Evidence for dynamic rearrangements but lack of fate or position restrictions in premigratory avian trunk neural crest

Identification and characterization of intermediate states in mammalian neural crest cell epithelial to mesenchymal transition and delamination

Epithelial to mesenchymal transition (EMT) is a cellular process that converts epithelial cells to mesenchymal cells with migratory potential in both developmental and pathological processes. Although originally considered a binary event, EMT in cancer progression involves intermediate states between a fully epithelial and a fully mesenchymal phenotype, which are characterized by distinct combinations of epithelial and mesenchymal markers. This phenomenon has been termed epithelial to mesenchymal plasticity (EMP), however, the intermediate states remain poorly described and it's unclear whether they exist during developmental EMT. Neural crest cells (NCC) are an embryonic progenitor cell population that gives rise to numerous cell types and tissues in vertebrates, and their formation is a classic example of developmental EMT. An important feature of NCC development is their delamination from the neuroepithelium via EMT, following which NCC migrate throughout the embryo and undergo differentiation. NCC delamination shares similar changes in cellular state and structure with cancer cell invasion. However, whether intermediate states also exist during NCC EMT and delamination remains unknown. Through single cell RNA sequencing, we identified intermediate NCC states based on their transcriptional signature and then spatially defined their locations in situ in the dorsolateral neuroepithelium. Our results illustrate the progressive transcriptional and spatial transitions from premigratory to migratory cranial NCC during EMT and delamination. Of note gene expression and trajectory analysis indicate that distinct intermediate populations of NCC delaminate in either S phase or G2/M phase of the cell cycle, and the importance of cell cycle regulation in facilitating mammalian cranial NCC delamination was confirmed through cell cycle inhibition studies. Additionally, transcriptional knockdown revealed a functional role for the intermediate stage marker Dlc1 in regulating NCC delamination and migration. Overall, our work identifying and characterizing the intermediate cellular states, processes, and molecular signals that regulate mammalian NCC EMT and delamination furthers our understanding of developmental EMP and may provide new insights into mechanisms regulating pathological EMP.

Colec12 and Trail signaling confine cranial neural crest cell trajectories and promote collective cell migration

BACKGROUND

Collective and discrete neural crest cell (NCC) migratory streams are crucial to vertebrate head patterning. However, the factors that confine NCC trajectories and promote collective cell migration remain unclear.

RESULTS

Computational simulations predicted that confinement is required only along the initial one-third of the cranial NCC migratory pathway. This guided our study of Colec12 (Collectin-12, a transmembrane scavenger receptor C-type lectin) and Trail (tumor necrosis factor-related apoptosis-inducing ligand, CD253) which we show expressed in chick cranial NCC-free zones. NCC trajectories are confined by Colec12 or Trail protein stripes in vitro and show significant and distinct changes in cell morphology and dynamic migratory characteristics when cocultured with either protein. Gain- or loss-of-function of either factor or in combination enhanced NCC confinement or diverted cell trajectories as observed in vivo with three-dimensional confocal microscopy, respectively, resulting in disrupted collective migration.

CONCLUSIONS

These data provide evidence for Colec12 and Trail as novel NCC microenvironmental factors playing a role to confine cranial NCC trajectories and promote collective cell migration.

Generation of Schwann cell derived melanocytes from hPSCs identifies pro-metastatic factors in melanoma

The neural crest (NC) is highly multipotent and generates diverse lineages in the developing embryo. However, spatiotemporally distinct NC populations display differences in fate potential, such as increased gliogenic and parasympathetic potential from later migrating, nerve-associated Schwann cell precursors (SCPs). Interestingly, while melanogenic potential is shared by both early migrating NC and SCPs, differences in melanocyte identity resulting from differentiation through these temporally distinct progenitors have not been determined. Here, we leverage a human pluripotent stem cell (hPSC) model of NC temporal patterning to comprehensively characterize human NC heterogeneity, fate bias, and lineage development. We captured the transition of NC differentiation between temporally and transcriptionally distinct melanogenic progenitors and identified modules of candidate transcription factor and signaling activity associated with this transition. For the first time, we established a protocol for the directed differentiation of melanocytes from hPSCs through a SCP intermediate, termed trajectory 2 (T2) melanocytes. Leveraging an existing protocol for differentiating early NC-derived melanocytes, termed trajectory 1 (T1), we performed the first comprehensive comparison of transcriptional and functional differences between these distinct melanocyte populations, revealing differences in pigmentation and unique expression of transcription factors, ligands, receptors and surface markers. We found a significant link between the T2 melanocyte transcriptional signature and decreased survival in melanoma patients in the cancer genome atlas (TCGA). We performed an in vivo CRISPRi screen of T1 and T2 melanocyte signature genes in a human melanoma cell line and discovered several T2-specific markers that promote lung metastasis in mice. We further demonstrated that one of these factors, SNRPB, regulates the splicing of transcripts involved in metastasis relevant functions such as migration, cell adhesion and proliferation. Overall, this study identifies distinct developmental trajectories as a source of diversity in melanocytes and implicates the unique molecular signature of SCP-derived melanocytes in metastatic melanoma.

Single-cell transcriptomic analysis of zebrafish cranial neural crest reveals spatiotemporal regulation of lineage decisions during development

Neural crest (NC) cells migrate throughout vertebrate embryos to give rise to a huge variety of cell types, but when and where lineages emerge and their regulation remain unclear. We have performed single-cell RNA sequencing (RNA-seq) of cranial NC cells from the first pharyngeal arch in zebrafish over several stages during migration. Computational analysis combining pseudotime and real-time data reveals that these NC cells first adopt a transitional state, becoming specified mid-migration, with the first lineage decisions being skeletal and pigment, followed by neural and glial progenitors. In addition, by computationally integrating these data with RNA-seq data from a transgenic Wnt reporter line, we identify gene cohorts with similar temporal responses to Wnts during migration and show that one, Atp6ap2, is required for melanocyte differentiation. Together, our results show that cranial NC cell lineages arise progressively and uncover a series of spatially restricted cell interactions likely to regulate such cell-fate decisions.

Tatarakis et al. provide a single-cell transcriptomic timeline of cranial neural crest (NC) development in zebrafish and address long-standing questions surrounding the integration of NC cell migration and lineage specification. They find that lineages are specified mid-migration. These fate decisions correspond to shifts in Wnt signaling, and lineages rapidly segregate.

Neural crest multipotency and specification: power and limits of single cell transcriptomic approaches

The neural crest is a unique population of multipotent cells forming in vertebrate embryos. Their vast cell fate potential enables the generation of a diverse array of differentiated cell types in vivo. These include, among others, connective tissue, cartilage and bone of the face and skull, neurons and glia of the peripheral nervous system (including enteric nervous system), and melanocytes. Following migration, these derivatives extensively populate multiple germ layers. Within the competent neural border ectoderm, an area located at the junction between the neural and non-neural ectoderm during embryonic development, neural crest cells form in response to a series of inductive secreted cues including BMP, Wnt, and FGF signals. As cells become progressively specified, they express transcriptional modules conducive with their stage of fate determination or cell state. Those sequential states include the neural border state, the premigratory neural crest state, the epithelium-to-mesenchyme transitional state, and the migratory state to end with post-migratory and differentiation states. However, despite the extensive knowledge accumulated over 150 years of neural crest biology, many key questions remain open, in particular the timing of neural crest lineage determination, the control of potency during early developmental stages, and the lineage relationships between different subpopulations of neural crest cells. In this review, we discuss the recent advances in understanding early neural crest formation using cutting-edge high-throughput single cell sequencing approaches. We will discuss how this new transcriptomic data, from 2017 to 2021, has advanced our knowledge of the steps in neural crest cell lineage commitment and specification, the mechanisms driving multipotency, and diversification. We will then discuss the questions that remain to be resolved and how these approaches may continue to unveil the biology of these fascinating cells.

From Neural Crest to Definitive Roof Plate: The Dynamic Behavior of the Dorsal Neural Tube

Research on the development of the dorsal neural tube is particularly challenging. In this highly dynamic domain, a temporal transition occurs between early neural crest progenitors that undergo an epithelial-to-mesenchymal transition and exit the neural primordium, and the subsequent roof plate, a resident epithelial group of cells that constitutes the dorsal midline of the central nervous system. Among other functions, the roof plate behaves as an organizing center for the generation of dorsal interneurons. Despite extensive knowledge of the formation, emigration and migration of neural crest progenitors, little is known about the mechanisms leading to the end of neural crest production and the transition into a roof plate stage. Are these two mutually dependent or autonomously regulated processes? Is the generation of roof plate and dorsal interneurons induced by neural tube-derived factors throughout both crest and roof plate stages, respectively, or are there differences in signaling properties and responsiveness as a function of time? In this review, we discuss distinctive characteristics of each population and possible mechanisms leading to the shift between the above cell types.

Live Imaging of the Neural Crest Cell Epithelial-to-Mesenchymal Transition in the Chick Embryo

Live embryo imaging may provide a wealth of information on intact cell and tissue dynamics, but can be technically challenging to sustain embryo orientation and health for long periods under a microscope. In this protocol, we describe an in vivo method to mount and image cell movements during the epithelial-to-mesenchymal transition (EMT) of neural crest cells within the chick dorsal neural tube. We focus on describing the collection of images and data preparation for image analysis throughout the developmental stages HH15-21 in the chick trunk. Trunk neural crest cell EMT is crucial to development of the peripheral nervous system and pigment cell patterning. The methods we describe may also be applied to other cell and tissue phenomena at various chick developmental stages with some modifications.

On the road again: Establishment and maintenance of stemness in the neural crest from embryo to adulthood

Unique to vertebrates, the neural crest (NC) is an embryonic stem cell population that contributes to a greatly expanding list of derivatives ranging from neurons and glia of the peripheral nervous system, facial cartilage and bone, pigment cells of the skin to secretory cells of the endocrine system. Here, we focus on what is specifically known about establishment and maintenance of NC stemness and ultimate fate commitment mechanisms, which could help explain its exceptionally high stem cell potential that exceeds the "rules set during gastrulation." In fact, recent discoveries have shed light on the existence of NC cells that coexpress commonly accepted pluripotency factors like Nanog, Oct4/PouV, and Klf4. The coexpression of pluripotency factors together with the exceptional array of diverse NC derivatives encouraged us to propose a new term "pleistopotent" (Greek for abundant, a substantial amount) to be used to reflect the uniqueness of the NC as compared to other post-gastrulation stem cell populations in the vertebrate body, and to differentiate them from multipotent lineage restricted stem cells. We also discuss studies related to the maintenance of NC stemness within the challenging context of being a transient and thus a constantly changing population of stem cells without a permanent niche. The discovery of the stem cell potential of Schwann cell precursors as well as multiple adult NC-derived stem cell reservoirs during the past decade has greatly increased our understanding of how NC cells contribute to tissues formed after its initial migration stage in young embryos.

Development of the Autonomic Nervous System: Clinical Implications

Investigations of the cellular and molecular mechanisms that mediate the development of the autonomic nervous system have identified critical genes and signaling pathways that, when disrupted, cause disorders of the autonomic nervous system. This review summarizes our current understanding of how the autonomic nervous system emerges from the organized spatial and temporal patterning of precursor cell migration, proliferation, communication, and differentiation, and discusses potential clinical implications for developmental disorders of the autonomic nervous system, including familial dysautonomia, Hirschsprung disease, Rett syndrome, and congenital central hypoventilation syndrome.

Cardiac Neural Crest Cells: Their Rhombomeric Specification, Migration, and Association with Heart and Great Vessel Anomalies

Outflow tract abnormalities are the most frequent congenital heart defects. These are due to the absence or dysfunction of the two main cell types, i.e., neural crest cells and secondary heart field cells that migrate in opposite directions at the same stage of development. These cells directly govern aortic arch patterning and development, ascending aorta dilatation, semi-valvular and coronary artery development, aortopulmonary septation abnormalities, persistence of the ductus arteriosus, trunk and proximal pulmonary arteries, sub-valvular conal ventricular septal/rotational defects, and non-compaction of the left ventricle. In some cases, depending on the functional defects of these cells, additional malformations are found in the expected spatial migratory area of the cells, namely in the pharyngeal arch derivatives and cervico-facial structures. Associated non-cardiovascular anomalies are often underestimated, since the multipotency and functional alteration of these cells can result in the modification of multiple neural, epidermal, and cervical structures at different levels. In most cases, patients do not display the full phenotype of abnormalities, but congenital cardiac defects involving the ventricular outflow tract, ascending aorta, aortic arch and supra-aortic trunks should be considered as markers for possible impaired function of these cells. Neural crest cells should not be considered as a unique cell population but on the basis of their cervical rhombomere origins R3-R5 or R6-R7-R8 and specific migration patterns: R3-R4 towards arch II, R5-R6 arch III and R7-R8 arch IV and VI. A better understanding of their development may lead to the discovery of unknown associated abnormalities, thereby enabling potential improvements to be made to the therapeutic approach.

Neural crest cells bulldoze through the microenvironment using Aquaporin 1 to stabilize filopodia

Neural crest migration requires cells to move through an environment filled with dense extracellular matrix and mesoderm to reach targets throughout the vertebrate embryo. Here, we use high-resolution microscopy, computational modeling, and in vitro and in vivo cell invasion assays to investigate the function of Aquaporin 1 (AQP-1) signaling. We find that migrating lead cranial neural crest cells express AQP-1 mRNA and protein, implicating a biological role for water channel protein function during invasion. Differential AQP-1 levels affect neural crest cell speed and direction, as well as the length and stability of cell filopodia. Furthermore, AQP-1 enhances matrix metalloprotease activity and colocalizes with phosphorylated focal adhesion kinases. Colocalization of AQP-1 with EphB guidance receptors in the same migrating neural crest cells has novel implications for the concept of guided bulldozing by lead cells during migration.

Mechanisms of Neural Crest Migration

Neural crest cells are a transient embryonic cell population that migrate collectively to various locations throughout the embryo to contribute a number of cell types to several organs. After induction, the neural crest delaminates and undergoes an epithelial-to-mesenchymal transition before migrating through intricate yet characteristic paths. The neural crest exhibits a variety of migratory behaviors ranging from sheet-like mass migration in the cephalic regions to chain migration in the trunk. During their journey, neural crest cells rely on a range of signals both from their environment and within the migrating population for navigating through the embryo as a collective. Here we review these interactions and mechanisms, including chemotactic cues of neural crest cells' migration.

The diverse neural crest: from embryology to human pathology

We review here some of the historical highlights in exploratory studies of the vertebrate embryonic structure known as the neural crest. The study of the molecular properties of the cells that it produces, their migratory capacities and plasticity, and the still-growing list of tissues that depend on their presence for form and function, continue to enrich our understanding of congenital malformations, paediatric cancers and evolutionary biology. Developmental biology has been key to our understanding of the neural crest, starting with the early days of experimental embryology and through to today, when increasingly powerful technologies contribute to further insight into this fascinating vertebrate cell population.

Establishment of a murine culture system for modeling the temporal progression of cranial and trunk neural crest cell differentiation

The neural crest (NC) is a transient population of embryonic progenitors that are implicated in a diverse range of congenital birth defects and pediatric syndromes. The broad spectrum of NC-related disorders can be attributed to the wide variety of differentiated cell types arising from the NC. In vitro models of NC development provide a powerful platform for testing the relative contributions of intrinsic and extrinsic factors mediating NC differentiation under normal and pathogenic conditions. Although differentiation is a dynamic process that unfolds over time, currently, there is no well-defined chronology that characterizes the in vitro progression of NC differentiation towards specific cell fates. In this study, we have optimized culture conditions for expansion of primary murine NC cells that give rise to both ectodermal and mesoectodermal derivatives, even after multiple passages. Significantly, we have delineated highly reproducible timelines that include distinct intermediate stages for lineage-specific NC differentiation in vitro. In addition, isolating both cranial and trunk NC cells from the same embryos enabled us to make direct comparisons between the two cell populations over the course of differentiation. Our results define characteristic changes in cell morphology and behavior that track the temporal progression of NC cells as they differentiate along the neuronal, glial and chondrogenic lineages in vitro. These benchmarks constitute a chronological baseline for assessing how genetic or environmental disruptions may facilitate or impede NC differentiation. Introducing a temporal dimension substantially increases the power of this platform for screening drugs or chemicals for developmental toxicity or therapeutic potential.

This article has an associated First Person interview with the first author of the paper.

Summary: A novel method for isolating and expanding primary neural crest cells, and establishment of reproducible temporal benchmarks of differentiation, provides a potential screening platform for developmental toxicity or therapeutic capacity.

Neural crest delamination and migration: Looking forward to the next 150 years

Neural crest (NC) cells were described for the first time in 1868 by Wilhelm His. Since then, this amazing population of migratory stem cells has been intensively studied. It took a century to fully unravel their incredible abilities to contribute to nearly every organ of the body. Yet, our understanding of the cell and molecular mechanisms controlling their migration is far from complete. In this review, we summarize the current knowledge on epithelial-mesenchymal transition and collective behavior of NC cells and propose further stops at which the NC train might be calling in the near future.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

DAN (NBL1) promotes collective neural crest migration by restraining uncontrolled invasion

Neural crest cells are both highly migratory and significant to vertebrate organogenesis. However, the signals that regulate neural crest cell migration remain unclear. In this study, we identify DAN as a novel factor that inhibits uncontrolled neural crest and metastatic melanoma invasion in a manner consistent with the inhibition of BMP signaling.

Neural crest cells are both highly migratory and significant to vertebrate organogenesis. However, the signals that regulate neural crest cell migration remain unclear. In this study, we test the function of differential screening-selected gene aberrant in neuroblastoma (DAN), a bone morphogenetic protein (BMP) antagonist we detected by analysis of the chick cranial mesoderm. Our analysis shows that, before neural crest cell exit from the hindbrain, DAN is expressed in the mesoderm, and then it becomes absent along cell migratory pathways. Cranial neural crest and metastatic melanoma cells avoid DAN protein stripes in vitro. Addition of DAN reduces the speed of migrating cells in vivo and in vitro, respectively. In vivo loss of function of DAN results in enhanced neural crest cell migration by increasing speed and directionality. Computer model simulations support the hypothesis that DAN restrains cell migration by regulating cell speed. Collectively, our results identify DAN as a novel factor that inhibits uncontrolled neural crest and metastatic melanoma invasion and promotes collective migration in a manner consistent with the inhibition of BMP signaling.

Illuminating developmental biology through photochemistry

Developmental biology has been continually shaped by technological advances, evolving from a descriptive science into one immersed in molecular and cellular mechanisms. Most recently, genome sequencing and 'omics' profiling have provided developmental biologists with a wealth of genetic and biochemical information; however, fully translating this knowledge into functional understanding will require new experimental capabilities. Photoactivatable probes have emerged as particularly valuable tools for investigating developmental mechanisms, as they can enable rapid, specific manipulations of DNA, RNA, proteins, and cells with spatiotemporal precision. In this Perspective, we describe optochemical and optogenetic systems that have been applied in multicellular organisms, insights gained through the use of these probes, and their current limitations. We also suggest how chemical biologists can expand the reach of photoactivatable technologies and bring new depth to our understanding of organismal development.

Dynamic transcriptional signature and cell fate analysis reveals plasticity of individual neural plate border cells

The ‘neural plate border’ of vertebrate embryos contains precursors of neural crest and placode cells, both defining vertebrate characteristics. How these lineages segregate from neural and epidermal fates has been a matter of debate. We address this by performing a fine-scale quantitative temporal analysis of transcription factor expression in the neural plate border of chick embryos. The results reveal significant overlap of transcription factors characteristic of multiple lineages in individual border cells from gastrula through neurula stages. Cell fate analysis using a Sox2 (neural) enhancer reveals that cells that are initially Sox2+ cells can contribute not only to neural tube but also to neural crest and epidermis. Moreover, modulating levels of Sox2 or Pax7 alters the apportionment of neural tube versus neural crest fates. Our results resolve a long-standing question and suggest that many individual border cells maintain ability to contribute to multiple ectodermal lineages until or beyond neural tube closure.

DOI:http://dx.doi.org/10.7554/eLife.21620.001

Prospect of Human Pluripotent Stem Cell-Derived Neural Crest Stem Cells in Clinical Application

Neural crest stem cells (NCSCs) represent a transient and multipotent cell population that contributes to numerous anatomical structures such as peripheral nervous system, teeth, and cornea. NCSC maldevelopment is related to various human diseases including pigmentation abnormalities, disorders affecting autonomic nervous system, and malformations of teeth, eyes, and hearts. As human pluripotent stem cells including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) can serve as an unlimited cell source to generate NCSCs, hESC/hiPSC-derived NCSCs can be a valuable tool to study the underlying mechanisms of NCSC-associated diseases, which paves the way for future therapies for these abnormalities. In addition, hESC/hiPSC-derived NCSCs with the capability of differentiating to various cell types are highly promising for clinical organ repair and regeneration. In this review, we first discuss NCSC generation methods from human pluripotent stem cells and differentiation mechanism of NCSCs. Then we focus on the clinical application potential of hESC/hiPSC-derived NCSCs on peripheral nerve injuries, corneal blindness, tooth regeneration, pathological melanogenesis, Hirschsprung disease, and cardiac repair and regeneration.

Angiopoietin 2 signaling plays a critical role in neural crest cell migration

BACKGROUND

Collective neural crest cell migration is critical to the form and function of the vertebrate face and neck, distributing bone, cartilage, and nerve cells into peripheral targets that are intimately linked with head vasculature. The vasculature and neural crest structures are ultimately linked, but when and how these patterns develop in the early embryo are not well understood.

RESULTS

Using in vivo imaging and sophisticated cell behavior analyses, we show that quail cranial neural crest and endothelial cells share common migratory paths, sort out in a dynamic multistep process, and display multiple types of motion. To better understand the underlying molecular signals, we examined the role of angiopoietin 2 (Ang2), which we found expressed in migrating cranial neural crest cells. Overexpression of Ang2 causes neural crest cells to be more exploratory as displayed by invasion of off-target locations, the widening of migratory streams into prohibitive zones, and differences in cell motility type. The enhanced exploratory phenotype correlates with increased phosphorylated focal adhesion kinase activity in migrating neural crest cells. In contrast, loss of Ang2 function reduces neural crest cell exploration. In both gain and loss of function of Ang2, we found disruptions to the timing and interplay between cranial neural crest and endothelial cells.

CONCLUSIONS

Together, these data demonstrate a role for Ang2 in maintaining collective cranial neural crest cell migration and suggest interdependence with endothelial cell migration during vertebrate head patterning.

Neural crest stem cells and their potential therapeutic applications

The neural crest (NC) is a remarkable transient structure generated during early vertebrate development. The neural crest progenitors have extensive migratory capacity and multipotency, harboring stem cell-like characteristics such as self-renewal. They can differentiate into a variety of cell types from craniofacial skeletal tissues to the trunk peripheral nervous system (PNS). Multiple regulators such as signaling factors, transcription factors, and migration machinery components are expressed at different stages of NC development. Gain- and loss-of-function studies in various vertebrate species revealed epistatic relationships of these molecules that could be assembled into a gene regulatory network defining the processes of NC induction, specification, migration, and differentiation. These basic developmental studies led to the subsequent establishment and molecular validation of neural crest stem cells (NCSCs) derived by various strategies. We provide here an overview of the isolation and characterization of NCSCs from embryonic, fetal, and adult tissues; the experimental strategies for the derivation of NCSCs from embryonic stem cells, induced pluripotent stem cells, and skin fibroblasts; and recent developments in the use of patient-derived NCSCs for modeling and treating neurocristopathies. We discuss future research on further refinement of the culture conditions required for the differentiation of pluripotent stem cells into axial-specific NC progenitors and their derivatives, developing non-viral approaches for the generation of induced NC cells (NCCs), and using a genomic editing approach to correct genetic mutations in patient-derived NCSCs for transplantation therapy. These future endeavors should facilitate the therapeutic applications of NCSCs in the clinical setting.

Analysis of neural crest-derived clones reveals novel aspects of facial development

Cranial neural crest cells populate the future facial region and produce ectomesenchyme-derived tissues, such as cartilage, bone, dermis, smooth muscle, adipocytes, and many others. However, the contribution of individual neural crest cells to certain facial locations and the general spatial clonal organization of the ectomesenchyme have not been determined. We investigated how neural crest cells give rise to clonally organized ectomesenchyme and how this early ectomesenchyme behaves during the developmental processes that shape the face. Using a combination of mouse and zebrafish models, we analyzed individual migration, cell crowd movement, oriented cell division, clonal spatial overlapping, and multilineage differentiation. The early face appears to be built from multiple spatially defined overlapping ectomesenchymal clones. During early face development, these clones remain oligopotent and generate various tissues in a given location. By combining clonal analysis, computer simulations, mouse mutants, and live imaging, we show that facial shaping results from an array of local cellular activities in the ectomesenchyme. These activities mostly involve oriented divisions and crowd movements of cells during morphogenetic events. Cellular behavior that can be recognized as individual cell migration is very limited and short-ranged and likely results from cellular mixing due to the proliferation activity of the tissue. These cellular mechanisms resemble the strategy behind limb bud morphogenesis, suggesting the possibility of common principles and deep homology between facial and limb outgrowth.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

Quantitative single cell gene expression profiling in the avian embryo

BACKGROUND

Single cell gene profiling has been successfully applied to cultured cells. However, isolation and preservation of a cell's native gene expression state from an intact embryo remain problematic.

RESULTS

Here, we present a strategy for in vivo single cell profiling that optimizes cell identification, isolation and amplification of nucleic acids with nominal bias and sufficient material detection. We first tested several photoconvertible fluorescent proteins to selectively mark a cell(s) of interest in living chick embryos then accurately identify and isolate the same cell(s) in fixed tissue slices. We determined that the dual color mDendra2 provided the optimal signal/noise ratio for this purpose. We developed proper procedures to minimize cell death and preserve gene expression, and suggest nucleic acid amplification strategies for downstream analysis by microfluidic reverse transcriptase quantitative polymerase chain reaction or RNAseq. Lastly, we compared methods for single cell isolation and found that our fluorescence-activated cell sorting (FACS) protocol was able to preserve native transcripts and generate expression profiles with much higher efficiency than laser capture microdissection (LCM).

CONCLUSIONS

Quantitative single cell gene expression profiling may be accurately applied to interrogate complex cell dynamics events during embryonic development by combining photoconversion cell labeling, FACS, proper handling of isolated cells, and amplification strategies.

Epithelial-Mesenchymal Transitions during Neural Crest and Somite Development

Epithelial-to-mesenchymal transition (EMT) is a central process during embryonic development that affects selected progenitor cells of all three germ layers. In addition to driving the onset of cellular migrations and subsequent tissue morphogenesis, the dynamic conversions of epithelium into mesenchyme and vice-versa are intimately associated with the segregation of homogeneous precursors into distinct fates. The neural crest and somites, progenitors of the peripheral nervous system and of skeletal tissues, respectively, beautifully illustrate the significance of EMT to the above processes. Ongoing studies progressively elucidate the gene networks underlying EMT in each system, highlighting the similarities and differences between them. Knowledge of the mechanistic logic of this normal ontogenetic process should provide important insights to the understanding of pathological conditions such as cancer metastasis, which shares some common molecular themes.

TrkB/BDNF signalling patterns the sympathetic nervous system

The sympathetic nervous system is essential for maintaining mammalian homeostasis. How this intricately connected network, composed of preganglionic neurons that reside in the spinal cord and post-ganglionic neurons that comprise a chain of vertebral sympathetic ganglia, arises developmentally is incompletely understood. This problem is especially complex given the vertebral chain of sympathetic ganglia derive secondarily from the dorsal migration of ‘primary' sympathetic ganglia that are initially located several hundred microns ventrally from their future pre-synaptic partners. Here we report that the dorsal migration of discrete ganglia is not a simple migration of individual cells but a much more carefully choreographed process that is mediated by extensive interactions of pre-and post-ganglionic neurons. Dorsal migration does not occur in the absence of contact with preganglionic axons, and this is mediated by BDNF/TrkB signalling. Thus BDNF released by preganglionic axons acts chemotactically on TrkB-positive sympathetic neurons, to pattern the developing peripheral nervous system.

The signals that pattern the sympathetic nervous system are not fully understood. Here the authors show that the dorsal migration of the primary sympathetic ganglia in chick embryos is orchestrated by BDNF/TrkB signalling and requires contact with preganglionic axons.

VEGF signals induce trailblazer cell identity that drives neural crest migration

Embryonic neural crest cells travel in discrete streams to precise locations throughout the head and body. We previously showed that cranial neural crest cells respond chemotactically to vascular endothelial growth factor (VEGF) and that cells within the migratory front have distinct behaviors and gene expression. We proposed a cell-induced gradient model in which lead neural crest cells read out directional information from a chemoattractant profile and instruct trailers to follow. In this study, we show that migrating chick neural crest cells do not display distinct lead and trailer gene expression profiles in culture. However, exposure to VEGF in vitro results in the upregulation of a small subset of genes associated with an in vivo lead cell signature. Timed addition and removal of VEGF in culture reveals the changes in neural crest cell gene expression are rapid. A computational model incorporating an integrate-and-switch mechanism between cellular phenotypes predicts migration efficiency is influenced by the timescale of cell behavior switching. To test the model hypothesis that neural crest cellular phenotypes respond to changes in the VEGF chemoattractant profile, we presented ectopic sources of VEGF to the trailer neural crest cell subpopulation and show diverted cell trajectories and stream alterations consistent with model predictions. Gene profiling of trailer cells that diverted and encountered VEGF revealed upregulation of a subset of 'lead' genes. Injection of neuropilin1 (Np1)-Fc into the trailer subpopulation or electroporation of VEGF morpholino to reduce VEGF signaling failed to alter trailer neural crest cell trajectories, suggesting trailers do not require VEGF to maintain coordinated migration. These results indicate that VEGF is one of the signals that establishes lead cell identity and its chemoattractant profile is critical to neural crest cell migration.

Vascular Mural Cells Promote Noradrenergic Differentiation of Embryonic Sympathetic Neurons

The sympathetic nervous system controls smooth muscle tone and heart rate in the cardiovascular system. Postganglionic sympathetic neurons (SNs) develop in close proximity to the dorsal aorta (DA) and innervate visceral smooth muscle targets. Here, we use the zebrafish embryo to ask whether the DA is required for SN development. We show that noradrenergic (NA) differentiation of SN precursors temporally coincides with vascular mural cell (VMC) recruitment to the DA and vascular maturation. Blocking vascular maturation inhibits VMC recruitment and blocks NA differentiation of SN precursors. Inhibition of platelet-derived growth factor receptor (PDGFR) signaling prevents VMC differentiation and also blocks NA differentiation of SN precursors. NA differentiation is normal in cloche mutants that are devoid of endothelial cells but have VMCs. Thus, PDGFR-mediated mural cell recruitment mediates neurovascular interactions between the aorta and sympathetic precursors and promotes their noradrenergic differentiation.

From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate

During vertebrate development, the central (CNS) and peripheral nervous systems (PNS) arise from the neural plate. Cells at the margin of the neural plate give rise to neural crest cells, which migrate extensively throughout the embryo, contributing to the majority of neurons and all of the glia of the PNS. The rest of the neural plate invaginates to form the neural tube, which expands to form the brain and spinal cord. The emergence of molecular cloning techniques and identification of fluorophores like Green Fluorescent Protein (GFP), together with transgenic and electroporation technologies, have made it possible to easily visualize the cellular and molecular events in play during nervous system formation. These lineage-tracing techniques have precisely demonstrated the migratory pathways followed by neural crest cells and increased knowledge about their differentiation into PNS derivatives. Similarly, in the spinal cord, lineage-tracing techniques have led to a greater understanding of the regional organization of multiple classes of neural progenitor and post-mitotic neurons along the different axes of the spinal cord and how these distinct classes of neurons assemble into the specific neural circuits required to realize their various functions. Here, we review how both classical and modern lineage and marker analyses have expanded our knowledge of early peripheral nervous system and spinal cord development.

Neural crest cell evolution: how and when did a neural crest cell become a neural crest cell

As vertebrates evolved from protochordates, they shifted to a more predatory lifestyle, and radiated and adapted to most niches of the planet. This process was largely facilitated by the generation of novel vertebrate head structures, which were derived from neural crest cells (NCC). The neural crest is a unique vertebrate cell population that is frequently termed the "fourth germ layer" because it forms in conjunction with the other germ layers and contributes to a diverse array of cell types and tissues including the craniofacial skeleton, the peripheral nervous system, and pigment cells among many other tissues and cell types. NCC are defined by their origin at the neural plate border, via an epithelial-to-mesenchymal transition (EMT), together with multipotency and polarized patterns of migration. These defining characteristics, which evolved independently in the germ layers of invertebrates, were subsequently co-opted through their gene regulatory networks to form NCC in vertebrates. Moreover, recent data suggest that the ability to undergo an EMT was one of the latter features co-opted by NCC. In this review, we discuss the potential origins of NCC and how they evolved to contribute to nearly all tissues and organs throughout the body, based on paleontological evidence together with an evaluation of the evolution of molecules involved in NCC development and their migratory cell paths.

Sympathoadrenal neural crest cells: the known, unknown and forgotten?

Neural crest cells (NCCs) are highly migratory progenitor cells that give rise to a vast array of differentiated cell types. One of their key derivatives is the autonomic nervous system (ANS) that is comprised in part from chromaffin cells of the adrenal medulla and organ of Zuckerkandl, the sympathetic chain and additional prevertebral ganglia such as the celiac ganglia, suprarenal ganglia and mesenteric ganglia. In this review we discuss recent advances toward our understanding of how the NCC precursors of the ANS migrate to their target regions, how they are instructed to differentiate into the correct cell types, and the morphogenetic signals controlling their development. Many of these processes remain enigmatic to developmental biologists worldwide. Taking advantage of lineage tracing mouse models one of our own aims is to address the morphogenetic events underpinning the formation of the ANS and to identify the molecular mechanisms that help to segregate a mixed population of NCCs into pathways specific for the sympathetic ganglia, sensory ganglia or adrenal medulla.

DNA methyltransferase 3B regulates duration of neural crest production via repression of Sox10

Neural crest stem cells arise within the central nervous system but then undergo an epithelial-to-mesenchymal transition to migrate away and contribute to the peripheral nervous system and craniofacial skeleton. Here we show that DNA methyltransferase 3B (DNMT3B) is responsible for the loss of competence of dorsal neural tube cells to generate emigrating neural crest cells. DNMT3B knockdown results in up-regulation of neural crest markers, prolonged neural crest emigration, and subsequent precocious neuronal differentiation of the trigeminal ganglion. We find that DNMT3B binds to the promoter of Sox10, known to be important for neural crest emigration and lineage acquisition. Bisulfite sequencing further reveals methylation of the Sox10 promoter region upon cessation of emigration in normal embryos, whereas this mark is reduced after DNMT3B loss. Taken together, these results reveal the importance of DNA methylation in regulating the ability of neural tube cells to produce neural crest cells and the timing of peripheral neuron differentiation.

Slit molecules prevent entrance of trunk neural crest cells in developing gut

Neural crest cells emerge from the dorsal neural tube early in development and give rise to sensory and sympathetic ganglia, adrenal cells, teeth, melanocytes and especially enteric nervous system. Several inhibitory molecules have been shown to play important roles in neural crest migration, among them are the chemorepulsive Slit1-3. It was known that Slits chemorepellants are expressed at the entry to the gut, and thus could play a role in the differential ability of vagal but not trunk neural crest cells to invade the gut and form enteric ganglia. Especially since trunk neural crest cells express Robo receptor while vagal do not. Thus, although we know that Robo mediates migration along the dorsal pathway in neural crest cells, we do not know if it is responsible in preventing their entry into the gut. The goal of this study was to further corroborate a role for Slit molecules in keeping trunk neural crest cells away from the gut. We observed that when we silenced Robo receptor in trunk neural crest, the sympathoadrenal (somites 18-24) were capable of invading gut mesenchyme in larger proportion than more rostral counterparts. The more rostral trunk neural crest tended not to migrate beyond the ventral aorta, suggesting that there are other repulsive molecules keeping them away from the gut. Interestingly, we also found that when we silenced Robo in sacral neural crest they did not wait for the arrival of vagal crest but entered the gut and migrated rostrally, suggesting that Slit molecules are the ones responsible for keeping them waiting at the hindgut mesenchyme. These combined results confirm that Slit molecules are responsible for keeping the timeliness of colonization of the gut by neural crest cells.

Neuropilins define distinct populations of neural crest cells

Background

Neural crest cells (NCCs) are a transient embryonic cell type that give rise to a wide spectrum of derivatives, including neurons and glia of the sensory and autonomic nervous system, melanocytes and connective tissues in the head. Lineage-tracing and functional studies have shown that trunk NCCs migrate along two distinct paths that correlate with different developmental fates. Thus, NCCs migrating ventrally through the anterior somite form sympathetic and sensory ganglia, whereas NCCs migrating dorsolaterally form melanocytes. Although the mechanisms promoting migration along the dorsolateral path are well defined, the molecules providing positional identity to sympathetic and sensory-fated NCCs that migrate along the same ventral path are ill defined. Neuropilins (Nrp1 and Nrp2) are transmembrane glycoproteins that are essential for NCC migration. Nrp1 and Nrp2 knockout mice have disparate phenotypes, suggesting that these receptors may play a role in sorting NCCs biased towards sensory and sympathetic fates to appropriate locations.

Results

Here we have combined in situ hybridisation, immunohistochemistry and lineage-tracing analyses to demonstrate that neuropilins are expressed in a non-overlapping pattern within NCCs. Whereas Nrp1 is expressed in NCCs emigrating from hindbrain rhombomere 4 (r4) and within trunk NCCs giving rise to sympathetic and sensory ganglia, Nrp2 is preferentially expressed in NCCs emigrating from r2 and in trunk NCCs giving rise to sensory ganglia. By generating a tamoxifen-inducible lineage-tracing system, we further demonstrate that Nrp2-expressing NCCs specifically populate sensory ganglia including the trigeminal ganglia (V) in the head and the dorsal root ganglia in the trunk.

Conclusions

Taken together, our results demonstrate that Nrp1 and Nrp2 are expressed in different populations of NCCs, and that Nrp2-expressing NCCs are strongly biased towards a sensory fate. In the trunk, Nrp2-expressing NCCs specifically give rise to sensory ganglia, whereas Nrp1-expressing NCCs likely give rise to both sensory and sympathetic ganglia. Our findings therefore suggest that neuropilins play an essential role in coordinating NCC migration with fate specification.

The neural crest: a versatile organ system

The neural crest is the name given to the strip of cells at the junction between neural and epidermal ectoderm in neurula-stage vertebrate embryos, which is later brought to the dorsal neural tube as the neural folds elevate. The neural crest is a heterogeneous and multipotent progenitor cell population whose cells undergo EMT then extensively and accurately migrate throughout the embryo. Neural crest cells contribute to nearly every organ system in the body, with derivatives of neuronal, glial, neuroendocrine, pigment, and also mesodermal lineages. This breadth of developmental capacity has led to the neural crest being termed the fourth germ layer. The neural crest has occupied a prominent place in developmental biology, due to its exaggerated migratory morphogenesis and its remarkably wide developmental potential. As such, neural crest cells have become an attractive model for developmental biologists for studying these processes. Problems in neural crest development cause a number of human syndromes and birth defects known collectively as neurocristopathies; these include Treacher Collins syndrome, Hirschsprung disease, and 22q11.2 deletion syndromes. Tumors in the neural crest lineage are also of clinical importance, including the aggressive melanoma and neuroblastoma types. These clinical aspects have drawn attention to the selection or creation of neural crest progenitor cells, particularly of human origin, for studying pathologies of the neural crest at the cellular level, and also for possible cell therapeutics. The versatility of the neural crest lends itself to interlinked research, spanning basic developmental biology, birth defect research, oncology, and stem/progenitor cell biology and therapy.

The neural crest, a multifaceted structure of the vertebrates

In this review, several features of the cells originating from the lateral borders of the primitive neural anlagen, the neural crest (NC) are considered. Among them, their multipotentiality, which together with their migratory properties, leads them to colonize the developing body and to participate in the development of many tissues and organs. The in vitro analysis of the developmental capacities of single NC cells (NCC) showed that they present several analogies with the hematopoietic cells whose differentiation involves the activity of stem cells endowed with different arrays of developmental potentialities. The permanence of such NC stem cells in the adult organism raises the problem of their role at that stage of life. The NC has appeared during evolution in the vertebrate phylum and is absent in their Protocordates ancestors. The major role of the NCC in the development of the vertebrate head points to a critical role for this structure in the remarkable diversification and radiation of this group of animals.

Axolotls with an under- or oversupply of neural crest can regulate the sizes of their dorsal root ganglia to normal levels

How animals adjust the size of their organs is a fundamental, enduring question in biology. Here we manipulate the amount of neural crest (NC) precursors for the dorsal root ganglia (DRG) in axolotl. We produce embryos with an under- or over-supply of pre-migratory NC in order to find out if DRG can regulate their sizes during development. Axolotl embryos are perfectly suitable for this research. Firstly, they are optimal for microsurgical manipulations and tissue repair. Secondly, they possess, unlike most other vertebrates, only one neural crest string located on top of the neural tube. This condition and position enables NC cells to migrate to either side of the embryo and participate in the regulation of NC cell distribution. We show that size compensation of DRG in axolotl occurs in 2 cm juveniles after undersupply of NC (up-regulation) and in 5 cm juveniles after oversupply of NC (down-regulation). The size of DRG is likely to be regulated locally within the DRG and not via adaptations of the pre-migratory NC or during NC cell migration. Ipsi- and contralateral NC cell migration occurs both in embryos with one and two neural folds, and contralateral migration of NC is the only source for contralateral DRG formation in embryos with only one neural fold. Compensatory size increase is accompanied by an increase in cell division of a DRG precursor pool (PCNA+/SOX2-), rather than by DRG neurons or glial cells. During compensatory size decrease, increased apoptosis and reduced proliferation of DRG cells are observed.

The neural crest cell cycle is related to phases of migration in the head

Embryonic cells that migrate long distances must critically balance cell division in order to maintain stream dynamics and population of peripheral targets. Yet details of individual cell division events and how cell cycle is related to phases of migration remain unclear. Here, we examined these questions using the chick cranial neural crest (NC). In vivo time-lapse imaging revealed that a typical migrating NC cell division event lasted ~1 hour and included four stereotypical steps. Cell tracking showed that dividing NC cells maintained position relative to non-dividing neighbors. NC cell division orientation and the time and distance to first division after neural tube exit were stochastic. To address how cell cycle is related to phases of migration, we used FACs analysis to identify significant spatiotemporal differences in NC cell cycle profiles. Two-photon photoconversion of single and small numbers of mKikGR-labeled NC cells confirmed that lead NC cells exhibited a nearly fourfold faster doubling time after populating the branchial arches. By contrast, Ki-67 staining showed that one out of every five later emerging NC cells exited the cell cycle after reaching proximal head targets. The relatively quiescent mitotic activity during NC cell migration to the branchial arches was altered when premigratory cells were reduced in number by tissue ablation. Together, our results provide the first comprehensive details of the pattern and dynamics of cell division events during cranial NC cell migration.

Combination of in ovo electroporation and time-lapse imaging to study migrational events in chicken embryos

BACKGROUND

During embryonic development cell migration plays a principal role in several processes. In past decades, many studies were performed to investigate migrational events, occurring during embryonic organogenesis, neurogenesis, gliogenesis or myogenesis, just to name a few. Although different common techniques are already used for this purpose, one of their major limitations is the static character. However, cell migration is a sophisticated and highly dynamic process, wherefore new appropriate technologies are required to investigate this event in all its complexity.

RESULTS AND CONCLUSIONS

Here we report a novel approach for dynamic analysis of cell migration within embryonic tissue. We combine the modern transfection method of in ovo electroporation with the use of tissue slice culture and state-of-the-art imaging techniques, such as confocal laser scanning microscopy or spinning disc confocal microscopy, and thus, develop a method to study live the migration of myogenic precursors in chicken embryos. The conditions and parameters used in this study allow long-term imaging for up to 24 hr. Our protocol can be easily adapted for investigations of a variety of other migrational events and provides a novel conception for dynamic analysis of migration during embryonic development.

Developmental imaging: the avian embryo hatches to the challenge

The avian embryo provides a multifaceted model to study developmental mechanisms because of its accessibility to microsurgery, fluorescence cell labeling, in vivo imaging, and molecular manipulation. Early two-dimensional planar growth of the avian embryo mimics human development and provides unique access to complex cell migration patterns using light microscopy. Later developmental events continue to permit access to both light and other imaging modalities, making the avian embryo an excellent model for developmental imaging. For example, significant insights into cell and tissue behaviors within the primitive streak, craniofacial region, and cardiovascular and peripheral nervous systems have come from avian embryo studies. In this review, we provide an update to recent advances in embryo and tissue slice culture and imaging, fluorescence cell labeling, and gene profiling. We focus on how technical advances in the chick and quail provide a clearer understanding of how embryonic cell dynamics are beautifully choreographed in space and time to sculpt cells into functioning structures. We summarize how these technical advances help us to better understand basic developmental mechanisms that may lead to clinical research into human birth defects and tissue repair.

Cell delamination in the mesencephalic neural fold and its implication for the origin of ectomesenchyme

The neural crest is a transient structure unique to vertebrate embryos that gives rise to multiple lineages along the rostrocaudal axis. In cranial regions, neural crest cells are thought to differentiate into chondrocytes, osteocytes, pericytes and stromal cells, which are collectively termed ectomesenchyme derivatives, as well as pigment and neuronal derivatives. There is still no consensus as to whether the neural crest can be classified as a homogenous multipotent population of cells. This unresolved controversy has important implications for the formation of ectomesenchyme and for confirmation of whether the neural fold is compartmentalized into distinct domains, each with a different repertoire of derivatives. Here we report in mouse and chicken that cells in the neural fold delaminate over an extended period from different regions of the cranial neural fold to give rise to cells with distinct fates. Importantly, cells that give rise to ectomesenchyme undergo epithelial-mesenchymal transition from a lateral neural fold domain that does not express definitive neural markers, such as Sox1 and N-cadherin. Additionally, the inference that cells originating from the cranial neural ectoderm have a common origin and cell fate with trunk neural crest cells prompted us to revisit the issue of what defines the neural crest and the origin of the ectomesenchyme.