The migration of neural crest cells

Accurate staging of chick embryonic tissues via deep learning of salient features

Recent work shows that the developmental potential of progenitor cells in the HH10 chick brain changes rapidly, accompanied by subtle changes in morphology. This demands increased temporal resolution for studies of the brain at this stage, necessitating precise and unbiased staging. Here, we investigated whether we could train a deep convolutional neural network to sub-stage HH10 chick brains using a small dataset of 151 expertly labelled images. By augmenting our images with biologically informed transformations and data-driven preprocessing steps, we successfully trained a classifier to sub-stage HH10 brains to 87.1% test accuracy. To determine whether our classifier could be generally applied, we re-trained it using images (269) of randomised control and experimental chick wings, and obtained similarly high test accuracy (86.1%). Saliency analyses revealed that biologically relevant features are used for classification. Our strategy enables training of image classifiers for various applications in developmental biology with limited microscopy data.

Col11a1a Expression Is Required for Zebrafish Development

The autosomal dominant chondrodystrophies, the Stickler type 2 and Marshall syndromes, are characterized by facial abnormalities, vision deficits, hearing loss, and articular joint issues resulting from mutations in COL11A1. Zebrafish carry two copies of the Col11a1 gene, designated Col11a1a and Col11a1b. Col11a1a is located on zebrafish chromosome 24 and Col11a1b is located on zebrafish chromosome 2. Expression patterns are distinct for Col11a1a and Col11a1b and Col11a1a is most similar to COL11A1 that is responsible for human autosomal chondrodystrophies and the gene responsible for changes in the chondrodystrophic mouse model cho/cho. We investigated the function of Col11a1a in craniofacial and axial skeletal development in zebrafish using a knockdown approach. Knockdown revealed abnormalities in Meckel's cartilage, the otoliths, and overall body length. Similar phenotypes were observed using a CRISPR/Cas9 gene-editing approach, although the CRISPR/Cas9 effect was more severe compared to the transient effect of the antisense morpholino oligonucleotide treatment. The results of this study provide evidence that the zebrafish gene for Col11a1a is required for normal development and has similar functions to the mammalian COL11A1 gene. Due to its transparency, external fertilization, the Col11a1a knockdown, and knockout zebrafish model systems can, therefore, contribute to filling the gap in knowledge about early events during vertebrate skeletal development that are not as tenable in mammalian model systems and help us understand Col11a1-related early developmental events.

Development of mesenteric tissues

Mesothelial, neurovascular, lymphatic, adipose and mesenchymal tissues make up the mesentery. These tissues are pathobiologically important for numerous reasons. Collectively, they form a continuous, discrete and substantive organ. Additionally, they maintain abdominal digestive organs in position and in continuity with other systems. Furthermore, as they occupy a central position, they mediate transmission of signals between the abdominal digestive system and the remainder of the body. Despite this physiologic centrality, mesenteric tissue development has received little investigatory focus. However, recent advances in our understanding of anatomy demonstrate continuity between all mesenteric tissues, thereby linking previously unrelated studies. In this review, we examine the development of mesenteric tissue in normality and in the setting of congenital abnormalities.

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.

Exact Solutions of Coupled Multispecies Linear Reaction-Diffusion Equations on a Uniformly Growing Domain

Embryonic development involves diffusion and proliferation of cells, as well as diffusion and reaction of molecules, within growing tissues. Mathematical models of these processes often involve reaction–diffusion equations on growing domains that have been primarily studied using approximate numerical solutions. Recently, we have shown how to obtain an exact solution to a single, uncoupled, linear reaction–diffusion equation on a growing domain, 0 < x < L(t), where L(t) is the domain length. The present work is an extension of our previous study, and we illustrate how to solve a system of coupled reaction–diffusion equations on a growing domain. This system of equations can be used to study the spatial and temporal distributions of different generations of cells within a population that diffuses and proliferates within a growing tissue. The exact solution is obtained by applying an uncoupling transformation, and the uncoupled equations are solved separately before applying the inverse uncoupling transformation to give the coupled solution. We present several example calculations to illustrate different types of behaviour. The first example calculation corresponds to a situation where the initially–confined population diffuses sufficiently slowly that it is unable to reach the moving boundary at x = L(t). In contrast, the second example calculation corresponds to a situation where the initially–confined population is able to overcome the domain growth and reach the moving boundary at x = L(t). In its basic format, the uncoupling transformation at first appears to be restricted to deal only with the case where each generation of cells has a distinct proliferation rate. However, we also demonstrate how the uncoupling transformation can be used when each generation has the same proliferation rate by evaluating the exact solutions as an appropriate limit.

Exact solutions of linear reaction-diffusion processes on a uniformly growing domain: criteria for successful colonization

Many processes during embryonic development involve transport and reaction of molecules, or transport and proliferation of cells, within growing tissues. Mathematical models of such processes usually take the form of a reaction-diffusion partial differential equation (PDE) on a growing domain. Previous analyses of such models have mainly involved solving the PDEs numerically. Here, we present a framework for calculating the exact solution of a linear reaction-diffusion PDE on a growing domain. We derive an exact solution for a general class of one-dimensional linear reaction—diffusion process on 0<x<L(t), where L(t) is the length of the growing domain. Comparing our exact solutions with numerical approximations confirms the veracity of the method. Furthermore, our examples illustrate a delicate interplay between: (i) the rate at which the domain elongates, (ii) the diffusivity associated with the spreading density profile, (iii) the reaction rate, and (iv) the initial condition. Altering the balance between these four features leads to different outcomes in terms of whether an initial profile, located near x = 0, eventually overcomes the domain growth and colonizes the entire length of the domain by reaching the boundary where x = L(t).

Parameters affecting efficiency of in ovo electroporation of the avian neural tube and crest

BACKGROUND

Many variations in avian in ovo transfection of the neural tube/crest have been reported, but never compared quantitatively.

RESULTS

Genome integrating pT2K-CAGGS-GFP and pCAGGS-T2TP transposase plasmids were co-electroporated into quail E2 embryo trunk neural tube and the proportion of GFP-expressing neural cells was counted 1 and 7 days later. Electroporation efficiency increased with plasmid concentration and pulse number but plateaued at, respectively, above 1.25 µg/µL and 3 pulses. Bilateral electroporation transfected more cells than unilateral but less than that anticipated by doubling the unilateral treatment. Holding the concentration of GFP plasmid constant and varying the transposase plasmid concentration revealed an optimum ratio of, in this case, 4:1 (1.2 µg/µL:0.3 µg/µL). Leaving transfected embryos to E9 confirmed that expression was maintained in vivo with the transposase system, but declined with non-integrated plasmid. Transfection of neural crest cells was low if electroporated less than 6-8 hr before emigration. We propose this indicates loss of epithelial integrity well prior to exit. We suggest this event be termed epithelio-mesenchymal transition sensu stricto, whereas the term delamination be reserved for the later emigration from the neural epithelium.

CONCLUSIONS

Co-electroporation in ovo must take into account plasmid(s) concentration and ratio, pulse number, pulse directionality, and timing.

Neurotrophic factor NT-3 displays a non-canonical cell guidance signaling function for cephalic neural crest cells

Chemotactic cell migration is triggered by extracellular concentration gradients of molecules segregated by target fields. Neural crest cells (NCCs), paradigmatic as an accurately moving cell population, undergo wide dispersion along multiple pathways, invading with precision defined sites of the embryo to differentiate into many derivatives. This report addresses the involvement of NT-3 in early colonization by cephalic NCCs invading the optic vesicle region. The results of in vitro and in vivo approaches showed that NCCs migrate directionally up an NT-3 concentration gradient. We also demonstrated the expression of NT-3 in the ocular region as well as their functional TrkB, TrkC and p75 receptors on cephalic NCCs. On whole-mount embryo, a perturbed distribution of NCCs colonizing the optic vesicle target field was shown after morpholino cancelation of cephalic NT-3 or TrkC receptor on NCCs, as well as in situ blocking of TrkC receptor of mesencephalic NCCs by specific antibody released from inserted microbeads. The present results strongly suggest that, among other complementary cell guidance factor(s), the chemotactic response of NCCs toward the ocular region NT-3 gradient is essential for spatiotemporal cell orientation, amplifying the functional scope of this neurotrophic factor as a molecular guide for the embryo cells, besides its well-known canonical functions.

Neural crest and somitic mesoderm as paradigms to investigate cell fate decisions during development

The dorsal domains of the neural tube and somites are transient embryonic epithelia; they constitute the source of neural crest progenitors that generate the peripheral nervous system, pigment cells and ectomesenchyme, and of the dermomyotome that develops into myocytes, dermis and vascular cells, respectively. Based on the variety of derivatives produced by each type of epithelium, a classical yet still highly relevant question is whether these embryonic epithelia are composed of homogeneous multipotent progenitors or, alternatively, of subsets of fate-restricted cells. Growing evidence substantiates the notion that both the dorsal tube and the dermomyotome are heterogeneous epithelia composed of multipotent as well as fate-restricted precursors that emerge as such in a spatio-temporally regulated manner. Elucidation of the state of commitment of the precedent progenitors is of utmost significance for deciphering the mechanisms that regulate fate segregation during embryogenesis. In addition, it will contribute to understanding the nature of well documented neural crest-somite interactions shown to modulate the timing of neural crest cell emigration, their segmental migration, and myogenesis.

Diversity in the molecular and cellular strategies of epithelium-to-mesenchyme transitions: Insights from the neural crest

Although epithelial to mesenchymal transitions (EMT) are often viewed as a unique event, they are characterized by a great diversity of cellular processes resulting in strikingly different outcomes. They may be complete or partial, massive or progressive, and lead to the complete disruption of the epithelium or leave it intact. Although the molecular and cellular mechanisms of EMT are being elucidated owing chiefly from studies on transformed epithelial cell lines cultured in vitro or from cancer cells, the basis of the diversity of EMT processes remains poorly understood. Clues can be collected from EMT occuring during embryonic development and which affect equally tissues of ectodermal, endodermal or mesodermal origins. Here, based on our current knowledge of the diversity of processes underlying EMT of neural crest cells in the vertebrate embryo, we propose that the time course and extent of EMT do not depend merely on the identity of the EMT transcriptional regulators and their cellular effectors but rather on the combination of molecular players recruited and on the possible coordination of EMT with other cellular processes.

Expression of the transcription factor snail and its target gene twist are associated with malignancy in pheochromocytomas

BACKGROUND

One of the best known functions of the zinc-finger transcription factor Snail is to induce epithelial-mesenchymal transition (EMT). Twist, a target genes of Snail, is known to promote the development of distant metastases in mice. Increasing evidence suggests that EMT plays a pivotal role in tumor progression and metastatic spread.

METHODS

Snail, Twist, and E-cadherin expression were assessed by immunohistochemistry and real-time quantitative reverse transcriptase-polymerase chain reaction in 12 malignant and 35 benign pheochromocytomas (PCC). Data were correlated with clinical characteristics and genetics.

RESULTS

We found Snail expression in 13 (28%) of 47 primary PCC samples. Twist was expressed in 31 (66%) of 47 cases. Only one of 47 PCC showed E-cadherin expression. We observed Snail expression in 7 (58%) of 12 malignant PCC, whereas only 6 (17%) of 35 apparently benign PCC revealed Snail expression (P = 0.01). Furthermore, 11 (92%) of 12 malignant PCC, but only 20 (57%) of 35 benign PCC, revealed Twist expression (P = 0.03). Interestingly, all five metastases showed Snail and Twist expression. In normal adrenal medulla, Snail, Twist, and E-cadherin expression could not be detected.

CONCLUSIONS

We describe for the first time that EMT markers Snail and Twist are expressed in PCC and that their expression is associated with malignancy. Our study supports a role for EMT in the malignant transformation of PCC.

Spatio-temporal control of neural epithelial cell migration and epithelium-to-mesenchyme transition during avian neural tube development

As opposed to the neural crest, the neural epithelium is generally viewed as a static and cohesive structure. Here, using an ex vivo system free of the environmental influences and physical constraints encountered in the embryo, we show that neural epithelial cells are on the contrary intrinsically motile, although they do not undergo spontaneous epithelium-to-mesenchyme transition and display molecular and cellular characteristics distinct from those of neural crest cells. However, they can be instructed to undergo epithelium-to-mesenchyme conversion independently of the acquisition of neural crest traits. Migration potentialities of neural epithelial cells are transient and are progressively restricted during neural tube development. Restriction of cell migration is irreversible and can be in part accounted for by increase in N-cadherin in cellular junctions and in cell polarity. In conclusion, our study reveals that the neural epithelium is a highly flexible tissue in which cells are maintained cohesive under the control of a combination of extrinsic factors and physical constraints.

A Sox10 expression screen identifies an amino acid essential for Erbb3 function

The neural crest (NC) is a population of embryonic stem cells that gives rise to numerous cell types, including the glia and neurons of the peripheral and enteric nervous systems and the melanocytes of the skin and hair. Mutations in genes and genetic pathways regulating NC development lead to a wide spectrum of human developmental disorders collectively called neurocristopathies. To identify molecular pathways regulating NC development and to understand how alterations in these processes lead to disease, we established an N-ethyl-N-nitrosourea (ENU) mutagenesis screen utilizing a mouse model sensitized for NC defects, Sox10^LacZ/+. Out of 71 pedigrees analyzed, we identified and mapped four heritable loci, called modifier of Sox10 expression pattern 1–4 (msp1–4), which show altered NC patterning. In homozygous msp1 embryos, Sox10^LacZ expression is absent in cranial ganglia, cranial nerves, and the sympathetic chain; however, the development of other Sox10-expressing cells appears unaffected by the mutation. Linkage analysis, sequencing, and complementation testing confirmed that msp1 is a new allele of the receptor tyrosine kinase Erbb3, Erbb3^msp1, that carries a single amino acid substitution in the extracellular region of the protein. The ENU-induced mutation does not alter protein expression, however, it is sufficient to impair ERBB3 signaling such that the embryonic defects observed in msp1 resemble those of Erbb3 null alleles. Biochemical analysis of the mutant protein showed that ERBB3 is expressed on the cell surface, but its ligand-induced phosphorylation is dramatically reduced by the msp1 mutation. These findings highlight the importance of the mutated residue for ERBB3 receptor function and activation. This study underscores the utility of using an ENU mutagenesis to identify genetic pathways regulating NC development and to dissect the roles of discrete protein domains, both of which contribute to a better understanding of gene function in a cellular and developmental setting.

Genome-wide mutagenesis screens provide a valuable tool to identify genes and genetic pathways regulating complex developmental processes. The neural crest is a population of multipotent cells that gives rise to many different tissue and organ systems. Alterations in the pathways coordinating neural crest formation lead to human developmental disorders. To identify genetic components involved in neural crest development, we combined a whole-genome chemical mutagenesis approach with a mouse strain, Sox10^LacZ/+, that marks neural crest progenitors during early embryogenesis. We identified and determined the chromosomal location of four mutant lines that display impaired neural crest patterning. One of the mutant lines identified carries a single amino acid change that is sufficient to alter neural crest development and cause embryonic lethality without impeding upon protein expression, highlighting the importance of the mutated residue for gene function. This study demonstrates the feasibility of mutagenesis screens to identify the molecular players required for neural crest development as well as to dissect protein domain functions.

Epithelial--mesenchymal and mesenchymal--epithelial transitions in carcinoma progression

Like a set of bookends, cellular, molecular, and genetic changes of the beginnings of life mirror those of one of the most common cause of death--metastatic cancer. Epithelial to mesenchymal transition (EMT) is an important change in cell phenotype which allows the escape of epithelial cells from the structural constraints imposed by tissue architecture, and was first recognized by Elizabeth Hay in the early to mid 1980's to be a central process in early embryonic morphogenesis. Reversals of these changes, termed mesenchymal to epithelial transitions (METs), also occur and are important in tissue construction in normal development. Over the last decade, evidence has mounted for EMT as the means through which solid tissue epithelial cancers invade and metastasize. However, demonstrating this potentially rapid and transient process in vivo has proven difficult and data connecting the relevance of this process to tumor progression is still somewhat limited and controversial. Evidence for an important role of MET in the development of clinically overt metastases is starting to accumulate, and model systems have been developed. This review details recent advances in the knowledge of EMT as it occurs in breast development and carcinoma and prostate cancer progression, and highlights the role that MET plays in cancer metastasis. Finally, perspectives from a clinical and translational viewpoint are discussed.

Exposure to chlorinated biphenyls causes polymorphonucleocytes to induce progenitor cell toxicity in culture

Progenitor cells (PC) are the precursors for many developmental structures and are sensitive to a variety of toxic agents including the environmental contaminants, polychlorinated biphenyls (PCBs). The mechanism(s) that contributes to the development of PCB-induced progenitor cell-related fetotoxicities are not completely understood. However, several studies have demonstrated an important role for neutrophils (polymorphonucleocytes) in the development of PCB induced toxicities. Our recent findings have indicated that conditioned medium collected from PC (CMPC) exposed to a single dose of the PCB mixture, Aroclor 1248, can activate isolated neutrophil populations. Because of our recent findings, this study was conducted to determine if conditioned medium from PC treated with a PCB mixture causes neutrophils to injure PC in culture. Isolated PC were cultured and treated with different concentrations of Aroclor 1248 for 24 hours. The resulting PC-derived conditioned media was collected and its affect on neutrophil activity was analyzed. Conditioned medium from PC treated with Aroclor 1248 was chemotactic for neutrophils. The conditioned medium from Aroclor 1248 treated-PC also stimulated neutrophils to release super oxide anion, cathepsin G and elastase into culture medium. Furthermore, the conditioned medium from Aroclor 1248 treated- PC was able to stimulate neutrophils to cause progenitor cell toxicity in co-cultures. The conditioned medium from Aroclor 1248 treated-PC was not toxic to individual neutrophil cultures or PC cultures. Moreover, the addition of a protease inhibitor to the co-cultures containing neutrophils and PC, afforded protection against neutrophil-induced cytotoxicity of PC. These data suggest that a PCB mixture can cause progenitor cells to produce a factor(s) that activates neutrophils and stimulates them to damage PC populations in culture.

Neural crest motility and integrin regulation are distinct in cranial and trunk populations

The neural crest is a transient cell population that travels long distances through the embryo to form a wide range of derivatives. The extensive migration of the neural crest is highly unusual and incompletely understood. We examined the ability of neural crest cells (NCCs) to migrate under different conditions in vitro. Unlike most motile cell types, avian NCCs migrate efficiently on a wide range of fibronectin concentrations. Strikingly, the migration of NCCs on laminin depends on the axial level from which the crest is derived. On high concentrations of laminin, cranial NCCs migrate at approximately twice the rate of trunk NCCs and show greater persistence, a higher percentage of migratory cells, and a less organized cytoskeleton. The difference in migration between cranial and trunk neural crest is not due to transcriptional differences in integrin mRNA, but rather to differences in posttranslational regulation. Overexpression of a single integrin is sufficient to significantly slow the migration velocity of cranial neural crest cultured on high laminin densities. These results demonstrate that neural crest cells accommodate a wide range of ECM concentrations in vitro and suggest that differences in integrin regulation along the anterior-posterior axis may contribute to differences in neural crest migration and cell fate.

Neurochemical differentiation of functionally distinct populations of autonomic neurons

The coeliac ganglion of guinea pigs displays a unique topographical arrangement of neurochemically and functionally distinct populations of sympathetic neurons. The authors used multiple-labeling immunohistochemistry to investigate the neurochemical differentiation of these neurons during embryonic and fetal development. Sympathoadrenal precursors, located on either side of the abdominal aorta, were intensely immunoreactive for tyrosine hydroxylase (TH-IR), neurofilament, and the human natural killer 1 antibody at midembryonic stages (Carnegie stages 16-19). During late embryonic stages (stages 20-23), a single bilobed ganglion had formed. At this time, neuropeptide Y immunoreactivity (NPY-IR) was widely expressed in sympathetic neurons (with moderate TH-IR) and chromaffin cells (with intense TH-IR). The onset of somatostatin (Som-IR) expression followed that of NPY-IR and was restricted to sympathetic neurons. However, at late embryonic stages, most TH-IR neurons with Som-IR also expressed NPY-IR (a combination of peptides not found in the mature coeliac ganglion). Between late embryonic stages and the end of the early fetal period, there was a significant increase in the proportion of neurons in lateral regions that had both NPY-IR and TH-IR. At the same time, there was an increase in the proportion of neurons in medial regions that had both Som-IR and TH-IR. Neurons expressing both Som-IR and TH-IR were rarely observed in lateral regions of the coeliac ganglion. Thus, a clear topography within the coeliac ganglion is established during late embryonic and early fetal stages of development and reflects that found in the mature animal by the end of the early fetal period.

Immunohistochemical demonstration of hyaluronan and its possible involvement in axolotl neural crest cell migration

Hyaluronan (HA), an extracellular matrix component, is involved mainly in the control of cell proliferation, neural crest and tumor cell migration, and wound repair. We investigated the effect of hyaluronan on neural crest (NC) cell migration and its ultrastructural localization in dark (wild-type) and white mutant embryos of the Mexican axolotl (Ambystoma mexicanum, Amphibia). The axolotl system is an accepted model for studying mechanisms of NC cell migration. Using a biotinylated hyaluronan binding protein (HABP), major extracellular matrix (ECM) spaces, including those of NC cell migration, reacted equally positive on cryosections through dark and white embryos. Since neural crest-derived pigment cells migrate only in subepidermal spaces of dark embryos, HA does not seem to influence crest cell migration in vivo. However, when tested on different alternating substrates in vitro, migrating NC cells in dark and white embryos prefer HA to fibronectin. In vivo, such an HA migration stimulating effect might exist as well, but be counteracted to differing degrees in dark and white embryos. The ultrastructural localization of HA was studied by means of transmission electron microscopic immunohistochemistry using HABP and different protocols of standard chemical fixation, cryofixation, embedding, and immunolabeling. The binding reaction of HA to HABP was strong and showed an equal distribution throughout ECM spaces after both standard chemical fixation/freeze substitution and cryofixation. A preference for the somite or subepidermal side was not observed. Following standard fixation/freeze substitution HABP-labeled "honeycomb"-like networks reminiscent of fixation artifacts were more prominent than labeled fibrillar or irregular net-like structures. The latter predominated in adequately frozen specimens following high-pressure freezing/freeze substitution. For this reason fibrillar or irregular net-like structures very likely represent hyaluronan in the complex subepidermal matrix of the axolotl embryo in its native arrangement.

Lumbo-sacral neural crest contributes to the avian enteric nervous system independently of vagal neural crest

Most of the avian enteric nervous system is derived from the vagal neural crest, but a minority of the neural cells in the hindgut, and to an even lesser extent in the midgut, are of lumbo-sacral crest origin. Since the lumbo-sacral contribution was not detected or deemed negligible in the absence of vagal cells, it had been hypothesised that lumbo-sacral neural crest cells require vagal crest cells to contribute to the enteric nervous system. In contrast, zonal aganglionosis, a rare congenital human bowel disease led to the opposite suggestion, that lumbo-sacral cells could compensate for the absence of vagal cells to construct a complete enteric nervous system. To test these notions, we combined E4 chick midgut and hindgut, isolated prior to arrival of neural precursors, with E1. 7 chick vagal and/or E2.7 quail lumbo-sacral neural tube as crest donors, and grafted these to the chorio-allantoic membrane of E9 chick hosts. Double and triple immuno-labelling for quail cells (QCPNA), neural crest cells (HNK-1), neurons and neurites (neurofilament) and glial cells (GFAP) indicated that vagal crest cells produced neurons and glia in large ganglia throughout the entire intestinal tissues. Lumbo-sacral crest contributed small numbers of neurons and glial cells in the presence or absence of vagal cells, chiefly in colorectum, but not in nearby small intestinal tissue. Thus for production of enteric neural cells the avian lumbo-sacral neural crest neither requires the vagal neural crest, nor significantly compensates for its lack. However, enteric neurogenesis of lumbo-sacral cells requires the hindgut microenvironment, whereas that of vagal cells is not restricted to a particular intestinal region.

Induction and patterning of the neural crest, a stem cell-like precursor population

The neural crest is a multipotent precursor population which ultimately generates much of the peripheral nervous system, epidermal pigment cells, and a variety of mesectodermal derivatives. Individual multipotent neural crest cells are capable of some self-renewing divisions, and based upon this criteria can be considered stem cells. Considerable progress has been made in recent years toward understanding how this important population of progenitor cells is initially established in the early embryo, and how cell-intrinsic and non-cell-intrinsic factors mediate their subsequent lineage segregation and differentiation.

Cell death in the avian sclerotome

In this study the occurrence of apoptotic cells in chick embryo trunk somites, between 2.5 and 4 days of development, has been examined using an in situ nick-end-labeling method (TUNEL) to identify nuclei in which DNA is undergoing fragmentation. At 2.5 days of development, apoptotic cells were found in the sclerotome with a distribution that depended on the rostrocaudal level in the trunk. At the most rostral levels (somites 1-18), dying cells were present primarily in the rostral half of the ventral sclerotome; at midlevels (somites 19-26), they were present throughout the ventral sclerotome; and at caudal levels (somites 27-32), no dying cells were present. By 4 days of development, the number of dying cells in the sclerotome was sharply reduced, and those present were primarily distributed to the caudal side of the intrasclerotomal fissure. Double labeling of cells for both TUNEL and the HNK-1 epitope, at 2.5 days, indicated that the majority of the dying cells were not neural crest cells. Further, dying cells in the rostral somite half were present largely in regions of the sclerotome that labeled poorly for HNK-1. It was confirmed that apoptotic neural crest cells retain the HNK-1 epitope and therefore would have been observed if present. Neural crest cells only appeared to be apoptotic in relatively small numbers and only at the ventral border of the sclerotome. Examination of DiI-labeled neural crest cells confirmed that the dying cells in the body of the somite were not primarily neural crest cells. Two hypotheses regarding the TUNEL-positive cells in the sclerotome were experimentally tested. First, that they originate from the somitocoel compartment of the somite, because their distribution patterns at 4 days were similar to those of somitocoel cells. To test this, somitocoel cells were labeled with carboxyfluorescein and grafted into host embryos in ovo. Results showed that these cells did not become apoptotic and that the dying cells were therefore not derived from the somitocoel. Second, the hypothesis was tested that the distribution patterns of the dying cells in the sclerotome are determined by factors outside the somite itself. Somites and segmental plates were transplanted into hosts in ovo with reversed orientation, after which the patterns of dying cells were examined using nile blue sulfate staining. The results indicated that the patterns were unchanged after a further 2 days incubation, suggesting that the patterns of cell death in the sclerotome are not determined solely from within the somite. The distribution of the cell death-associated gene products, bcl-2, bax, and interleukin-1 beta converting enzyme, indicates that although these proteins are segmentally distributed in the dermomyotome and in the rostrodorsal quadrant of the sclerotome, their patterns are not directly correlated with the distribution of dying cells.

Role of the extracellular matrix in neural crest cell migration

Development of the neural crest involves a remarkable feat of coordinated cell migration in which cells detach from the neural tube, take varying routes of migration through the embryonic tissues and then differentiate at the end of their journey to participate in the formation of a number of organ systems. In general, neural crest cells appear to migrate without the guidance of long-range physical or chemical cues, but rather they respond to heterogeneity in the extracellular matrix that forms their migration substrate. Molecules such as fibronectin and laminin act as permissive substrate components, encouraging neural crest cell attachment and spreading, whereas chondroitin sulphate proteoglycans are nonpermissive for migration. A balance between permissive and nonpermissive substrate components seems to be necessary to ensure successful migration, as indicated by a number of studies in mouse mutant systems where nonpermissive molecules are over-expressed, leading to inhibition of neural crest migration. The neural crest expresses cell surface receptors that permit interaction with the extracellular matrix and may also modify the matrix by secretion of proteases. Thus the principles that govern the complex migration of neural crest cells are beginning to emerge.

A role for the roof plate and its resident TGFbeta-related proteins in neuronal patterning in the dorsal spinal cord

Distinct neuronal cell types are generated at characteristic times and positions in the dorsal horn of the spinal cord. We provide evidence that the identity and pattern of generation of dorsal neurons depend initially on BMP-mediated signals that derive from the epidermal ectoderm and induce dorsal midline cells of the roof plate. Roof plate cells provide a secondary source of TGFbeta-related signals that are required for the generation of distinct classes of dorsal interneurons. These inductive interactions involve both qualitative and quantitative differences in signaling by TGFbeta-related factors and temporal changes in the response of neural progenitor cells.

Cross talk between adhesion molecules: control of N-cadherin activity by intracellular signals elicited by beta1 and beta3 integrins in migrating neural crest cells

During embryonic development, cell migration and cell differentiation are associated with dynamic modulations both in time and space of the repertoire and function of adhesion receptors, but the nature of the mechanisms responsible for their coordinated occurrence remains to be elucidated. Thus, migrating neural crest cells adhere to fibronectin in an integrin-dependent manner while maintaining reduced N-cadherin-mediated intercellular contacts. In the present study we provide evidence that, in these cells, the control of N-cadherin may rely directly on the activity of integrins involved in the process of cell motion. Prevention of neural crest cell migration using RGD peptides or antibodies to fibronectin and to beta1 and beta3 integrins caused rapid N-cadherin-mediated cell clustering. Restoration of stable intercellular contacts resulted essentially from the recruitment of an intracellular pool of N-cadherin molecules that accumulated into adherens junctions in tight association with the cytoskeleton and not from the redistribution of a preexisting pool of surface N-cadherin molecules. In addition, agents that cause elevation of intracellular Ca2+ after entry across the plasma membrane were potent inhibitors of cell aggregation and reduced the N-cadherin- mediated junctions in the cells. Finally, elevated serine/ threonine phosphorylation of catenins associated with N-cadherin accompanied the restoration of intercellular contacts. These results indicate that, in migrating neural crest cells, beta1 and beta3 integrins are at the origin of a cascade of signaling events that involve transmembrane Ca2+ fluxes, followed by activation of phosphatases and kinases, and that ultimately control the surface distribution and activity of N-cadherin. Such a direct coupling between adhesion receptors by means of intracellular signals may be significant for the coordinated interplay between cell-cell and cell-substratum adhesion that occurs during embryonic development, in wound healing, and during tumor invasion and metastasis.

Improved preservation of the subepidermal extracellular matrix in axolotl embryos using electron microscopical techniques based on cryoimmobilization

The purpose of this metholdological survey was to find optimal methods for the fixation and demonstration of glycosaminoglycans, mainly hyaluronan, and proteoglycans, in subepidermal extracellular matrix (ECM) regions of axolotl embryos. We compared living ECM in the laser-scanning microscope (LSM) with chemically fixed or cryoimmobilized extracellular matrix in the transmission (TEM) and scanning electron microscope (SEM). The gel-like structure of living extracellular matrix in the LSM undoubtedly provides the most natural state, whereas shrinkage of the extracellular matrix occurs during conventional fixation and dehydration for TEM or SEM. Among the methods used for fixation and processing of subepidermal extracellular matrices for SEM, plunge-freezing/freeze-drying is to be preferred. Still more satisfying, however, are results obtained with high-pressure frozen/freeze-substituted ECM material in the TEM, for which 10% polyvinyl pyrrolidon +7% methanol was used as a cryoprotectant before high-pressure freezing. In these specimens, no freeze-damage could be observed and they could be regarded as adequately frozen. Conversely, the yield in adequately frozen specimens without cryoprotection was insufficient. In these specimens, the ECM contained honeycomb-like structures which, in the current literature, are regarded as hyaluronan.

Restriction in cell fates of developing spinal cord cells transplanted to neural crest pathways

At early neural tube stages, individual stem cells can generate neural crest cells as well as dorsal or ventral spinal cord cells. To determine whether this pluripotency is lost as development proceeds, we back-transplanted quail spinal cells from different developmental stages and different spinal locations into the crest migratory pathways of st 16-20 chicken host embryos. The transplanted spinal cells from st 27 dorsal cord and st 18 ventral cord differentiated within the new crest environment into sensory and sympathetic neurons, satellite and Schwann cells, and melanocytes. St 27 ventral cells still generated several crest derivatives but not sensory or sympathetic neurons. This loss in ability to produce neurons correlates with the end of neurogenesis in ventral cord. The end of neurogenesis in the cord, therefore, results from an intrinsic change in the potential of spinal neuroepithelial cells to generate neurons.

Late emigrating neural crest cells migrate specifically to the exit points of cranial branchiomotor nerves

Morphological segmentation of the avian hindbrain into rhombomeres is also reflected by the emergent organisation of branchiomotor nerves. In each case, the motor neurons of these nerves lie in two adjacent rhombomeres (e.g. of the Vth nerve in r2 and r3, VIIth in r4 and r5 etc.), and their outgrowing axons emerge into the periphery through defined exit points in rhombomeres r2, r4 and r6, respectively. Sensory axons of the cranial ganglia also enter the neuroepithelium at the same points. Motor axon outgrowth through experimentally rotated rhombomeres has suggested that a chemoattractive mechanism, involving the exit points, may form a component of their guidance. Yet so far, nothing is known about the establishment of the exit points or the identity of the cells that form them. In this study, we describe a group of late emigrating cranial neural crest cells which populate specifically the prospective exit points. Using chimaeras in which premigratory chick neural crest had been replaced orthotopically by quail cells, a population of neural crest was found to leave the cranial neural tube from about stage 10+ onwards and to migrate directly to the prospective exit points. These cells define the exit points by stage 12+, long before either motor or sensory axons have grown through them. The entire neural crest population of exit point cells expresses the recently described cell adhesion molecule c-cad7. Further, heterotopic grafting experiments show that midbrain and spinal cord crest, grafted at late stages in place of r4 crest, share the same migratory behaviour to the facial nerve exit points and express the same markers as cells contributed by the native r4 crest. It was not possible to generate new exit points in odd numbered rhombomeres simply by experimentally increasing their (normally insignificant) amount of crest production. Initiation of the exit point region probably lies, therefore, in the neuroepithelium.

Foetal human melanocytes: in situ detection, in vitro culture and differentiation characteristics at 6-11 weeks EGA

In vivo, melanocytes were detected in epidermis from human tissue of 6.5 weeks estimated gestinational age (EGA) and older. We have successfully established melanocyte monocultures from tissue of 9 to 10 weeks EGA. To our knowledge, this is the first report on physiology of human foetal melanocytes in monoculture. In culture, such melanocytes retained foetal characteristics. Proliferation rates noted were markedly higher (approximately 2.7-fold) when compared to those in cultures of neonatal melanocytes. Moreover, when analyzing cellular phenotypes by markers for cells of the melanocytic lineage, foetal cells isolated from tissue of 9 weeks EGA reproducibly showed expression of the high molecular weight (HMW) antigen and c-kit to an extent intermediate to that found in neonatal melanocytes and M14 melanoma cells. Such differential expression was not observed if cells were isolated from tissue of 10 weeks EGA, indicating that the foetal environment provides essential differentiation stimuli during the 10th week of gestation. Moreover, these results are supportive of the theory that malignant transformation involves a process of dedifferentiation. In all, human foetal melanocyte culture provides a useful model to investigate pigment cell differentiation.

Stage-dependent effects of ethanol on cranial neural crest cell development: partial basis for the phenotypic variations observed in fetal alcohol syndrome

Fetal alcohol syndrome (FAS) is characterized by growth retardation, mental deficiencies, and numerous craniofacial and neuronal anomalies; the type and severity of these defects may be related to the time and dose of maternal ethanol exposure. Ethanol administered during presomitic stages results in the typical FAS craniofacial phenotype and is accompanied by a loss of cranial neural crest cells (CNCCs) through ethanol-induced cell death. However, the stage-specific effects of ethanol on the CNCC population is unknown. We examined the effects of ethanol on CNCC populations by treating in ovo chick embryos with a single ethanol dose (0.43 mmol/egg) at various stages of CNCC development, and corresponding to the first 3-4 weeks of human gestation. Ethanol treatment induced cell death and reduced CNCC populations in patterns consistent with observed dysmorphologies of CNCC-derived cranial structures. The precise population affected was dependent on the timing of ethanol exposure. Treatment at gastrulation or neurulation induced cell death and losses of CNCC populations, particularly those in rostral positions, and resulted in more severe craniofacial defects. In contrast, treatment at early somitic stages (4-16 somites) induced cell death, primarily within caudal CNCC populations, but resulted in less severe craniofacial defects, suggesting an increased capacity for recovery. These results suggest that there are distinct developmental windows during which the CNCCs may be particularly susceptible to ethanol-induced cell death. We conclude that ethanol exposure seems to affect specific events adversely during neural crest development. The timing of embryonic ethanol exposure relative to CNCC development could account, in part, for the heterogenous craniofacial defects observed in FAS.

Control of N-cadherin-mediated intercellular adhesion in migrating neural crest cells in vitro

Dispersion of neural crest cells and their ultimate regroupment into peripheral ganglia are associated with precisely coordinated regulations both in time and space of the expression and function of cell adhesion receptors. In particular, the disappearance of N-cadherin from the cell surface at the onset of migration and its reexpression during cell aggregation suggest that, during migration, N-cadherin expression is repressed in neural crest cells. In the present study, we have analyzed in vitro the mechanism of control of N-cadherin expression and function in migrating neural crest cells. Although these cells moved as a dense population, each individual did not establish extensive and permanent intercellular contacts with its neighbors. However, cells synthesized and expressed mature N-cadherin molecules at levels comparable to those found in cells that exhibit stable intercellular contacts, but in contrast to them, the bulk of N-cadherin molecules was not connected with the cytoskeleton. We next determined which intracellular events are responsible for the instability of the N-cadherin junctions in neural crest cells using various chemical agents known to affect signal transduction processes. Agents that block a broad spectrum of serine-threonine kinases (6-dimethylaminopurine, H7 and staurosporine) or that affect selectively protein kinases C (bisindolylmaleimide and sphingosine), inhibitors of protein tyrosine kinases (erbstatin, herbimycin A, and tyrphostins), and inhibitors of phosphatases (vanadate) all restored tight cell-cell associations among neural crest cells, accompanied by a slight increase in the overall cellular content of N-cadherin and its accumulation to the regions of intercellular contacts. The effect of the kinase and phosphatase blockers was inhibitable by agents known to affect protein synthesis (cycloheximide) and exportation (brefeldin A), indicating that the restored cell-cell contacts were mediated chiefly by an intracellular pool of N-cadherin molecules recruited to the membrane. Finally, N-cadherin molecules were constitutively phosphorylated in migrating neural crest cells, but their level and state of phosphorylation were apparently not modified in the presence of kinase and phosphatase inhibitors. These observations therefore suggest that N-cadherin-mediated cell-cell interactions are not stable in neural crest cells migrating in vitro, and that they are under the control of a complex cascade of intracellular signals involving kinases and phosphatases and probably elicited by surface receptors.

Differential regulation of transcription factor gene expression and phenotypic markers in developing sympathetic neurons

We have examined the regulation of transcription factor gene expression and phenotypic markers in developing chick sympathetic neurons. Sympathetic progenitor cells first express the bHLH transcriptional regulator Cash-1 (a chicken achaete-scute homologue), followed by coordinate expression of Phox2, a paired homeodomain protein, and GATA-2, a zinc finger protein. SCG10, a pan-neuronal membrane protein, is first detected one stage later, followed by the catecholaminergic neurotransmitter enzyme tyrosine hydroxylase (TH). We have used these markers to ask two questions: (1) is their expression dependent upon inductive signals derived from the notochord or floor plate?; (2) does their sequential expression reflect a single linear pathway or multiple parallel pathways? Notochord ablation experiments indicate that the floor plate is essential for induction of GATA-2, Phox2 and TH, but not for that of Cash-1 and SCG10. Taken together these data suggest that the development of sympathetic neurons involves multiple transcriptional regulatory cascades: one, dependent upon notochord or floor plate-derived signals and involving Phox2 and GATA-2, is assigned to the expression of the neurotransmitter phenotype; the other, independent of such signals and involving Cash-1, is assigned to the expression of pan-neuronal properties. The parallel specification of different components of the terminal neuronal phenotype is likely to be a general feature of neuronal development.

Changes in the fibronectin-specific integrin expression pattern modify the migratory behavior of sarcoma S180 cells in vitro and in the embryonic environment

The molecules that mediate cell-matrix recognition, such as fibronectins (FN) and integrins, modulate cell behavior. We have previously demonstrated that FN and the beta 1-integrins are used during neural crest cell (NCC) migration in vitro as well as in vivo, and that the FN cell-binding domains I and II exhibit functional specificity in controlling either NCC attachment, spreading, or motility in vitro. In the present study, we have analyzed the effect of changes in the integrin expression patterns on migratory cell behavior in vivo. We have generated, after stable transfection, S180 cells expressing different levels of alpha 4 beta 1 or alpha 5 beta 1 integrins, two integrins that recognize distinct FN cell-binding domains. Murine S180 cells were chosen because they behave similarly to NCC after they are grafted into the NCC embryonic pathways in the chicken embryo. Thus, they provide a model system with which to investigate the mechanisms controlling in vitro and in vivo migratory cell behavior. We have observed that either the overexpression of alpha 5 beta 1 integrin or the induction of alpha 4 beta 1 expression in transfected S180 cells enhances their motility on FN in vitro. These genetically modified S180 cells also exhibit different migratory properties when grafted into the early trunk NCC migratory pathways. We observe that alpha 5 and low alpha 4 expressors migrate in both the ventral and dorsolateral paths simultaneously, in contrast to the parental S180 cells or the host NCC, which are delayed by 24 h in their invasion of the dorsolateral path. Moreover, the alpha 4 expressors exhibit different migratory properties according to their level of alpha 4 expression at the cell surface. Cells of the low alpha 4 expressor line invade both the ventral and dorsolateral pathways. In contrast, the high expressors remain as an aggregate at the graft site, possibly the result of alpha 4 beta 1-dependent homotypic aggregation. Thus, changes in the repertoire of FN-specific integrins enable the S180 cells to exploit different pathways in the embryo and regulate the speed with which they disperse in vivo and in culture. Our studies correlate well with known changes in integrin expression during neural crest morphogenesis and strongly suggest that neural crest cells that migrate into the dorsolateral path, i.e., melanoblasts, do so only after they have upregulated the expression of FN receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo dynamic MRI tracking of rat T-cells labeled with superparamagnetic iron-oxide particles

Dynamic MRI tracking of rat T-cells in vivo is performed in rat testicles after labeling isolated rat T-cells in vitro with superparamagnetic dextran-coated iron-oxide particles, BMS180549. Tissue inflammation induced by the local injection of the calcium ionophore, A23187, is used to attract labeled T-cells. Gradient-echo MR images of rat testicles show a statistically significant decrease (4%) of the signal intensity in areas of injection of A23187 as early as 30 min after intravenous infusion of 2 x 10(8) labeled T-cells. The signal change reaches its maximum (6-7% decrease) at about 60-120 min after cell infusion. T2-mapping also shows a decrease of T2 in the areas with A23187. Image quantitation, which includes a chemical-shift effect, significantly enhances the sensitivity for detection of superparamagnetically labeled T-cells. Localization of labeled T-cells in rat testicles has been verified by fluorescence microscopy studies of T-cells co-labeled with a lipophilic fluorescent carbocyanine dye, 1,1-dioctadecyl-3,3,3',3'-tetramethyl-lindocarbocyanine perchlorate. These results represent the first successful demonstration of dynamic tracking of specific cells in vivo using MRI.

Biology of hypothalamic neurons and pituitary cells

Results obtained by examining hypothalamic neurons producing precursors to neurohormones, and pituitary cells synthesizing peptide and glycoprotein families of hormones, and recent advances in comparative endocrinology, have been summarized and considered from the following viewpoints: species specificity in the organization and communication of the hypothalamic neurons with different brain areas lying inside the BBB and with CVOs; sensitivity of hypothalamic neurons and pituitary cells to the environmental stimuli; gonadal steroids as modulators of gene expression needed for neuronal differentiation and synaptogenesis; dose(s)-dependent pituitary cell proliferation and differentiation; an inverse relationship between PRL and GH synthesis and release and also between degree of hyperplasia and hypertrophy of PRL cells and retardation of GTH cell differentiation; and responsiveness of neurons producing CRH, and of neurons and pituitary cells synthesizing POMC hormones, to stress and glucocorticosteroids. These data show that growth of the animals may be stimulated, retarded, or inhibited; reproductive properties and behavior may be under hormonal control; and character of responsiveness in reaction to stress, and ability for adaptation and other related functions, may be controlled.

Late-migrating neuroepithelial cells from the spinal cord differentiate into sensory ganglion cells and melanocytes

During embryonic development, neural crest cells give rise to many structures in peripheral tissues. Other neural tube cells are thought to contribute only to structures within the CNS. In contrast to this idea, we report a second wave of migration of cells away from the spinal cord occurring after the emigration of crest cells is complete. Neuroepithelial cells from spinal cords in E5 chicken embryos migrate into the periphery and differentiate into neurons and satellite cells within sensory ganglia and into melanocytes in skin and feathers. These results show that some cell types previously considered to be the descendants exclusively of neural crest cells are also derived from neuroepithelial cells in the spinal cord.

Specific roles of the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins in avian neural crest cell adhesion and migration on vitronectin

To identify potentially important extracellular matrix adhesive molecules in neural crest cell migration, the possible role of vitronectin and its corresponding integrin receptors was examined in the adhesion and migration of avian neural crest cells in vitro. Adhesion and migration on vitronectin were comparable to those found on fibronectin and could be almost entirely abolished by antibodies against vitronectin and by RGD peptides. Immunoprecipitation and immunocytochemistry analyses revealed that neural crest cells expressed primarily the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins as possible vitronectin receptors. Inhibition assays of cellular adhesion and migration with function-perturbing antibodies demonstrated that adhesion of neural crest cells to vitronectin was mediated essentially by one or more of the different alpha V integrins, with a possible preeminence of alpha V beta 1, whereas cell migration involved mostly the alpha V beta 3 and alpha V beta 5 integrins. Immunofluorescence labeling of cultured motile neural crest cells revealed that the alpha V integrins are differentially distributed on the cell surface. The beta 1 and alpha V subunits were both diffuse on the surface of cells and in focal adhesion sites in association with vinculin, talin and alpha-actinin, whereas the alpha V beta 3 and alpha V beta 5 integrins were essentially diffuse on the cell surface. Finally, vitronectin could be detected by immunoblotting and immunohistochemistry in the early embryo during the ontogeny of the neural crest. It was in particular closely associated with the surface of migrating neural crest cells. In conclusion, our study indicates that neural crest cells can adhere to and migrate on vitronectin in vitro by an RGD-dependent mechanism involving at least the alpha V beta 1, alpha V beta 3 and alpha V beta 5 integrins and that these integrins may have specific roles in the control of cell adhesion and migration.

Molecular mechanisms of neural crest cell attachment and migration on types I and IV collagen

We have examined the mechanisms involved in the interaction of avian neural crest cells with collagen types I and IV (Col I and IV) during their adhesion and migration in vitro. For this purpose native Col IV was purified from chicken tissues, characterized biochemically and ultrastructurally. Purified chicken Col I and Col IV, and various proteolytic fragments of the collagens, were used in quantitative cell attachment and migration assays in conjunction with domain-specific collagen antibodies and antibodies to avian integrin subunits. Neural crest cells do not distinguish between different macromolecular arrangements of Col I during their initial attachment, but do so during their migration, showing a clear preference for polymeric Col I. Interaction with Col I is mediated by the alpha 1 beta 1 integrin, through binding to a segment of the alpha 1(I) chain composed of fragment CNBr3. Neural crest cell attachment and migration on Col IV involves recognition of conformation-dependent sites within the triple-helical region and the noncollagenous, carboxyl-terminal NC1 domain. This recognition requires integrity of inter- and intrachain disulfide linkages and correct folding of the molecule. Moreover, there also is evidence that interaction sites within the NC1 domain may be cryptic, being exposed during migration of the cells in the intact collagen as a result of the prolonged cell-substratum contact. In contrast to Col I, neural crest cell interaction with Col IV is mediated by beta 1-class integrins other than alpha 1 beta 1.

Morphogenesis of the avian trunk neural crest: use of morphological techniques in elucidating the process

Morphological data generated from light and electron microscopy form the basis of our understanding of avian morphogenesis. Because chicken embryos are readily and cheaply obtained and are easily accessible for experimental manipulation, morphogenetic processes have been studied extensively in this species. Such studies have allowed us to identify the cells involved during morphogenesis, observe the shape changes or cellular translocations that accompany a morphogenetic process, and determine the timing of these events. Elucidation of the molecular basis of morphogenesis has awaited the integration of several additional approaches. Among these are experimental embryology, which has allowed us to understand cellular behavior associated with morphogenesis; immunocytochemistry, which has identified the macromolecular cues that regulate cell movements and the environmental factors that control them; and molecular techniques, which will permit us eventually to clarify the genetic regulation of morphogenesis. Although current research in development is heavily biased towards molecular biology, morphological studies continue to frame the questions that are now being addressed using molecular techniques. This review focuses on the cells of the neural crest as a model system where questions of avian morphogenesis have been profitably addressed.