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

A targeted CRISPR-Cas9 mediated F0 screen identifies genes involved in establishment of the enteric nervous system

The vertebrate enteric nervous system (ENS) is a crucial network of enteric neurons and glia resident within the entire gastrointestinal tract (GI). Overseeing essential GI functions such as gut motility and water balance, the ENS serves as a pivotal bidirectional link in the gut-brain axis. During early development, the ENS is primarily derived from enteric neural crest cells (ENCCs). Disruptions to ENCC development, as seen in conditions like Hirschsprung disease (HSCR), lead to the absence of ENS in the GI, particularly in the colon. In this study, using zebrafish, we devised an in vivo F0 CRISPR-based screen employing a robust, rapid pipeline integrating single-cell RNA sequencing, CRISPR reverse genetics, and high-content imaging. Our findings unveil various genes, including those encoding opioid receptors, as possible regulators of ENS establishment. In addition, we present evidence that suggests opioid receptor involvement in the neurochemical coding of the larval ENS. In summary, our work presents a novel, efficient CRISPR screen targeting ENS development, facilitating the discovery of previously unknown genes, and increasing knowledge of nervous system construction.

Genetic regulation of enteric nervous system development in zebrafish

The enteric nervous system (ENS) is a complex series of interconnected neurons and glia that reside within and along the entire length of the gastrointestinal tract. ENS functions are vital to gut homeostasis and digestion, including local control of peristalsis, water balance, and intestinal cell barrier function. How the ENS develops during embryological development is a topic of great concern, as defects in ENS development can result in various diseases, the most common being Hirschsprung disease, in which variable regions of the infant gut lack ENS, with the distal colon most affected. Deciphering how the ENS forms from its progenitor cells, enteric neural crest cells, is an active area of research across various animal models. The vertebrate animal model, zebrafish, has been increasingly leveraged to understand early ENS formation, and over the past 20 years has contributed to our knowledge of the genetic regulation that underlies enteric development. In this review, I summarize our knowledge regarding the genetic regulation of zebrafish enteric neuronal development, and based on the most current literature, present a gene regulatory network inferred to underlie its construction. I also provide perspectives on areas for future zebrafish ENS research.

A targeted CRISPR-Cas9 mediated F0 screen identifies genes involved in establishment of the enteric nervous system

The vertebrate enteric nervous system (ENS) is a crucial network of enteric neurons and glia resident within the entire gastrointestinal tract (GI). Overseeing essential GI functions such as gut motility and water balance, the ENS serves as a pivotal bidirectional link in the gut-brain axis. During early development, the ENS is primarily derived from enteric neural crest cells (ENCCs). Disruptions to ENCC development, as seen in conditions like Hirschsprung disease (HSCR), lead to absence of ENS in the GI, particularly in the colon. In this study, using zebrafish, we devised an in vivo F0 CRISPR-based screen employing a robust, rapid pipeline integrating single-cell RNA sequencing, CRISPR reverse genetics, and high-content imaging. Our findings unveil various genes, including those encoding for opioid receptors, as possible regulators of ENS establishment. In addition, we present evidence that suggests opioid receptor involvement in neurochemical coding of the larval ENS. In summary, our work presents a novel, efficient CRISPR screen targeting ENS development, facilitating the discovery of previously unknown genes, and increasing knowledge of nervous system construction.

Reprogramming of trunk neural crest to a cranial crest-like identity alters their transcriptome and developmental potential

Neural crest cells along the body axis of avian embryos differ in their developmental potential, such that the cranial neural crest forms cartilage and bone whereas the trunk neural crest is unable to do so. Previous studies have identified a cranial crest-specific subcircuit that can imbue the trunk neural crest with the ability to form cartilage after grafting to the head. Here, we examine transcriptional and cell fate changes that accompany this reprogramming. First, we examined whether reprogrammed trunk neural crest maintain the ability to form cartilage in their endogenous environment in the absence of cues from the head. The results show that some reprogrammed cells contribute to normal trunk neural crest derivatives, whereas others migrate ectopically to the forming vertebrae and express cartilage markers, thus mimicking heterotypically transplanted cranial crest cells. We find that reprogrammed trunk neural crest upregulated more than 3000 genes in common with cranial neural crest, including numerous transcriptional regulators. In contrast, many trunk neural crest genes are downregulated. Together, our findings show that reprogramming trunk neural crest with cranial crest subcircuit genes alters their gene regulatory program and developmental potential to be more cranial crest-like.

The transcriptional portraits of the neural crest at the individual cell level

Since the discovery of this cell population by His in 1850, the neural crest has been under intense study for its important role during vertebrate development. Much has been learned about the function and regulation of neural crest cell differentiation, and as a result, the neural crest has become a key model system for stem cell biology in general. The experiments performed in embryology, genetics, and cell biology in the last 150 years in the neural crest field has given rise to several big questions that have been debated intensely for many years: "How does positional information impact developmental potential? Are neural crest cells individually multipotent or a mixed population of committed progenitors? What are the key factors that regulate the acquisition of stem cell identity, and how does a stem cell decide to differentiate towards one cell fate versus another?" Recently, a marriage between single cell multi-omics, statistical modeling, and developmental biology has shed a substantial amount of light on these questions, and has paved a clear path for future researchers in the field.

Seq Your Destiny: Neural Crest Fate Determination in the Genomic Era

Neural crest stem/progenitor cells arise early during vertebrate embryogenesis at the border of the forming central nervous system. They subsequently migrate throughout the body, eventually differentiating into diverse cell types ranging from neurons and glia of the peripheral nervous system to bones of the face, portions of the heart, and pigmentation of the skin. Along the body axis, the neural crest is heterogeneous, with different subpopulations arising in the head, neck, trunk, and tail regions, each characterized by distinct migratory patterns and developmental potential. Modern genomic approaches like single-cell RNA- and ATAC-sequencing (seq) have greatly enhanced our understanding of cell lineage trajectories and gene regulatory circuitry underlying the developmental progression of neural crest cells. Here, we discuss how genomic approaches have provided new insights into old questions in neural crest biology by elucidating transcriptional and posttranscriptional mechanisms that govern neural crest formation and the establishment of axial level identity. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Between Fate Choice and Self-Renewal-Heterogeneity of Adult Neural Crest-Derived Stem Cells

Stem cells of the neural crest (NC) vitally participate to embryonic development, but also remain in distinct niches as quiescent neural crest-derived stem cell (NCSC) pools into adulthood. Although NCSC-populations share a high capacity for self-renewal and differentiation resulting in promising preclinical applications within the last two decades, inter- and intrapopulational differences exist in terms of their expression signatures and regenerative capability. Differentiation and self-renewal of stem cells in developmental and regenerative contexts are partially regulated by the niche or culture condition and further influenced by single cell decision processes, making cell-to-cell variation and heterogeneity critical for understanding adult stem cell populations. The present review summarizes current knowledge of the cellular heterogeneity within NCSC-populations located in distinct craniofacial and trunk niches including the nasal cavity, olfactory bulb, oral tissues or skin. We shed light on the impact of intrapopulational heterogeneity on fate specifications and plasticity of NCSCs in their niches in vivo as well as during in vitro culture. We further discuss underlying molecular regulators determining fate specifications of NCSCs, suggesting a regulatory network including NF-κB and NC-related transcription factors like SLUG and SOX9 accompanied by Wnt- and MAPK-signaling to orchestrate NCSC stemness and differentiation. In summary, adult NCSCs show a broad heterogeneity on the level of the donor and the donors' sex, the cell population and the single stem cell directly impacting their differentiation capability and fate choices in vivo and in vitro. The findings discussed here emphasize heterogeneity of NCSCs as a crucial parameter for understanding their role in tissue homeostasis and regeneration and for improving their applicability in regenerative medicine.

An atlas of neural crest lineages along the posterior developing zebrafish at single-cell resolution

Neural crest cells (NCCs) are vertebrate stem cells that give rise to various cell types throughout the developing body in early life. Here, we utilized single-cell transcriptomic analyses to delineate NCC-derivatives along the posterior developing vertebrate, zebrafish, during the late embryonic to early larval stage, a period when NCCs are actively differentiating into distinct cellular lineages. We identified several major NCC/NCC-derived cell-types including mesenchyme, neural crest, neural, neuronal, glial, and pigment, from which we resolved over three dozen cellular subtypes. We dissected gene expression signatures of pigment progenitors delineating into chromatophore lineages, mesenchyme cells, and enteric NCCs transforming into enteric neurons. Global analysis of NCC derivatives revealed they were demarcated by combinatorial hox gene codes, with distinct profiles within neuronal cells. From these analyses, we present a comprehensive cell-type atlas that can be utilized as a valuable resource for further mechanistic and evolutionary investigations of NCC differentiation.

Neural crest lineage analysis: from past to future trajectory

Since its discovery 150 years ago, the neural crest has intrigued investigators owing to its remarkable developmental potential and extensive migratory ability. Cell lineage analysis has been an essential tool for exploring neural crest cell fate and migration routes. By marking progenitor cells, one can observe their subsequent locations and the cell types into which they differentiate. Here, we review major discoveries in neural crest lineage tracing from a historical perspective. We discuss how advancing technologies have refined lineage-tracing studies, and how clonal analysis can be applied to questions regarding multipotency. We also highlight how effective progenitor cell tracing, when combined with recently developed molecular and imaging tools, such as single-cell transcriptomics, single-molecule fluorescence in situ hybridization and high-resolution imaging, can extend the scope of neural crest lineage studies beyond development to regeneration and cancer initiation.

Pulmonary Function in Patients With Multiple Endocrine Neoplasia 2B

CONTEXT

Multiple endocrine neoplasia type 2B (MEN2B) is a rare cancer predisposition syndrome resulting from an autosomal-dominant germline mutation of the RET proto-oncogene. No prior studies have investigated pulmonary function in patients with MEN2B.

OBJECTIVE

This study characterized the pulmonary function of patients with MEN2B.

DESIGN

This is a retrospective analysis of pulmonary function tests (PFTs) and chest imaging of patients enrolled in the Natural History Study of Children and Adults with MEN2A or MEN2B at the National Institutes of Health.

RESULTS

Thirty-six patients with MEN2B (18 males, 18 females) were selected based on the availability of PFTs; 27 patients underwent at least 2 PFTs and imaging studies. Diffusion abnormalities were observed in 94% (33/35) of the patients, with 63% (22/35) having moderate to severe defects. A declining trend in diffusion capacity was seen over time, with an estimated slope of -2.9% per year (P = 0.0001). Restrictive and obstructive abnormalities were observed in 57% (20/35) and 39% (14/36), respectively. Computed tomography imaging revealed pulmonary thin-walled cavities (lung cysts) in 28% (9/32) of patients and metastatic lung disease in 34% (11/32) of patients; patients with metastatic lung lesions also tended to have thin-walled cavities (P = 0.035).

CONCLUSIONS

This study characterized pulmonary function within a MEN2B cohort. Diffusion, restrictive, and obstructive abnormalities were evident, and lung cysts were present in 28% of patients. Further research is required to determine the mechanism of the atypical pulmonary features observed in this cohort.

Immunohistochemical and ultrastructural analysis of the maturing larval zebrafish enteric nervous system reveals the formation of a neuropil pattern

The gastrointestinal tract is constructed with an intrinsic series of interconnected ganglia that span its entire length, called the enteric nervous system (ENS). The ENS exerts critical local reflex control over many essential gut functions; including peristalsis, water balance, hormone secretions and intestinal barrier homeostasis. ENS ganglia exist as a collection of neurons and glia that are arranged in a series of plexuses throughout the gut: the myenteric plexus and submucosal plexus. While it is known that enteric ganglia are derived from a stem cell population called the neural crest, mechanisms that dictate final neuropil plexus organization remain obscure. Recently, the vertebrate animal, zebrafish, has emerged as a useful model to understand ENS development, however knowledge of its developing myenteric plexus architecture was unknown. Here, we examine myenteric plexus of the maturing zebrafish larval fish histologically over time and find that it consists of a series of tight axon layers and long glial cell processes that wrap the circumference of the gut tube to completely encapsulate it, along all levels of the gut. By late larval stages, complexity of the myenteric plexus increases such that a layer of axons is juxtaposed to concentric layers of glial cells. Ultrastructurally, glial cells contain glial filaments and make intimate contacts with one another in long, thread-like projections. Conserved indicators of vesicular axon profiles are readily abundant throughout the larval plexus neuropil. Together, these data extend our understanding of myenteric plexus architecture in maturing zebrafish, thereby enabling functional studies of its formation in the future.

The molecular basis of neural crest axial identity

The neural crest is a migratory cell population that contributes to multiple tissues and organs during vertebrate embryonic development. It is remarkable in its ability to differentiate into an array of different cell types, including melanocytes, cartilage, bone, smooth muscle, and peripheral nerves. Although neural crest cells are formed along the entire anterior-posterior axis of the developing embryo, they can be divided into distinct subpopulations based on their axial level of origin. These groups of cells, which include the cranial, vagal, trunk, and sacral neural crest, display varied migratory patterns and contribute to multiple derivatives. While these subpopulations have been shown to be mostly plastic and to differentiate according to environmental cues, differences in their intrinsic potentials have also been identified. For instance, the cranial neural crest is unique in its ability to give rise to cartilage and bone. Here, we examine the molecular features that underlie such developmental restrictions and discuss the hypothesis that distinct gene regulatory networks operate in these subpopulations. We also consider how reconstructing the phylogeny of the trunk and cranial neural crest cells impacts our understanding of vertebrate evolution.

The neural crest and evolution of the head/trunk interface in vertebrates

The migration and distribution patterns of neural crest (NC) cells reflect the distinct embryonic environments of the head and trunk: cephalic NC cells migrate predominantly along the dorsolateral pathway to populate the craniofacial and pharyngeal regions, whereas trunk crest cells migrate along the ventrolateral pathways to form the dorsal root ganglia. These two patterns thus reflect the branchiomeric and somitomeric architecture, respectively, of the vertebrate body plan. The so-called vagal NC occupies a postotic, intermediate level between the head and trunk NC. This level of NC gives rise to both trunk- and cephalic-type (circumpharyngeal) NC cells. The anatomical pattern of the amphioxus, a basal chordate, suggests that somites and pharyngeal gills coexist along an extensive length of the body axis, indicating that the embryonic environment is similar to that of vertebrate vagal NC cells and may have been ancestral for vertebrates. The amniote-like condition in which the cephalic and trunk domains are distinctly separated would have been brought about, in part, by anteroposterior reduction of the pharyngeal domain.

Retinoic acid temporally orchestrates colonization of the gut by vagal neural crest cells

The enteric nervous system arises from neural crest cells that migrate as chains into and along the primitive gut, subsequently differentiating into enteric neurons and glia. Little is known about the mechanisms governing neural crest migration en route to and along the gut in vivo. Here, we report that Retinoic Acid (RA) temporally controls zebrafish enteric neural crest cell chain migration. In vivo imaging reveals that RA loss severely compromises the integrity and migration of the chain of neural crest cells during the window of time window when they are moving along the foregut. After loss of RA, enteric progenitors accumulate in the foregut and differentiate into enteric neurons, but subsequently undergo apoptosis resulting in a striking neuronal deficit. Moreover, ectopic expression of the transcription factor meis3 and/or the receptor ret, partially rescues enteric neuron colonization after RA attenuation. Collectively, our findings suggest that retinoic acid plays a critical temporal role in promoting enteric neural crest chain migration and neuronal survival upstream of Meis3 and RET in vivo.

Enteric nervous system development in avian and zebrafish models

Our current understanding of the developmental biology of the enteric nervous system (ENS) and the genesis of ENS diseases is founded almost entirely on studies using model systems. Although genetic studies in the mouse have been at the forefront of this field over the last 20 years or so, historically it was the easy accessibility of the chick embryo for experimental manipulations that allowed the first descriptions of the neural crest origins of the ENS in the 1950s. More recently, studies in the chick and other non-mammalian model systems, notably zebrafish, have continued to advance our understanding of the basic biology of ENS development, with each animal model providing unique experimental advantages. Here we review the basic biology of ENS development in chick and zebrafish, highlighting conserved and unique features, and emphasising novel contributions to our general understanding of ENS development due to technical or biological features.

Meis3 is required for neural crest invasion of the gut during zebrafish enteric nervous system development

Loss of Meis3 leads to defects in enteric neural crest cell migration, number, and proliferation during colonization of the gut. This leads to colonic aganglionosis, in which the hindgut is devoid of neurons, identifying it as a novel candidate factor in the etiology of Hirschsprung’s disease during enteric nervous system development.

During development, vagal neural crest cells fated to contribute to the enteric nervous system migrate ventrally away from the neural tube toward and along the primitive gut. The molecular mechanisms that regulate their early migration en route to and entry into the gut remain elusive. Here we show that the transcription factor meis3 is expressed along vagal neural crest pathways. Meis3 loss of function results in a reduction in migration efficiency, cell number, and the mitotic activity of neural crest cells in the vicinity of the gut but has no effect on neural crest or gut specification. Later, during enteric nervous system differentiation, Meis3-depleted embryos exhibit colonic aganglionosis, a disorder in which the hindgut is devoid of neurons. Accordingly, the expression of Shh pathway components, previously shown to have a role in the etiology of Hirschsprung’s disease, was misregulated within the gut after loss of Meis3. Taken together, these findings support a model in which Meis3 is required for neural crest proliferation, migration into, and colonization of the gut such that its loss leads to severe defects in enteric nervous system development.

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.

Two patterns of development of interstitial cells of Cajal in the human duodenum

At the end of the embryonic period of human development, c-kit immunoreactive (c-kit IR) cells identifiable as interstitial cells of Cajal (ICC) are present in the oesophagus and stomach wall. In the small and large bowel, c-kit-IR cells appear later (in the small bowel at 9 weeks, and in the colon at 10-12 weeks), also in the MP region. The object of this study was to determine the timing of appearance and distribution of c-kit IR cells in the human embryonic and foetal duodenum. I used immunohistochemistry to examine the embryonic and foetal duodenum for cells expressing CD117 (Kit), expressed by mature ICC and ICC progenitor cells and CD34 to identify presumed ICC progenitors. Enteric plexuses were examined by way of antineuron-specific enolase and the differentiation of smooth muscle cells was studied using antidesmin antibodies. At the end of the embryonic period of development, c-kit IR cells were solely present in the proximal duodenum in the form of a wide belt of densely packed cells around the inception of the myenteric plexus (MP) ganglia. In the distal duodenum, c-kit IR cells emerged at the beginning of the foetal period in the form of thin rows of pleomorphic cells at the level of the MP. From the beginning of the fourth month, the differences in the distribution of ICC in the different portions of the duodenum were established, and this relationship was still present in later developmental stages. In fact, in the proximal duodenum, ICC of the MP (ICC-MP), ICC of the circular muscle (ICC-CM) and ICC of the septa (ICC-SEP) were present, and in the distal duodenum ICC-MP and ICC-SEP only. In conclusion, in the humans there is a difference in the timing and patterns of development of ICC in the proximal duodenum compared to the distal duodenum.

Can mesenchymal cells undergo collective cell migration? The case of the neural crest

Cell migration is critical for proper development of the embryo and is also used by many cell types to perform their physiological function. For instance, cell migration is essential for immune cells to monitor the body and for epithelial cells to heal a wound whereas, in cancer cells, acquisition of migratory capabilities is a critical step towards malignancy. Migratory cells are often categorized into two groups: mesenchymal cells, produced by an epithelium-to-mesenchyme transition, that undergo solitary migration and epithelial-like cells which migrate collectively. However, on some occasions, mesenchymal cells may travel in large, dense groups and exhibit key features of collectively migrating cells such as coordination and cooperation. Here, using data published on Neural Crest cells, a highly invasive mesenchymal cell population that extensively migrate throughout the embryo, we explore the idea that other mesenchymal cells, including cancer cells, might be able to undergo collective cell migration under certain conditions and discuss how they could do so.

Vagal neural crest cell migratory behavior: a transition between the cranial and trunk crest

Migration and differentiation of cranial neural crest cells are largely controlled by environmental cues, whereas pathfinding at the trunk level is dictated by cell-autonomous molecular changes owing to early specification of the premigratory crest. Here, we investigated the migration and patterning of vagal neural crest cells. We show that (1) vagal neural crest cells exhibit some developmental bias, and (2) they take separate pathways to the heart and to the gut. Together these observations suggest that prior specification dictates initial pathway choice. However, when we challenged the vagal neural crest cells with different migratory environments, we observed that the behavior of the anterior vagal neural crest cells (somite-level 1-3) exhibit considerable migratory plasticity, whereas the posterior vagal neural crest cells (somite-level 5-7) are more restricted in their behavior. We conclude that the vagal neural crest is a transitional population that has evolved between the head and the trunk.

The migration of autonomic precursor cells in the embryo

The neural crest is an excellent model system to study cell fate and cell guidance signaling. Neural crest cells emerge from a common multipotent subpopulation and follow stereotypical migratory pathways to contribute to many diverse peripheral structures throughout the vertebrate embryo. The neural tube and diverse embryonic microenvironments from which the neural crest originate and migrate through are important sources of signals, yet it is still unclear how a common pool of neural crest stem and progenitor cells diversify and become distributed along specific stereotypical migratory paths. In the post-otic hindbrain and trunk, the neural crest emerge and contribute to the autonomic nervous system, and failure of proper cell navigation and differentiation often leads to congenital disorders that include dysautonomias, Hirschprung's disease, and neuroblastoma cancer. Recent exciting studies of neural crest cell behaviors have revealed the interplay of several molecular signaling pathways that guide and shape autonomic precursor cells to and into proper target structures, suggesting further work may help to better understand autonomic nervous system assembly, derived from a convergence of time-lapse imaging and molecular analyses. In this mini-review, we summarize recent fluorescent cell labeling strategies and cell behavior analyses that elucidate the role of molecular signals on the migration of autonomic precursor cells. We highlight advances in our understanding of the autonomic precursor cell behaviors and fate determination studied within the embryonic microenvironment.

A bird's eye view of enteric nervous system development: lessons from the avian embryo

The avian embryo has been an important model system for studying enteric nervous system (ENS) development for over 50 y. Since the initial demonstration in chick embryos that the ENS is derived from the neural crest, investigators have used the avian model to reveal the cellular origins and migratory pathways of enteric neural crest-derived cells, with more recent work focusing on the molecular mechanisms regulating ENS development. Seminal contributions have been made in this field by researchers who have taken advantage of the strengths of the avian model system. These strengths include in vivo accessibility throughout development, ability to generate quail-chick chimeras, and the capacity to modulate gene expression in vivo in a spatially and temporally targeted manner. The recent availability of the chicken genome further enhances this model system, allowing investigators to combine classic embryologic methods with current genetic techniques. The strengths and versatility of the avian embryo continue to make it a valuable experimental system for studying the development of the ENS.

Critical numbers of neural crest cells are required in the pathways from the neural tube to the foregut to ensure complete enteric nervous system formation

The enteric nervous system (ENS) is mainly derived from vagal neural crest cells (NCC) that arise at the level of somites 1-7. To understand how the size and composition of the NCC progenitor pool affects ENS development, we reduced the number of NCC by ablating the neural tube adjacent to somites 3-6 to produce aganglionic gut. We then back-transplanted various somite lengths of quail neural tube into the ablated region to determine the `tipping point',whereby sufficient progenitors were available for complete ENS formation. The addition of one somite length of either vagal, sacral or trunk neural tube into embryos that had the neural tube ablated adjacent to somites 3-6,resulted in ENS formation along the entire gut. Although these additional cells contributed to the progenitor pool, the quail NCC from different axial levels retained their intrinsic identities with respect to their ability to form the ENS; vagal NCC formed most of the ENS, sacral NCC contributed a limited number of ENS cells, and trunk NCC did not contribute to the ENS. As one somite length of vagal NCC was found to comprise almost the entire ENS, we ablated all of the vagal neural crest and back-transplanted one somite length of vagal neural tube from the level of somite 1 or somite 3 into the vagal region at the position of somite 3. NCC from somite 3 formed the ENS along the entire gut, whereas NCC from somite 1 did not. Intrinsic differences, such as an increased capacity for proliferation, as demonstrated in vitro and in vivo,appear to underlie the ability of somite 3 NCC to form the entire ENS.

A role for RhoA in the two-phase migratory pattern of post-otic neural crest cells

Neural crest (NC) cells have been elegantly traced to follow stereotypical migratory pathways throughout the vertebrate embryo, yet we still lack complete information on individual cell migratory behaviors and how molecular mechanisms direct NC cell guidance. Here, we analyze the spatio-temporal migratory pattern of post-otic NC and the in vivo role of the small Rho GTPase, RhoA, using fluorescent cell labeling, molecular perturbation, and intravital 4D (3D+ time) confocal imaging in the intact chick embryo. We find that the post-otic NC cell migratory pattern is established in two phases with distinct cell migratory behaviors. An initial wide front of lateral-directed NC cells, led by NC from rhombomere 7 (r7), move as a distinct subpopulation. This is followed in time by fewer NC cells that migrate collectively from r7 to r8 in a follow-the-leader manner with extensive cellular extensions between cells. We show that post-otic migratory NC cells express RhoA, using RT-PCR on isolated, flow cytometry sorted NC cells and in neural tube culture explants. When RhoA function is altered by expression of a dominant negative or constitutively active form, or injection of C3, there are two major consequences. RhoA constitutively active expressing NC cells are less directional, slower and form fewer follow-the-leader chain assemblies. NC cells expressing RhoA-DN are less affective in retracting filopodia, migrate slower and also form fewer follow-the-leader chain assemblies. Together, these alterations to NC cell intrinsic signaling and cell-cell contact disrupt the precise spatio-temporal post-otic NC cell migratory pattern.

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.

Lineage specification in neural crest cell pathfinding

There are two principal models to explain neural crest patterning. One assumes that neural crest cells are multipotent precursors that migrate throughout the embryo and differentiate according to cues present in the local environment. A second proposes that the neural crest is a population of cells that becomes restricted to particular fates early in its existence and migrates along particular pathways dependent on unique cell-autonomous properties. Although it is now evident that the neural crest cell population, as a whole, is actually heterogenous (composed of both multipotent and restricted progenitors), evidence supporting the model of prespecification has increased over the past few years. This review will begin by telling the story of melanoblasts: a neural crest subpopulation that is biased toward a single fate and subsequently acquires intrinsic properties that guide cells of this lineage to their final destination. The remainder of this review will explore whether this model is exclusive to melanoblasts or if it can also be used to explain the patterning of other neural crest cells like those of the sensory, sympathoadrenal, and enteric lineages.

Vagal neural crest contribution to the chick embryo cloaca

Intrinsic innervation of the developing chick cloaca is provided by the enteric nervous system, a network of neurons and glia that lies within its walls. The enteric nervous system originates from neural crest cells that migrate from the vagal and sacral regions of the neural tube during the early stages of development. Abnormal cloacal development can cause a number of anorectal anomalies including persistent cloaca. Our study aimed to investigate the contribution of vagal neural crest cells to the total population of enteric neurons and glia within the chick embryo cloaca, using quail-chick chimeras. Chicken embryos were incubated until the 10-12 somite stage (ss). The vagal neural tube, corresponding to somites 1-7, was then microsurgically ablated in ovo and isochronic and isotopic quail grafts were performed. The eggs were then reincubated until embryos were harvested at E12. Whole embryos were fixed in Bouin's fluid, embedded in paraffin wax and sectioned. Immunohistochemistry was carried out using the HNK-1 antibody to label all neural crest cells, and the quail-specific antibody, QCPN, to label quail cells. QCPN-immunoreactive cells were seen to make up a large proportion of enteric neurons and glia within the walls of the embryonic cloaca. HNK-1 labelled all neural crest cells in the myenteric and submucosal plexuses as well as the sacral crest-derived nerve of Remak, while QCPN-positive cells were evident in both plexuses but mostly in the submucosal plexus, where they appeared to make up the majority of neurons. Results show that the chick embryo cloaca is primarily innervated by vagal neural crest cells. Further studies to investigate the contribution of sacral neural crest cells to the same region will give further insight into the development of the enteric nervous system within the embryonic cloaca.

Cell proliferation drives neural crest cell invasion of the intestine

A general mathematical model of cell invasion is developed and validated with an experimental system. The model incorporates two basic cell functions: non-directed (diffusive) motility and proliferation to a carrying capacity limit. The model is used here to investigate cell proliferation and motility differences along the axis of an invasion wave. Mathematical simulations yield surprising and counterintuitive predictions. In this general scenario, cells at the invasive front are proliferative and migrate into previously unoccupied tissues while those behind the front are essentially nonproliferative and do not directly migrate into unoccupied tissues. These differences are not innate to the cells, but are a function of proximity to uninvaded tissue. Therefore, proliferation at the invading front is the critical mechanism driving apparently directed invasion. An appropriate system to experimentally validate these predictions is the directional invasion and colonization of the gut by vagal neural crest cells that establish the enteric nervous system. An assay using gut organ culture with chick-quail grafting is used for this purpose. The experimental results are entirely concordant with the mathematical predictions. We conclude that proliferation at the wavefront is a key mechanism driving the invasive process. This has important implications not just for the neural crest, but for other invasion systems such as epidermal wound healing, carcinoma invasion and other developmental cell migrations.

Identification of the lacZ insertion site and beta-galactosidase expression in transgenic chickens

The quail:chick chimera system is a classical research model in developmental biology. An improvement over the quail:chick chimera system would be a line of transgenic chickens expressing a reporter gene. Transgenic chickens carrying lacZ and expressing bacterial beta-galactosidase have been generated, but complete characterization of the insertion event and characterization of beta-galactosidase expression have not previously been available. The genomic sequences flanking the retroviral insertion site have now been identified by using inverse polymerase chain reaction (PCR), homozygous individuals have been identified by using PCR-based genotyping, and beta-galactosidase expression has been evaluated by using Western analysis and histochemistry. Based upon the current draft of the chicken genome, the viral insertion carrying the lacZ gene has been located on chromosome 11 within the predicted gene for neurotactin/fractalkine (CX3CL1); neurotactin mRNA expression appears to be missing from the brain of homozygous individuals. When Generation 2 (G2) lacZ-positive individuals were inter-mated, they generated 361 G3 progeny; 82 were homozyous for lacZ (22.7%), 97 were wild-type non-transgenic (26.9%), and 182 (50.4%) were hemizygous for lacZ. Western analysis revealed the highest expression in the muscle and liver. With the identification of homozygous birds, the line of chickens is now designated NCSU-Blue1.

Phenotypes of neural-crest-derived cells in vagal and sacral pathways

Enteric neurons arise from vagal and sacral level neural crest cells. To examine the phenotype of neural-crest-derived cells in vagal and sacral pathways, we used antisera to Sox10, p75, Phox2b, and Hu, and transgenic mice in which the expression of green fluorescent protein was under the control of the Ret promoter. Sox10 was expressed prior to the emigration of vagal cells, whereas p75 was expressed shortly after their emigration. Most crest-derived cells that emigrated adjacent to somites 1-4 migrated along a pathway that was later followed by the vagus nerve. A sub-population of these vagal cells coalesced to form vagal ganglia, whereas others continued their migration towards the heart and gut. Cells that coalesced into vagal ganglia showed a different phenotype from cells in the migratory streams proximal and distal to the ganglia. Only a sub-population of the vagal cells that first entered the foregut expressed Phox2b or Ret. Sacral neural crest cells gave rise to pelvic ganglia and some neurons in the hindgut. The pathways of sacral neural crest cells were examined by using DbetaH-nlacZ mice. Sacral cells appeared to enter the distal hindgut around embryonic day 14.5. Very few of the previously demonstrated, but rare, neurons that were present in the large intestine of Ret null mutants and that presumably arose from the sacral neural crest expressed nitric oxide synthase, unlike their counterparts in Ret heterozygous mice.

The effect of vagal neural crest ablation on the chick embryo cloaca

The cloaca, the caudal limit of the avian gastrointestinal tract, acts as a collecting chamber into which the gastrointestinal, urinary, and genital tracts discharge. It is intrinsically innervated by the enteric nervous system, which is derived from neural crest emigres that migrate from the vagal and sacral regions of the neural tube. Abnormal cloacal development can cause a number of anorectal anomalies, including persistent cloaca. Ablation of the vagal neural crest has previously been shown to result in an aganglionic hindgut to the extent of the colorectum. The aim of our study was to investigate the effect of vagal neural crest ablation on the cloaca, the limit of the hindgut in the developing chick embryo. Chick embryos were incubated until the 10-12 somite stage. The vagal neural tube corresponding to the level of somites 3-6 was then ablated, and eggs were incubated until harvested on embryonic day 11 (E11). Whole chick embryos were fixed, embedded in paraffin, and sectioned. Immunohistochemistry was then carried out using the HNK-1 monoclonal antibody to label neural crest cells, and results were assessed by light microscopy. Vagal neural crest ablation resulted in a dramatic decrease in the number of neural crest cells colonizing the chick embryo cloaca compared with control embryos. Ablated embryos contained only a small number of HNK-1-positive neural crest cells, which were scattered within the myenteric plexus in a disorganised pattern. Hypoganglionosis was also evident in other regions of the hindgut in ablated embryos. Ablation of the vagal neural crest results in a hypoganglionic cloaca in addition to hypoganglionosis of the hindgut. These results suggest that the cloaca is largely innervated by vagal neural crest emigres. Further studies involving quail-chick chimeras to investigate the exact contribution provided by both vagal and sacral neural crest cells to the cloaca should increase our understanding of the pathophysiology of conditions like persistent cloaca.

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.

Neural crest cell origin for intrinsic ganglia of the developing chicken lung

The development of intrinsic ganglia, comprised of neurons and glia cells that innervate airway smooth muscle, is a recognized component of the growing lung. However, the embryological origin of these neurons and glia is unclear. The lung buds develop as an outgrowth of the foregut, which contains migrating neural crest cells (NCC) that ultimately give rise to the enteric nervous system (ENS) along the entire length of the gut. It has therefore been proposed that the intrinsic ganglia of the lung arise from a subset of NCC that leave the gut and migrate into the lung buds during early development. We have tested this hypothesis using quail-chick interspecies grafting to selectively label the hindbrain-derived neural crest cell population that colonizes the gut. In conjunction with antibody labeling and in situ hybridization, we demonstrate that: (i) lung ganglia arise from vagal NCC that migrate from the foregut into the lung buds; (ii) like ENS precursors, these NCC express the transcription factor Sox10, and the receptors EDNRB and RET; (iii) the co-receptor for RET, GFRalpha1, is expressed in the lung mesenchyme and in ganglia; (iv) ganglia persist within the lung throughout development and contain cells immunopositive for the pan-neuronal markers ANNA-1 and PGP9.5, the inhibitory neurotransmitter NO, as shown by NADPH-diaphorase staining, and the glial marker GFAP.

Somatic transgenesis using retroviral vectors in the chicken embryo

The avian embryo is an excellent model system for experimental studies because of its accessibility and ease of microsurgical manipulations. While the complete chicken genome sequence will soon be determined, a comprehensive germ cell transmission-based genetic approach is not available for this animal model. Several techniques of somatic cell transgenesis have been developed in the past decade. Of these, the retroviral shuttle vector system provides both (1) stable integration of exogenous genes into the host cell genome, and (2) constant expression levels in a target cell population over the course of development. This review summarizes retroviral vectors available for the avian model and outlines the uses of retroviral-mediated gene transfer for cell lineage analysis as well as functional studies of genes and proteins in the chick embryo.

The neural crest: Basic biology and clinical relationships in the craniofacial and enteric nervous systems

The highly migratory, mesenchymal neural crest cell population was discovered over 100 years ago. Proposals of these cells' origin within the neuroepithelium, and of the tissues they gave rise to, initiated decades-long heated debates, since these proposals challenged the powerful germ-layer theory. Having survived this storm, the neural crest is now regarded as a pluripotent stem cell population that makes vital contributions to an astounding array of both neural and non-neural organ systems. The earliest model systems for studying the neural crest were amphibian, and these pioneering contributions have been ably refined and extended by studies in the chick, mouse, and more recently the fish to provide detailed understanding of the cellular and molecular mechanisms regulating and regulated by the neural crest. The key questions regarding control of craniofacial morphogenesis and innervation of the gut illustrate the wide range of developmental contexts in which the neural crest plays an important role. These questions also focus attention on common issues such as the role of growth factor signaling in neural crest cell development and highlight the central role of the neural crest in human congenital disease.

Development of transgenic chickens expressing bacterial beta-galactosidase

Replication-defective retroviral vectors are efficient vehicles for the delivery of exogenous genes, and they may be used in the generation of transgenic animals. The replication-defective retroviral SNTZ vector carrying the lacZ gene with a nuclear localized signal was injected into the subgerminal cavity of freshly laid eggs. Subsequently, the eggs were allowed to hatch, and the chickens were screened for the lacZ gene by using the polymerase chain reaction. Eight of 15 male chickens that survived to sexual maturity contained the lacZ gene in their semen. Subsequently, these males were mated with wild-type female chickens. From one of the eight lacZ-positive G(0) males, two lacZ-positive male chickens were produced from a total of 224 G(1) progeny for a germline transmission rate of 0.89%. Both G(1) male chickens carrying the lacZ gene were mated with wild-type female chickens and 46.5% of the G(2) progeny contained the lacZ gene, which is consistent with the expected Mendelian 50% ratio for a heterozygous dominant allele. The product of the lacZ gene, nuclear localized beta-galactosidase, was expressed in primary myoblast cultures derived from G(2) chickens, and it was also expressed in whole G(2) chicken embryos.

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.

Enteric nervous system: development and developmental disturbances--part 2

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. Accurate descriptions of the phenotype of ENS dysplasias, and knowledge of genes which, when mutated, give rise to the disorders (see Part 1 in the previous issue of this journal), are not sufficient to give a real understanding of how these abnormalities arise. The often indirect link between genotype and phenotype must be sought in the early embryonic development of the ENS. Therefore, in this, the second part, we provide a description of the development of the ENS, concentrating mainly on the origin of the ENS precursor cells and on the cell migration by which they become distributed throughout the gastrointestinal tract. This section also includes experimental evidence on the controls of ENS formation derived from classic embryological, cell culture, and molecular genetic approaches. In addition, for reasons of completeness, we also briefly describe the origins of the interstitial cells of Cajal, a cell population closely related anatomically and functionally to the ENS. Finally, a brief sketch is presented of current notions on the developmental processes between the genes and the morphogenesis of the ENS, and of the means by which the known genetic abnormalities might result in the ENS phenotype observed in Hirschsprung's disease.

In ovo transplantation of enteric nervous system precursors from vagal to sacral neural crest results in extensive hindgut colonisation

The enteric nervous system (ENS) is derived from vagal and sacral neural crest cells (NCC). Within the embryonic avian gut, vagal NCC migrate in a rostrocaudal direction to form the majority of neurons and glia along the entire length of the gastrointestinal tract, whereas sacral NCC migrate in an opposing caudorostral direction, initially forming the nerve of Remak, and contribute a smaller number of ENS cells primarily to the distal hindgut. In this study, we have investigated the ability of vagal NCC, transplanted to the sacral region of the neuraxis, to colonise the chick hindgut and form the ENS in an experimentally generated hypoganglionic hindgut in ovo model. Results showed that when the vagal NC was transplanted into the sacral region of the neuraxis, vagal-derived ENS precursors immediately migrated away from the neural tube along characteristic pathways, with numerous cells colonising the gut mesenchyme by embryonic day (E) 4. By E7, the colorectum was extensively colonised by transplanted vagal NCC and the migration front had advanced caudorostrally to the level of the umbilicus. By E10, the stage at which sacral NCC begin to colonise the hindgut in large numbers, myenteric and submucosal plexuses in the hindgut almost entirely composed of transplanted vagal NCC, while the migration front had progressed into the pre-umbilical intestine, midway between the stomach and umbilicus. Immunohistochemical staining with the pan-neuronal marker, ANNA-1, revealed that the transplanted vagal NCC differentiated into enteric neurons, and whole-mount staining with NADPH-diaphorase showed that myenteric and submucosal ganglia formed interconnecting plexuses, similar to control animals. Furthermore, using an anti-RET antibody, widespread immunostaining was observed throughout the ENS, within a subpopulation of sacral NC-derived ENS precursors, and in the majority of transplanted vagal-to-sacral NCC. Our results demonstrate that: (1) a cell autonomous difference exists between the migration/signalling mechanisms used by sacral and vagal NCC, as transplanted vagal cells migrated along pathways normally followed by sacral cells, but did so in much larger numbers, earlier in development; (2) vagal NCC transplanted into the sacral neuraxis extensively colonised the hindgut, migrated in a caudorostral direction, differentiated into neuronal phenotypes, and formed enteric plexuses; (3) RET immunostaining occurred in vagal crest-derived ENS cells, the nerve of Remak and a subpopulation of sacral NCC within hindgut enteric ganglia.

Zebrafish deadly seven functions in neurogenesis

In a genetic screen, we isolated a mutation that perturbed motor axon outgrowth, neurogenesis, and somitogenesis. Complementation tests revealed that this mutation is an allele of deadly seven (des). By creating genetic mosaics, we demonstrate that the motor axon defect is non-cell autonomous. In addition, we show that the pattern of migration for some neural crest cell populations is aberrant and crest-derived dorsal root ganglion neurons are misplaced. Furthermore, our analysis reveals that des mutant embryos exhibit a neurogenic phenotype. We find an increase in the number of primary motoneurons and in the number of three hindbrain reticulospinal neurons: Mauthner cells, RoL2 cells, and MiD3cm cells. We also find that the number of Rohon-Beard sensory neurons is decreased whereas neural crest-derived dorsal root ganglion neurons are increased in number supporting a previous hypothesis that Rohon-Beard neurons and neural crest form an equivalence group during development. Mutations in genes involved in Notch-Delta signaling result in defects in somitogenesis and neurogenesis. We found that overexpressing an activated form of Notch decreased the number of Mauthner cells in des mutants indicating that des functions via the Notch-Delta signaling pathway to control the production of specific cell types within the central and peripheral nervous systems.

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.

Environmental signals and cell fate specification in premigratory neural crest

Neural crest cells are multipotent progenitors, capable of producing diverse cell types upon differentiation. Recent studies have identified significant heterogeneity in both the fates produced and genes expressed by different premigratory crest cells. While these cells may be specified toward particular fates prior to migration, transplant studies show that some may still be capable of respecification at this time. Here we summarize evidence that extracellular signals in the local environment may act to specify premigratory crest and thus generate diversity in the population. Three main classes of signals-Wnts, BMP2/BMP4 and TGFbeta1,2,3-have been shown to directly influence the production of particular neural crest cell fates, and all are expressed near the premigratory crest. This system may therefore provide a good model for integration of multiple signaling pathways during embryonic cell fate specification.

Delta signaling mediates segregation of neural crest and spinal sensory neurons from zebrafish lateral neural plate

We examined the role of Delta signaling in specification of two derivatives in zebrafish neural plate: Rohon-Beard spinal sensory neurons and neural crest. deltaA-expressing Rohon-Beard neurons are intermingled with premigratory neural crest cells in the trunk lateral neural plate. Embryos homozygous for a point mutation in deltaA, or with experimentally reduced delta signalling, have supernumerary Rohon-Beard neurons, reduced trunk-level expression of neural crest markers and lack trunk neural crest derivatives. Fin mesenchyme, a putative trunk neural crest derivative, is present in deltaA mutants, suggesting it segregates from other neural crest derivatives as early as the neural plate stage. Cranial neural crest derivatives are also present in deltaA mutants, revealing a genetic difference in regulation of trunk and cranial neural crest development.

Factors controlling lineage specification in the neural crest

The neural crest is a transitory tissue of the vertebrate embryo that originates in the neural folds, populates the embryo, and gives rise to many different cell types and tissues of the adult organism. When neural crest cells initiate their migration, a large fraction of them are still pluripotent, that is, capable of generating progeny that consists of two or more distinct phenotypes. To elucidate the cellular and molecular mechanisms by which neural crest cells become committed to a particular lineage is therefore crucial to the understanding of neural crest development and represents a major challenge in current neural crest research. This chapter discusses selected aspects of neural crest cell differentiation into components of the peripheral nervous system. Topics include sympathetic neurons, the adrenal medulla, primary sensory neurons of the spinal ganglia, some of their mechanoreceptive and proprioceptive end organs, and the enteric nervous system.

Transdifferentiation of esophageal smooth to skeletal muscle is myogenic bHLH factor-dependent

Previously, coexpression of smooth and skeletal differentiation markers, but not myogenic regulatory factors (MRFs), was observed from E16.5 mouse fetuses in a small percentage of diaphragm level esophageal muscle cells, suggesting that MRFs are not involved in the process of initiation of developmentally programmed transdifferentiation in the esophagus. To investigate smooth-to-skeletal esophageal muscle transition, we analyzed Myf5nlacZ knock-in mice, MyoD-lacZ and myogenin-lacZ transgenic embryos with a panel of the antibodies reactive with myogenic regulatory factors (MRFs) and smooth and skeletal muscle markers. We observed that lacZ-expressing myogenic precursors were not detected in the esophagus before E15.5, arguing against the hypothesis that muscle precursor cells populate the esophagus at an earlier stage of development. Rather, the expression of the MRFs initiated in smooth muscle cells in the upper esophagus of E15.5 mouse embryos and was immediately followed by the expression of skeletal muscle markers. Moreover, transdifferentiation was markedly delayed or absent only in the absence of Myf5, suggesting that appropriate initiation and progression of smooth-to-skeletal muscle transdifferentiation is Myf5-dependent. Accordingly, the esophagus of Myf5(−/−):MyoD(−/−)embryos completely failed to undergo skeletal myogenesis and consisted entirely of smooth muscle. Lastly, extensive proliferation of muscularis precursor cells, without programmed cell death, occurred concomitantly with esophageal smooth-to-skeletal muscle transdifferentiation. Taken together, these results indicate that transdifferentiation is the fate of all smooth muscle cells in the upper esophagus and is normally initiated by Myf5.

Sacral neural crest cells colonise aganglionic hindgut in vivo but fail to compensate for lack of enteric ganglia

The vagal neural crest is the origin of majority of neurons and glia that constitute the enteric nervous system, the intrinsic innervation of the gut. We have recently confirmed that a second region of the neuraxis, the sacral neural crest, also contributes to the enteric neuronal and glial populations of both the myenteric and the submucosal plexuses in the chick, caudal to the level of the umbilicus. Results from this previous study showed that sacral neural crest-derived precursors colonised the gut in significant numbers only 4 days after vagal-derived cells had completed their migration along the entire length of the gut. This observation suggested that in order to migrate into the hindgut and differentiate into enteric neurons and glia, sacral neural crest cells may require an interaction with vagal-derived cells or with factors or signalling molecules released by them or their progeny. This interdependence may also explain the inability of sacral neural crest cells to compensate for the lack of ganglia in the terminal hindgut of Hirschsprung's disease in humans or aganglionic megacolon in animals. To investigate the possible interrelationship between sacral and vagal-derived neural crest cells within the hindgut, we mapped the contribution of various vagal neural crest regions to the gut and then ablated appropriate sections of chick vagal neural crest to interrupt the migration of enteric nervous system precursor cells and thus create an aganglionic hindgut model in vivo. In these same ablated animals, the sacral level neural axis was removed and replaced with the equivalent tissue from quail embryos, thus enabling us to document, using cell-specific antibodies, the migration and differentiation of sacral crest-derived cells. Results showed that the vagal neural crest contributed precursors to the enteric nervous system in a regionalised manner. When quail-chick grafts of the neural tube adjacent to somites 1-2 were performed, neural crest cells were found in enteric ganglia throughout the preumbilical gut. These cells were most numerous in the esophagus, sparse in the preumbilical intestine, and absent in the postumbilical gut. When similar grafts adjacent to somites 3-5 or 3-6 were carried out, crest cells were found within enteric ganglia along the entire gut, from the proximal esophagus to the distal colon. Vagal neural crest grafts adjacent to somites 6-7 showed that crest cells from this region were distributed along a caudal-rostral gradient, being most numerous in the hindgut, less so in the intestine, and absent in the proximal foregut. In order to generate aneural hindgut in vivo, it was necessary to ablate the vagal neural crest adjacent to somites 3-6, prior to the 13-somite stage of development. When such ablations were performed, the hindgut, and in some cases also the cecal region, lacked enteric ganglionated plexuses. Sacral neural crest grafting in these vagal neural crest ablated chicks showed that sacral cells migrated along normal, previously described hindgut pathways and formed isolated ganglia containing neurons and glia at the levels of the presumptive myenteric and submucosal plexuses. Comparison between vagal neural crest-ablated and nonablated control animals demonstrated that sacral-derived cells migrated into the gut and differentiated into neurons in higher numbers in the ablated animals than in controls. However, the increase in numbers of sacral neural crest-derived neurons within the hindgut did not appear to be sufficiently high to compensate for the lack of vagal-derived enteric plexuses, as ganglia containing sacral neural crest-derived neurons and glia were small and infrequent. Our findings suggest that the neuronal fate of a relatively fixed subpopulation of sacral neural crest cells may be predetermined as these cells neither require the presence of vagal-derived enteric precursors in order to colonise the hindgut, nor are capable of dramatically altering their proliferation or d

Distribution of different regions of cardiac neural crest in the extrinsic and the intrinsic cardiac nervous system

In this study we focused upon whether different levels of postotic neural crest as well as the right and left cardiac neural crest show a segmented or mixed distribution in the extrinsic and intrinsic cardiac nervous system. Different parts of the postotic neural crest were labeled by heterospecific replacement of chick neural tube by its quail counterpart. Quail-chick chimeras (n = 21) were immunohistochemically evaluated at stage HH28+, HH29+, and between HH34-37. In another set of embryos, different regions of cardiac neural crest were tagged with a retrovirus containing the LacZ reporter gene and evaluated between HH35-37 (n = 13). The results show a difference in distribution between the right- and left-sided cardiac neural crest cells at the arterial pole and ventral cardiac plexus. In the dorsal cardiac plexus, the right and left cardiac neural crest cells mix. In general, the extrinsic and intrinsic cardiac nerves receive a lower contribution from the right cardiac neural crest compared with the left cardiac neural crest. The right-sided neural crest from the level of somite 1 seeds only the cranial part of the vagal nerve and the ventral cardiac plexus. Furthermore, the results show a nonsegmented overlapping contribution of neural crest originating from S1 to S3 to the Schwann cells of the cranial and recurrent nerves and the intrinsic cardiac plexus. Also the Schwann cells along the distal intestinal part of the vagal nerve are derived exclusively from the cardiac neural crest region. These findings and the smaller contribution of the more cranially emanating cardiac neural crest to the dorsal cardiac plexus compared with more caudal cardiac neural crest levels, suggests an initial segmented distribution of cardiac neural crest cells in the circumpharyngeal region, followed by longitudinal migration along the vagal nerve during later stages.