Signals derived from the underlying mesoderm are dispensable for zebrafish neural crest induction

Review: The Role of Wnt/β-Catenin Signalling in Neural Crest Development in Zebrafish

The neural crest (NC) is a multipotent cell population in vertebrate embryos with extraordinary migratory capacity. The NC is crucial for vertebrate development and forms a myriad of cell derivatives throughout the body, including pigment cells, neuronal cells of the peripheral nervous system, cardiomyocytes and skeletogenic cells in craniofacial tissue. NC induction occurs at the end of gastrulation when the multipotent population of NC progenitors emerges in the ectodermal germ layer in the neural plate border region. In the process of NC fate specification, fate-specific markers are expressed in multipotent progenitors, which subsequently adopt a specific fate. Thus, NC cells delaminate from the neural plate border and migrate extensively throughout the embryo until they differentiate into various cell derivatives. Multiple signalling pathways regulate the processes of NC induction and specification. This review explores the ongoing role of the Wnt/β-catenin signalling pathway during NC development, focusing on research undertaken in the Teleost model organism, zebrafish (Danio rerio). We discuss the function of the Wnt/β-catenin signalling pathway in inducing the NC within the neural plate border and the specification of melanocytes from the NC. The current understanding of NC development suggests a continual role of Wnt/β-catenin signalling in activating and maintaining the gene regulatory network during NC induction and pigment cell specification. We relate this to emerging models and hypotheses on NC fate restriction. Finally, we highlight the ongoing challenges facing NC research, current gaps in knowledge, and this field's potential future directions.

BMP2 Is Related to Hirschsprung's Disease and Required for Enteric Nervous System Development

The enteric nervous system (ENS) is derived from neural crest cells (NCCs). Defects in ENS NCCs colonizing in the intestines lead to an absence of enteric ganglia in the colon and results in Hirschsprung’s disease (HSCR). Bone morphogenetic proteins (BMPs) play diverse roles in the proliferation, migration and survival of ENS NCCs; however, whether BMPs are involved in HSCR and the underlying mechanism remains largely unknown. In this study, we found that BMP2 expression is significantly decreased in HSCR patients. Further experiments demonstrated that BMP2 is involved in the regulation of NCC proliferation, migration and differentiation. In a detailed analysis of the role of BMP2 in HSCR development in vivo, we demonstrated that BMP2b regulates the proliferation, migration and differentiation of vagal NCCs in zebrafish and that BMP2b is required for intestinal smooth muscle development. In addition, we showed that BMP2b is involved in regulating the expression of glial cell line-derived neurotrophic factor (GDNF) in the intestine, which mediates the regulation of ENS development by BMP2b in zebrafish. These results highlight a central role of the BMP-GDNF cascade in intestinal patterning and ENS development. Our results further demonstrate the key role of BMP2 in the etiology of HSCR in vitro and in vivo.

Specification and formation of the neural crest: Perspectives on lineage segregation

The neural crest is a fascinating embryonic population unique to vertebrates that is endowed with remarkable differentiation capacity. Thought to originate from ectodermal tissue, neural crest cells generate neurons and glia of the peripheral nervous system, and melanocytes throughout the body. However, the neural crest also generates many ectomesenchymal derivatives in the cranial region, including cell types considered to be of mesodermal origin such as cartilage, bone, and adipose tissue. These ectomesenchymal derivatives play a critical role in the formation of the vertebrate head, and are thought to be a key attribute at the center of vertebrate evolution and diversity. Further, aberrant neural crest cell development and differentiation is the root cause of many human pathologies, including cancers, rare syndromes, and birth malformations. In this review, we discuss the current findings of neural crest cell ontogeny, and consider tissue, cell, and molecular contributions toward neural crest formation. We further provide current perspectives into the molecular network involved during the segregation of the neural crest lineage.

Identification of transcripts potentially involved in neural tube closure using RNA sequencing

Anencephaly is a fatal human neural tube defect (NTD) in which the anterior neural tube remains open. Zebrafish embryos with reduced Nodal signaling display an open anterior neural tube phenotype that is analogous to anencephaly. Previous work from our laboratory suggests that Nodal signaling acts through induction of the head mesendoderm and mesoderm. Head mesendoderm/mesoderm then, through an unknown mechanism, promotes formation of the polarized neuroepithelium that is capable of undergoing the movements required for closure. We compared the transcriptome of embryos treated with a Nodal signaling inhibitor at sphere stage, which causes NTDs, to embryos treated at 30% epiboly, which does not cause NTDs. This screen identified over 3,000 transcripts with potential roles in anterior neurulation. Expression of several genes encoding components of tight and adherens junctions was significantly reduced, supporting the model that Nodal signaling regulates formation of the neuroepithelium. mRNAs involved in Wnt, FGF, and BMP signaling were also differentially expressed, suggesting these pathways might regulate anterior neurulation. In support of this, we found that pharmacological inhibition of FGF-receptor function causes an open anterior NTD as well as loss of mesodermal derivatives. This suggests that Nodal and FGF signaling both promote anterior neurulation through induction of head mesoderm.

Signaling pathways and tissue interactions in neural plate border formation

The neural crest is a transient cell population that gives rise to various cell types of multiple tissues and organs in the vertebrate embryo. Neural crest cells arise from the neural plate border, a region localized at the lateral borders of the prospective neural plate. Temporally and spatially coordinated interaction with the adjacent tissues, the non-neural ectoderm, the neural plate and the prospective dorsolateral mesoderm, is required for neural plate border specification. Signaling molecules, namely BMP, Wnt and FGF ligands and corresponding antagonists are derived from these tissues and interact to induce the expression of neural plate border specific genes. The present mini-review focuses on the current understanding of how the NPB territory is formed and accentuates the need for coordinated interaction of BMP and Wnt signaling pathways and precise tissue communication that are required for the definition of the prospective NC in the competent ectoderm.

Neural crest development and craniofacial morphogenesis is coordinated by nitric oxide and histone acetylation

Cranial neural crest (CNC) cells are patterned and coalesce to facial prominences that undergo convergence and extension to generate the craniofacial form. We applied a chemical genetics approach to identify pathways that regulate craniofacial development during embryogenesis. Treatment with the nitric oxide synthase inhibitor 1-(2-[trifluoromethyl] phenyl) imidazole (TRIM) abrogated first pharyngeal arch structures and induced ectopic ceratobranchial formation. TRIM promoted a progenitor CNC fate and inhibited chondrogenic differentiation, which were mediated through impaired nitric oxide (NO) production without appreciable effect on global protein S-nitrosylation. Instead, TRIM perturbed hox gene patterning and caused histone hypoacetylation. Rescue of TRIM phenotype was achieved with overexpression of histone acetyltransferase kat6a, inhibition of histone deacetylase, and complementary NO. These studies demonstrate that NO signaling and histone acetylation are coordinated mechanisms that regulate CNC patterning, differentiation, and convergence during craniofacial morphogenesis.

FGF signaling transforms non-neural ectoderm into neural crest

The neural crest arises at the border between the neural plate and the adjacent non-neural ectoderm. It has been suggested that both neural and non-neural ectoderm can contribute to the neural crest. Several studies have examined the molecular mechanisms that regulate neural crest induction in neuralized tissues or the neural plate border. Here, using the chick as a model system, we address the molecular mechanisms by which non-neural ectoderm generates neural crest. We report that in response to FGF the non-neural ectoderm can ectopically express several early neural crest markers (Pax7, Msx1, Dlx5, Sox9, FoxD3, Snail2, and Sox10). Importantly this response to FGF signaling can occur without inducing ectopic mesodermal tissues. Furthermore, the non-neural ectoderm responds to FGF by expressing the prospective neural marker Sox3, but it does not express definitive markers of neural or anterior neural (Sox2 and Otx2) tissues. These results suggest that the non-neural ectoderm can launch the neural crest program in the absence of mesoderm, without acquiring definitive neural character. Finally, we report that prior to the upregulation of these neural crest markers, the non-neural ectoderm upregulates both BMP and Wnt molecules in response to FGF. Our results provide the first effort to understand the molecular events leading to neural crest development via the non-neural ectoderm in amniotes and present a distinct response to FGF signaling.

Bmps and id2a act upstream of Twist1 to restrict ectomesenchyme potential of the cranial neural crest

Cranial neural crest cells (CNCCs) have the remarkable capacity to generate both the non-ectomesenchyme derivatives of the peripheral nervous system and the ectomesenchyme precursors of the vertebrate head skeleton, yet how these divergent lineages are specified is not well understood. Whereas studies in mouse have indicated that the Twist1 transcription factor is important for ectomesenchyme development, its role and regulation during CNCC lineage decisions have remained unclear. Here we show that two Twist1 genes play an essential role in promoting ectomesenchyme at the expense of non-ectomesenchyme gene expression in zebrafish. Twist1 does so by promoting Fgf signaling, as well as potentially directly activating fli1a expression through a conserved ectomesenchyme-specific enhancer. We also show that Id2a restricts Twist1 activity to the ectomesenchyme lineage, with Bmp activity preferentially inducing id2a expression in non-ectomesenchyme precursors. We therefore propose that the ventral migration of CNCCs away from a source of Bmps in the dorsal ectoderm promotes ectomesenchyme development by relieving Id2a-dependent repression of Twist1 function. Together our model shows how the integration of Bmp inhibition at its origin and Fgf activation along its migratory route would confer temporal and spatial specificity to the generation of ectomesenchyme from the neural crest.

Current perspectives of the signaling pathways directing neural crest induction

The neural crest is a migratory population of embryonic cells with a tremendous potential to differentiate and contribute to nearly every organ system in the adult body. Over the past two decades, an incredible amount of research has given us a reasonable understanding of how these cells are generated. Neural crest induction involves the combinatorial input of multiple signaling pathways and transcription factors, and is thought to occur in two phases from gastrulation to neurulation. In the first phase, FGF and Wnt signaling induce NC progenitors at the border of the neural plate, activating the expression of members of the Msx, Pax, and Zic families, among others. In the second phase, BMP, Wnt, and Notch signaling maintain these progenitors and bring about the expression of definitive NC markers including Snail2, FoxD3, and Sox9/10. In recent years, additional signaling molecules and modulators of these pathways have been uncovered, creating an increasingly complex regulatory network. In this work, we provide a comprehensive review of the major signaling pathways that participate in neural crest induction, with a focus on recent developments and current perspectives. We provide a simplified model of early neural crest development and stress similarities and differences between four major model organisms: Xenopus, chick, zebrafish, and mouse.

Induction of the neural crest state: control of stem cell attributes by gene regulatory, post-transcriptional and epigenetic interactions

Neural crest cells are a population of multipotent stem cell-like progenitors that arise at the neural plate border in vertebrates, migrate extensively, and give rise to diverse derivatives such as melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia. The neural crest gene regulatory network (NC-GRN) includes a number of key factors that are used reiteratively to control multiple steps in the development of neural crest cells, including the acquisition of stem cell attributes. It is therefore essential to understand the mechanisms that control the distinct functions of such reiteratively used factors in different cellular contexts. The context-dependent control of neural crest specification is achieved through combinatorial interaction with other factors, post-transcriptional and post-translational modifications, and the epigenetic status and chromatin state of target genes. Here we review the current understanding of the NC-GRN, including the role of the neural crest specifiers, their links to the control of "stemness," and their dynamic context-dependent regulation during the formation of neural crest progenitors.

Specification and regionalisation of the neural plate border

During early vertebrate development, the embryonic ectoderm becomes subdivided into neural, neural plate border (border) and epidermal regions. The nervous system is derived from the neural and border domains which, respectively, give rise to the central and peripheral nervous systems. To better understand the functional nervous system we need to know how individual neurons are specified and connected. Our understanding of the early development of the peripheral nervous system has been lagging compared to knowledge regarding central nervous system and epidermal cell lineage decision. Recent advances have shown when and how the specification of border cells is initiated. One important insight is that border specification is already initiated at blastula stages, and can be molecularly and temporally distinguished from rostrocaudal regionalisation of the border. From findings in several species, it is clear that Wnt, Bone Morphogenetic Protein and Fibroblast Growth Factor signals play important roles during the specification and regionalisation of the border. In this review, we highlight the individual roles of these signals and compare models of border specification, including a new model that describes how temporal coordination and epistatic interactions of extracellular signals result in the specification and regionalisation of border cells.

Tfap2a and Foxd3 regulate early steps in the development of the neural crest progenitor population

The neural crest is a stem cell-like population exclusive to vertebrates that gives rise to many different cell types including chondrocytes, neurons and melanocytes. Arising from the neural plate border at the intersection of Wnt and Bmp signaling pathways, the complexity of neural crest gene regulatory networks has made the earliest steps of induction difficult to elucidate. Here, we report that tfap2a and foxd3 participate in neural crest induction and are necessary and sufficient for this process to proceed. Double mutant tfap2a (mont blanc, mob) and foxd3 (mother superior, mos) mob;mos zebrafish embryos completely lack all neural crest-derived tissues. Moreover, tfap2a and foxd3 are expressed during gastrulation prior to neural crest induction in distinct, complementary, domains; tfap2a is expressed in the ventral non-neural ectoderm and foxd3 in the dorsal mesendoderm and ectoderm. We further show that Bmp signaling is expanded in mob;mos embryos while expression of dkk1, a Wnt signaling inhibitor, is increased and canonical Wnt targets are suppressed. These changes in Bmp and Wnt signaling result in specific perturbations of neural crest induction rather than general defects in neural plate border or dorso-ventral patterning. foxd3 overexpression, on the other hand, enhances the ability of tfap2a to ectopically induce neural crest around the neural plate, overriding the normal neural plate border limit of the early neural crest territory. Although loss of either Tfap2a or Foxd3 alters Bmp and Wnt signaling patterns, only their combined inactivation sufficiently alters these signaling gradients to abort neural crest induction. Collectively, our results indicate that tfap2a and foxd3, in addition to their respective roles in the differentiation of neural crest derivatives, also jointly maintain the balance of Bmp and Wnt signaling in order to delineate the neural crest induction domain.

FGF/MAPK signaling is required in the gastrula epiblast for avian neural crest induction

Neural crest induction involves the combinatorial inputs of the FGF, BMP and Wnt signaling pathways. Recently, a two-step model has emerged where BMP attenuation and Wnt activation induces the neural crest during gastrulation, whereas activation of both pathways maintains the population during neurulation. FGF is proposed to act indirectly during the inductive phase by activating Wnt ligand expression in the mesoderm. Here, we use the chick model to investigate the role of FGF signaling in the amniote neural crest for the first time and uncover a novel requirement for FGF/MAPK signaling. Contrary to current models, we demonstrate that FGF is required within the prospective neural crest epiblast during gastrulation and is unlikely to operate through mesodermal tissues. Additionally, we show that FGF/MAPK activity in the prospective neural plate prevents the ectopic expression of lateral ectoderm markers, independently of its role in neural specification. We then investigate the temporal participation of BMP/Smad signaling and suggest a later involvement in neural plate border development, likely due to widespread FGF/MAPK activity in the gastrula epiblast. Our results identify an early requirement for FGF/MAPK signaling in amniote neural crest induction and suggest an intriguing role for FGF-mediated Smad inhibition in ectodermal development.

Snail2 controls mesodermal BMP/Wnt induction of neural crest

The neural crest is an induced tissue that is unique to vertebrates. In the clawed frog Xenopus laevis, neural crest induction depends on signals secreted from the prospective dorsolateral mesodermal zone during gastrulation. The transcription factors Snail2 (Slug), Snail1 and Twist1 are expressed in this region. It is known that Snail2 and Twist1 are required for both mesoderm formation and neural crest induction. Using targeted blastomere injection, morpholino-based loss of function and explant studies, we show that: (1) Snail1 is also required for mesoderm and neural crest formation; (2) loss of snail1, snail2 or twist1 function in the C2/C3 lineage of 32-cell embryos blocks mesoderm formation, but neural crest is lost only in the case of snail2 loss of function; (3) snail2 mutant loss of neural crest involves mesoderm-derived secreted factors and can be rescued synergistically by bmp4 and wnt8 RNAs; and (4) loss of snail2 activity leads to changes in the RNA levels of a number of BMP and Wnt agonists and antagonists. Taken together, these results identify Snail2 as a key regulator of the signals involved in mesodermal induction of neural crest.

Kzp controls canonical Wnt8 signaling to modulate dorsoventral patterning during zebrafish gastrulation

During vertebrate embryonic development, the body axis formation requires the action of Wnt signals and their antagonists. Zygotic canonical wnt8 expression appears exclusively at the ventrolateral margin and mediates Wnt/β-catenin activities to promote posterior and ventral cell fate. However, the mechanisms involved in the initiation of zygotic wnt8 signals are poorly understood. Here, we identify a novel, maternally derived transcription factor, Kzp (Kaiso zinc finger-containing protein), as an important determinant for the initiation of zygotic Wnt signals in zebrafish. Kzp is a DNA-binding transcription factor that recognizes specific consensus DNA sequences, 5'-(t/a/g)t(a/t/g)nctgcca-3', through zinc fingers and controls the initiation of zygotic wnt8 expression by directly binding to the wnt8 promoter during zebrafish embryonic development. Depletion of Kzp strongly dorsalized embryos, which was characterized by the expansion of dorsal gene expression. Overexpression of Kzp caused posteriorization. These phenotypes were highly similar to ones induced by wnt8 depletion or overexpression and were rescued by alteration of wnt8 activity. Thus, our results provide the first insight into the mechanism involved in the initiation of zygotic canonical Wnt signals by a maternally derived transcription factor.

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been is critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular, and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play, and equally importantly, where disparities in experimental results suggest areas of profitable study.

Midkine-b regulates cell specification at the neural plate border in zebrafish

Midkines compose a family of secreted, heparin-binding growth factors with neurotrophic activity in vitro, but largely unknown functions in mammals in vivo. Here we show that one member of this family, Midkine-b (Mdkb), is responsible for establishment of the neural plate border in zebrafish. We propose that MdkB acts downstream of several signaling factors, most notably retinoic acid, implicated in neural crest cell (ncc) induction and refines a zone of competence for ncc and Rohon-Beard (RB) sensory neuron formation. Overexpression of Mdkb expands the cell numbers of various ncc precursor subtypes and results in a significant increase in the number of RB neurons at the neural plate border. On the other hand, Morpholino-mediated knockdown of Mdkb leads to a dramatic reduction of ncc and a loss of sensory neurons. Our results imply that Mdkb is required for the earliest steps of cell specification at the neural plate border in zebrafish.

The BMP signaling gradient patterns dorsoventral tissues in a temporally progressive manner along the anteroposterior axis

Patterning of the vertebrate anteroposterior (AP) axis proceeds temporally from anterior to posterior. How dorsoventral (DV) axial patterning relates to AP temporal patterning is unknown. We examined the temporal activity of BMP signaling in patterning ventrolateral cell fates along the AP axis, using transgenes that rapidly turn "off" or "on" BMP signaling. We show that BMP signaling patterns rostral DV cell fates at the onset of gastrulation, whereas progressively more caudal DV cell fates are patterned at progressively later intervals during gastrulation. Increased BMP signal duration is not required to pattern more caudal DV cell fates; rather, distinct temporal intervals of signaling are required. This progressive action is regulated downstream of, or in parallel to, BMP signal transduction at the level of Smad1/5 phosphorylation. We propose that a temporal cue regulates a cell's competence to respond to BMP signaling, allowing the acquisition of a cell's DV and AP identity simultaneously.

Neural crest inducing signals

The formation of the neural crest has been traditionally considered a classic example of secondary induction, where signals form one tissue elicit a response in a competent responding tissue. Interactions of the neural plate with paraxial mesoderm or nonneural ectoderm can generate neural crest. Several signaling pathways converge at the border between neural and nonneural ectoderm where the neural crest will form. Among the molecules identified in this process are members of the BMP, Wnt, FGF and Notch signaling pathways. The concerted action of these signals and their downstream targets will define the identity of the neural crest.

Development of the neural crest: achieving specificity in regulatory pathways

Recent studies have revealed the signaling pathways and downstream effectors involved in the specification of the neural crest. Neural crest cells are generated from a zone at the neurectoderm border in response to Wnt and BMP signals. BMP signals are involved in establishing a competency zone at the border of the neurectoderm, while subsequent Wnt signals specify neural crest cells. Combinations of transcription factors, including pax and msx gene products, act downstream of these pathways to integrate signals and establish the neural crest. Mechanisms are emerging for how specificity is generated from reiterated signals and effectors.

The regulation of mesodermal progenitor cell commitment to somitogenesis subdivides the zebrafish body musculature into distinct domains

The vertebrate musculature is produced from a visually uniform population of mesodermal progenitor cells (MPCs) that progressively bud off somites populating the trunk and tail. How the MPCs are regulated to continuously release cells into the presomitic mesoderm throughout somitogenesis is not understood. Using a genetic approach to study the MPCs, we show that a subset of MPCs are set aside very early in zebrafish development, and programmed to cell-autonomously enter the tail domain beginning with the 16th somite. Moreover, we show that the trunk is subdivided into two domains, and that entry into the anterior trunk, posterior trunk, and tail is regulated by interactions between the Nodal and bone morphogenetic protein (Bmp) pathways. Finally, we show that the tail MPCs are held in a state we previously called the Maturation Zone as they wait for the signal to begin entering somitogenesis.

Relations and interactions between cranial mesoderm and neural crest populations

The embryonic head is populated by two robust mesenchymal populations, paraxial mesoderm and neural crest cells. Although the developmental histories of each are distinct and separate, they quickly establish intimate relations that are variably important for the histogenesis and morphogenesis of musculoskeletal components of the calvaria, midface and branchial regions. This review will focus first on the genesis and organization within nascent mesodermal and crest populations, emphasizing interactions that probably initiate or augment the establishment of lineages within each. The principal goal is an analysis of the interactions between crest and mesoderm populations, from their first contacts through their concerted movements into peripheral domains, particularly the branchial arches, and continuing to stages at which both the differentiation and the integrated three-dimensional assembly of vascular, connective and muscular tissues is evident. Current views on unresolved or contentious issues, including the relevance of head somitomeres, the processes by which crest cells change locations and constancy of cell-cell relations at the crest-mesoderm interface, are addressed.

Reiterated Wnt and BMP signals in neural crest development

Common signaling pathways such as those for Wnts and BMPs are used many times during embryogenesis. During the development of the neural crest, Wnt and BMP signals are used repeatedly at different stages to influence initial induction, segregation from the neuroepithelium and cell fate determination. This review considers how specificity is generated within the neural crest for these reiterated signals, discussing examples of how the outcomes of signaling events are modulated by context.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.

Early steps in neural crest specification

The neural crest is a multipotent cell population that arise at the border of the neural plate and non-neural ectoderm. Studies conducted in a number of model organisms including chickens, frogs, zebrafish and mice have been instrumental in elucidating this molecular mechanisms underlying neural crest formation. Signaling molecules of the Wnt, BMP, and FGF families and their downstream effectors have been shown to mediate neural crest induction. Transcription factors including members of the Snail and SoxE gene families as well as FoxD3, c-Myc and others have been implicated in specification of the neural crest. These studies represent an important step in understanding the regulatory interactions involved in generating this complex and interesting cell type.

Specification and Patterning of Neural Crest Cells During Craniofacial Development

Craniofacial evolution is considered fundamental to the origin of vertebrates and central to this process was the formation of a migratory, multipotent cell population known as the neural crest. The number of cell types that arise from the neural crest is truly astonishing as is the number of tissues and organs to which the neural crest contributes. In addition to forming melanocytes as well as many neurons and glia in the peripheral nervous system, neural crest cells also contribute much of the cartilage, bone and connective tissue of the face. These multipotent migrating cells are capable of self renewing decisions and based upon these criteria are often considered stem cells or stem cell-like. Rapid advances in our understanding of neural crest cell patterning continue to shape our appreciation of the evolution of neural crest cells and their impact on vertebrate craniofacial morphogenesis.