in situ hybridization technology. By amplifying target specific signals, this technique enables the detection of low abundance, tightly regulated RNA content as is the case for Wnt-signaling components. Additionally, we compared our data to previously published physiological single cell RNA and RiboTag-based RNA sequencing analyses of enteric gliosis using data-mining approaches.

Results

Our descriptive analysis shows that several components of the multidi-mensional regulatory network of the Wnt-signaling pathway are present in the murine ENS. The transport and secretion protein for Wnt-ligands Wntless as well as canonical (Wnt3a and Wnt2b) and non-canonical Wnt-ligands (Wnt5a, Wnt7a, Wnt8b and Wnt11) are detectable within submucosal and myenteric plexus. Further, corresponding Frizzled receptors (Fzd1, Fzd3, Fzd6, and Fzd7) and regulatory signaling mediators like R-Spondin/DKK ligands are present in the ENS of the small and large intestine. Further, data mining approaches revealed, that several Wnt-related molecules are expressed by enteric glial cell clusters and are dynamically regulated during the inflammatory manifestation of enteric gliosis.

Discussion

Our results suggest, that canonical and non-canonical Wnt-signaling has a much broader impact on the mature ENS and its cellular homeostasis in health and inflammation, than previously anticipated.

Harnessing the Power of Enteric Glial Cells' Plasticity and Multipotency for Advancing Regenerative Medicine

The enteric nervous system (ENS), known as the intrinsic nervous system of the gastrointestinal tract, is composed of a diverse array of neuronal and glial cell subtypes. Fascinating questions surrounding the generation of cellular diversity in the ENS have captivated ENS biologists for a considerable time, particularly with recent advancements in cell type-specific transcriptomics at both population and single-cell levels. However, the current focus of research in this field is predominantly restricted to the study of enteric neuron subtypes, while the investigation of enteric glia subtypes significantly lags behind. Despite this, enteric glial cells (EGCs) are increasingly recognized as equally important regulators of numerous bowel functions. Moreover, a subset of postnatal EGCs exhibits remarkable plasticity and multipotency, distinguishing them as critical entities in the context of advancing regenerative medicine. In this review, we aim to provide an updated overview of the current knowledge on this subject, while also identifying key questions that necessitate future exploration.

The transition zone in Hirschsprung's bowel contains abnormal hybrid ganglia with characteristics of extrinsic nerves

The aganglionic bowel in short-segment Hirschsprung's disease is characterized both by the absence of enteric ganglia and the presence of extrinsic thickened nerve bundles (TNBs). The relationship between the TNBs and the loss of enteric ganglia is unknown. Previous studies have described decreasing numbers of ganglia with increasing density of TNBs within the transition zone (TZ) between ganglionic and aganglionic gut, and there is some evidence of spatial contact between them in this region. To determine the cellular interactions involved, we have analysed the expression of perineurial markers of TNBs and enteric ganglionic markers for both neural cells and their ensheathing telocytes across four cranio-caudal segments consisting of most proximal ganglionic to most distal aganglionic from pull-through resected colon. We show that in the TZ, enteric ganglia are abnormal, being surrounded by perineurium cells characteristic of TNBs. Furthermore, short processes of ganglionic neurons extend caudally towards the aganglionic region, where telocytes in the TNB are located between the perineurium and nerve fibres into which they project telopodes. Thus, enteric ganglia within the TZ have abnormal structural characteristics, the cellular relationships of which are shared by the TNBs. These findings will help towards elucidation of the cellular mechanisms involved in the aetiology of Hirschsprung's disease.

Neonatal development of intestinal neuroimmune interactions

Interactions between the enteric nervous system (ENS), immune system, and gut microbiota regulate intestinal homeostasis in adults, but their development and role(s) in early life are relatively underexplored. In early life, these interactions are dynamic, because the mucosal immune system, microbiota, and the ENS are developing and influencing each other. Moreover, disrupting gut microbiota and gut immune system development, and potentially ENS development, by early-life antibiotic exposure increases the risk of diseases affecting the gut. Here, we review the development of the ENS and immune/epithelial cells, and identify potential critical periods for their interactions and development. We also highlight knowledge gaps that, when addressed, may help promote intestinal homeostasis, including in the settings of early-life antibiotic exposure.

Neonatal antibiotics have long term sex-dependent effects on the enteric nervous system

Infants and young children receive the highest exposures to antibiotics globally. Although there is building evidence that early life exposure to antibiotics increases susceptibility to various diseases including gut disorders later in life, the lasting impact of early life antibiotics on the physiology of the gut and its enteric nervous system (ENS) remains unclear. We treated neonatal mice with the antibiotic vancomycin during their first 10 postnatal days, then examined potential lasting effects of the antibiotic treatment on their colons during young adulthood (6 weeks old). We found that neonatal vancomycin treatment disrupted the gut functions of young adult female and male mice differently. Antibiotic-exposed females had significantly longer whole gut transit while antibiotic-treated males had significantly lower faecal weights compared to controls. Both male and female antibiotic-treated mice had greater percentages of faecal water content. Neonatal vancomycin treatment also had sexually dimorphic impacts on the neurochemistry and Ca2+ activity of young adult myenteric and submucosal neurons. Myenteric neurons of male mice were more disrupted than those of females, while opposing changes in submucosal neurons were seen in each sex. Neonatal vancomycin also induced sustained changes in colonic microbiota and lasting depletion of mucosal serotonin (5-HT) levels. Antibiotic impacts on microbiota and mucosal 5-HT were not sex-dependent, but we propose that the responses of the host to these changes are sex-specific. This first demonstration of long-term impacts of neonatal antibiotics on the ENS, gut microbiota and mucosal 5-HT has important implications for gut function and other physiological systems of the host. KEY POINTS: Early life exposure to antibiotics can increase susceptibility to diseases including functional gastrointestinal (GI) disorders later in life. Yet, the lasting impact of this common therapy on the gut and its enteric nervous system (ENS) remains unclear. We investigated the long-term impact of neonatal antibiotic treatment by treating mice with the antibiotic vancomycin during their neonatal period, then examining their colons during young adulthood. Adolescent female mice given neonatal vancomycin treatment had significantly longer whole gut transit times, while adolescent male and female mice treated with neonatal antibiotics had significantly wetter stools. Effects of neonatal vancomycin treatment on the neurochemistry and Ca2+ activity of myenteric and submucosal neurons were sexually dimorphic. Neonatal vancomycin also had lasting effects on the colonic microbiome and mucosal serotonin biosynthesis that were not sex-dependent. Different male and female responses to antibiotic-induced disruptions of the ENS, microbiota and mucosal serotonin biosynthesis can lead to sex-specific impacts on gut function.

Interaction of the Microbiota and the Enteric Nervous System During Development

The gastrointestinal tract contains the enteric nervous system within its walls and a large community of microbial symbionts (microbiota) in its lumen. In recent years, studies have shown that these two systems that lie adjacent to each other interact. This review will summarize new data using mouse models demonstrating the concurrent development of the enteric nervous system and microbiota during key pre- and postnatal stages. It will also discuss the possible roles that microbiota play on influencing enteric nervous system development and implications of antibiotic exposure during developmental windows.

Optical Approaches to Understanding Enteric Circuits Along the Radial Axis

The gastrointestinal tract operates in a highly dynamic environment. The gut is typically exposed to continually changing and highly convoluted luminal compositions comprising not only ingested content but also a multitude of resident microbes and microbial factors. It is therefore critical that the gut is capable of distinguishing between nutritious components from noxious substances. This is facilitated by specialized cellular sensory machinery that are in place in the intestinal epithelium and the ENS. However, the specific chemosensory processes and enteric neuronal pathways that enable the gut to discern and respond appropriately to different chemicals remain unclear. A major hurdle in studying the neural processing of luminal information has been the complex spatial organization of the mucosal structures and their innervation along the radial axis. Much of our current knowledge of enteric neuronal responses to luminal stimuli stems from studies that used semi-dissected guinea pig small intestine preparations with the mucosa and submucosa removed in one-half in order to record electrical activity from exposed myenteric neurons or in the circular muscle. Building on this, we ultimately strive to work towards integrated systems with all the gut layers intact. With advanced microscopy techniques including multiphoton intravital imaging, together with transgenic technologies utilizing cell-type specific activity-dependent reporters, we stand in good stead for studying the ENS in more intact preparations and even in live animals. In this chapter, we highlight recent contributions to the knowledge of sensory gut innervation by the developing and mature ENS. We also revisit established work examining the functional connectivity between the myenteric and submucosal plexus, and discuss the methodologies that can help advance our understanding of the enteric circuitry and signaling along the mucosa-serosa axis.

Luminal short-chain fatty acids and 5-HT acutely activate myenteric neurons in the mouse proximal colon

BACKGROUND

Gastrointestinal (GI) function is critically dependent on the control of the enteric nervous system (ENS), which is situated within the gut wall and organized into two ganglionated nerve plexuses: the submucosal and myenteric plexus. The ENS is optimally positioned and together with the intestinal epithelium, is well-equipped to monitor the luminal contents such as microbial metabolites and to coordinate appropriate responses accordingly. Despite the heightened interest in the gut microbiota and its influence on intestinal physiology and pathophysiology, how they interact with the host ENS remains unclear.

METHODS

Using full-thickness proximal colon preparations from transgenic Villin-CreERT2;R26R-GCaMP3 and Wnt1-Cre;R26R-GCaMP3 mice, which express a fluorescent Ca²⁺ indicator in their intestinal epithelium or in their ENS, respectively, we examined the effects of key luminal microbial metabolites (SCFAs and 5-HT) on the mucosa and underlying enteric neurons.

KEY RESULTS

We show that the SCFAs acetate, propionate, and butyrate, as well as 5-HT can, to varying extents, acutely elicit epithelial and neuronal Ca²⁺ responses. Furthermore, SCFAs exert differential effects on submucosal and myenteric neurons. Additionally, we found that submucosal ganglia are predominantly aligned along the striations of the transverse mucosal folds in the proximal colon.

CONCLUSIONS & INFERENCES

Taken together, our study demonstrates that different microbial metabolites, including SCFAs and 5-HT, can acutely stimulate Ca²⁺ signaling in the mucosal epithelium and in enteric neurons.

Gut innervation and enteric nervous system development: a spatial, temporal and molecular tour de force

During embryonic development, the gut is innervated by intrinsic (enteric) and extrinsic nerves. Focusing on mammalian ENS development, in this Review we highlight how important the different compartments of this innervation are to assure proper gut function. We specifically address the three-dimensional architecture of the innervation, paying special attention to the differences in development along the longitudinal and circumferential axes of the gut. We review recent information about the formation of both intrinsic innervation, which is fairly well-known, as well as the establishment of the extrinsic innervation, which, despite its importance in gut-brain signaling, has received much less attention. We further discuss how external microbial and nutritional cues or neuroimmune interactions may influence development of gut innervation. Finally, we provide summary tables, describing the location and function of several well-known molecules, along with some newer factors that have more recently been implicated in the development of gut innervation.

Influence of bacterial components on the developmental programming of enteric neurons

BACKGROUND

Intestinal bacteria have been increasingly shown to be involved in early postnatal development. Previous work has shown that intestinal bacteria are necessary for the structural development and intrinsic function of the enteric nervous system in early postnatal life. Furthermore, colonization with a limited number of bacteria appears to be sufficient for the formation of a normal enteric nervous system. We tested the hypothesis that common bacterial components could influence the programming of developing enteric neurons.

METHODS

The developmental programming of enteric neurons was studied by isolating enteric neural crest-derived cells from the fetal gut of C57Bl/6 mice at embryonic day 15.5. After the establishment of the cell line, cultured enteric neuronal precursors were exposed to increasing concentrations of a panel of bacterial components including lipopolysaccharide, flagellin, and components of peptidoglycan.

KEY RESULT

Exposure to bacterial components consistently affected proportions of enteric neuronal precursors that developed into nitrergic neurons. Furthermore, flagellin and D-gamma-Glu-mDAP were found to promote the development of serotonergic neurons. Proportions of dopaminergic neurons remained unchanged. Proliferation of neuronal precursor cells was significantly increased upon exposure to lipopolysaccharide and flagellin, while no significant changes were observed in the proportion of apoptotic neuronal precursors compared to baseline with exposure to any bacterial component.

CONCLUSIONS AND INTERFACES

These findings suggest that bacterial components may influence the development of enteric neurons.

Extrinsic innervation of the pelvic organs in the lesser pelvis of human embryos

Realistic models to understand the developmental appearance of the pelvic nervous system in mammals are scarce. We visualized the development of the inferior hypogastric plexus and its preganglionic connections in human embryos at 4-8 weeks post-fertilization, using Amira 3D reconstruction and Cinema 4D-remodelling software. We defined the embryonic lesser pelvis as the pelvic area caudal to both umbilical arteries and containing the hindgut. Neural crest cells (NCCs) appeared dorsolateral to the median sacral artery near vertebra S1 at ~5 weeks and had extended to vertebra S5 1 day later. Once para-arterial, NCCs either formed sympathetic ganglia or continued to migrate ventrally to the pre-arterial region, where they formed large bilateral inferior hypogastric ganglionic cell clusters (IHGCs). Unlike more cranial pre-aortic plexuses, both IHGCs did not merge because the 'pelvic pouch', a temporary caudal extension of the peritoneal cavity, interposed. Although NCCs in the sacral area started to migrate later, they reached their pre-arterial position simultaneously with the NCCs in the thoracolumbar regions. Accordingly, the superior hypogastric nerve, a caudal extension of the lumbar splanchnic nerves along the superior rectal artery, contacted the IHGCs only 1 day later than the lumbar splanchnic nerves contacted the inferior mesenteric ganglion. The superior hypogastric nerve subsequently splits to become the superior hypogastric plexus. The IHGCs had two additional sources of preganglionic innervation, of which the pelvic splanchnic nerves arrived at ~6.5 weeks and the sacral splanchnic nerves only at ~8 weeks. After all preganglionic connections had formed, separate parts of the inferior hypogastric plexus formed at the bladder neck and distal hindgut.

Cerebral dopamine neurotrophic factor-deficiency leads to degeneration of enteric neurons and altered brain dopamine neuronal function in mice

Cerebral dopamine neurotrophic factor (CDNF) is neuroprotective for nigrostriatal dopamine neurons and restores dopaminergic function in animal models of Parkinson’s disease (PD). To understand the role of CDNF in mammals, we generated CDNF knockout mice (Cdnf^−/−), which are viable, fertile, and have a normal life-span. Surprisingly, an age-dependent loss of enteric neurons occurs selectively in the submucosal but not in the myenteric plexus. This neuronal loss is a consequence not of increased apoptosis but of neurodegeneration and autophagy. Quantitatively, the neurodegeneration and autophagy found in the submucosal plexus in duodenum, ileum and colon of the Cdnf^−/− mouse are much greater than in those of Cdnf^+/+ mice. The selective vulnerability of submucosal neurons to the absence of CDNF is reminiscent of the tendency of pathological abnormalities to occur in the submucosal plexus in biopsies of patients with PD. In contrast, the number of substantia nigra dopamine neurons and dopamine and its metabolite concentrations in the striatum are unaltered in Cdnf^−/− mice; however, there is an age-dependent deficit in the function of the dopamine system in Cdnf^−/− male mice analyzed. This is observed as D-amphetamine-induced hyperactivity, aberrant dopamine transporter function, and as increased D-amphetamine-induced dopamine release demonstrating that dopaminergic axon terminal function in the striatum of the Cdnf^−/− mouse brain is altered. The deficiencies of Cdnf^−/− mice, therefore, are reminiscent of those seen in early stages of Parkinson’s disease.

Age-associated changes of the intrinsic nervous system in relation with interstitial cells in the pre-weaning goat rumen

In this study, we investigated the neural changes and their relationships with interstitial cells (ICs) in the rumen of pre-weaning goats by transmission electron microscopy, western blot and immunofluorescence (antibody: general neuronal marker-Protein Gene Product (PGP9.5)/ IC marker-vimentin). The immunofluorescence results showed that PGP9.5-positive reaction was widely distributed in neuronal soma (NS) and nerve fibre (NF). The NSs were observed in the ganglia of the myenteric plexus (MP) but not in the submucosal plexus. The mean optical density (MOD) of the whole of PGP9.5-positive nerves and the protein expression level of PGP.5 in the rumen wall both decreased significantly with age. However an obvious increase MOD of PGP.5-positive NFs within the rumen epithelium were observed. In the MP, the nerves and ICs were interwoven to form two complex networks that gradually tightened with age. Furthermore, NSs and nerve trunks were surrounded by a ring-boundary layer consisting of several ICs that became physically closer with aging. Moreover, ICs were located nearby NFs within the ML, forming connections between ICs, smooth muscle cells and axons. This study describes the pattern of neural distribution and its association with ICs in the developing rumen which shed light on the postpartum development of ruminants.

Waardenburg syndrome with isolated deficiency of myenteric ganglion cells at the sigmoid colon and rectum

Waardenburg syndrome (WS) has the characteristic clinical features caused by the embryologic abnormality of neural crest cells. WS patients sometimes suffer from functional intestinal obstruction. When it is Hirschsprung disease (HD), the WS is diagnosed as type 4 WS. We report a case of WS which did not have myenteric ganglion cells in the sigmoid colon and rectum. Whether to diagnosis this case as type 1 or 4 WS is controversial. Moreover, this is the third report which has peristalsis failure caused by abnormal myenteric plexus. In all three cases, the eosinophils had aggregated in the myenteric layer of the transition zone. During embryonic life, enteric ganglion cells migrate to the myenteric layer from the proximal to the distal side sequentially and, subsequently, to the submucosal layer through the circular muscle. Therefore, we hypothesize that myenteric ganglion cells that had already migrated were eliminated by an eosinophil-mediated mechanism in these three cases. We believe this report may be helpful to elucidate the pathogenesis of some types of HD.

Involvement of the enteroendocrine system in intestinal obstruction

INTRODUCTION

Intestinal atresia, a rare congenital condition, is often associated with intestinal motility disorders despite adequate neonatal surgery. Previous studies have focused on changes in the enteric nervous system (ENS). We hypothesized that other components of the digestive tract could be involved in this condition.

MATERIAL AND METHODS

In a rat model of surgically-induced intestinal obstruction, a transcriptome analysis was performed to measure the global gene expression. Then, analyzes were focused on genes expressed in ENS and neuroendocrine cells. Rat fetus small intestines at different developmental stages (ED15, ED17, ED19 and ED21, (n = 22)) were studied as controls and compared to the upper and lower segments of small intestines from rat fetuses with surgically-induced obstruction (n = 14; ligature at ED18). The gene expression pattern was confirmed by immunohistochemistry, electron microscopy and RT-qPCR.

RESULTS

From ED15 to ED21, there was a physiological decrease in the gene expression of ENS markers and an increase in that of neuroendocrine genes. Regarding operated embryos, the changes in global gene expression were significantly higher in the proximal segment compared to the distal segment (18% vs. 9%). More precisely, a decrease in ENS gene expression and an increase in neuroendocrine gene expression were observed in the proximal segment compared to controls, indicating an accelerated maturation pattern. Immunohistochemistry and electron microscopy confirmed these findings.

CONCLUSION

Fetal intestinal obstruction seems to induce an accelerated maturation in the proximal segment. Moreover, neuroendocrine cells undergo significant unexpected changes, suggesting that ENS changes could be associated with other changes to induce intestinal motility disorders.

Milk Fat Globule Membrane Supplementation in Formula Modulates the Neonatal Gut Microbiome and Normalizes Intestinal Development

Breast milk has many beneficial properties and unusual characteristics including a unique fat component, termed milk fat globule membrane (MFGM). While breast milk yields important developmental benefits, there are situations where it is unavailable resulting in a need for formula feeding. Most formulas do not contain MFGM, but derive their lipids from vegetable sources, which differ greatly in size and composition. Here we tested the effects of MFGM supplementation on intestinal development and the microbiome as well as its potential to protect against Clostridium difficile induced colitis. The pup-in-a-cup model was used to deliver either control or MFGM supplemented formula to rats from 5 to 15 days of age; with mother’s milk (MM) reared animals used as controls. While CTL formula yielded significant deficits in intestinal development as compared to MM littermates, addition of MFGM to formula restored intestinal growth, Paneth and goblet cell numbers, and tight junction protein patterns to that of MM pups. Moreover, the gut microbiota of MFGM and MM pups displayed greater similarities than CTL, and proved protective against C. difficile toxin induced inflammation. Our study thus demonstrates that addition of MFGM to formula promotes development of the intestinal epithelium and microbiome and protects against inflammation.

Advances in understanding functional variations in the Hirschsprung disease spectrum (variant Hirschsprung disease)

Hirschsprung disease (HSCR) is a fairly well understood congenital, genetically based functional obstruction due to the congenital absence of ganglion cells in the distal bowel. However, although over 90% of Hirschsprung cases conform to the normally accepted histological diagnostic criteria, it has become increasingly clear that in addition to HSCR, there is a group of functional disturbances relating to a number of other congenital neurodysplastic conditions causing some degree of gastrointestinal tract malfunction. Although these represent a variety of possibly separate conditions of the enteric nervous system, this spectrum it would appear to be also influenced by similar developmental processes. The term "variant Hirschsprung" is commonly used to describe these conditions, but ganglion cells are mostly present if abnormal in number and distribution. These conditions are a problem group being amongst the most difficult to diagnose and treat with possible practical and legal consequences. The problem appears to be possibly one of definition which has proven difficult in the relative paucity of normal values, especially when correlated to age and gestation. It is the purpose of this paper to review the current position on these conditions and to explore possible shared common pathogenetic and genetic mechanisms. This article explores those conditions where a similar pathogenetic mechanisms to HSCR can be demonstrated (e.g. hypoganglionosis) as well as other neural features, which appear to represent separate conditions possibly linked to certain syndromes.

Enteric nervous system development: A crest cell's journey from neural tube to colon

The enteric nervous system (ENS) is comprised of a network of neurons and glial cells that are responsible for coordinating many aspects of gastrointestinal (GI) function. These cells arise from the neural crest, migrate to the gut, and then continue their journey to colonize the entire length of the GI tract. Our understanding of the molecular and cellular events that regulate these processes has advanced significantly over the past several decades, in large part facilitated by the use of rodents, avians, and zebrafish as model systems to dissect the signals and pathways involved. These studies have highlighted the highly dynamic nature of ENS development and the importance of carefully balancing migration, proliferation, and differentiation of enteric neural crest-derived cells (ENCCs). Proliferation, in particular, is critically important as it drives cell density and speed of migration, both of which are important for ensuring complete colonization of the gut. However, proliferation must be tempered by differentiation among cells that have reached their final destination and are ready to send axonal extensions, connect to effector cells, and begin to produce neurotransmitters or other signals. Abnormalities in the normal processes guiding ENCC development can lead to failure of ENS formation, as occurs in Hirschsprung disease, in which the distal intestine remains aganglionic. This review summarizes our current understanding of the factors involved in early development of the ENS and discusses areas in need of further investigation.

Development of the intrinsic and extrinsic innervation of the gut

The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic efferent and afferent nerves. The enteric (intrinsic) nervous system (ENS) in most regions of the gut consists of two main ganglionated layers; myenteric and submucosal ganglia, containing numerous types of enteric neurons and glial cells. Axons arising from the ENS and from extrinsic neurons innervate most layers of the gut wall and regulate many gut functions. The majority of ENS cells are derived from vagal neural crest cells (NCCs), which proliferate, colonize the entire gut, and first populate the myenteric region. After gut colonization by vagal NCCs, the extrinsic nerve fibers reach the GI tract, and Schwann cell precursors (SCPs) enter the gut along the extrinsic nerves. Furthermore, a subpopulation of cells in myenteric ganglia undergoes a radial (inward) migration to form the submucosal plexus, and the intrinsic and extrinsic innervation to the mucosal region develops. Here, we focus on recent progress in understanding the developmental processes that occur after the gut is colonized by vagal ENS precursors, and provide an up-to-date overview of molecular mechanisms regulating the development of the intrinsic and extrinsic innervation of the GI tract.

White paper on guidelines concerning enteric nervous system stem cell therapy for enteric neuropathies

Over the last 20 years, there has been increasing focus on the development of novel stem cell based therapies for the treatment of disorders and diseases affecting the enteric nervous system (ENS) of the gastrointestinal tract (so-called enteric neuropathies). Here, the idea is that ENS progenitor/stem cells could be transplanted into the gut wall to replace the damaged or absent neurons and glia of the ENS. This White Paper sets out experts' views on the commonly used methods and approaches to identify, isolate, purify, expand and optimize ENS stem cells, transplant them into the bowel, and assess transplant success, including restoration of gut function. We also highlight obstacles that must be overcome in order to progress from successful preclinical studies in animal models to ENS stem cell therapies in the clinic.

ENS Development Research Since 1983: Great Strides but Many Remaining Challenges

The first enteric nervous system (ENS) conference, organized by Marcello Costa and John Furness, was held in Adelaide, Australia in 1983. In this article, we review what was known about the development of the ENS in 1983 and then summarize some of the major advances in the field since 1983.

Development of myenteric plexus in human foetuses: a quantitative study

Maturation of neurons of the myenteric plexus (MP) of human fetal sigmoid colon was studied at various weeks of gestation (WG). There is abundant literature on the development of MP in various segments of the gut but there are fewer reports on the development of MP in human sigmoid colon which is a site of various disorders. Sigmoid colonic segments from 12 aborted foetuses aged 14-23WG were processed for NADPH histochemistry. Stereologic evaluation of the neuronal cell profiles, numerical density, number of neurons per ganglion and myenteric fraction was conducted using using imageJ software. According to gestational age, foetuses were assigned into two groups (group 1 [n=7], less than <17WG and group 2 [n=5], more than >17WG). The overall size of neuronal cell profiles in the MP was significantly increased (P<0.05). The numerical density of neurons decreased in group 2 in comparison to group 1, the number of neurons per ganglion and myenteric fraction were increased in group 2 but all these were not statistically significant. This study revealed that the maturational event increases after 17WG and extensive innervations is established at 23WG. During prenatal life there is an increase in the neuronal cell size from 14-23WG signifying maturational process. Such studies are essential for clinicians and surgeons to correlate the normal and pathologic development of the enteric nervous system.

Enteric neural crest-derived cells promote their migration by modifying their microenvironment through tenascin-C production

The enteric nervous system (ENS) is derived from vagal and sacral neural crest cells that migrate, proliferate, and differentiate into enteric neurons and glia within the gut wall. The mechanisms regulating enteric neural crest-derived cell (ENCC) migration are poorly characterized despite the importance of this process in gut formation and function. Characterization of genes involved in ENCC migration is essential to understand ENS development and could provide targets for treatment of human ENS disorders. We identified the extracellular matrix glycoprotein tenascin-C (TNC) as an important regulator of ENCC development. We find TNC dynamically expressed during avian gut development. It is absent from the cecal region just prior to ENCC arrival, but becomes strongly expressed around ENCCs as they enter the ceca and hindgut. In aganglionic hindguts, TNC expression is strong throughout the outer mesenchyme, but is absent from the submucosal region, supporting the presence of both ENCC-dependent and independent expression within the gut wall. Using rat-chick coelomic grafts, neural tube cultures, and gut explants, we show that ENCCs produce TNC and that this ECM protein promotes their migration. Interestingly, only vagal neural crest-derived ENCCs express TNC, whereas sacral neural crest-derived cells do not. These results demonstrate that vagal crest-derived ENCCs actively modify their microenvironment through TNC expression and thereby help to regulate their own migration.

Transplanted progenitors generate functional enteric neurons in the postnatal colon

Cell therapy has the potential to treat gastrointestinal motility disorders caused by diseases of the enteric nervous system. Many studies have demonstrated that various stem/progenitor cells can give rise to functional neurons in the embryonic gut; however, it is not yet known whether transplanted neural progenitor cells can migrate, proliferate, and generate functional neurons in the postnatal bowel in vivo. We transplanted neurospheres generated from fetal and postnatal intestinal neural crest-derived cells into the colon of postnatal mice. The neurosphere-derived cells migrated, proliferated, and generated neurons and glial cells that formed ganglion-like clusters within the recipient colon. Graft-derived neurons exhibited morphological, neurochemical, and electrophysiological characteristics similar to those of enteric neurons; they received synaptic inputs; and their neurites projected to muscle layers and the enteric ganglia of the recipient mice. These findings show that transplanted enteric neural progenitor cells can generate functional enteric neurons in the postnatal bowel and advances the notion that cell therapy is a promising strategy for enteric neuropathies.

Immunophenotypic characterization of enteric neural crest cells in the developing avian colorectum

BACKGROUND

The enteric nervous system (ENS) develops from neural crest-derived cells that migrate along the intestine to form two plexuses of neurons and glia. While the major features of ENS development are conserved across species, minor differences exist, especially in the colorectum. Given the embryologic and disease-related importance of the distal ENS, the aim of this study was to characterize the migration and differentiation of enteric neural crest-derived cells (ENCCs) in the colorectum of avian embryos.

RESULTS

Using normal chick embryos and vagal neural tube transplants from green fluorescent protein (GFP) -transgenic chick embryos, we find ENCCs entering the colon at embryonic day (E) 6.5, with colonization complete by E8. Undifferentiated ENCCs at the wavefront express HNK-1, N-cadherin, Sox10, p75, and L1CAM. By E7, differentiation begins in the proximal colon, with L1CAM and Sox10 becoming restricted to neuronal and glial lineages, respectively. By E8, multiple markers of differentiation are expressed along the entire colorectum.

CONCLUSIONS

Our results establish the pattern of ENCC migration and differentiation in the chick colorectum, demonstrate the conservation of marker expression across species, highlight a range of markers, including neuronal cell adhesion molecules, which label cells at the wavefront, and provide a framework for future studies in avian ENS development.

Evidence for neural progenitor cells in the human adult enteric nervous system

Putative neural stem cells have been identified within the enteric nervous system (ENS) of adult rodents and cultured from human myenteric plexus. We conducted studies to identify neural stem cells or progenitor cells within the submucosa of adult human ENS. Jejunum tissue was removed from adult human subjects undergoing gastric bypass surgery. The tissue was immunostained, and confocal images of ganglia in the submucosal plexus were collected to identify protein gene product 9.5 (PGP 9.5) - immunoractive neurons and neuronal progenitor cells that coexpress PGP 9.5 and nestin. In addition to PGP-9.5-positive/nestin-negative neuronal cells within ganglia, we observed two other types of cells: (1) cells in which PGP 9.5 and nestin were co-localized, (2) cells negative for both PGP 9.5 and nestin. These observations suggest that the latter two types of cells are related to a progenitor cell population and are consistent with the concept that the submucosa of human adult ENS contains stem cells capable of maintenance and repair within the peripheral nervous system.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Effects of tissue age, presence of neurones and endothelin-3 on the ability of enteric neurone precursors to colonize recipient gut: implications for cell-based therapies

BACKGROUND Most enteric neurones arise from neural crest cells that originate in the post-otic hindbrain, and migrate into and along the developing gastrointestinal tract. There is currently great interest in the possibility of cell therapy to replace diseased or absent enteric neurones in patients with enteric neuropathies, such as Hirschsprung's disease. However, it is unclear whether neural crest stem/progenitor cells will be able to colonize colon (i) in which the mesenchyme has differentiated into distinct layers, (ii) that already contains enteric neurones or (iii) that lacks a gene expressed by the gut mesenchyme, such as endothelin-3 (Et-3). METHODS Co-cultures were used to examine the ability of enteric neural crest-derived cells (ENCCs) from E11.5 mouse gut to colonize a variety of recipient hindguts. KEY RESULTS Enteric neural crest-derived cells migrated and gave rise to neurones in E14.5 and E16.5 aneural colon in which the external muscle layers had differentiated, but they did not migrate as far as in younger colon. There was no evidence of altered ENCC proliferation, cell death or neuronal differentiation in older recipient explants. Enteric neural crest-derived cells failed to enter most recipient E14.5 and E16.5 colon explants already containing enteric neurones, and the few that did showed very limited migration. Finally, ENCCs migrated a shorter distance and a higher proportion expressed the pan-neuronal marker, Hu, in recipient E11.5 Et-3(-/-) colon compared to wild-type recipient colon. CONCLUSIONS & INFERENCES Age and an absence of Et-3 from the recipient gut both significantly reduced but did not prevent ENCC migration, but the presence of neurones almost totally prevented ENCC migration.

Development of enteric neuron diversity

The mature enteric nervous system (ENS) is composed of many different neuron subtypes and enteric glia, which all arise from the neural crest. How this diversity is generated from neural crest-derived cells is a central question in neurogastroenterology, as defects in these processes are likely to underlie some paediatric motility disorders. Here we review the developmental appearance (the earliest age at which expression of specific markers can be localized) and birthdates (the age at which precursors exit the cell cycle) of different enteric neuron subtypes, and their projections to some targets. We then focus on what is known about the mechanisms underlying the generation of enteric neuron diversity and axon pathfinding. Finally, we review the development of the ENS in humans and the etiologies of a number of paediatric motility disorders.

Postnatal and diet-dependent increases in enteric glial cells and VIP-containing neurones in preterm pigs

A mature enteric nervous system (ENS) is required to ensure a normal pattern of intestinal motility in order to regulate digestion after birth. We hypothesized that neuronal and glial components of the ENS would mature during the first postnatal days in preterm pigs that are a sensitive animal model of food intolerance and necrotizing enterocolitis (NEC). Stereological volume densities of the general neuronal population [assessed by betaIII-tubulin immunoreactivity (IR)] and subsets of neuronal (VIP-IR and nitrergic IR) and glial cells (GFAP-IR and S100-IR) were determined in the small intestine of newborn preterm piglets (93% gestation), after 3 days of receiving total parenteral nutrition (TPN) and after 3 days of TPN plus 2 days of enteral feeding with sow's colostrum or milk formula. Following TPN, VIP in the myenteric and inner submucous plexus and GFAP in the inner submucous plexus increased, while the relative volume of the total neuronal population remained constant. Introduction of enteral food induced variable degrees of food intolerance and NEC, especially after formula feeding, a diet that gave rise to a higher myenteric VIP and GFAP content in the inner submucous plexus than colostrum feeding. However, the ENS seemed unaffected by the presence of NEC-like intestinal lesions. Nevertheless, this study shows that the ENS is highly plastic during the first days after premature birth and adapts in an age- and diet-dependent manner. The observed postnatal adaptation in enteric VIP and GFAP may help to maintain intestinal homeostasis during suboptimal feeding regimens in preterm neonates.

Functional development of the enteric nervous system--from migration to motility

The enteric nervous system (ENS) consists of many different types of enteric neurones forming complex reflex circuits that underlie or regulate many gut functions. Studies of humans with Hirschsprung's disease (distal aganglionosis), and of animal models of Hirschsprung's disease, have led to the identification of many of the genetic, molecular and cellular mechanisms responsible for the colonization of the gut by enteric neurone precursors. However, later events in the ENS development are still poorly understood, including the development of functioning ENS circuits. This article is a personal view of the current state of play in our understanding of the ENS development and of the future of the field.

Neural crest and the development of the enteric nervous system

The formation of the enteric nervous system (ENS) is a particularly interesting example of the migratory ability of the neural crest and of the complexity of structures to which neural crest cells contribute. The distance that neural crest cells migrate to colonize the entire length of the gastrointestinal tract exceeds that of any other neural crest cell population. Furthermore, this migration takes a long time--over 25% of the gestation period for mice and around 3 weeks in humans. After colonizing the gut, neural crest-derived cells within the gut wall then differentiate into glial cells plus many different types of neurons, and generate the most complex part of the peripheral nervous system.

Effects of different regions of the developing gut on the migration of enteric neural crest-derived cells: A role for Sema3A, but not Sema3F

The enteric nervous system arises from vagal (caudal hindbrain) and sacral level neural crest-derived cells that migrate into and along the developing gut. Data from previous studies have suggested that (i) there may be gradients along the gut that induce the caudally directed migration of vagal enteric neural precursors (ENPs), (ii) exposure to the caecum might alter the migratory ability of vagal ENPs and (iii) Sema3A might regulate the entry into the hindgut of ENPs derived from sacral neural crest. Using co-cultures we show that there is no detectable gradient of chemoattractive molecules along the pre-caecal gut that specifically promotes the caudally directed migration of vagal ENPs, although vagal ENPs migrate faster caudally than rostrally along explants of hindgut. Exposure to the caecum did not alter the rate at which ENPs colonized explants of hindgut, but it did alter the ability of ENPs to colonize the midgut. The co-cultures also revealed that there is localized expression of a repulsive cue in the distal hindgut, which might delay the entry of sacral ENPs. We show that Sema3A is expressed by the hindgut mesenchyme and its receptor, neuropilin-1, is expressed by migrating ENPs. Furthermore, there is premature entry of sacral ENPs and extrinsic axons into the distal hindgut of fetal mice lacking Sema3A. These data show that Sema3A expressed by the distal hindgut regulates the entry of sacral ENPs and extrinsic axons into the hindgut. ENPs did not express neuropilin-2 and there was no detectable change in the timetable by which ENPs colonize the gut in mice lacking neuropilin-2.

BMP signaling regulates murine enteric nervous system precursor migration, neurite fasciculation, and patterning via altered Ncam1 polysialic acid addition

The enteric nervous system (ENS) forms from migrating neural crest-derived precursors that differentiate into neurons and glia, aggregate into ganglion cell clusters, and extend neuronal processes to form a complex interacting network that controls many aspects of intestinal function. Bone morphogenetic proteins (BMPs) have diverse roles in development and influence the differentiation, proliferation, and survival of ENS precursors. We hypothesized that BMP signaling might also be important for the ENS precursor migration, ganglion cell aggregation, and neurite fasciculation necessary to form the enteric nervous system. We now demonstrate that BMP signaling restricts murine ENS precursors to the outer bowel wall during migration. In addition, blocking BMP signaling causes faster colonization of the murine colon, reduces ganglion cell aggregation, and reduces neurite fasciculation. BMP signaling also influences patterns of neurite extension within the developing bowel wall. These effects on ENS precursor migration and neurite fasciculation appear to be mediated at least in part by increased polysialic acid addition to neural cell adhesion molecule (Ncam1) in response to BMP. Removing PSA enzymatically reverses the BMP effects on ENS precursor migration and neurite fasciculation. These studies demonstrate several novel roles for BMP signaling and highlight new functions for sialyltransferases in the developing ENS.

Advances in ontogeny of the enteric nervous system

The neurons and glia that comprise the enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, are derived from vagal and sacral regions of the neural crest. In order to form the ENS, neural crest-derived precursors undergo a number of processes including survival, migration and proliferation, prior to differentiation into neuronal subtypes, some of which form functional connections with the gut smooth muscle. Investigation of the developmental processes that underlie ENS formation has progressed dramatically in recent years, in no small part due to the attention of scientists from a range of disciplines on the genesis of Hirschsprung's disease (aganglionic megacolon), the major congenital abnormality of the ENS. This review summarizes recent advances in the field of early ENS ontogeny and focuses on: (i) the spatiotemporal migratory pathways followed by vagal and sacral neural crest-derived ENS precursors, including recent in vivo imaging of migrating crest cells within the gut, (ii) the roles of the RET and EDNRB signalling pathways and how these pathways interact to control ENS development, and (iii) how perpendicular migrations of neural crest cells within the gut lead to the formation of the myenteric and submucosal plexi located between the smooth muscle layers of the gut wall.

Intestinal coelomic transplants: a novel method for studying enteric nervous system development

Normal development of the enteric nervous system (ENS) requires the coordinated activity of multiple proteins to regulate the migration, proliferation, and differentiation of enteric neural crest cells. Much of our current knowledge of the molecular regulation of ENS development has been gained from transgenic mouse models and cultured neural crest cells. We have developed a method for studying the molecular basis of ENS formation complementing these techniques. Aneural quail or mouse hindgut, isolated prior to the arrival of neural crest cells, was transplanted into the coelomic cavity of a host chick embryo. Neural crest cells from the chick host migrated to and colonized the grafted hindgut. Thorough characterization of the resulting intestinal chimeras was performed by using immunohistochemistry and vital dye labeling to determine the origin of the host-derived cells, their pattern of migration, and their capacity to differentiate. The formation of the ENS in the intestinal chimeras was found to recapitulate many aspects of normal ENS development. The host-derived cells arose from the vagal neural crest and populated the graft in a rostral-to-caudal wave of migration, with the submucosal plexus being colonized first. These crest-derived cells differentiated into neurons and glial cells, forming ganglionated plexuses grossly indistinguishable from normal ENS. The resulting plexuses were specific to the grafted hindgut, with quail grafts developing two ganglionated plexuses, but mouse grafts developing only a single myenteric plexus. We discuss the advantages of intestinal coelomic transplants for studying ENS development.

Lack of beta1 integrins in enteric neural crest cells leads to a Hirschsprung-like phenotype

The enteric nervous system arises mainly from vagal and sacral neural crest cells that colonise the gut between 9.5 and 14 days of development in mice. Using the Cre-LoxP system, we removed beta1 integrins in the neural crest cells when they emerge from the neural tube. beta1-null enteric neural crest cells fail to colonise the gut completely, leading to an aganglionosis of the descending colon, which resembles the human Hirschsprung's disease. Moreover, beta1-null enteric neural crest cells form abnormal aggregates in the gut wall, leading to a severe alteration of the ganglia network organisation. Organotypic cultures of gut explants reveal that beta1-null enteric neural crest cells show impaired adhesion on extracellular matrix and enhanced intercellular adhesion properties. They display migration defects in collagen gels and gut tissue environments. We also provide evidence that beta1 integrins are required for the villi innervation in the small intestine. Our findings highlight the crucial roles played by beta1 integrins at various steps of enteric nervous system development.

Three-dimensional slice cultures from murine fetal gut for investigations of the enteric nervous system

Three-dimensional intestinal cultures offer new possibilities for the examination of growth potential, analysis of time specific gene expression, and spatial cellular arrangement of enteric nervous system in an organotypical environment. We present an easy to produce in vitro model of the enteric nervous system for analysis and manipulation of cellular differentiation processes. Slice cultures of murine fetal colon were cultured on membrane inserts for up to 2 weeks without loss of autonomous contractility. After slice preparation, cultured tissue reorganized within the first days in vitro. Afterward, the culture possessed more than 35 cell layers, including high prismatic epithelial cells, smooth muscle cells, glial cells, and neurons analyzed by immunohistochemistry. The contraction frequency of intestinal slice culture could be modulated by the neurotransmitter serotonin and the sodium channel blocker tetrodotoxin. Coculture experiments with cultured neurospheres isolated from enhanced green fluorescent protein (eGFP) transgenic mice demonstrated that differentiating eGFP-positive neurons were integrated into the intestinal tissue culture. This slice culture model of enteric nervous system proved to be useful for studying cell-cell interactions, cellular signaling, and cell differentiation processes in a three-dimensional cell arrangement.

Gangliogenesis in the enteric nervous system: Roles of the polysialylation of the neural cell adhesion molecule and its regulation by bone morphogenetic protein-4

The neural crest-derived cells that colonize the fetal bowel become patterned into two ganglionated plexuses. The hypothesis that bone morphogenetic proteins (BMPs) promote ganglionation by regulating neural cell adhesion molecule (NCAM) polysialylation was tested. Transcripts encoding the sialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), which polysialylate NCAM, were detectable in fetal rat gut by E12 but were downregulated postnatally. PSA-NCAM-immunoreactive neuron numbers, but not those of NCAM, were developmentally regulated similarly. Circular smooth muscle was transiently (E16-20) PSA-NCAM-immunoreactive when it is traversed by migrating precursors of submucosal neurons. Neurons developing in vitro from crest-derived cells immunoselected at E12 with antibodies to p75(NTR) expressed NCAM and PSA-NCAM. BMP-4 promoted neuronal NCAM polysialylation and clustering. N-butanoylmannosamine, which blocks NCAM polysialylation, but not N-propanoylmannosamine, which does not, interfered with BMP-4-induced neuronal clustering. Observations suggest that BMP signaling enhances NCAM polysialylation, which allows precursors to migrate and form ganglionic aggregates during the remodeling of the developing ENS.

Expression of intermediate filament proteins and neuronal markers in the human fetal gut

The human enteric nervous system (ENS) derives from migrating neural crest cells (NCC) and is structured into different plexuses embedded in the gastrointestinal tract wall. During development of the NCC, a rearrangement of various cytoskeletal intermediate filaments such as nestin, peripherin, or alpha-internexin takes place. Although all are related to developing neurons, nestin is also used to identify neural stem cells. Until now, information about the prenatal development of the human ENS has been very restricted, especially concerning potential stem cells. In this study the expression of nestin, peripherin, and alpha-internexin, but also of neuronal markers such as protein gene product (PGP) 9.5 and tyrosine hydroxylase, were investigated in human fetal and postnatal gut. The tissue samples were rapidly removed and subsequently processed for immunohistochemistry or immunoblotting. Nestin could be detected in all samples investigated with the exception of the 9th and the 12th week of gestation (WOG). Although the neuronal marker PGP9.5 was coexpressed with nestin at the 14th WOG, this could no longer be observed at later time points. Alpha-internexin and peripherin expression also did not appear before the 14th WOG, where they were coexpressed with PGP9.5. This study reveals that the intermediate filament markers investigated are not suitable to detect early neural crest stem cells.

Structural development of PGP9.5-immunopositive myenteric plexus in embryonic rats

To investigate relationships between changes in the 3-dimensional structure of the myenteric plexus and the time at which functional movement of intestine begins in mammalian embryos, whole mounts of embryonic rat intestine were examined under confocal laser scanning microscopy on spacer-equipped glass slides after immunostaining with antiprotein gene product 9.5 antibody. At embryonic day 12.5, very few nerve cells were scattered throughout the small intestine, but no immunostained structures were apparent on the anal side of the large intestine. At embryonic day 13.5, immunostained fibers appeared on the oral side of the large intestine. Nerve cells and associated fiber bundles formed neuronal networks with large meshes in both intestines. Marked increases in number of nerve fibers and decreases in mesh size were seen in the small intestine between embryonic days 13.5 and 15.5. Similar changes were found in the large intestine between embryonic days 13.5 and 16.5. After embryonic day 16.5, nerve cells were arranged parallel to circular muscle fibers, and networks formed by cell fibers elongated until the neonatal period in both intestines. Meconium passed through the large intestine from embryonic day 17.5. Thin fiber bundles extended from the ganglion to the inner side of the myenteric layer, parallel (and occasionally extending) to the circular muscle fibers. Formation of nerve fiber networks and arrangement of nerve cells parallel to circular muscle fibers probably relate to movement coordination for inner circular muscle fibers in the intestinal wall, and development of this neural network may be important for acquiring intestinal movements before birth.

Development of the enteric nervous system, smooth muscle and interstitial cells of Cajal in the human gastrointestinal tract

The generation of functional neuromuscular activity within the pre-natal gastrointestinal tract requires the coordinated development of enteric neurons and glial cells, concentric layers of smooth muscle and interstitial cells of Cajal (ICC). We investigated the genesis of these different cell types in human embryonic and fetal gut material ranging from weeks 4-14. Neural crest cells (NCC), labelled with antibodies against the neurotrophin receptor p75NTR, entered the foregut at week 4, and migrated rostrocaudally to reach the terminal hindgut by week 7. Initially, these cells were loosely distributed throughout the gut mesenchyme but later coalesced to form ganglia along a rostrocaudal gradient of maturation; the myenteric plexus developed primarily in the foregut, then in the midgut, and finally in the hindgut. The submucosal plexus formed approximately 2-3 weeks after the myenteric plexus, arising from cells that migrated centripetally through the circular muscle layer from the myenteric region. Smooth muscle differentiation, as evidenced by the expression of alpha-smooth muscle actin, followed NCC colonization of the gut within a few weeks. Gut smooth muscle also matured in a rostrocaudal direction, with a large band of alpha-smooth muscle actin being present in the oesophagus at week 8 and in the hindgut by week 11. Circular muscle developed prior to longitudinal muscle in the intestine and colon. ICC emerged from the developing gut mesenchyme at week 9 to surround and closely appose the myenteric ganglia by week 11. By week 14, the intestine was invested with neural cells, longitudinal, circular and muscularis mucosae muscle layers, and an ICC network, giving the fetal gut a mature appearance.

Hoxb3 vagal neural crest-specific enhancer element for controlling enteric nervous system development

The neural and glial cells of the intrinsic ganglia of the enteric nervous system (ENS) are derived from the hindbrain neural crest at the vagal level. The Hoxb3 gene is expressed in the vagal neural crest and in the enteric ganglia of the developing gut during embryogenesis. We have identified a cis-acting enhancer element b3IIIa in the Hoxb3 gene locus. In this study, by transgenic mice analysis, we examined the tissue specificity of the b3IIIa enhancer element using the lacZ reporter gene, with emphasis on the vagal neural crest cells and their derivatives in the developing gut. We found that the b3IIIa-lacZ transgene marks only the vagal region and not the trunk or sacral region. Using cellular markers, we showed that the b3IIIa-lacZ transgene was expressed in a subset of enteric neuroblasts during early development of the gut, and the expression was maintained in differentiated neurons of the myenteric plexus at later stages. The specificity of the b3IIIa enhancer in directing gene expression in the developing ENS was further supported by genetic analysis using the Dom mutant, a spontaneous mouse model of Hirschsprung's disease characterized by the absence of enteric ganglia in the distal gut. The colonization of lacZ-expressing cells in the large intestine was incomplete in all the Dom/b3IIIa-lacZ hybrid mutants we examined. To our knowledge, this is the only vagal neural crest-specific genetic regulatory element identified to date. This element could be used for a variety of genetic manipulations and in establishing transgenic mouse models for studying the development of the ENS.

Embryonic development of the myenteric nerve plexus in the rat small intestine as revealed by light and scanning electron microscopy

This study was performed to clarify developmental changes in the structure of the myenteric nerve plexus in the rat small intestine. The small intestines of fetal and neonatal rats were examined by light microscopic immunohistochemistry for protein gene product 9.5 (PGP-9.5) and alpha-smooth muscle actin (alpha-SMA). Direct observation of the three-dimensional structure of the neuronal and muscular elements was accomplished by scanning electron microscopy of tissues after KOH treatment. At the embryonic day 14 (E14), PGP-9.5 immunopositive (neuronal) cells first appeared in the mesenchymal tissue outside an alpha-SMA positive (muscle) cell layer. These cells increased in number to form a thin and continuous sheet, which was sandwiched between the inner and outer muscular layers at E17. This neuronal layer consisted of immature ganglion cells associated with supportive (enteroglial) cells, and was gradually segregated into groups by incomplete separation through slit-like interruptions at E18. With the widening of these slits at E19, the neuronal sheet was observed as a ladder composed of transversely elongated ganglia and their connecting nerve strands, resulting in the typical structure of the myenteric nerve plexus. Fibroblastic cells of a peculiar shape appeared on the outer surface of the myenteric plexus at E19. These cells (presumably interstitial cells) interconnected their long branching projections to form a cellular network on the myenteric plexus until birth.