Avian neural crest-derived neurogenic precursors undergo apoptosis on the lateral migration pathway

Photobiomodulation of avian embryos by red laser

The current research focuses on the effect of variable doses of red laser on the chick embryonic development. He-Ne laser of 632-nm wavelength was used as an irradiation source in the first 48 h post-laying of chicken eggs. We have used five different doses: 2, 1, 0.3, 0.2, and 0.1 mJ/cm² that needed a time range for about 400-20 s. Those irradiated embryos were left for additional 11 days for incubation in normal conditions, where they are blindly studied after the 11^th day. Light microscopy was used in this study to investigate the histological and pathological features of the different experimental groups compared to the control one. However, electron microcopy was utilized to trace the apoptotic distribution in the developmental embryos. Minor abnormalities that are dependent on the laser dose have been shown in the irradiated embryos when compared to the sham group, where the highest laser dose showed about 12% embryonic development anomalies when related to the other irradiated groups. Irradiated embryos were found to express more INF-γ and IL-2 as circulating cytokines relative to the unexposed group, where the levels of IL-2 were highly significantly increased by all laser doses (0.3 mJ/cm² light dose recipient group showed significant increase only when compared to the control group). IFN-γ levels were significantly increased as well by light doses above 0.2 mJ/cm². This IFN-γ increase trend seemed to be laser dose-dependent. Simultaneously, these combined results propose the ability of high laser doses in inducing incurable changes in the embryonic development and consequently such alterations can have potential therapeutic applications through what is known as photobiomodulation.

WITHDRAWN: Effect of He-Ne laser irradiation on embryonic development in chicken eggs

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). These articles are retracted at the request of the authors. The joint Editors-in-Chief agree with this decision.

Inhibition of Apoptosis Overcomes Stage-Related Compatibility Barriers to Chimera Formation in Mouse Embryos

Cell types more advanced in development than embryonic stem cells, such as EpiSCs, fail to contribute to chimeras when injected into pre-implantation-stage blastocysts, apparently because the injected cells undergo apoptosis. Here we show that transient promotion of cell survival through expression of the anti-apoptotic gene BCL2 enables EpiSCs and Sox17⁺ endoderm progenitors to integrate into blastocysts and contribute to chimeric embryos. Upon injection into blastocyst, BCL2-expressing EpiSCs contributed to all bodily tissues in chimeric animals while Sox17⁺ endoderm progenitors specifically contributed in a region-specific fashion to endodermal tissues. In addition, BCL2 expression enabled rat EpiSCs to contribute to mouse embryonic chimeras, thereby forming interspecies chimeras that could survive to adulthood. Our system therefore provides a method to overcome cellular compatibility issues that typically restrict chimera formation. Application of this type of approach could broaden the use of embryonic chimeras, including region-specific chimeras, for basic developmental biology research and regenerative medicine.

Extended multipotency of neural crest cells and neural crest-derived cells

Neural crest cells (NCC) are migratory multipotent cells that give rise to diverse derivatives. They generate various cell types during embryonic development, including neurons and glial cells of the peripheral sensory and autonomic ganglia, Schwann cells, melanocytes, endocrine cells, smooth muscle, and skeletal and connective tissue cells of the craniofacial complex. The multipotency of NCC is thought to be transient at the early stage of NCC generation; once NCC emerge from the neural tube, they change into lineage-restricted precursors. Although many studies have described the clear segregation of NCC lineages right after their delamination from the neural tube, recent reports suggest that multipotent neural crest stem cells (NCSC) are present not only in migrating NCC in the embryo, but also in their target tissues in the fetus and adult. Furthermore, fully differentiated NCC-derived cells such as glial cells and melanocytes have been shown to dedifferentiate or transdifferentiate into other NCC derivatives. The multipotency of migratory and postmigratory NCC-derived cells was found to be similar to that of NCSC. Collectively, these findings support the multipotency or plasticity of NCC and NCC-derived cells.

Accumulation of melanin in the peritoneum causes black abdomens in broilers

A suspected case of localized visceral hyperpigmentation was described for a breed of broiler in China. Using optical microscopy, the accumulation of pigments in the abdominal skin and visceral peritoneum was observed. Electron microscopy was used to further study the ultrastructure of the pigmented peritoneum, and pigment granules resembling melanosomes at different stages were found, and melanocytes were present in this tissue. Infrared spectroscopy was used to analyze the physical-chemical properties of pigments extracted from these broilers. Using synthetic melanin as a reference and the melanin from the peritoneum of Silkie fowls as a control, the pigments in the peritonea of these broilers were found to be melanin, and it had a chemical structure similar to that of melanin from the Silkie fowl peritoneum. In this way, the black abdomens of these broilers were found to have been caused by accumulation of melanin produced by melanocytes in visceral peritonea.

Neural crest-derived cells sustain their multipotency even after entry into their target tissues

BACKGROUND

Neural crest cells (NC cells) are highly migratory multipotent cells. Their multipotency is transient at the early stage of their generation; soon after emerging from the neural tube, these cells turn into lineage-restricted precursors. However, recent studies have disputed this conventionally believed paradigm. In this study, we analyzed the differentiation potency of NC-derived cells after their arrival at target tissues.

RESULTS

Using Sox10-IRES-Venus mice, we found that the NC-derived cells in the skin, DRG, and inner ear could be divided into two populations: Sox10-positive/Kit-negative cells (Sox10+/Kit- cells) and Sox10- and Kit-positive cells (Sox10+/Kit+ cells). Only the Sox10+/Kit- cells were detected in the intestines. Unexpectedly, the Sox10+/Kit+ cells differentiated into neurons, glial cells, and melanocytes, showing that they had maintained their multipotency even after having entered the target tissues. The Sox10+/Kit+ cells in the DRG maintained their multipotency for a restricted period during the earlier embryonic stages, whereas those in the skin and inner ear were multipotent yet even in later embryonic stages.

CONCLUSIONS

We showed that NC-derived Sox10+/Kit+ cells maintained their multipotency even after entry into the target tissues. This unexpected differentiation potency of these cells in tissues seems to have been strictly restricted by the tissue microenvironment.

Penetration and differentiation of cephalic neural crest-derived cells in the developing mouse telencephalon

Neural crest (NC) cells originate from the neural folds and migrate into the various embryonic regions where they differentiate into multiple cell types. A population of cephalic neural crest-derived cells (NCDCs) penetrates back into the developing forebrain to differentiate into microvascular pericytes, but little is known about when and how cephalic NCDCs invade the telencephalon and differentiate into pericytes. Using a transgenic mouse line in which NCDCs are genetically labeled with enhanced green fluorescent protein (EGFP), we observed that NCDCs started to invade the telencephalon together with endothelial cells from embryonic day (E) 9.5. A majority of NCDCs located in the telencephalon expressed pericyte markers, that is, PDGFRβ and NG2, and differentiated into pericytes around E11.5. Surprisingly, many of the NC-derived pericytes express p75, an undifferentiated NCDC marker at E11.5, as well as NCDCs in the mesenchyme. At the same time, a minor population of NCDCs that located separately from blood vessels in the telencephalon were NG2-negative and some of these NCDCs also expressed p75. Proliferation and differentiation of pericytes appeared to occur in a specific mesenchymal region where blood vessels penetrated into the telencephalon. These results indicate that (i) NCDCs penetrate back into the telencephalon in parallel with angiogenesis, (ii) many NC-derived pericytes may be still in pre-mature states even though after differentiation into pericytes in the early developing stages, (iii) a small minority of NCDCs may retain undifferentiated states in the developing telencephalon, and (iv) a majority of NCDCs proliferate and differentiate into pericytes in the mesenchyme around the telencephalon.

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.

Active participation of cell death in development and organismal homeostasis

The precise, regional execution of programmed cell death is required for the proper patterning and sculpting of the embryonic primordium during animal development. In addition, cell death that is not directly involved in sculpting is also widely observed. The most abundant morphological form of programmed cell death in developing animals is apoptosis, and identification of the apoptotic genetic pathways has enabled the study of apoptosis' regulation and roles during development. Genetic and bio-imaging studies have permitted the study of the active roles of cell death in development and organismal homeostasis.

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively, and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: 1) what establishes the pathways of migration and 2) what controls the final destination and differentiation of various neural crest subpopulations. These questions will be addressed in this review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube, and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted, or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk. The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: 1) the vagal-level neural crest cells exhibit modest developmental bias; 2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively; 3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.

Unexpected multipotency of melanoblasts isolated from murine skin

Melanoblasts, precursor of melanocytes, are generated from the neural crest and differentiate into melanocytes during their migration throughout the entire body. The melanoblasts are thought to be progenitor cells that differentiate only into melanocyte. Here, we show that melanoblasts, even after they have already migrated throughout the skin, are multipotent, being able to generate neurons, glial cells, and smooth muscle cells in addition to melanocytes. We isolated Kit-positive and CD45-negative (Kit+/CD45-) cells from both embryonic and neonate skin by flow cytometry and cultured them on stromal cells. The Kit+/CD45- cells formed colonies containing neurons, glial cells, and smooth muscle cells, together with melanocytes. The Kit+/CD45- cells expressed Mitf-M, Sox10, and Trp-2, which are genes known to be expressed in melanoblasts. Even a single Kit+/CD45- cell formed colonies that contained neurons, glial cells, and melanocytes, confirming their multipotential cell fate. The colonies formed from Kit+/CD45- cells retained Kit+/CD45- cells even after 21 days in culture and these retained cells also differentiated into neurons, glial cells, and melanocytes, confirming their self-renewal capability. When the Kit signal was inhibited by the antagonist ACK2, the Kit+/CD45- cells did not form colonies that contained multidifferentiated cells. These results indicate that melanoblasts isolated from skin have multipotency and self-renewal capabilities.

Stripes and belly-spots -- a review of pigment cell morphogenesis in vertebrates

Pigment patterns in the integument have long-attracted attention from both scientists and non-scientists alike since their natural attractiveness combines with their excellence as models for the general problem of pattern formation. Pigment cells are formed from the neural crest and must migrate to reach their final locations. In this review, we focus on our current understanding of mechanisms underlying the control of pigment cell migration and patterning in diverse vertebrates. The model systems discussed here - chick, mouse, and zebrafish - each provide unique insights into the major morphogenetic events driving pigment pattern formation. In birds and mammals, melanoblasts must be specified before they can migrate on the dorsolateral pathway. Transmembrane receptors involved in guiding them onto this route include EphB2 and Ednrb2 in chick, and Kit in mouse. Terminal migration depends, in part, upon extracellular matrix reorganization by ADAMTS20. Invasion of the ectoderm, especially into the feather germ and hair follicles, requires specific signals that are beginning to be characterized. We summarize our current understanding of the mechanisms regulating melanoblast number and organization in the epidermis. We note the apparent differences in pigment pattern formation in poikilothermic vertebrates when compared with birds and mammals. With more pigment cell types, migration pathways are more complex and largely unexplored; nevertheless, a role for Kit signaling in melanophore migration is clear and indicates that at least some patterning mechanisms may be highly conserved. We summarize the multiple factors thought to contribute to zebrafish embryonic pigment pattern formation, highlighting a recent study identifying Sdf1a as one factor crucial for regulation of melanophore positioning. Finally, we discuss the mechanisms generating a second, metamorphic pigment pattern in adult fish, emphasizing recent studies strengthening the evidence that undifferentiated progenitor cells play a major role in generating adult pigment cells.

Multipotent skin-derived precursors: adult neural crest-related precursors with therapeutic potential

We previously made the surprising finding that cultures of multipotent precursors can be grown from the dermis of neonatal and adult mammalian skin. These skin-derived precursors (SKPs) display multi-lineage differentiation potential, producing both neural and mesodermal progeny in vitro, and are an apparently novel precursor cell type that is distinct from other known precursors within the skin. In this review, we begin by placing these findings within the context of the rapidly evolving stem cell field. We then describe our recent efforts focused on understanding the developmental biology of SKPs, discussing the idea that SKPs are neural crest-related precursors that (i) migrate into the skin during embryogenesis, (ii) persist within a specific dermal niche, and (iii) play a key role in the normal physiology, and potentially pathology, of the skin. We conclude by highlighting some of the therapeutic implications and unresolved questions raised by these studies.

The secreted metalloprotease ADAMTS20 is required for melanoblast survival

ADAMTS20 (Adisintegrin-like and metalloprotease domain with thrombospondin type-1 motifs) is a member of a family of secreted metalloproteases that can process a variety of extracellular matrix (ECM) components and secreted molecules. Adamts20 mutations in belted (bt) mice cause white spotting of the dorsal and ventral torso, indicative of defective neural crest (NC)-derived melanoblast development. The expression pattern of Adamts20 in dermal mesenchymal cells adjacent to migrating melanoblasts led us to initially propose that Adamts20 regulated melanoblast migration. However, using a Dct-LacZ transgene to track melanoblast development, we determined that melanoblasts were distributed normally in whole mount E12.5 bt/bt embryos, but were specifically reduced in the trunk of E13.5 bt/bt embryos due to a seven-fold higher rate of apoptosis. The melanoblast defect was exacerbated in newborn skin and embryos from bt/bt animals that were also haploinsufficient for Adamts9, a close homolog of Adamts20, indicating that these metalloproteases functionally overlap in melanoblast development. We identified two potential mechanisms by which Adamts20 may regulate melanoblast survival. First, skin explant cultures demonstrated that Adamts20 was required for melanoblasts to respond to soluble Kit ligand (sKitl). In support of this requirement, bt/bt;Kit(tm1Alf)/+ and bt/bt;Kitl(Sl)/+ mice exhibited synergistically increased spotting. Second, ADAMTS20 cleaved the aggregating proteoglycan versican in vitro and was necessary for versican processing in vivo, raising the possibility that versican can participate in melanoblast development. These findings reveal previously unrecognized roles for Adamts proteases in cell survival and in mediating Kit signaling during melanoblast colonization of the skin. Our results have implications not only for understanding mechanisms of NC-derived melanoblast development but also provide insights on novel biological functions of secreted metalloproteases.

The genetic regulation of pigment cell development

Pigment cells in developing vertebrates are derived from a transient and pluripotent population of cells called neural crest. The neural crest delaminates from the developing neural tube and overlying ectoderm early in development. The pigment cells are the only derivative to migrate along the dorso-lateral pathway. As they migrate, the precursor pigment cell population differentiates and expands through proliferation and pro-survival processes, ultimately contributing to the coloration of organisms. The types of pigment cells that develop, timing of these processes, and final destination can vary between organisms. Studies from mice, chick, Xenopus, zebrafish, and medaka have led to the identification of many genes that regulate pigment cell development. These include several classes of proteins: transcription factors, transmembrane receptors, and extracellular ligands. This chapter discusses an overview of pigment cell development and the genes that regulate this important process.

Lineage specification in neural crest cell pathfinding

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

Migratory patterns and developmental potential of trunk neural crest cells in the axolotl embryo

Using cell markers and grafting, we examined the timing of migration and developmental potential of trunk neural crest cells in axolotl. No obvious differences in pathway choice were noted for DiI-labeling at different lateral or medial positions of the trunk neural folds in neurulae, which contributed not only to neural crest but also to Rohon-Beard neurons. Labeling wild-type dorsal trunks at pre- and early-migratory stages revealed that individual neural crest cells migrate away from the neural tube along two main routes: first, dorsolaterally between the epidermis and somites and, later, ventromedially between the somites and neural tube/notochord. Dorsolaterally migrating crest primarily forms pigment cells, with those from anterior (but not mid or posterior) trunk neural folds also contributing glia and neurons to the lateral line. White mutants have impaired dorsolateral but normal ventromedial migration. At late migratory stages, most labeled cells move along the ventromedial pathway or into the dorsal fin. Contrasting with other anamniotes, axolotl has a minor neural crest contribution to the dorsal fin, most of which arises from the dermomyotome. Taken together, the results reveal stereotypic migration and differentiation of neural crest cells in axolotl that differ from other vertebrates in timing of entry onto the dorsolateral pathway and extent of contribution to some derivatives.

The cleavage of microphthalmia-associated transcription factor, MITF, by caspases plays an essential role in melanocyte and melanoma cell apoptosis

Microphthalmia-associated transcription factor (MITF) M-form is a melanocyte-specific transcription factor that plays a key role in melanocyte development, survival, and differentiation. Here, we identified MITF as a new substrate of caspases and we characterized the cleavage site after Asp 345 in the C-terminal domain. We show that expression of a noncleavable form of MITF renders melanoma cells resistant to apoptotic stimuli, and we found that the C-terminal fragment generated upon caspase cleavage is endowed with a proapoptotic activity that sensitizes melanoma cells to death signals. The proapoptotic function gained by MITF following its processing by caspases provides a tissue-restricted means to modulate death in melanocyte and melanoma cells.

Cardiac neural crest cells contribute to the dormant multipotent stem cell in the mammalian heart

Arodent cardiac side population cell fraction formed clonal spheroids in serum-free medium, which expressed nestin, Musashi-1, and multi-drug resistance transporter gene 1, markers of undifferentiated neural precursor cells. These markers were lost following differentiation, and were replaced by the expression of neuron-, glial-, smooth muscle cell-, or cardiomyocyte-specific proteins. Cardiosphere-derived cells transplanted into chick embryos migrated to the truncus arteriosus and cardiac outflow tract and contributed to dorsal root ganglia, spinal nerves, and aortic smooth muscle cells. Lineage studies using double transgenic mice encoding protein 0-Cre/Floxed-EGFP revealed undifferentiated and differentiated neural crest-derived cells in the fetal myocardium. Undifferentiated cells expressed GATA-binding protein 4 and nestin, but not actinin, whereas the differentiated cells were identified as cardiomyocytes. These results suggest that cardiac neural crest-derived cells migrate into the heart, remain there as dormant multipotent stem cells-and under the right conditions-differentiate into cardiomyocytes and typical neural crest-derived cells, including neurons, glia, and smooth muscle.

Sox10 overexpression induces neural crest-like cells from all dorsoventral levels of the neural tube but inhibits differentiation

SoxE genes (Sox8, Sox9, and Sox10) are early response genes to neural crest induction. Although the early role of Sox9 has been examined in chick and frog, later roles in neural crest migration and differentiation remain largely unexplored. We first examined which SoxE genes were expressed in trunk neural crest cells and then investigated their function using in ovo electroporation. The results of this analysis reveal that Sox10 is present in migrating neural crest cells, whereas other SoxE genes are only expressed transiently after induction. Ectopic expression of Sox10 in the neural tube at trunk level induced expression of HNK-1 in neuroepithelial cells followed by extensive emigration from all levels of the dorsoventral neuraxis, including the floor plate. Sox10-expressing cells failed to express neuronal, Schwann, or melanocyte markers up to 6 days posttransfection (E8), suggesting these cells were maintained in an undifferentiated state. Overexpression of Sox8 or Sox9 had similar but not identical effects on neuroepithelial cells. These results show that high levels of Sox10, Sox9, or Sox8 expression in the neural tube are capable of inducing a migratory neural crest-like phenotype even in the absence of dorsal signals and can maintain these cells in an undifferentiated state.

Gli3 and Plzf cooperate in proximal limb patterning at early stages of limb development

The vertebrate limb initially develops as a bud of mesenchymal cells that subsequently aggregate in a proximal to distal (P-D) sequence to give rise to cartilage condensations that prefigure all limb skeletal components. Of the three cardinal limb axes, the mechanisms that lead to establishment and patterning of skeletal elements along the P-D axis are the least understood. Here we identify a genetic interaction between Gli3 (GLI-Kruppel family member 3) and Plzf (promyelocytic leukaemia zinc finger, also known as Zbtb16 and Zfp145), which is required specifically at very early stages of limb development for all proximal cartilage condensations in the hindlimb (femur, tibia, fibula). Notably, distal condensations comprising the foot are relatively unperturbed in Gli3(-/-);Plzf(-/-) mouse embryos. We demonstrate that the cooperative activity of Gli3 and Plzf establishes the correct temporal and spatial distribution of chondrocyte progenitors in the proximal limb-bud independently of known P-D patterning markers and overall limb-bud size. Moreover, the limb defects in Gli3(-/-);Plzf(-/-) embryos correlate with the transient death of a specific subset of proximal mesenchymal cells that express bone morphogenetic protein receptor, type 1B (Bmpr1b) at the onset of limb development. These findings suggest that the development of proximal and distal skeletal elements is distinctly regulated early during limb-bud formation. The initial division of the vertebrate limb into two distinct molecular domains is consistent with fossil evidence indicating that the upper and lower extremities of the limb have different evolutionary origins.

High-level activation of cyclic AMP signaling attenuates bone morphogenetic protein 2-induced sympathoadrenal lineage development and promotes melanogenesis in neural crest cultures

The intensity of cyclic AMP (cAMP) signaling is a differential instructive signal in neural crest (NC) cell specification. By an unknown mechanism, sympathoadrenal lineage specification is suppressed by high-level activation of cAMP signaling. In NC cultures, high-level activation of cAMP signaling mediates protein kinase A (PKA)-dependent Rap1-B-Raf-ERK1/2 activation, leading to cytoplasmic accumulation of phospho-Smad1, thus terminating bone morphogenetic protein 2 (BMP2)-induced sympathoadrenal cell development. Concurrently, cAMP signaling induces transcription of the melanocyte-determining transcription factor Mitf and melanogenesis. dnACREB and E1A inhibit Mitf expression and melanogenesis, supporting the notion that CREB activation is necessary for melanogenesis. However, constitutively active CREB(DIEDML) without PKA activation is insufficient for Mitf expression and melanogenesis, indicating PKA regulates additional aspects of Mitf transcription. Thus, high-level activation of cAMP signaling plays a dual role in NC cell differentiation: attenuation of BMP2-induced sympathoadrenal cell development and induction of melanogenesis. We conclude the intensity of activation of signal transduction cascades determines cell lineage segregation mechanisms.

Disruption of actin cytoskeleton and anchorage-dependent cell spreading induces apoptotic death of mouse neural crest cells cultured in vitro

In vertebrate embryos, neural crest cells emigrate out of the neural tube and contribute to the formation of a variety of neural and nonneural tissues. Some neural crest cells undergo apoptotic death during migration, but its biological significance and the underlying mechanism are not well understood. We carried out an in vitro study to examine how the morphology and survival of cranial neural crest (CNC) cells of the mouse embryo are affected when their actin cytoskeleton or anchorage-dependent cell spreading is perturbed. Disruption of actin fiber organization by cytochalasin D (1 microg/ml) and inhibition of cell attachment by matrix metalloproteinase-2 (MMP-2; 2.0 units/ml) were followed by morphologic changes and apoptotic death of cultured CNC cells. When the actin cytoskeleton was disrupted by cytochalasin D, the morphologic changes of cultured CNC cells preceded DNA fragmentation. These results indicate that the maintenance of cytoskeleton and anchorage-dependent cell spreading are required for survival of CNC cells. The spatially and temporally regulated expression of proteinases may be essential for the differentiation and migration of neural crest cells.

Expression of a novel secreted factor, Seraf indicates an early segregation of Schwann cell precursors from neural crest during avian development

The neural crest gives rise to glial cells in the peripheral nervous system. Among the peripheral glia, Schwann cells form the myelin often wrapping the peripheral axons. Compared to other crest-derived cell lineages such as neurons, the analysis of fate determination and subsequent differentiation of Schwann cells is not well advanced, partly due to the lack of early markers of this phenotype. In this study, we have identified a gene, uniquely expressed in avian embryo Schwann cell precursors, which encodes a novel secreted factor, designated Seraf (Schwann cell-specific EGF-like repeat autocrine factor). Expression of Seraf and P0 delineates the earliest phase of Schwann cell differentiation. Seraf binds to neural crest cells and Schwann cells, and affects the distribution of Schwann cells, when introduced to chicken embryos during neural crest migration. Our results suggest an autocrine function of Seraf and provide a significant step to understand the developmental processes of Schwann cell lineage.

Neural crest and the origin of ectomesenchyme: neural fold heterogeneity suggests an alternative hypothesis

The striking similarity between mesodermally derived fibroblasts and ectomesenchyme cells, which are thought to be derivatives of the neural crest, has long been a source of interest and controversy. In mice, the gene encoding the alpha subunit of the platelet-derived growth factor receptor (PDGFRalpha) is expressed both by mesodermally derived mesenchymal cells and by ectomesenchyme. Whole-mount immunostaining previously revealed that PDGFRalpha is present in the cephalic neural fold epithelium of early murine embryos (Takakura et al. [1997] J Histochem Cytochem 45:883-893). We now show that, within the neural fold, a sharp boundary exists between E-cadherin-expressing non-neural epithelium and the neural epithelium of the dorsal ridge. In addition, we found that cells coexpressing E-cadherin and PDGFRalpha are present in the non-neural epithelium of the neural folds. These observations raise the possibility that at least some PDGFRalpha(+) ectomesenchyme originates from the lateral non-neural domain of neural fold epithelium. This inference is consistent with previous reports (Nichols [ 1981] J Embryol Exp Morphol 64:105-120; Nichols [ 1986] Am J Anat 176:221-231) that mesenchymal cells emerge precociously from an epithelial neural fold domain resembling the primitive streak in the early embryonic epiblast. Therefore, we propose the name "metablast" for this non-neural epithelial domain to indicate that it is the site of a delayed local delamination of mesenchyme similar to involution of mesoderm during gastrulation. We further propose the testable hypothesis that neural crest and ectomesenchyme are developmentally distinct progenitor populations and that at least some ectomesenchyme is metablast-derived rather than neural crest-derived tissue. Developmental Dynamics 229:118-130, 2004.

Multiple roles of Sox2, an HMG-box transcription factor in avian neural crest development

Expression of Sox2, which encodes an HMG-box-type transcription factor, is down-regulated in the neural plate when neural crest segregates from dorsal neural tube and remains low during crest cell migration. Sox2 expression is subsequently up-regulated in some crest-derived cells in the developing peripheral nervous system and is later restricted to glial sublineages. Misexpression of Sox2 and mutant forms of Sox2 both in neural plate explants and in embryonic ectoderm reveals that Sox2 inhibits neural crest formation as a transcriptional activator. Similar manipulation of Sox2 function in migratory and postmigratory neural crest-derived cells indicates that Sox2 regulates proliferation and differentiation in developing peripheral nervous system. Developmental Dynamics 229:74-86, 2004.

The endothelin receptor-B is required for the migration of neural crest-derived melanocyte and enteric neuron precursors

Mutations in the genes encoding endothelin receptor-B (Ednrb) and its ligand endothelin-3 (Edn3) affect the development of two neural crest-derived cell types, melanocytes and enteric neurons. EDNRB signaling is exclusively required between E10.5 and E12.5 during the migratory phase of melanoblast and enteric neuroblast development. To determine the fate of Ednrb-expressing cells during this critical period, we generated a strain of mice with the bacterial beta-galactosidase (lacZ) gene inserted downstream of the endogenous Ednrb promoter. The expression of the lacZ gene was detected in melanoblasts and precursors of the enteric neuron system (ENS), as well as other neural crest cells and nonneural crest-derived lineages. By comparing Ednrb(lacZ)/+ and Ednrb(lacZ)/Ednrb(lacZ) embryos, we determined that the Ednrb pathway is not required for the initial specification and dispersal of melanoblasts and ENS precursors from the neural crest progenitors. Rather, the EDNRB-mediated signaling is required for the terminal migration of melanoblasts and ENS precursors, and this pathway is not required for the survival of the migratory cells.

Cell migration and programmed cell death of Drosophila germ cells

Cell migration and programmed cell death are essential components of animal development and homeostasis, and the germ cells of Drosophila provide a simple genetic system to study the molecular mechanisms that govern these important cellular processes. Detailed descriptions of germ cell migration in Drosophila were accomplished long ago, but most genetic and molecular analyses of the process have occurred within the past 10 years. A few of the genes required for germ cell migration have been identified, and a very interesting picture is emerging. However, a process as complex as cell migration must involve the functions of many more molecules. In addition, cell migration and cell death mechanisms are often linked, as it is important to eliminate cells that are misplaced and could present a danger to the organism. In Drosophila, genes involved in germ cell migration can also affect programmed cell death. Currently, very little is known about how germ cells ectopic to the gonads are eliminated. To date, only four genes have been reported with roles in germ cell death, and three of these have additional functions in germ cell pathfinding. The nature of the cell death program has not been elucidated. Here, I provide a brief review of Drosophila germ cell migration and programmed cell death at both the descriptive and molecular levels. Many questions remain to be answered, but advances made in recent years are providing useful insights into these critical biological phenomena.

Long-distance cue from emerging dermis stimulates neural crest melanoblast migration

Neural crest melanoblasts display unique navigational abilities enabling them to colonize the dorsal path between ectoderm and somite. One signal shown here to elicit melanoblast migration is a chemotactic cue supplied by the emerging dermis. Until dermis emerges, melanoblasts fail to enter the dorsal path. The dermis emerges from a site that is too distant to stimulate migration by cell contact. Instead, surgeries show that dermis elicits migration from a distance. When dermis is grafted distally, neural crest cells enter the path precociously. Moreover, large grafts recruit melanoblasts from the control sides (without increasing crest cell numbers) as well as a few crest cells from ventral somite. Because other grafted tissues fail to stimulate migration, the dermis stimulus is specific. This report is the first documentation that trunk neural crest cells can be guided chemotactically. It also extends evidence that migration is exquisitely sensitive to temporal-spatial patterns of somite morphogenesis. Developmental Dynamics 229:99-108, 2004.

Plumage pigmentation and expression of its regulatory genes during quail development--histochemical analysis using Bh (black at hatch) mutants

The plumage on the dorsal trunk of normal quail embryos exhibits longitudinal black and brown stripes of pigments produced by melanocytes. However, this pigmentation pattern disappeared in Bh (black at hatch) heterozygous and homozygous embryos because of overall black and brown pigmentation of plumages, respectively. To investigate the mechanisms of the pigment pattern formation of plumage and clarify the roles of the Bh locus in the pattern formation, we examined the expression pattern of genes relating to melanocyte development (Mitf, MelEM antigen, Kitl, Kit and EdnrB2) and melanin pigment production (Dct, Tyrp1, Tyr and Mmp115) in Bh mutant and wild-type embryos throughout development. As a result, we found that MelEM antigen was expressed in melanoblasts committed to produce black pigment before apparent melanogenic gene expression, and that Bh heterozygotes and homozygotes showed abnormal expression patterns of the MelEM antigen. These results indicate that MelEM antigen is a good marker for melanoblasts committed to produce black pigment, and suggests that the Bh locus directs melanocytes to produce eumelanin in proper positions.

Identification of X-Linked Genes Required for Migration and Programmed Cell Death of Drosophila melanogaster Germ Cells

Drosophila germ cells form at the posterior pole of the embryo and migrate to the somatic gonad. Approximately 50% of the germ cells that form reach their target. The errant cells within the embryo undergo developmentally regulated cell death. Prior studies have identified some autosomal genes that regulate germ cell migration, but the genes that control germ cell death are not known. To identify X-linked genes required for germ cell migration and/or death, we performed a screen for mutations that disrupt these processes. Here we report the identification of scattershot and outsiders, two genes that regulate the programmed death of germ cells. The scattershot gene is defined by a mutation that disrupts both germ cell migration and the death of germ cells ectopic to the gonad. Maternal and zygotic expression of scattershot is required, but the migration and cell death functions can be genetically uncoupled. Zygotic expression of wild-type scattershot rescues germ cell pathfinding, but does not restore the programmed death of errant cells. The outsiders gene is required zygotically. In outsiders mutant embryos, the appropriate number of germ cells is incorporated into the gonad, but germ cells ectopic to the gonad persist.

Cell death and neuronal replacement during formation of the avian ciliary ganglion

Programmed cell death is a prominent feature of embryonic development and is essential in matching the number of neurons to the target tissues that are innervated. Although a decrease in neuronal number which coincides with peripheral synaptogenesis has been well documented in the avian ciliary ganglion, it has not been clear whether cell death also occurs earlier. We observed TUNEL-positive neurons as early as stage 24, with a large peak at stage 29. This cell death at stage 29 was followed by a statistically significant (P < 0.0001) decrease in total neuron number at stage 31. The total number of neurons was recovered by stage 33/34. This suggested that dying neurons were replaced by new neurons. This replacement process did not involve proliferation because bromodeoxyuridine applied at stages 29 and 31 was unable to label neurons harvested at stage 33/34. The peak of cell death at stage 29 was increased 2.3-fold by removal of the optic vesicle and was reduced by 50% when chCNTF was overexpressed. Taken together, these results suggest that the regulation of neuron number in the ciliary ganglion is a dynamic process involving both cell death and neural replacement from postmitotic precursors prior to differentiation and innervation of target tissues.

Analysis of melanocyte precursors in Nf1 mutants reveals that MGF/KIT signaling promotes directed cell migration independent of its function in cell survival

Neural crest-derived melanocyte precursors (MPs) in avian and murine embryos emerge from the dorsal neural tube into a migration staging area (MSA). MPs subsequently migrate from the MSA on a dorsolateral pathway between the dermamyotome and the overlying epithelium. In mouse embryos, MPs express the receptor tyrosine kinase, KIT, and require its cognate ligand, Mast cell growth factor (MGF), for survival and differentiation. Prior to the onset of MP migration, MGF is expressed on the dorsolateral pathway at some distance from cells in the MSA and appears to be required for normal MP development. To learn if MGF is required solely for MP survival on this pathway, or if it also provides directional cues for migration, we uncoupled survival from chemoattractive or motogenic functions of this ligand using mice that carry a targeted mutation at the Neurofibromin (Nf1) locus and consequently lack RAS-GAP function. We show that Nf1-mutant MPs survive in the absence of MGF in vitro and in vivo and that Nf1-mutant MPs disperse normally on the lateral migration pathway in the presence of MGF. In contrast, Nf1-mutant MPs persist in the location of the MSA but are not observed on the lateral migration pathway in double-mutant mice that also lack MGF. We conclude that MGF/KIT function provides a signal required for directed migration of the MPs on the lateral pathway in vivo, independent of its function in survival. We further suggest that the MGF mediates MP migration through a signaling pathway that does not involve RAS.

The winged-helix transcription factor FoxD3 is important for establishing the neural crest lineage and repressing melanogenesis in avian embryos

The winged-helix or forkhead class of transcription factors has been shown to play important roles in cell specification and lineage segregation. We have cloned the chicken homolog of FoxD3, a member of the winged-helix class of transcription factors, and analyzed its expression. Based on its expression in the dorsal neural tube and in all neural crest lineages except the late-emigrating melanoblasts, we predicted that FoxD3 might be important in the segregation of the neural crest lineage from the neural epithelium, and for repressing melanogenesis in early-migrating neural crest cells. Misexpression of FoxD3 by electroporation in the lateral neural epithelium early in neural crest development produced an expansion of HNK1 immunoreactivity throughout the neural epithelium, although these cells did not undergo an epithelial/mesenchymal transformation. To test whether FoxD3 represses melanogenesis in early migrating neural crest cells, we knocked down expression in cultured neural crest with antisense oligonucleotides and in vivo by treatment with morpholino antisense oligonucleotides. Both experimental approaches resulted in an expansion of the melanoblast lineage, probably at the expense of neuronal and glial lineages. Conversely, persistent expression of FoxD3 in late-migrating neural crest cells using RCAS viruses resulted in the failure of melanoblasts to develop. We suggest that FoxD3 plays two important roles in neural crest development. First, it is involved in the segregation of the neural crest lineage from the neuroepithelium. Second, it represses melanogenesis, thereby allowing other neural crest derivatives to differentiate during the early stages of neural crest patterning.

Hyperpigmentation in the Silkie fowl correlates with abnormal migration of fate-restricted melanoblasts and loss of environmental barrier molecules

In most homeothermic vertebrates, pigment cells are confined to the skin. Recent studies show that the fate-restricted melanoblast (pigment cell precursor) is the only neural crest lineage that can exploit the dorsolateral path between the ectoderm and somite into the dermis, thereby excluding neurons and glial cells from the skin. This does not explain why melanoblasts do not generally migrate ventrally into the region where neurons and glial cell derivatives of the neural crest differentiate, or why melanoblasts do not escape from the dorsolateral path once they have arrived at this destination. To answer these questions we have studied melanogenesis in the Silkie fowl, which is a naturally occurring chicken mutant in which pigment cells occupy most connective tissues, thereby giving them a dramatic blue-black cast. By using markers for neural crest cells (HNK-1) and melanoblasts (Smyth line serum), we have documented the development of the Silkie pigment pattern. The initial dispersal of melanoblasts is the same in the Silkie fowl as in Lightbrown Leghorn (LBL), White Leghorn (WLH), and quail embryos. However, by stage 22, when all ventral neural crest cell migration has ceased in the WLH, melanoblasts in the Silkie embryo continue to migrate between the neural tube and somites to occupy the sclerotome. This late ventral migration was confirmed by filling the lumen of the neural tube with DiI at stage 19 and observing the embryos at stage 26. No DiI-labeled cells were observed in the sclerotome of LBL embryos, whereas in the Silkie embryos DiI-filled cells were found as far ventral as the mesentery. In addition to this extensive ventral migration, we also observed considerable migration of melanoblasts from the distal end of the dorsolateral space into the somatic mesoderm (the future parietal peritoneum), and into the more medioventral regions where they accumulated around the dorsal aorta and the kidney. The ability of melanoblasts in the Silkie embryos to migrate ventrally along the neural tube and medially from the dorsolateral space is correlated with a lack of peanut agglutinin (PNA) -binding barrier tissues, which are present in the LBL embryo. The abnormal pattern of melanoblast migration in the Silkie embryo is further exaggerated by the fact that the melanoblasts continue to divide, as evidenced by BrdU incorporation (but the rate of incorporation is not greater than seen in the LBL). Results from heterospecific grafting studies and cell cultures of WLH and Silkie neural crest cells support the notion that the Silkie phenotype is brought about by an environmental difference rather than a neural crest-specific defect. We conclude that melanoblasts are normally constrained to migrate only in the dorsolateral path, and once in that path they generally do not escape it. We further conclude that the barriers that normally restrain melanoblast migration in the chicken are not present in the Silkie fowl. We are now actively investigating the nature of this barrier molecule to complete our understanding of melanoblast migration and patterning.

Cell interactions within nascent neural crest cell populations transiently promote death of neurogenic precursors

We have previously shown that cultured trunk neural crest cell populations irreversibly lose neurogenic ability when dispersal is prevented or delayed, while the ability to produce other crest derivatives is retained (Vogel, K. S. and Weston, J. A. (1988) Neuron 1, 569–577). Here, we show that when crest cells are prevented from dispersing, cell death is increased and neurogenesis is decreased in the population, as a result of high cell density. Control experiments to characterize the effects of high cell density on environmental conditions in culture suggest that reduced neurogenesis is the result of cell-cell interactions and not changes (conditioning or depletion) of the culture medium. Additionally, we show that the caspase inhibitor zVAD-fmk, which blocks developmentally regulated cell death, rescues the neurogenic ability of high density cultures, without any apparent effect on normal, low-density cultures. We conclude, therefore, that increased cell interaction at high cell densities results in the selective death of neurogenic precursors in the nascent crest population. Furthermore, we show that neurogenic cells in cultured crest cell populations that have dispersed immediately are not susceptible to contact-mediated death, even if they are subsequently cultured at high cell density. Since most early migrating avian crest cells express Notch1, and a subset expresses Delta1 (Wakamatsu, Y., Maynard, T. M. and Weston, J. A. (2000) Development 127, 2811–2821), we tested the possibility that the effects of cell contact were mediated by components of a Notch signaling pathway. We found that neurogenic precursors are eliminated when crest cells are co-cultured with exogenous Delta1-expressing cells immediately after they segregate from the neural tube, although not after they have previously dispersed. We conclude that early and prolonged cell interactions, mediated at least in part by Notch signaling, can regulate the survival of neurogenic cells within the nascent crest population. We suggest that a transient episode of cell contact-mediated death of neurogenic cells may serve to eliminate fate-restricted neurogenic cells that fail to disperse promptly in vivo.

Environmental signals and cell fate specification in premigratory neural crest

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

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.

Factors controlling lineage specification in the neural crest

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

Avian transitin expression mirrors glial cell fate restrictions during neural crest development

During development, trunk neural crest cells give rise to three primary classes of derivatives: glial cells, melanocytes, and neurons. As part of an effort to learn how neural crest diversification is regulated, we have produced monoclonal antibodies (MAbs) that recognize antigens expressed by neural crest cells early in development. One of these, MAb 7B3 (7B3), was found to recognize an avian transitin-like protein by co-immunostaining with a series of transitin-specific monoclonal antibodies and by Western blot analysis. In neural crest cell cultures, we found that 7B3 initially recognizes the majority of neural crest cells as they emerge from the neural tube. Subsequently, 7B3-immunoreactivity (IR) is progressively restricted to a smaller subpopulation of cells. In fully differentiated trunk neural crest cell cultures, 7B3-IR is expressed only by cells that do not express neuronal markers and lack melanin granules. During development in vivo, 7B3-IR is evident in neural crest cells on the medial, but not the lateral migration pathway, suggesting that it is not expressed by melanocyte precursors. Later, the antigen is detected in non-neuronal, presumptive glial cells in dorsal root ganglia (DRG) and sympathetic ganglia, as well as along ventral roots. Cultures of E5 DRG confirm that 7B3-IR is restricted to non-neuronal cells of ganglia, many of which closely associate with neuronal processes. Therefore, of the three major classes of differentiated trunk neural crest derivatives, 7B3 exclusively recognizes glial cells, including both satellite glia and Schwann cells. Since the pattern of 7B3 expression in vitro mirrors the pattern of glial cell fate-restrictions in the trunk neural crest lineage, and is expressed by neural crest-derived glia in vivo, we conclude that 7B3 is an early pan-glial marker for neural crest-derived glial cells and their precursors.

The concentric structure of the developing gut is regulated by Sonic hedgehog derived from endodermal epithelium

The embryonic gut of vertebrates consists of endodermal epithelium, surrounding mesenchyme derived from splanchnic mesoderm and enteric neuronal components derived from neural crest cells. During gut organogenesis, the mesenchyme differentiates into distinct concentric layers around the endodermal epithelium forming the lamina propria, muscularis mucosae, submucosa and lamina muscularis (the smooth muscle layer). The smooth muscle layer and enteric plexus are formed at the outermost part of the gut, always some distance away from the epithelium. How this topographical organization of gut mesenchyme is established is largely unknown. Here we show the following: (1) Endodermal epithelium inhibits differentiation of smooth muscle and enteric neurons in adjacent mesenchyme. (2) Endodermal epithelium activates expression of patched and BMP4 in adjacent non-smooth muscle mesenchyme, which later differentiates into the lamina propria and submucosa. (3) Sonic hedgehog (Shh) is expressed in endodermal epithelium and disruption of Shh-signaling by cyclopamine induces differentiation of smooth muscle and a large number of neurons even in the area adjacent to epithelium. (4) Shh can mimic the effect of endodermal epithelium on the concentric stratification of the gut. Taken together, these data suggest that endoderm-derived Shh is responsible for the patterning across the radial axis of the gut through induction of inner components and inhibition of outer components, such as smooth muscle and enteric neurons.

Extracellular-regulated kinase activation and CAS/Crk coupling regulate cell migration and suppress apoptosis during invasion of the extracellular matrix

Regulation of cell migration/invasion is important for embryonic development, immune function, and angiogenesis. However, migratory cells must also coordinately activate survival mechanisms to invade the extracellular matrix and colonize foreign sites in the body. Although invasive cells activate protective programs to survive under diverse and sometimes hostile conditions, the molecular signals that regulate these processes are poorly understood. Evidence is provided that signals that induce cell invasion also promote cell survival by suppressing apoptosis of migratory cells. Extracellular-regulated kinase (ERK) activation and molecular coupling of the adaptor proteins p130 Crk-associated substrate (CAS) and c-CrkII (Crk) represent two distinct pathways that induce cell invasion and protect cells from apoptosis in a three-dimensional collagen matrix. CAS/Crk-mediated cell invasion and survival requires activation of the small GTPase Rac, whereas ERK-induced cell invasion, but not survival requires myosin light chain kinase activation and myosin light chain phosphorylation. Uncoupling CAS from Crk or inhibition of ERK activity prevents migration and induces apoptosis of invasive cells. These findings provide molecular evidence that during invasion of the extracellular matrix, cells coordinately regulate migration and survival mechanisms through ERK activation and CAS/Crk coupling.

Zebrafish sparse corresponds to an orthologue of c-kit and is required for the morphogenesis of a subpopulation of melanocytes, but is not essential for hematopoiesis or primordial germ cell development

The relative roles of the Kit receptor in promoting the migration and survival of amniote melanocytes are unresolved. We show that, in the zebrafish, Danio rerio, the pigment pattern mutation sparse corresponds to an orthologue of c-kit. This finding allows us to further elucidate morphogenetic roles for this c-kit-related gene in melanocyte morphogenesis. Our analyses of zebrafish melanocyte development demonstrate that the c-kit orthologue identified in this study is required both for normal migration and for survival of embryonic melanocytes. We also find that, in contrast to mouse, the zebrafish c-kit gene that we have identified is not essential for hematopoiesis or primordial germ cell development. These unexpected differences may reflect evolutionary divergence in c-kit functions following gene duplication events in teleosts.