In ovo time-lapse analysis of chick hindbrain neural crest cell migration shows cell interactions during migration to the branchial arches

In vivo evidence for short- and long-range cell communication in cranial neural crest cells

The proper assembly of craniofacial structures and the peripheral nervous system requires neural crest cells to emerge from the neural tube and navigate over long distances to the branchial arches. Cell and molecular studies have shed light on potential intrinsic and extrinsic cues, which, in combination,are thought to ensure the induction and specification of cranial neural crest cells. However, much less is known about how migrating neural crest cells interpret and integrate signals from the microenvironment and other neural crest cells to sort into and maintain the stereotypical pattern of three spatially segregated streams. Here, we explore the extent to which cranial neural crest cells use cell-to-cell and cell-environment interactions to pathfind. The cell membrane and cytoskeletal elements in chick premigratory neural crest cells were labeled in vivo. Three-dimensional reconstructions of migrating neural crest cells were then obtained using confocal static and time-lapse imaging. It was found that neural crest cells maintained nearly constant contact with other migrating neural crest cells, in addition to the microenvironment. Cells used lamellipodia or short, thin filopodia (1-2 μm wide) for local contacts (<20 μm). Non-local, long distance contact (up to 100 μm) was initiated by filopodia that extended and retracted, extended and tracked, or tethered two non-neighboring cells. Intriguingly, the cell-to-cell contacts often stimulated a cell to change direction in favor of a neighboring cell's trajectory. In summary, our results present in vivo evidence for local and long-range neural crest cell interactions, suggesting a possible role for these contacts in directional guidance.

Developmental imaging: Insights into the avian embryo

The study of embryonic events using different animal model systems is crucial for gaining insights into human development and birth defects. Biological imaging plays a major role in this effort by providing a spatiotemporal framework to link complex cell movements with molecular data. However, depending on the age of the embryo and the location of a morphogenetic event, visualization often requires the design of novel culture and imaging techniques. One of the primary model systems for biological imaging is the avian embryo, due to its accessibility to manipulation, relatively two-dimensional morphogenesis early on, and viability when grown in culture. Significant work in avian embryo culture and cell labeling, together with advances in imaging technology, now make it possible to monitor many developmental events within the period from egg laying to hatching. Here, we present the latest in avian developmental imaging, focusing on cell labeling, embryo culture, and imaging technologies.

Dynamics of neural crest-derived cell migration in the embryonic mouse gut

Neural crest-derived cells that form the enteric nervous system undergo an extensive migration from the caudal hindbrain to colonize the entire gastrointestinal tract. Mice in which the expression of GFP is under the control of the Ret promoter were used to visualize neural crest-derived cell migration in the embryonic mouse gut in organ culture. Time-lapse imaging revealed that GFP(+) crest-derived cells formed chains that displayed complicated patterns of migration, with sudden and frequent changes in migratory speed and trajectories. Some of the leading cells and their processes formed a scaffold along which later cells migrated. To examine the effect of population size on migratory behavior, a small number of the most caudal GFP(+) cells were isolated from the remainder of the population. The isolated cells migrated slower than cells in large control populations, suggesting that migratory behavior is influenced by cell number and cell-cell contact. Previous studies have shown that neurons differentiate among the migrating cell population, but it is unclear whether they migrate. The phenotype of migrating cells was examined. Migrating cells expressed the neural crest cell marker, Sox10, but not neuronal markers, indicating that the majority of migratory cells observed did not have a neuronal phenotype.

Current progress in neural crest cell motility and migration and future prospects for the zebrafish model system

The neural crest is a unique population of cells that contributes to the formation of diverse cell types, including craniofacial cartilage, peripheral neurons, the cardiac outflow tract, and pigment cells. Neural crest cells (NCCs) are specified within the neuroepithelium, undergo an epithelial-to-mesenchymal transition, and migrate to target destinations throughout the embryo. Here, we review current understanding of two steps in NCC development, both of which involve NCC motility. The first is NCC delamination from the neuroepithelium and the changes in cell adhesion and the cytoskeleton necessary for the initiation of migration. The second is NCC migration and the signals that guide NCCs along specific migratory pathways. We illustrate the strength of the zebrafish, Danio rerio, as a model organism to study NCC motility. The zebrafish is particularly well suited for the study of neural crest motility because of the ability to combine genetic manipulation with live imaging of migrating NCCs.

Comparative analysis of neural crest cell death, migration, and function during vertebrate embryogenesis

Cranial neural crest cells are a multipotent, migratory population that generates most of the cartilage, bone, connective tissue and peripheral nervous system in the vertebrate head. Proper neural crest cell patterning is essential for normal craniofacial morphogenesis and is highly conserved among vertebrates. Neural crest cell patterning is intimately connected to the early segmentation of the neural tube, such that neural crest cells migrate in discrete segregated streams. Recent advances in live embryo imaging have begun to reveal the complex behaviour of neural crest cells which involve intricate cell-cell and cell-environment interactions. Despite the overall similarity in neural crest cell migration between distinct vertebrates species there are important mechanistic differences. Apoptosis for example, is important for neural crest cell patterning in chick embryos but not in mouse, frog or fish embryos. In this paper we highlight the potential evolutionary significance of such interspecies differences in jaw development and evolution. Developmental Dynamics 229:14-29, 2004.

Significance of the cranial neural crest

The cranial neural crest has long been viewed as being of particular significance. First, it has been held that the cranial neural crest has a morphogenetic role, acting to coordinate the development of the pharyngeal arches. By contrast, the trunk crest seems to play a more subservient role in terms of embryonic patterning. Second, the cranial crest not only generates neurons, glia, and melanocytes, but additionally forms skeletal derivatives (bones, cartilage, and teeth, as well as smooth muscle and connective tissue), and this potential was thought to be a unique feature of the cranial crest. Recently, however, several studies have suggested that the cranial neural crest may not be so influential in terms of patterning, nor so exceptional in the derivatives that it makes. It is now becoming clear that the morphogenesis of the pharyngeal arches is largely driven by the pharyngeal endoderm. Furthermore, it is now apparent that trunk neural crest cells have skeletal potential. However, it has now been demonstrated that a key role for the cranial neural crest streams is to organise the innervation of the hindbrain by the cranial sensory ganglia. Thus, in the past few years, our views of the significance of the cranial neural crest for head development have been altered. Developmental Dynamics 229:5-13, 2004.

Combined intrinsic and extrinsic influences pattern cranial neural crest migration and pharyngeal arch morphogenesis in axolotl

Cranial neural crest cells migrate in a precisely segmented manner to form cranial ganglia, facial skeleton and other derivatives. Here, we investigate the mechanisms underlying this patterning in the axolotl embryo using a combination of tissue culture, molecular markers, scanning electron microscopy and vital dye analysis. In vitro experiments reveal an intrinsic component to segmental migration; neural crest cells from the hindbrain segregate into distinct streams even in the absence of neighboring tissue. In vivo, separation between neural crest streams is further reinforced by tight juxtapositions that arise during early migration between epidermis and neural tube, mesoderm and endoderm. The neural crest streams are dense and compact, with the cells migrating under the epidermis and outside the paraxial and branchial arch mesoderm with which they do not mix. After entering the branchial arches, neural crest cells conduct an "outside-in" movement, which subsequently brings them medially around the arch core such that they gradually ensheath the arch mesoderm in a manner that has been hypothesized but not proven in zebrafish. This study, which represents the most comprehensive analysis of cranial neural crest migratory pathways in any vertebrate, suggests a dual process for patterning the cranial neural crest. Together with an intrinsic tendency to form separate streams, neural crest cells are further constrained into channels by close tissue apposition and sorting out from neighboring tissues.

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

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

Time-lapse microscopy of brain development

Zebrafish embryos represent an ideal vertebrate model organism for noninvasive intravital imaging because of their optical clarity, external embryogenesis, and fast development. Many different labeling techniques have been adopted from other model organisms or newly developed to address a wealth of different developmental questions directly inside the living organism. The parallel advancements in the field of optical imaging let us now observe dynamic processes at the cellular and subcellular resolution. Combined with the repertoire of available surgical and genetic manipulations, zebrafish embryos provide the powerful and almost unique possibility to observe the interplay of molecular signals with cellular, morphological, and behavioral changes directly within a living and developing vertebrate organism. A bright future for zebrafish is yet to come, let there be light.

Spatiotemporally separated cardiac neural crest subpopulations that target the outflow tract septum and pharyngeal arch arteries

We used lacZ-retrovirus labeling combined with neural crest ablation in chick embryos to determine whether the cardiac neural crest cells constitute one group of multipotent cells, or they emigrate from the neural tube in time-dependent groups with different fates in the developing cardiovascular system. We demonstrated that early-migrating cardiac neural crest cells (HH9-10) massively target the aorticopulmonary septum and pharyngeal arch arteries, while the late-migrating cardiac neural crest cells (HH12) are restricted to the proximal part of the pharyngeal arch arteries. These results suggest a prominent role for early-migrating cells in outflow tract septation, and a function for late-migrating cells in pharyngeal arch artery remodeling. We demonstrated in cultures of neural tube explants an intrinsic difference between the early and late populations. However, by performing heterochronic transplantations we showed that the late-migrating cardiac neural crest cells were not developmentally restricted, and could contribute to the condensed mesenchyme of the aorticopulmonary septum when transplanted to a younger environment. Our findings on the exact timing and migratory behavior of cardiac neural crest cells will help narrow the range of factors and genes that are involved in neural crest-related congenital heart diseases.

Dual function of Slit2 in repulsion and enhanced migration of trunk, but not vagal, neural crest cells

Neural crest precursors to the autonomic nervous system form different derivatives depending upon their axial level of origin; for example, vagal, but not trunk, neural crest cells form the enteric ganglia of the gut. Here, we show that Slit2 is expressed at the entrance of the gut, which is selectively invaded by vagal, but not trunk, neural crest. Accordingly, only trunk neural crest cells express Robo receptors. In vivo and in vitro experiments demonstrate that trunk, not vagal, crest cells avoid cells or cell membranes expressing Slit2, thereby contributing to the differential ability of neural crest populations to invade and innervate the gut. Conversely, exposure to soluble Slit2 significantly increases the distance traversed by trunk neural crest cells. These results suggest that Slit2 can act bifunctionally, both repulsing and stimulating the motility of trunk neural crest cells.

The role of cell mixing in branchial arch development

Compartmental structures are the basis of a number of developing systems, including parts of the vertebrate head. One of the characteristics of a series of compartments is that mixing between cells in adjacent units is restricted. This is a consequence of differential chemoaffinity between neighbouring cells in adjacent compartments. We set out to determine whether mesenchymal cells in the branchial arches and their precursors show cell-mixing properties consistent with a compartmental organisation. In chimaeric avian embryos we found no evidence of preferential association or segregation of neural crest cells when surrounded by cells derived from a different axial level. In reassociation assays using mesenchymal cells isolated from chick branchial arches at stage 18, cells reformed into clusters without exhibiting a preferential affinity for cells derived from the same branchial arch. We find no evidence for differential chemoaffinity in vivo or in vitro between mesenchymal cells in different branchial arches. Our findings suggest that branchial arch mesenchyme is not organised into a series of compartments.

c-jun regulation and function in the developing hindbrain

Hindbrain development is a well-characterised segmentation process in vertebrates. The bZip transcription factor MafB/kreisler is specifically expressed in rhombomeres (r) 5 and 6 of the developing vertebrate hindbrain and is required for proper caudal hindbrain segmentation. Here, we provide evidence that the mouse protooncogene c-jun, which encodes a member of the bZip family, is coexpressed with MafB in prospective r5 and r6. Analysis of mouse mutants suggests that c-jun expression in these territories is dependent on MafB but independent of the zinc-finger transcription factor Krox20, another essential determinant of r5 development. Loss- and gain-of-function studies, performed in mouse and chick embryos, respectively, demonstrate that c-Jun participates, together with MafB and Krox20, in the transcriptional activation of the Hoxb3 gene in r5. The action of c-Jun is likely to be direct, since c-Jun homodimers and c-Jun/MafB heterodimers can bind to essential regulatory elements within the transcriptional enhancer responsible for Hoxb3 expression in r5. These data indicate that c-Jun acts both as a downstream effector and a cofactor of MafB and belongs to the complex network of factors governing hindbrain patterning.

Cranial skeletal biology

To artists, the face is a mirror of the soul. To biologists, the face reflects remarkable structural diversity--think of bulldogs and wolfhounds or galapagos finches. How do such variations in skeletal form arise? Do the same mechanisms control skeletogenesis elsewhere in the body? The answers lie in the molecular machinery that generates neural crest cells, controls their migration, and guides their differentiation to cartilage and bone.

Direct visualization of a stratified epithelium reveals that wounds heal by unified sliding of cell sheets

Observing cells in their original niche is a key link between the information gleaned from planar culture and in vivo physiology and pathology. A new approach combining the transparency of the cornea, Hoffman modulation optics, and digital imaging allowed movements of individual corneal cells to be viewed directly in situ. 3-Dimensional time-lapse movies imaging unstained cells within the stratified corneal epithelium during wound healing were made. Tracking cell movements dynamically provided a definitive answer to the long-standing question: does a stratified epithelium heal by "sliding" of cell sheets as a coherent unit or do individual cells "leap frog" each other at the wound margin? A wound in the corneal epithelium healed primarily by sliding of the whole epithelium, with approximately 95% of cells moving with similar speed and trajectories and with little change in their relative position. Only 5% of cells changed layers, with equal proportions moving up or down. Epithelial healing in situ occurred in three phases: a latency, migration, and reconstruction phase. This model provides a unique system to study the behaviors of individual cells in their original niche. It shows that cells slide into a wound as a unified unit to heal a stratified epithelium.

Pathogenic pathways in fluconazole-induced branchial arch malformations

BACKGROUND

A widely-used antimycotic agent, bis-triazole fluconazole (FLUCO), is able to produce abnormalities to the branchial apparatus (hypoplasia, agenesis, and fusion) in postimplantation rodent embryos cultured in vitro. The branchial apparatus is a complex and transient structure in vertebrate embryos and is essential for the development of the face skeleton. Branchial arch mesenchyme is formed by two different cellular populations: paraxial mesenchyme and ectomesenchyme, which originate from rhombencephalic neural crest cell (NCC) migration. We investigated the possible pathogenic pathways involved in FLUCO-related branchial arch abnormalities. Perturbations in physiological apoptosis, cell proliferation, NCC migration and branchial mesenchyme induction have been considered.

METHODS

Rat embryos (9.5-day postcoitum; 1-3 somites) were exposed in vitro to 0 or 500 microM FLUCO. After 24, 36, or 48 hr of culture, embryos were examined for apoptosis (acridine orange method) and cell proliferation (BrdU incorporation and detection method). Rhombencephalic NCC migration was analyzed using immunostaining of NCC (using anti-CRABP antibodies) and the extracellular matrix (using anti-fibronectin antibodies). The differentiative capability of the branchial mesenchymes was investigated using anti-endothelin and anti-endothelin-receptor antibodies.

RESULTS

During the whole culture period, no alterations in physiological apoptosis, cell proliferation, and mesenchymal cell induction were observed in FLUCO-exposed embryos in comparison to controls. On the contrary, severe alterations in NCC migration pathways were observed in FLUCO-exposed embryos.

CONCLUSIONS

The findings suggest that FLUCO produces teratogenic effects by interfering with the cellular and molecular mechanisms that control NCC migration.

Border-cell migration: the race is on

The conversion of stationary epithelial cells into migratory, invasive cells is important for normal embryonic development and tumour metastasis. Border-cell migration in the ovary of Drosophila melanogaster has emerged as a simple, genetically tractable model for studying this process. Three distinct signals, which are also upregulated in cancer, control border-cell migration, so identifying further genes that are involved in border-cell migration could provide new insights into tumour invasion.

Apoptosis of premigratory neural crest cells in rhombomeres 3 and 5: consequences for patterning of the branchial region

In the avian hindbrain, premigratory neural crest cells undergo programmed cell death (apoptosis) in rhombomeres 3 and 5 (r3, r5). Here, we have attempted to analyze the significance of the loss of neural crest cells from these odd-numbered rhombomeres. When apoptosis is prevented in r3 and r5, r3 crest migrate into the first arch and r5 into the third arch. Interestingly, these extra neural crest cells contributed to the formation of ectopic muscle attachment sites that are also found in those species in which r3 and r5 neural crest cells do not undergo apoptosis. Thus, apoptosis in the odd-numbered rhombomeres appears to be an evolutionarily derived mechanism that is required to eliminate r3 and r5 crest migration into first and third arches and thereby remove these muscle attachment sites.

Initiating and guiding migration: lessons from border cells

Cell migration occurs in many different contexts. Amoebae and other isolated cells migrate in culture. In animals, 'professional' migratory cells of the immune system constantly survey the body for intruders, whereas other cell types perform specific developmentally regulated migrations. One simple model for the latter type of event is migration of border cells during Drosophila oogenesis. Recent findings have shed light on how border cell fate is induced and on how the migration is guided. This article discusses the implications of these studies and compares (invasive) migration through a tissue with what is known about cells crawling on a flat substratum.

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

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

Multi-field 3D scanning light microscopy of early embryogenesis

A computer-controlled microscopy system was devised to allow the observation of avian embryo development over an extended time period. Parallel experiments, as well as extended specimen volumes, can be recorded at cellular resolution using a three-dimensional scanning procedure. The resulting large set of data is processed automatically into registered, focal- and positional-drift corrected mosaic images, assembled as montages of adjacent microscopic fields. The configuration of the incubator and a sterile embryo chamber prevents condensation of the humidified culturing atmosphere in the optical path and is compatible with both differential interference contrast and epifluorescence optics. As a demonstration, recordings are presented showing the large-scale remodelling of the embryonic primordial vascular structure.

Using electroporation and lipid-mediated transfection of GFP-expressing plasmids to label embryonic avian cells for vital confocal and two-photon microscopy

Fluorescent proteins have emerged as an ideal fluorescent marker for studying cell morphologies in vital systems. These proteins were first applied in whole organisms with established germ-line transformation protocols, but now it is possible to label cells with fluorescent proteins in other organisms. Here we present two ways to introduce GFP expressing plasmids into avian embryos for vital confocal and two-photon imaging. First, electroporation is a powerful approach to introduce GFP into the developing neural tube, offering several advantages over dye labeling. Second, we introduce a new lipid-based transfection system for introducing plasmid DNA directly to a small group of injected cells within live, whole embryos. These complementary approaches make it possible to transfect a wide-range of cell types in the avian embryo and the bright, stable, uniform expression of GFP offers great advantages for vital fluorescence imaging.

Role of the isthmus and FGFs in resolving the paradox of neural crest plasticity and prepatterning

Cranial neural crest cells generate the distinctive bone and connective tissues in the vertebrate head. Classical models of craniofacial development argue that the neural crest is prepatterned or preprogrammed to make specific head structures before its migration from the neural tube. In contrast, recent studies in several vertebrates have provided evidence for plasticity in patterning neural crest populations. Using tissue transposition and molecular analyses in avian embryos, we reconcile these findings by demonstrating that classical manipulation experiments, which form the basis of the prepatterning model, involved transplantation of a local signaling center, the isthmic organizer. FGF8 signaling from the isthmus alters Hoxa2 expression and consequently branchial arch patterning, demonstrating that neural crest cells are patterned by environmental signals.

Cadherins in neural crest cell development and transformation

Cadherins constitute a superfamily of cell adhesion molecules involved in cell-cell interaction, histogenesis and cellular transformation. They have been implicated in the development of various lineages, including derivatives of the neural crest. Neural crest cells (NCC) emerge from the dorsal part of the neural tube after an epithelio-mesenchymal transition (EMT) and migrate through the embryo. After homing and differentiation, NCC give rise to many cell types, such as neurons, Schwann cells and melanocytes. During these steps, the pattern of expression of the various cadherins studied is very dynamic. Cadherins also display plasticity of expression during the transformation of neural crest cell derivatives. Here, we review the pattern of expression and the role of the main cadherins involved in the development and transformation of neural crest cell derivatives.

The neuronal naturalist: watching neurons in their native habitat

Dynamic processes of neural development, such as migrations of precursor cells, growth of axons and dendrites, and formation and modification of synapses, can be fully analyzed only with techniques that monitor changes over time. Although there has been long-standing motivation for following cellular and synaptic events in vivo (intravital microscopy), until recently few preparations have been studied, and then often only with great effort. Innovations in low-light and laser-scanning microscopies, coupled with developments of new dyes and of genetically encoded indicators, have increased both the breadth and depth of in situ imaging approaches. Here we present the motivations and challenges for dynamic imaging methods, offer some illustrative examples and point to future opportunities with emerging technologies.

A new approach to neural cell culture for long-term studies

We have developed a new method for culturing cells that maintains their health and sterility for many months. Using conventional techniques, primary neuron cultures seldom survive more than 2 months. Increases in the osmotic strength of media due to evaporation are a large and underappreciated contributor to the gradual decline in the health of these cultures. Because of this and the ever-present likelihood of contamination by airborne pathogens, repeated or extended experiments on any given culture have until now been difficult, if not impossible. We surmounted survival problems by using culture dish lids that form a gas-tight seal, and incorporate a transparent hydrophobic membrane (fluorinated ethylene-propylene) that is selectively permeable to oxygen (O(2)) and carbon dioxide (CO(2)), and relatively impermeable to water vapor. This prevents contamination and greatly reduces evaporation, allowing the use of a non-humidified incubator. We have employed this technique to grow dissociated cortical cultures from rat embryos on multi-electrode arrays. After more than a year in culture, the neurons still exhibit robust spontaneous electrical activity. The combination of sealed culture dishes with extracellular multi-electrode recording and stimulation enables study of development, adaptation, and very long-term plasticity, across months, in cultured neuronal networks. Membrane-sealed dishes will also be useful for the culture of many other cell types susceptible to evaporation and contamination.

Cell movements during vertebrate development: integrated tissue behaviour versus individual cell migration

Cell migration during development is fundamental to the establishment of the embryonic architecture. Depending on the context, cells may move either as integrated sheets of tissue or individually. Recently, molecules that are involved in both these types of cell behaviour have been identified, helping us to understand developmental processes as important as gastrulation and neural crest formation, and ultimately, the morphogenetic movements that shape the embryo.

Segmental organization of neural crest migration

Avian neural crest cells migrate on precise pathways to their target areas where they form a wide variety of cellular derivatives, including neurons, glia, pigment cells and skeletal components. In one portion of their pathway, trunk neural crest cells navigate in the somitic mesoderm in a segmental fashion, invading the rostral, while avoiding the caudal, half-sclerotome. This pattern of cell migration, imposed by the somitic mesoderm, contributes to the metameric organization of the peripheral nervous system, including the sensory and sympathetic ganglia. At hindbrain levels, neural crest cells also travel from the neural tube in a segmental manner via three migratory streams of cells that lie adjacent to even-numbered rhombomeres. In this case, the adjacent mesoderm does not possess an obvious segmental organization, compared to the somitic mesoderm at trunk levels. Thus, the mechanisms by which the embryo controls segmentally-organized cell migrations have been a fascinating topic over the past several years. Here, I discuss findings from classical and recent studies that have delineated several of the tissue, cellular and molecular elements that contribute to the segmental organization of neural crest migration, primarily in the avian embryo. One common theme is that neural crest cells are prohibited from entering particular territories in the embryo due to the expression of inhibitory factors. However, permissive, migration-promoting factors may also play a key role in coordinating neural crest migration.

FGF receptor signalling is required to maintain neural progenitors during Hensen's node progression

Previous analyses of labelled clones of cells within the developing nervous system of the mouse have indicated that descendants are initially dispersed rostrocaudally followed by more local proliferation, which is consistent with the progressing node's contributing descendants from a resident population of progenitor cells as it advances caudally. Here we electroporated an expression vector encoding green fluorescent protein into the chicken embryo near Hensen's node to test and confirm the pattern inferred in the mouse. This provides a model in which a proliferative stem zone is maintained in the node by a localized signal; those cells that are displaced out of the stem zone go on to contribute to the growing axis. To test whether fibroblast growth factor (FGF) signalling could be involved in the maintenance of the stem zone, we co-electroporated a dominant-negative FGF receptor with a lineage marker, and found that it markedly alters the elongation of the spinal cord primordium. The results indicate that FGF receptor signalling promotes the continuous development of the posterior nervous system by maintaining presumptive neural progenitors in the region near Hensen's node. This offers a potential explanation for the mixed findings on FGF in the growth and patterning of the embryonic axis.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

Cryptic responses to tissue manipulations in avian embryos

Experimental embryology performed on avian embryos combines tissue manipulations and cell-labeling methods with increasing opportunities and demands for critical assays of the results. These approaches continue to reveal unexpected complexities in the normal patterns of cell movement and tissue origins, documentation of which is critical to unraveling the intricacies of cell and tissue interactions during embryogenesis. Viktor Hamburger's many pioneering contributions helped launch and promote the philosophical as well as technical elements of avian experimental embryology. Furthermore, his scholarship and profoundly positive presence influenced not just those of us fortunate to have trained with him, but several generations of developmental biologists. The first part of this article presents examples of the opportunities and rewards that have occurred due to his influences. Surgical manipulation of avian embryonic tissues always introduces a greater number of variables than the experimenter can control for or, often, readily identify. We present the results of dorsal and ventral lesions of hindbrain segments, which include defects in structures within, beside, and also at a considerable distance from the site of lesion. Extramedullary loops of longitudinal tract axons exit and re-enter the neural tube, and intra-medullary proliferation of blood vessels is expanded. Peripherally, the coalescence of neural crest- and placode-derived neuroblasts is disrupted. As expected, motor neurons and their projections close to the sites of lesion are compromised. However, an unexpected finding is that the normal projections of cranial nerves located distant to the lesion site were also disrupted. Following brainstem lesions in the region of rhombomeres 3, 4 or 5, trigeminal or oculomotor axons penetrated the lateral rectus muscle. Surprisingly, the ability of VIth nerve axons to reach the lateral rectus muscle was not destroyed in most cases, even though the terrain through which they needed to pass was disrupted. These axons typically followed a more ventral course than normal, and usually, the axons emerging from individual roots failed to fasciculate into a common VIth nerve, which suggests that each rootlet contains pathfinder-competent axons. The lesson from these lesions is that surgical intervention in avian embryos may have substantial effects upon tissues within, adjacent to, and distant to those that are being manipulated.

Neural crest patterning and the evolution of the jaw

Here we present ideas connecting the behaviour of the cranial neural crest during development with the venerable, perhaps incorrect, view that gill-supporting cartilages of an ancient agnathan evolved into the skeleton of an early gnathostome's jaw. We discuss the pattern of migration of the cranial neural crest ectomesenchyme in zebrafish, along with the subsequent arrangement of postmigratory crest and head mesoderm in the nascent pharyngeal segments (branchiomeres), in diverse gnathostomes and in lampreys. These characteristics provide for a plausible von Baerian explanation for the problematic inside-outside change in topology of the gills and their supports between these 2 major groups of vertebrates. We consider it likely that the jaw supports did indeed arise from branchiomeric cartilages.

alpha4 integrin is expressed in a subset of cranial neural crest cells and in epicardial progenitor cells during early mouse development

By RNA in situ hybridization and immunohistochemical analyses of early stage mouse embryos, we find that alpha 4 integrin gene is expressed in migratory cranial neural crest cells originating from the presumptive forebrain, midbrain, and rhombomeres 1 and 2 of the presumptive hindbrain. alpha 4 is also expressed in epicardial progenitor cells in the septum transversum that migrate to the heart.

The WNT antagonist cSFRP2 modulates programmed cell death in the developing hindbrain

In the avian hindbrain, the loss of premigratory neural crest cells from rhombomeres 3 and 5 (r3, r5) through programmed cell death contributes to the patterning of emigrant crest cells into three discrete streams. Programmed cell death is induced by the upregulation of Bmp4 and Msx2 in r3 and r5. We show that cSFRP2, a WNT antagonist, is expressed in the even-numbered rhombomeres and that over-expression of cSfrp2 inhibits Bmp4 expression in r3 and r5, preventing programmed cell death. By contrast, depleting cSFRP2 function in r4 results in elevated levels of Msx2 expression and ectopic programmed cell death, as does overexpression of Wnt1. We propose that programmed cell death in the rhombencephalic neural crest is modulated by pre-patterned cSfrp2 expression and a WNT-BMP signalling loop.

Molecular control of neural crest formation, migration and differentiation

Induction, migration and differentiation of the neural crest are crucial for the development of the vertebrate embryo, and elucidation of the underlying mechanisms remains an important challenge. In the past year, a novel signal regulating the formation of neural crest cells has been identified, and advances have been made in uncovering roles for bone morphogenetic protein signals and for a transcription factor in the onset of neural crest migration. There have been new insights into the migration and plasticity of branchial neural crest cells. Important progress has been made in dissecting the roles of bone morphogenetic protein, Wnt and Notch signalling systems and their associated downstream transcription factors in the control of neural crest cell differentiation.

Role of the extracellular matrix during neural crest cell migration

Once specified to become neural crest (NC), cells occupying the dorsal portion of the neural tube disrupt their cadherin-mediated cell-cell contacts, acquire motile properties, and embark upon an extensive migration through the embryo to reach their ultimate phenotype-specific sites. The understanding of how this movement is regulated is still rather fragmentary due to the complexity of the cellular and molecular interactions involved. An additional intricate aspect of the regulation of NC cell movement is that the timings, modes and patterns of NC cell migration are intimately associated with the concomitant phenotypic diversification that cells undergo during their migratory phase and the fact that these changes modulate the way that moving cells interact with their microenvironment. To date, two interplaying mechanisms appear central for the guidance of the migrating NC cells through the embryo: one involves secreted signalling molecules acting through their cognate protein kinase/phosphatase-type receptors and the other is contributed by the multivalent interactions of the cells with their surrounding extracellular matrix (ECM). The latter ones seem fundamental in light of the central morphogenetic role played by the intracellular signals transduced through the cytoskeleton upon integrin ligation, and the convergence of these signalling cascades with those triggered by cadherins, survival/growth factor receptors, gap junctional communications, and stretch-activated calcium channels. The elucidation of the importance of the ECM during NC cell movement is presently favoured by the augmenting knowledge about the macromolecular structure of the specific ECM assembled during NC development and the functional assaying of its individual constituents via molecular and genetic manipulations. Collectively, these data propose that NC cell migration may be governed by time- and space-dependent alterations in the expression of inhibitory ECM components; the relative ratio of permissive versus non-permissive ECM components; and the supramolecular assembly of permissive ECM components. Six multidomain ECM constituents encoded by a corresponding number of genes appear to date the master ECM molecules in the control of NC cell movement. These are fibronectin, laminin isoforms 1 and 8, aggrecan, and PG-M/version isoforms V0 and V1. This review revisits a number of original observations in amphibian and avian embryos and discusses them in light of more recent experimental data to explain how the interaction of moving NC cells with these ECM components may be coordinated to guide cells toward their final sites during the process of organogenesis.

In ovo time-lapse analysis after dorsal neural tube ablation shows rerouting of chick hindbrain neural crest

Previous analyses of single neural crest cell trajectories have suggested important roles for interactions between neural crest cells and the environment, and amongst neural crest cells. To test the relative contribution of intrinsic versus extrinsic information in guiding cells to their appropriate sites, we ablated subpopulations of premigratory chick hindbrain neural crest and followed the remaining neural crest cells over time using a new in ovo imaging technique. Neural crest cell migratory behaviors are dramatically different in ablated compared with unoperated embryos. Deviations from normal migration appear either shortly after cells emerge from the neural tube or en route to the branchial arches, areas where cell-cell interactions typically occur between neural crest cells in normal embryos. Unlike the persistent, directed trajectories in normal embryos, neural crest cells frequently change direction and move somewhat chaotically after ablation. In addition, the migration of neural crest cells in collective chains, commonly observed in normal embryos, was severely disrupted. Hindbrain neural crest cells have the capacity to reroute their migratory pathways and thus compensate for missing neural crest cells after ablation of neighboring populations. Because the alterations in neural crest cell migration are most dramatic in regions that would normally foster cell-cell interactions, the trajectories reported here argue that cell-cell interactions have a key role in the shaping of the neural crest migration.