Formation of ganglia in the gut of the chick embryo

Development, Diversity, and Neurogenic Capacity of Enteric Glia

Enteric glia are a fascinating population of cells. Initially identified in the gut wall as the "support" cells of the enteric nervous system, studies over the past 20 years have unveiled a vast array of functions carried out by enteric glia. They mediate enteric nervous system signalling and play a vital role in the local regulation of gut functions. Enteric glial cells interact with other gastrointestinal cell types such as those of the epithelium and immune system to preserve homeostasis, and are perceptive to luminal content. Their functional versatility and phenotypic heterogeneity are mirrored by an extensive level of plasticity, illustrated by their reactivity in conditions associated with enteric nervous system dysfunction and disease. As one of the hallmarks of their plasticity and extending their operative relationship with enteric neurons, enteric glia also display neurogenic potential. In this review, we focus on the development of enteric glial cells, and the mechanisms behind their heterogeneity in the adult gut. In addition, we discuss what is currently known about the role of enteric glia as neural precursors in the enteric nervous system.

Cells as strain-cued automata

We argue in favor of representing living cells as automata and review demonstrations that autonomous cells can form patterns by responding to local variations in the strain fields that arise from their individual or collective motions. An autonomous cell's response to strain stimuli is assumed to be effected by internally-generated, internally-powered forces, which generally move the cell in directions other than those implied by external energy gradients. Evidence of cells acting as strain-cued automata have been inferred from patterns observed in nature and from experiments conducted in vitro. Simulations that mimic particular cases of pattern forming share the idealization that cells are assumed to pass information among themselves solely via mechanical boundary conditions, i.e., the tractions and displacements present at their membranes. This assumption opens three mechanisms for pattern formation in large cell populations: wavelike behavior, kinematic feedback in cell motility that can lead to sliding and rotational patterns, and directed migration during invasions. Wavelike behavior among ameloblast cells during amelogenesis (the formation of dental enamel) has been inferred from enamel microstructure, while strain waves in populations of epithelial cells have been observed in vitro. One hypothesized kinematic feedback mechanism, "enhanced shear motility", accounts successfully for the spontaneous formation of layered patterns during amelogenesis in the mouse incisor. Directed migration is exemplified by a theory of invader cells that sense and respond to the strains they themselves create in the host population as they invade it: analysis shows that the strain fields contain positional information that could aid the formation of cell network structures, stabilizing the slender geometry of branches and helping govern the frequency of branch bifurcation and branch coalescence (the formation of closed networks). In simulations of pattern formation in homogeneous populations and network formation by invaders, morphological outcomes are governed by the ratio of the rates of two competing time dependent processes, one a migration velocity and the other a relaxation velocity related to the propagation of strain information. Relaxation velocities are approximately constant for different species and organs, whereas cell migration rates vary by three orders of magnitude. We conjecture that developmental processes use rapid cell migration to achieve certain outcomes, and slow migration to achieve others. We infer from analysis of host relaxation during network formation that a transition exists in the mechanical response of a host cell from animate to inanimate behavior when its strain changes at a rate that exceeds 10-4-10-3s-1. The transition has previously been observed in experiments conducted in vitro.

Why are enteric ganglia so small? Role of differential adhesion of enteric neurons and enteric neural crest cells

The avian enteric nervous system (ENS) consists of a vast number of unusually small ganglia compared to other peripheral ganglia. Each ENS ganglion at mid-gestation has a core of neurons and a shell of mesenchymal precursor/glia-like enteric neural crest (ENC) cells. To study ENS cell ganglionation we isolated midgut ENS cells by HNK-1 fluorescence-activated cell sorting (FACS) from E5 and E8 quail embryos, and from E9 chick embryos. We performed cell-cell aggregation assays which revealed a developmentally regulated functional increase in ENS cell adhesive function, requiring both Ca ²⁺ -dependent and independent adhesion. This was consistent with N-cadherin and NCAM labelling. Neurons sorted to the core of aggregates, surrounded by outer ENC cells, showing that neurons had higher adhesion than ENC cells. The outer surface of aggregates became relatively non-adhesive, correlating with low levels of NCAM and N-cadherin on this surface of the outer non-neuronal ENC cells. Aggregation assays showed that ENS cells FACS selected for NCAM-high and enriched for enteric neurons formed larger and more coherent aggregates than unsorted ENS cells. In contrast, ENS cells of the NCAM-low FACS fraction formed small, disorganised aggregates.  This suggests a novel mechanism for control of ENS ganglion morphogenesis where i) differential adhesion of ENS neurons and ENC cells controls the core/shell ganglionic structure and ii) the ratio of neurons to ENC cells dictates the equilibrium ganglion size by generation of an outer non-adhesive surface.

Building additional complexity to in vitro-derived intestinal tissues

Gastrointestinal (GI) disorders affect up to 25% of the US population. Common intestinal disorders include malabsorption, irritable bowel syndrome and fecal incontinence. Some GI disorders such as Hirschsprung's disease have a genetic basis and are associated with an absence or paucity of enteric nerves. Current treatment plans for GI disorders range from changes in diet to bowel resection, and there are very few drugs available that target the primary deficiencies in intestinal function such as controlled peristalsis. While animal models can recapitulate the broad range of intestinal pathologies of the GI tract, they are intrinsically complicated and of low throughput. Several in vitro systems have been established, and these range from epithelial enteroids to more complex organoids, which contain most intestinal cell types. One of the more complex organoid systems was derived from adult mouse intestines and contains functional enteric nerves and smooth muscle capable of peristalsis. Establishing an equivalent human intestinal system is challenging due to limited access and variable quality of human intestinal tissues. However, owing to recent advances, it is possible to differentiate human induced and embryonic pluripotent stem cells, collectively called pluripotent stem cells, into human intestinal organoids (HIOs) in vitro. Although HIOs contain a significant degree of epithelial and mesenchymal complexity, they lack enteric nerves and thus are unable to model the peristaltic movements of the gut. The goal of this review is to discuss approaches to generate complex in vitro systems that can be used to more comprehensively model common intestinal pathologies. New and more biologically complete human models of the intestine would allow for unprecedented studies of the cellular and molecular basis of normal and pathological gut function. Furthermore, fully functional HIOs could serve as a platform for preclinical drug studies to model absorption and efficacy.

Simple rules for a "simple" nervous system? Molecular and biomathematical approaches to enteric nervous system formation and malformation

We review morphogenesis of the enteric nervous system from migratory neural crest cells, and defects of this process such as Hirschsprung disease, centering on cell motility and assembly, and cell adhesion and extracellular matrix molecules, along with cell proliferation and growth factors. We then review continuum and agent-based (cellular automata) models with rules of cell movement and logistical proliferation. Both movement and proliferation at the individual cell level are modeled with stochastic components from which stereotyped outcomes emerge at the population level. These models reproduced the wave-like colonization of the intestine by enteric neural crest cells, and several new properties emerged, such as colonization by frontal expansion, which were later confirmed biologically. These models predict a surprising level of clonal heterogeneity both in terms of number and distribution of daughter cells. Biologically, migrating cells form stable chains made up of unstable cells, but this is not seen in the initial model. We outline additional rules for cell differentiation into neurons, axon extension, cell-axon and cell-cell adhesions, chemotaxis and repulsion which can reproduce chain migration. After the migration stage, the cells re-arrange as a network of ganglia. Changes in cell adhesion molecules parallel this, and we describe additional rules based on Steinberg's Differential Adhesion Hypothesis, reflecting changing levels of adhesion in neural crest cells and neurons. This was able to reproduce enteric ganglionation in a model. Mouse mutants with disturbances of enteric nervous system morphogenesis are discussed, and these suggest future refinement of the models. The modeling suggests a relatively simple set of cell behavioral rules could account for complex patterns of morphogenesis. The model has allowed the proposal that Hirschsprung disease is mostly an enteric neural crest cell proliferation defect, not a defect of cell migration. In addition, the model suggests an explanations for zonal and skip segment variants of Hirschsprung disease, and also gives a novel stochastic explanation for the observed discordancy of Hirschsprung disease in identical twins.

Pleiotropic effects of the bone morphogenetic proteins on development of the enteric nervous system

Formation of the enteric nervous system (ENS) from migratory neural crest-derived cells that colonize the primordial gut involves a complex interplay among different signaling molecules. The bone morphogenetic proteins (BMPs), specifically BMP2 and BMP4, play a particularly important role in virtually every stage of gut and ENS development. BMP signaling helps to pattern both the anterior-posterior axis and the radial axis of the gut prior to colonization by migratory crest progenitor cells. BMP signaling then helps regulate the migration of enteric neural crest-derived precursors as they colonize the fetal gut and form ganglia. BMP2 and -4 promote differentiation of enteric neurons in early fetal ENS development and glia at later stages. A major role for BMP signaling in the ENS is regulation of responses to other growth factors. Thus BMP signaling first regulates neurogenesis by modulating responses to GDNF and later gliogenesis through its effects on GGF-2 responses. Furthermore, BMPs promote growth factor dependency for survival of ENS neurons (on NT-3) and glia (on GGF-2) by inducing TrkC (neurons) and ErbB3 (glia). BMP signaling limits total neuron numbers, favoring the differentiation of later born neuronal phenotypes at the expense of earlier born ones thus influencing the neuronal composition of the ENS and the glia/neuron ratio. BMP2 and -4 also promote gangliogenesis via modification of neural cell adhesion molecules and promote differentiation of the circular and then longitudinal smooth muscles. Disruption of BMP signaling leads to defects in the gut and in ENS function commensurate with these complex developmental roles.

Genetic background impacts developmental potential of enteric neural crest-derived progenitors in the Sox10Dom model of Hirschsprung disease

Abnormalities in the development of enteric neural crest-derived progenitors (ENPs) that generate the enteric nervous system (ENS) can lead to aganglionosis in a variable portion of the distal gastrointestinal tract. Cumulative evidence suggests that variation of aganglionosis is due to gene interactions that modulate the ability of ENPs to populate the intestine; however, the developmental processes underlying this effect are unknown. We hypothesized that differences in enteric ganglion deficits could be attributable to the effects of genetic background on early developmental processes, including migration, proliferation, or lineage divergence. Developmental processes were investigated in congenic Sox10(Dom) mice, an established Hirschsprung disease (HSCR) model, on distinct inbred backgrounds, C57BL/6J (B6) and C3HeB/FeJ (C3Fe). Immuno-staining on whole-mount fetal gut tissue and dissociated cell suspensions was used to assess migration and proliferation. Flow cytometry utilizing the cell surface markers p75 and HNK-1 was used to isolate live ENPs for analysis of developmental potential. Frequency of ENPs was reduced in Sox10(Dom) embryos relative to wild-type embryos, but was unaffected by genetic background. Both migration and developmental potential of ENPs in Sox10(Dom) embryos were altered by inbred strain background with the most highly significant differences seen for developmental potential between strains and genotypes. In vivo imaging of fetal ENPs and postnatal ganglia demonstrates that altered lineage divergence impacts ganglia in the proximal intestine. Our analysis demonstrates that genetic background alters early ENS development and suggests that abnormalities in lineage diversification can shift the proportions of ENP populations and thus may contribute to ENS deficiencies in vivo.

Early development of chick embryo respiratory nervous system: an immunohistochemical study

The extrinsic and intrinsic respiratory nervous systems receive specific contributions from the vagal and sympathetic components. Using specific markers for vagal and sympathetic structures, we studied the distribution patterns of immunoreactivity to galanin (GAL), pituitary adenylate cyclase-activating polypeptide-27 (PACAP) and the tachykinin substance P in extrinsic and intrinsic nerve of chick embryo respiratory system, during development from the very early age to hatching. All peptides studied appeared in the intrinsic and extrinsic nervous systems early. We found substance P in both the vagal and sympathetic systems, PACAP in vagal components alone and GAL mainly in the sympathetic system. The intrinsic nervous system showed high immunoreactivity for all peptides studied. These data accord with the well known early trophic functions that peptides have on the development of nervous networks and modulatory activity on the intrinsic nervous system. The GAL again proves to be the main peptide in chick embryo sympathetic respiratory system.

lessen encodes a zebrafish trap100 required for enteric nervous system development

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest. The developmental controls that govern the specification and patterning of the ENS are not well understood. To identify genes required for the formation of the vertebrate ENS, we preformed a genetic screen in zebrafish. We isolated the lessen (lsn) mutation that has a significant reduction in the number of ENS neurons as well as defects in other cranial neural crest derived structures. We show that the lsn gene encodes a zebrafish orthologue of Trap100, one of the subunits of the TRAP/mediator transcriptional regulation complex. A point mutation in trap100 causes a premature stop codon that truncates the protein, causing a loss of function. Antisense-mediated knockdown of trap100 causes an identical phenotype to lsn. During development trap100 is expressed in a dynamic tissue-specific expression pattern consistent with its function in ENS and jaw cartilage development. Analysis of neural crest markers revealed that the initial specification and migration of the neural crest is unaffected in lsn mutants. Phosphohistone H3 immunocytochemistry revealed that there is a significant reduction in proliferation of ENS precursors in lsn mutants. Using cell transplantation studies, we demonstrate that lsn/trap100 acts cell autonomously in the pharyngeal mesendoderm and influences the development of neural crest derived cartilages secondarily. Furthermore, we show that endoderm is essential for ENS development. These studies demonstrate that lsn/trap100 is not required for initial steps of cranial neural crest development and migration, but is essential for later proliferation of ENS precursors in the intestine.

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.

The pattern of neural crest advance in the cecum and colon

Neural crest cells leave the hindbrain, enter the gut mesenchyme at the pharynx, and migrate as strands of cells to the terminal bowel to form the enteric nervous system. We generated embryos containing fluorescent enteric neural crest-derived cells (ENCCs) by mating Wnt1-Cre mice with Rosa-floxed-YFP mice and investigated ENCC behavior in the intact gut of mouse embryos using time-lapse fluorescent microscopy. With respect to the entire gut, we have found that ENCCs in the cecum and proximal colon behave uniquely. ENCCs migrating caudally through either the ileum, or caudal colon, are gradually advancing populations of strands displaying largely unpredictable local trajectories. However, in the cecum, advancing ENCCs pause for approximately 12 h, and then display an invariable pattern of migration to distinct regions of the cecum and proximal colon. In addition, while most ENCCs migrating through other regions of the gut remain interconnected as strands; ENCCs initially migrating through the cecum and proximal colon fragment from the main population and advance as isolated single cells. These cells aggregate into groups isolated from the main network, and eventually extend strands themselves to reestablish a network in the mid-colon. As the advancing network of ENCCs reaches the terminal bowel, strands of sacral crest cells extend, and intersect with vagal crest to bridge the small space between. We found a relationship between ENCC number, interaction, and migratory behavior by utilizing endogenously isolated strands and by making cuts along the ENCC wavefront. Depending on the number of cells, the ENCCs aggregated, proliferated, and extended strands to advance the wavefront. Our results show that interactions between ENCCs are important for regulating behaviors necessary for their advancement.

Bone morphogenetic protein-2 and -4 limit the number of enteric neurons but promote development of a TrkC-expressing neurotrophin-3-dependent subset

The hypothesis that BMPs (bone morphogenetic proteins), which act early in gut morphogenesis, also regulate specification and differentiation in the developing enteric nervous system (ENS) was tested. Expression of BMP-2 and BMP-4, BMPR-IA (BMP receptor subunit), BMPR-IB, and BMPR-II, and the BMP antagonists, noggin, gremlin, chordin, and follistatin was found when neurons first appear in the primordial bowel at embryonic day 12 (E12). Agonists, receptors, and antagonists were detected in separated populations of neural crest- and noncrest-derived cells. When applied to immunopurified E12 ENS precursors, BMP-2 and BMP-4 induced nuclear translocation of phosphorylated Smad-1 (Sma and Mad-related protein). The number of neurons developing from these cells was increased by low concentrations and decreased by high concentrations of BMP-2 or BMP-4. BMPs induced the precocious appearance of TrkC-expressing neurons and their dependence on neurotrophin-3 for survival. BMP-4 interacted with glial cell line-derived neurotrophic factor (GDNF) to enhance neuronal development but limited GDNF-driven expansion of the precursor pool. BMPs also promoted development of smooth muscle from mesenchymal cells immunopurified at E12. To determine the physiological significance of these observations, the BMP antagonist noggin was overexpressed in the developing ENS of transgenic mice under the control of the neuron-specific enolase promoter. Neuronal numbers in both enteric plexuses and smooth muscle were increased throughout the postnatal small intestine. These increases were already apparent by E18. In contrast, TrkC-expressing neurons decreased in both plexuses of postnatal noggin-overexpressing animals, again an effect detectable at E18. BMP-2 and/or BMP-4 thus limit the size of the ENS but promote the development of specific subsets of enteric neurons, including those that express TrkC.

Neuronal differentiation and myenteric plexus organization are delayed in gastroschisis: an immunohistochemical study in a rat model

Gastroschisis is a malformation due to prenatal rupture of the abdominal wall and evisceration of the midgut. Intestinal loops are shortened, matted, and covered by a peel caused by the harmful effect of the amniotic fluid. Babies born with gastroschisis suffer from gastrointestinal dysmotility. The present aim was to verify whether the myenteric plexus is damaged in a rat model of gastroschisis. In the gastroschisis rat model fetus, the myenteric plexus was not yet organized in the well-defined ganglia and, in the most damaged loops, the neuronal cells were scattered or absent. Immunohistochemistry for alpha-internexin and peripherin (markers of neuronal maturity) gave results similar to those of earlier embryonic ages. These findings indicate a delay in neuronal differentiation and myenteric plexus organization that might play a role in the postnatal dysmotility observed in gastroschisis.

Appearance of neurons and glia with respect to the wavefront during colonization of the avian gut by neural crest cells

The enteric nervous system is formed by neural crest cells that migrate, proliferate, and differentiate into neurons and glia distributed in ganglia along the gastrointestinal tract. In the developing embryo some enteric crest cells cease their caudal movements, whereas others continue to migrate. Subsequently, the enteric neurons form a reticular network of ganglia interconnected by axonal projections. We studied the developing avian gut to characterize the pattern of migration of the crest cells, and the relationship between migration and differentiation. Crest cells at the leading edge of the migratory front appear as strands of cells; isolated individual crest cells are rarely seen. In the foregut and midgut, these strands are located immediately beneath the serosa. In contrast, crest cells entering the colon appear first in the deeper submucosal mesenchyme and later beneath the serosa. As the neural crest wavefront passes caudally, the crest cell cords become highly branched, forming a reticular lattice that presages the mature organization of the enteric nervous system. Neurons and glia first appear within the strands at the advancing wavefront. Later neurons are consistently located at the nodes where branches of the lattice intersect. In the most rostral foregut and in the colon, some neurons initially appear in close association with extrinsic nerve fibers from the vagus and Remak's nerve, respectively. We conclude that crest cells colonize the gut as chains of cells and that, within these chains, both neurons and glia appear close to the wavefront.

A second source of precursor cells for the developing enteric nervous system and interstitial cells of Cajal

The enteric nervous system is believed to be derived solely from the neural crest cells. This is partly based on the belief that the neural crest cells are the sole neural tube-derived cells colonizing the gastrointestinal tract. However, recent studies have shown that after the emigration of neural crest cells an additional population of cells emigrate from the cranial neural tube. These cells originate in the ventral part of the hindbrain, emigrate through the site of attachment of the cranial nerves, and colonize a variety of developing structures including the gastrointestinal tract. This cell population has been named the ventrally emigrating neural tube (VENT) cells. We followed the fate of these cells in the gastrointestinal tract. Ventral hindbrain neural tube cells of chick embryos were tagged with replication-deficient retroviral vectors containing the LacZ gene, after the emigration of neural crest from this region. In control embryos, the viral concentrate was dropped on the dorsal part of the neural tube. Embryos were sacrificed from embryonic days 3-12 and processed for the detection of LacZ positive ventrally emigrating neural tube cells. These cells colonized only the foregut, specifically the duodenum and stomach. Immunostaining with the neural crest cell marker HNK-1 showed that they were HNK-1 negative, indicating that they were not derived from neural crest. Cells were detected in three locations: (1). the myenteric and submucosal plexus of the enteric nervous system; (2). circular smooth muscle cell layer; and (3). mucosal lining of the lumen. A variety of specific markers were used to identify their fate. Some ventrally emigrating neural tube cells differentiated into neurons and glial cells, indicating that the enteric nervous system in the foregut develops from an additional source of precursor cells. It was also found that some of these cells differentiated into interstitial cells of Cajal, which mediate impulses between the enteric nervous system and smooth muscle cells, whereas others differentiated into epithelium. Altogether, these results indicate that the ventrally emigrating neural tube cells are multipotential. More importantly, they reveal a novel source of precursor cells for the neurons and glial cells of the enteric nervous system. The developmental and functional significance of the heterogeneous origin of the cell types remains to be established.

The development of colon innervation in trisomy 16 mice and Hirschsprungs disease

AIM: To study the colon innervation of trisomy 16 mouse, an animal model for Down’s syndrome, and the expression of protein gene product 9.5 (PGP 9.5) in the stenosed segment of colon in Hirschsprungs disease (HD).

METHODS: Trisomy 16 mouse breeding; cytogenetic analysis of trisomy 16 mice; and PGP 9.5 immunohistochemistry of colons of trisomy 16 mice and HD were carried out.

RESULTS: Compared with their normal littermates, the nervous system of colon in trisomy 16 mice was abnormally developed. There existed developmental delay of muscular plexuses of colon, no submucosal plexus was found in the colon, and there was 5 mm aganglionic bowel aparting from the anus in trisomy 16 mice. The mesentery nerve fibers were as well developed as shown in their normal littermates. Abundant proliferation of PGP 9.5 positive nerve fibers was evealed in the stenosed segment of HD colon.

CONCLUSION: Trisomy 16 mice could serve as an animal model for Hirschsprung’s disease for aganglionic bowel in the distal part of colon. Abundant proliferation of PGP 9.5 positive fibers resulted from extrinsic nerve compensation, since no ganglionic cells were observed in the stenosed segment of the colon in HD. HD has a genetic tendency.

Enteric nervous system development: analysis of the selective developmental potentialities of vagal and sacral neural crest cells using quail-chick chimeras

The majority of the enteric nervous system (ENS) is derived from vagal neural crest cells (NCC). For many years, the contribution from a second region of the neuraxis (the sacral neural crest) to the ENS has been less clear, with conflicting reports appearing in the literature. To resolve this longstanding issue, we documented the spatiotemporal migration and differentiation of vagal and sacral-derived NCC within the developing chick embryo using quail-chick grafting and antibody labelling. Results showed that vagal NCC colonised the entire length of the gut in a rostrocaudal direction. The hindgut, the region of the gastrointestinal tract most frequently affected in developmental disorders, was found to be colonised in a complex manner. Vagal NCC initially migrated within the submucosa, internal to the circular muscle layer, before colonising the myenteric plexus region. In contrast, sacral NCC, which colonised the hindgut in a caudorostral direction, were primarily located in the myenteric plexus region from where they subsequently migrated to the submucosa. We also observed that sacral NCC migrated into the hindgut in significant numbers only after vagal-derived cells had colonised the entire length of the gut. This suggested that to participate in ENS formation, sacral cells may require an interaction with vagal-derived cells, or with factors or signalling molecules released by them or their progeny. To investigate this possible inter-relationship, we ablated sections of vagal neural crest (NC) to prevent the rostrocaudal migration of ENS precursors and, thus, create an aganglionic hindgut model. In the same NC ablated animals, quail-chick sacral NC grafts were performed. In the absence of vagal-derived ganglia, sacral NCC migrated and differentiated in an apparently normal manner. Although the numbers of sacral cells within the hindgut was slightly higher in the absence of vagal-derived cells, the increase was not sufficient to compensate for the lack of enteric ganglia. As vagal NCC appear to be more invasive than sacral NCC, since they colonise the entire length of the gut, we investigated the ability of transplanted vagal cells to colonise the hindgut by grafting the vagal NC into the sacral region. We found that when transplanted, vagal cells retained their invasive capacity and migrated into the hindgut in large numbers. Although sacral-derived cells normally contribute a relatively small number of precursors to the post-umbilical gut, many heterotopic vagal cells were found within the hindgut enteric plexuses at much earlier stages of development than normal. Heterotopic grafting of invasive vagal NCC into the sacral neuraxis may, therefore, be a means of rescuing an aganglionic hindgut phenotype.

Synapse formation during neuron differentiation: an in situ study of the myenteric plexus during murine embryonic life

Ultrastructural steps characterizing synapse formation in vivo and appearance in neuroblasts of properties suggestive of synaptic function acquisition have scarcely been studied. Synapse formation and proteosynthetic apparatus organization were thus studied under transmission electron microscope in mouse myenteric neurons from embryonic day 12.5 (E12.5) until birth. Expression of Ret and p75(NTR), markers of neural crest cells, as well as that of neuron-specific enolase (NSE), synaptophysin (SY), and synaptosomal-associated protein (SNAP), markers of synaptic function acquisition, were immunohistochemically evaluated. At E12.5 many cells were Ret- and p75(NTR)-immunoreactive (IR), whereas a few were NSE-IR and had neuronal ultrastructural characteristics. Two types of contacts between poorly or nondifferentiated cells and axons of presumed extrinsic (synapse-like contacts) or local (immature synapses) origin were identified, along with SY-IR elements. By E16. 5, many cells had developed a proteosynthetic apparatus, synapse-like contacts were no longer present, and immature synapses were gradually differentiating. Concurrently, there was an increase in NSE-IR cells, some of which were also SNAP-IR, and in SY-IR varicosities. At E18.5, ultrastructurally mature neurons and synapses had increased in number as had NSE-IR and SNAP-IR cells and SY-IR varicosities. These data indicate that 1) one type of contact (synapse-like) is present at E12.5 between very immature cells and presumed vagal fibers, with a possible transient role for the onset of the differentiative process of these cells; and 2) another type of contact (typical synapses) lasts until E18.5, with a similar but long-lasting role that progressively shifts to the classical function (neurotransmission) as the synapse matures and the embryo reaches the day of birth.

A confocal microscopic study of the formation of ganglia in the airways of fetal pig lung

Fetal airway smooth muscle contracts to neural stimulation from early gestation. This study aimed to document the development of the nerves and ganglia within the bronchial tree of the fetal pig lung as the structural correlates for this function. The formation of these structures during lung development (pseudoglandular stage, canalicular stage, and saccular stage) was followed through to the postnatal period, using antibodies to protein gene product 9.5, a nonspecific nerve marker; synaptic vesicle protein 2, a marker of synaptic vesicle membranes; and neurofilament, a marker of filaments in the neuronal cytoskeleton. Glial cells were stained for glial fibrillary acidic protein (GFAP) and S-100, and the airway smooth muscle for alpha-actin. Whole mounts of the bronchial tree were imaged using confocal microscopy. The formation of ganglia commences in the pseudoglandular stage with patches of neuroblasts in the wall of the epithelial tubules. These ganglionic precursors are supplied with an abundance of nerve trunks and fibers that arise from the vagus and extend to the growing tips of the airways. These trunks show profiles of Schwann cells. As the airways grow, the ganglionic precursors condense at the nerve junctions. Nerve bundles in trunks and neurons in ganglia become increasingly enveloped by GFAP-positive sheaths. From midterm onward (canalicular stage), ganglia contain cholinergic neurons. In the third trimester (saccular stage) and postnatally, ganglia further increase in size and contain mainly nerve fibers in the center. Thus, neural tissue is a dominant feature of the primordial lung, which is enveloped by nerves and ganglia through gestation into postnatal life.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

Ontogeny of galanin-immunoreactive elements in the intrinsic nervous system of the chicken gut

Galanin is a brain-gut peptide that is present in the central and peripheral nervous systems. In the gut, it is contained exclusively in intrinsic and extrinsic nerve supplies, and it is involved overall in the regulation of gut motility. To obtain information about the ontogeny of galanin, we undertook an immunohistochemical study of chicken embryos. The time of first appearance and the distribution patterns of galanin were investigated with fluorescence and streptavidin-biotin-peroxidase (ABC) immunohistochemical protocols by using a galanin polyclonal antiserum. The various regions of the gut and the pancreas were obtained from chicken embryos aged from 3 days of incubation to hatching. All specimens were fixed in buffered picric acid-paraformaldehyde, frozen, and cut with a cryostat. Galanin-immunoreactive neuroblasts were first detected at 4 days in the mesenchyme of the proventriculus/gizzard primordium and within the Remak ganglion. They then extended cranially and caudally, reaching all of the other gut regions at 6.5 days. Galanin-immunoreactive nerve elements mainly occupied the sites of myenteric and submucous plexuses. From day 15, galanin-immunoreactive nerve fibers tended to invade the circular muscular layer and part of the lamina propria of the mucosa. In the pancreas, weak galanin-immunoreactive nerve elements were detected at 5.5 days. They tended to be distributed among the glandular lobules according to the organ differentiation. The widespread distribution during the earlier embryonic stages represents evidence indicating that the neuropeptide galanin may have a role as a differentiating or growth factor. From late embryonic life, its predominant presence in sympathetic nerves and in muscular layers fits with the functions demonstrated previously in adults of other vertebrates for galanin as a modulator of intestinal motility.

Alterations of the intramural nervous distributions in a chick intestinal atresia model

The postoperative intestinal dysmotility seen in intestinal atresia (IA) is usually found in association with a dilatation of the proximal intestinal segment, but the etiology of this disorder is not yet fully understood. A chick IA model was made by cutting the postumbilical midgut on d 11 in ovo. The operated chicks were euthanized 2 d after hatching. The samples were divided into two groups according to the extent of the dilatation of proximal ileal segments. Cryostat sections were processed for immunohistochemistry by the use of antisera to protein gene product 9.5, vasoactive intestinal polypeptide, substance-P, and alpha-smooth muscle actin and were also stained by NADPH-diaphorase. Tn highly dilated proximal segments, a decreased number of protein gene product 9.5-positive fibers was found in both the circular muscle and submucous layers. The number of nerve fibers positive for vasoactive intestinal polypeptide, substance-P, and NADPH-diaphorase also decreased in the circular muscle layer, particularly in the deep muscular plexus. Hypertrophy and an alteration of the staining intensities in the circular muscle layer were also revealed by a-smooth muscle actin staining. The nerve distribution of the distal segments was indistinguishable from that of the age-matched controls and the sham-operated group. Abnormalities in the intramural nerves are only found in the proximal ileal segment of the IA models. The abnormal nerve distribution of the proximal segment might thus be implicated in the postoperative dysmotility of the intestine in IA.

The sacral neural crest contributes neurons and glia to the post-umbilical gut: spatiotemporal analysis of the development of the enteric nervous system

The majority of the enteric nervous system is derived from vagal neural crest cells (NCC), which migrate to the developing gut, proliferate, form plexuses and differentiate into neurons and glia. However, for some time, controversy has existed as to whether cells from the sacral region of the neural crest also contribute to the enteric nervous system. The aim of this study was to investigate the spatiotemporal migration of vagal and sacral NCC within the developing gut and to determine whether the sacral neural crest contributes neurons and glia to the ENS. We utilised quail-chick chimeric grafting in conjunction with antibody labelling to identify graft-derived cells, neurons and glia. We found that vagal NCC migrated ventrally within the embryo and accumulated in the caudal branchial arches before entering the pharyngeal region and colonising the entire length of the gut in a proximodistal direction. During migration, vagal crest cells followed different pathways depending on the region of the gut being colonised. In the pre-umbilical intestine, NCC were evenly distributed throughout the splanchnopleural mesenchyme while, in the post-umbilical intestine, they occurred adjacent to the serosal epithelium. Behind this migration front, NCC became organised into the presumptive Auerbach's and Meissner's plexuses situated on either side of the developing circular muscle layer. The colorectum was found to be colonised in a complex manner. Vagal NCC initially migrated within the submucosa, internal to the circular muscle layer, before migrating outwards, adjacent to blood vessels, towards the myenteric plexus region. In contrast, sacral NCC, which also formed the entire nerve of Remak, were primarily located in the presumptive myenteric plexus region and subsequently migrated inwards towards the submucosal ganglia. Although present throughout the post-umbilical gut, sacral NCC were most numerous in the distal colorectum where they constituted up to 17% of enteric neurons, as identified by double antibody labelling using the quail-cell-specific marker, QCPN and the neuron-specific marker, ANNA-1. Sacral NCC were also immunopositive for the glial-specific antibody, GFAP, thus demonstrating that this region of the neural crest contributes neurons and glia to the enteric nervous system.

A single rostrocaudal colonization of the rodent intestine by enteric neuron precursors is revealed by the expression of Phox2b, Ret, and p75 and by explants grown under the kidney capsule or in organ culture

The colonization of the rodent gastrointestinal tract by enteric neuron precursors is controversial due to the lack of specific cellular markers at early stages. The transcription factor, Phox2b, is expressed by enteric neuron precursors (Pattyn et al. Development 124, 4065-4075, 1997). In this study, we have used an antiserum to Phox2b to characterize in detail the spatiotemporal expression of Phox2b in the gastrointestinal tract of adult mice and embryonic mice and rats. In adult mice, all enteric neurons (labeled with neuron-specific enolase antibodies), and a subpopulation of glial cells (labeled with GFAP antibodies), showed immunoreactivity to Phox2b. In embryonic mice, the appearance of Phox2b-immunoreactive cells was mapped during development of the gastrointestinal tract. At Embryonic Days 9.5-10 (E9.5-10), Phox2b-labeled cells were present only in the stomach, and during subsequent development, labeled cells appeared as a single rostrocaudal wave along the gastrointestinal tract; at E14 Phox2b-labeled cells were present along the entire length of the gastrointestinal tract. Ret and p75 have also been reported to label migratory-stage enteric neuron precursors. A unidirectional, rostral-to-caudal colonization of the gastrointestinal tract of embryonic mice by Ret- and p75-immunoreactive cells was also observed, and the locations of Ret- and p75-positive cells within the gut were very similar to that of Phox2b-positive cells. To verify the location of enteric neuron precursors within the gut, explants from spatiotemporally defined regions of embryonic intestine, 0.3-3 mm long, were grown in the kidney subcapsular space, or in catenary organ culture, and examined for the presence of neurons. The location and sequence of appearance of enteric neuron precursors deduced from the explants grown under the kidney capsule or in organ culture was very similar to that seen with the Phox2b, Ret, and p75 antisera. Previous studies have mapped the rostrocaudal colonization of the rat intestine by enteric neuron precursors using HNK-1 as a marker. In the current study, all HNK-1-labeled cells in the gastrointestinal tract of rat embryos showed immunoreactivity to Phox2b, but HNK-1 cells comprised only a small subpopulation of the Phox2b-labeled cells. In addition, in rats, Phox2b-labeled cells were present in advance of (more caudal to) the most caudal HNK-1-labeled cells by 600-700 microm in the hindgut at E15. We conclude that the neural crest cell population that arises from the vagal level of the neural axis and that populates the stomach, midgut, and hindgut expresses Phox2b, Ret, and p75. In contrast, the sacral-level neural crest cells that populate the hindgut either do not express, or show a delayed expression of, all of the known markers of vagal- and trunk-level neural crest cells.

Glial fibrillary acidic protein (GFAP) immunoreactivity in enteric ganglia of the chick embryo

We examined by immunohistochemistry the expression of glial fibrillary acidic protein (GFAP) in enteric ganglia of the chick embryo, using a polyclonal antibody. The morphology of enteric ganglion cells was examined by electron microscopy. Faint GFAP immunoreactivity was detected in ganglion cells and cell processes from around day 7 in ovo. Later in development the intensity of the immunofluorescence increased and it became more evident that immunoreactive small ganglion cells (interpreted as primitive glial cells), and their processes, surrounded larger negative cell profiles (interpreted as primitive neuronal cells); GFAP immunofluorescence was also evident in intramuscular and mucosal nerve trunks. In colocalization experiments, GFAP immunoreactivity was detected in a proportion of HNK-1/N-CAM immunoreactive ganglion cells, in both the myenteric and submucosal plexus. In addition, we observed GFAP immunoreactive nerves in wholemount preparations of chick gut from as early as day 4.5 in ovo. In the ganglionated nerve of Remak, GFAP immunoreactive satellite and Schwann cells were in evidence from day 5 of incubation. Neuronal markers, such as neurofilament, have been detected very early in development in neural crest cell populations in chick enteric ganglia. In contrast, the expression of markers of the glial phenotype has previously been observed only in the late stages of embryonic development. From our experiments, we conclude that neuronal and glial phenotypes are immunohistochemically distinct from as early as day 4.5 of incubation, even if by ultrastructural criteria glial cells are clearly distinguishable from neurons only after day 16 in ovo.

Transiently catecholaminergic cells in the fetal rat express mRNA for the glutamate NMDAR1 receptor

The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor has been shown to be vital to the development of the central nervous system. The purpose of this study was to determine if the neural crost-derived precursors which migrate to the primitive gut contain mRNA encoding for the NMDA receptor. Many of these enteric precursors briefly elaborate tyrosine hydroxylase (TH) and have been termed transiently catecholaminergic (TC) cells. TH-like immunoreactivity (TH-ir) serves as a marker for them. Immunocytochemistry combined with NMDAR1 in situ hybridization revealed that TH-ir cells in Day 14 rat embryos do express mRNA coding for the NMDAR1 receptor. However, the TC cells did not contain detectable levels of immunoreactivity for the NMDAR1 receptor peptide. The absence of detectable NMDAR1-like immunoreactivity might reflect some form of transcriptional or translational regulation, such that the onset of functional receptor activity is delayed until differentiation and/or synaptogenesis commence. Whether TC cell migration is glutamate-mediated remains unclear, since some of them successfully reached the gut without expressing NMDAR1 message. Characterizing TC cell NMDA receptor activity and determining exactly when it ensues will be of paramount importance to defining the role(s) of this receptor in ENS development. In conclusion, the expression of NMDAR1 mRNA by TH-ir cells suggests a possible developmental role for this receptor.

Differentiation of cholinergic cells in the rat gut during pre- and postnatal life

In the present work we studied the distribution and number of cholinergic neurons in the rat stomach, ileum and colon starting from prenatal life up to the adult animal. Cryo-cut sections of the three regions were incubated in the presence of the primary choline acetyltransferase (ChAT)-antibody and the immunoreaction was observed under an epifluorescence microscope and photographed. Our results demonstrate that cholinergic neurons are already present during prenatal life in the stomach and ileum, that several steps characterize cholinergic cell differentiation during postnatal life with a consistent delay in the appearance of ChAT-immunoreactivity (IR) in the submucous plexus compared to the myenteric plexus, and that the complete development is accomplished with weaning. In the colon the total number of ChAT-IR cells does not change from the suckling period to adulthood; a significantly larger number of ChAT-IR cells is found in the ileum of 5-day-old rats than in that of adult rats.

Expression of ret in the chicken embryo suggests roles in regionalisation of the vagal neural tube and somites and in development of multiple neural crest and placodal lineages

In a screen for receptor tyrosine kinase genes regionally expressed in the developing hindbrain, we cloned and characterised the chicken ret gene. Data derived from studies of congenital human disease and from disruption of murine ret have demonstrated roles for ret in development of the kidney and enteric nervous system; the latter has been most well-studied in the avian embryo. In agreement with studies of the mouse embryo, we find expression of ret in both the intermediate mesoderm and the enteric nervous system. However, we additionally detect transcripts specifically in the vagal neural tube prior to the migration of enteric crest precursors, suggesting a possible earlier function in regionalisation of the neural tube and vagal neural crest. This spatial restriction in the neural tube is modulated by retinoic acid, but is not coordinately regulated with RAR-beta which shares a common anterior limit of expression with ret in normal embryos. Widespread expression of ret in placodal and neural crest derivatives raises the possibility of other roles in the development of the peripheral nervous system. In addition, ret may function in the spatial organisation of the epithelial somite, where it might play a role in the specification of anterior dermamyotome.

Appearance of neurons in the developing chick gut

The enteric nervous system is formed from neural crest-derived cells. These cells enter the gut, migrate, proliferate, and ultimately differentiate into neurons and glia. We have used a human anti-neuronal autoantibody (ANNA-1), which recognizes neuron-specific RNA-binding proteins of the Hu family as an early marker of neuronal phenotype, to study the appearance of enteric neurons in the developing chicken gut. Immunoreactive cells appear first in the gizzard primordium at E3.5 and are found at progressively more caudal locations in the gut as development proceeds. Nascent neurons are present at the yolk stalk at E4.5, at the ileocecal junction at E6.5, and within the rectum at E7.5-8.5. Neurons appear slightly later in the esophagus. Aggregates of cells resembling developing ganglia were first observed at E6.5 in the distal esophagus and at E8.5 in the proximal esophagus. A small number of cells appeared in the vagus nerve trunks at E4.5 and that number increased at E7.5-8.5. Immunoreactive cells were also found in the sympatho-aortic plexus between the mesonephri and in the dorsal mesentery. These cells appeared to coalesce and form the ganglionated Nerve of Remak which contained positive cells at E3.5. This Nerve extended to the yolk stalk at E4.5 and to duodenum at E6.5. We conclude that the appearance of nascent neurons occurs first in the gizzard and proceeds more rapidly in a distal than proximal direction along the gut. Furthermore, cell that appear to be nascent neurons are found in the vagus and in the dorsal mesentery.

Structural study on the appearance of innervation in the stomach of mouse and rat embryos

The stomach of developing embryos was examined by light and electron microscopy on specimens taken at each embryonic day from 11 to 17 (rat) and from 10 to 16 (mouse). The aim of the study was to determine when the precursor cells of enteric neurons and endocrine cells colonize the stomach and when they begin to express morphologic features of mature cells. The findings show that the elements of the enteric nervous system are recognizable and functionally mature prior to the appearance of morphologically detectable gut epithelial endocrine cells. Some aspects of neuronal differentiation in the wall of the stomach are also discussed.

Mapping the origin of the avian enteric nervous system with a retroviral marker

The enteric nervous system is largely formed from the vagal neural crest which arises from the neuroaxis between somites 1-7. In order to evaluate the contribution of different regions of the vagal crest to the enteric nervous system, we marked crest cells by injecting somites 1-10 with a replication-defective spleen necrosis virus vector which contains the marker gene lacZ. After incubation in X-gal, lacZ-positive blue cells were found in the wall of the gut in three locations. Most were found at the peripheral edge of the developing circular muscle and within the developing submucosa, sites characteristic of developing ganglia. LacZ-positive cells in these ganglionic sites were always surrounded by HNK-1 immunostained cells, confirming their neural crest origin. LacZ-positive cells were also seen in a third location, the circular muscle layer of the esophagus and crop, and were separated from the HNK-1 positive ganglionic elements. These cells in the circular muscle are probably muscle cells derived from labeled mesodermal cells of the somite. Injection of somites 3, 4, 5, and 6 resulted in the largest percentage of preparations with lacZ-positive crest-derived cells and in the largest number of positive cells in the gut. After injection of these somites, lacZ-positive crest-derived cells were found in all regions of the gut from the proventriculus to the rectum. Very few positive crest-derived cells were found in the esophagus.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and birthdates of vasoactive intestinal peptide immunoreactive neurons in the chick proventriculus

To gain insight into the mechanisms regulating expression of transmitter phenotypes in the enteric nervous system, we have studied the development and birthdate of vasoactive intestinal peptide immunoreactive (VIP-IR) myenteric neurons in the chicken proventriculus (secretory portion of the avian stomach) by a combination of immunocytochemistry and radioautography. The appearance and numbers of VIP-IR neurons in whole mounts of the myenteric plexus from chick embryos and chickens were examined. We found that VIP-IR neurons first appeared at embryonic day (E) 5.5-6.5 in the distal part of the proventriculus. At E7.5, VIP-IR neurons were found singly, in pairs, or in small groups, which together with unlabeled cells formed primitive myenteric ganglia. VIP-IR fibers were found within the developing fiber tracts which connected the ganglia. The number of VIP-IR neurons was found to be maximum in the E15.5 embryo and to decline to 68% of maximum in the 4 week old chicken. Birthdate studies were performed by application of either single pulses or cumulative doses of [3H]-thymidine to embryos between E3 and E14. Whole mounts of the myenteric plexus from the proventriculus of these embryos were immunostained for VIP at E10 or E17. The whole mounts were subsequently sectioned and processed for radioautography. We found that VIP-IR myenteric neurons were born between E3 and E10 with a peak at E7. Most cells underwent terminal division between E5 and E9. These data will be useful in determining the time and conditions when cells make decisions about transmitter phenotypes.

Appearance of somatostatin and vasoactive intestinal peptide along the developing chicken gut

The appearance of somatostatin (SOM)-immunoreactive (IR) and vasoactive intestinal peptide (VIP)-IR neurons in different regions of the embryonic chicken gut was studied by immunostaining wholemounts. The patterns of expression of these peptides in myenteric neurons showed a number of similarities. Both peptides first appeared in the region of the proventriculus-gizzard: SOM at embryonic day (E)4, VIP at E5.5. At later times both peptides were found in positions both rostral and caudal to the gizzard. Both peptides appeared independently in cells at a second site, the cecum of the hindgut: SOM was observed at E6.5 and VIP at E7.5. VIP-IR and SOM-IR cells appear throughout the cecum, then in the rectum, and finally in the ileum. Differences in the patterns of expression were also found. SOM- and VIP-IR neurons appeared at different times along the length of the gut. VIP-IR cells populated the entire gut by E11.5, whereas SOM-IR cells were not present throughout the gut until E13.5. SOM-IR cells appeared in the terminal part of the ganglion of Remak at E4.0. At E6 these SOM-IR cells sent fibers into the wall of the hindgut and later into the midgut. No VIP-IR cells were found in the ganglion of Remak. These findings suggest that neural crest-derived cells first express SOM- and VIP-IR in particular regions of the gut, namely, the proventriculus-gizzard and the cecum. Certain conditions must exist at these sites which favor the expression of these neuropeptides by neural crest-derived cells. The observation of SOM- and VIP-IR cells in the cecum at a stage of development before cells are seen in the ileum supports the concept that sacral neural crest cells contribute precursors for enteric neurons of the avian hindgut.