Analysis of developmentally homogeneous neural crest cell populations in vitro. I. Formation, morphology and differentiative behavior

In Vitro Models in Developmental Toxicology

Developmental toxicity associated with exposure to exogenous compounds such as drugs and environmental chemicals can be assessed using a variety of different in vitro models, each with their own advantages and disadvantages. These models include cultured cells (Chapters 3 - 6 ), organ and tissue cultures (Chapters 7 and 8 ), and whole embryo cultures (Chapters 13 - 15 ) and typically support the guiding principles of the three Rs: replace, reduce, and refine. These models can be used in early chemical screens and have helped further our understanding into the mechanisms associated with developmental toxicity following exposure to many chemicals.

Differentiation of Odontoblast-Like Cells From Mouse Induced Pluripotent Stem Cells by Pax9 and Bmp4 Transfection

The field of tooth regeneration has progressed in recent years, and human tooth regeneration could become viable in the future. Because induced pluripotent stem (iPS) cells can differentiate into odontogenic cells given appropriate conditions, iPS cells are a potential cell source for tooth regeneration. However, a definitive method to induce iPS cell-derived odontogenic cells has not been established. We describe a novel method of odontoblast differentiation from iPS cells using gene transfection. We generated mouse iPS cell-derived neural crest-like cells (iNCLCs), which exhibited neural crest markers. Next, we differentiated iNCLCs into odontoblast-like cells by transfection of Pax9 and Bmp4 expression plasmids. Exogenous Pax9 upregulated expression of Msx1 and dentin matrix protein 1 (Dmp1) in iNCLCs but not bone morphogenetic protein 4 (Bmp4) or dentin sialophosphoprotein (Dspp). Exogenous Bmp4 upregulated expression of Msx1, Dmp1, and Dspp in iNCLCs, but not Pax9. Moreover, cotransfection of Pax9 and Bmp4 plasmids in iNCLCs revealed a higher expression of Pax9 than when Pax9 plasmid was used alone. In contrast, exogenous Pax9 downregulated Bmp4 overexpression. Cotransfection of Pax9 and Bmp4 synergistically upregulated Dmp1 expression; however, Pax9 overexpression downregulated exogenous Bmp4-induced Dspp expression. Together, these findings suggest that an interaction between exogenous Pax9- and Bmp4-induced signaling modulated Dmp1 and Dspp expression. In conclusion, transfection of Pax9 and Bmp4 expression plasmids in iNCLCs induced gene expression associated with odontoblast differentiation, suggesting that iNCLCs differentiated into odontoblast-like cells. The iPS cell-derived odontoblast-like cells could be a useful cell source for tooth regeneration.

SIGNIFICANCE

It has been reported that induced pluripotent stem (iPS) cells differentiate into odontogenic cells by administration of recombinant growth factors and coculture with odontogenic cells. Therefore, they can be potential cell sources for tooth regeneration. However, these previous methods still have problems, such as usage of other cell types, heterogeneity of differentiated cells, and tumorigenicity. In the present study, a novel method to differentiate iPS cells into odontoblast-like cells without tumorigenicity using gene transfection was established. It is an important advance in the establishment of efficient methods to generate homogeneous functional odontogenic cells derived from iPS cells.

Overview of in vitro models in developmental toxicology

In vitro methods used to study the modes of action of developmental toxicants range in biological complexity from cell monocultures to the culture of intact viable conceptuses. Primitive methods were static in nature and generally failed to provide for the adequate transfer of oxygen and nutrients to the cells and tissues being cultured. Modern methods have improved on these deficits to allow growth of cells, tissues, and whole conceptuses at rates that mirror those seen in vivo. The experimental challenges encountered with in vitro studies are also not unlike those seen in vivo where scale, accessibility, and developmental timing continue to persist as significant challenges. In vitro models have advantages in the ability to control environmental conditions and the direct exposure to toxicants so that specific effects on cell growth, differentiation, and morphology can be assessed directly in cells that have retained a sufficient degree of pluripotency. In vitro methods range in biological complexity from primary and immortalized cell cultures, organ and tissue cultures, and whole embryo cultures using intact, viable conceptuses explanted from a variety of species. Different experimental approaches are used for the various stages of development and cover the spectrum from preimplantation, periimplantation, and on to postimplantation periods of development.

FOXD3 regulates the lineage switch between neural crest-derived glial cells and pigment cells by repressing MITF through a non-canonical mechanism

The first neural crest cells to emigrate from the neural tube are specified as neurons and glial cells and are subsequently followed by melanocytes of the skin. We wished to understand how this fate switch is controlled. The transcriptional repressor FOXD3 is expressed exclusively in the neural/glial precursors and MITF is expressed only in melanoblasts. Moreover, FOXD3 represses melanogenesis. Here we show that avian MITF expression begins very early during melanoblast migration and that loss of MITF in melanoblasts causes them to transdifferentiate to a glial phenotype. Ectopic expression of FOXD3 represses MITF in cultured neural crest cells and in B16-F10 melanoma cells. We also show that FOXD3 does not bind directly to the MITF promoter, but instead interacts with the transcriptional activator PAX3 to prevent the binding of PAX3 to the MITF promoter. Overexpression of PAX3 is sufficient to rescue MITF expression from FOXD3-mediated repression. We conclude that FOXD3 controls the lineage choice between neural/glial and pigment cells by repressing MITF during the early phase of neural crest migration.

Fibronectin promotes differentiation of neural crest progenitors endowed with smooth muscle cell potential

The neural crest (NC) is a model system used to investigate multipotency during vertebrate development. Environmental factors control NC cell fate decisions. Despite the well-known influence of extracellular matrix molecules in NC cell migration, the issue of whether they also influence NC cell differentiation has not been addressed at the single cell level. By analyzing mass and clonal cultures of mouse cephalic and quail trunk NC cells, we show for the first time that fibronectin (FN) promotes differentiation into the smooth muscle cell phenotype without affecting differentiation into glia, neurons, and melanocytes. Time course analysis indicated that the FN-induced effect was not related to massive cell death or proliferation of smooth muscle cells. Finally, by comparing clonal cultures of quail trunk NC cells grown on FN and collagen type IV (CLIV), we found that FN strongly increased both NC cell survival and the proportion of unipotent and oligopotent NC progenitors endowed with smooth muscle potential. In contrast, melanocytic progenitors were prominent in clonogenic NC cells grown on CLIV. Taken together, these results show that FN promotes NC cell differentiation along the smooth muscle lineage, and therefore plays an important role in fate decisions of NC progenitor cells.

Slit/Robo signaling is necessary to confine early neural crest cells to the ventral migratory pathway in the trunk

Neural crest cells migrate along two discrete pathways within the trunk of developing embryos. In the chick, early migrating crest cells are confined to a ventral pathway medial to the dermamyotome while later cells migrate on a dorsal pathway lateral to the dermamyotome. Here we show that Slits are expressed in the dermamyotome, that early migrating crest cells express the Slit receptors Robo 1 and Robo 2, that Slit2 repels migrating crest cells in an in vitro assay, and that the misexpression of a dominant-negative Robo1 receptor induces a significant fraction of early crest cells to migrate ectopically in the dorso-lateral pathway. These findings suggest that Slits, most likely those expressed in the dermamyotome, help to confine the migration of early crest cells to the ventral pathway.

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.

MMP-2 plays an essential role in producing epithelial-mesenchymal transformations in the avian embryo

To investigate the roles that matrix-degrading proteases may have in development of the chicken embryo, we documented the expression pattern of matrix metalloprotease-2 (MMP-2, 72-kDa type IV collagenase or gelatinase A) and perturbed its function in vitro and in vivo. MMP-2 is expressed as neural crest cells detach from the neural epithelium during an epithelial-mesenchymal transformation (EMT) but is rapidly extinguished as they disperse. It is also expressed in the sclerotome and in the dermis at the time that the EMT is initiated, and also as these cells migrate, and is down-regulated once motility has ceased. These patterns suggest that MMP-2 plays a role in cell motility during the EMT and during later morphogenesis. Inhibitors of MMPs, including BB-94 and TIMP-2 (tissue inhibitor of metalloprotease-2), prevent the EMT that generates neural crest cells, both in tissue culture and in vivo, but do not affect migration of the cells that have already detached from the neural tube. Similarly, knockdown of MMP-2 expression in the dorsal neural tube using antisense morpholino oligos perturbs the EMT, but also does not affect migration of neural crest cells after they have detached from the neural tube. On the other hand, when somites in culture are treated with TIMP-2, some mesenchymal cells are produced, suggesting that they undergo the EMT, but show greatly reduced migration through the collagen gel. MMP-2 is also expressed in mesenchyme where tissue remodeling is in progress, such as in the developing feather germs, in the head mesenchyme, in the lateral plate mesoderm, and in the limb dermis, especially in the regions where tendons are developing. Comparisons of these expression patterns in multiple embryonic tissues suggest a probable role for MMP-2 in the migration phase of the EMT, in addition to mesenchyme dispersion and tissue remodeling. Developmental Dynamics 229:42-53, 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.

ADAM 10: an active metalloprotease expressed during avian epithelial morphogenesis

The ADAMs are a family of proteins containing multiple functional domains. We have cloned the avian orthologue of ADAM 10 and demonstrate that it has metalloprotease activity. Chick ADAM 10 is expressed in the developing dermatome and myotome of the somite, epidermis, gut endoderm, the epithelial tissues of the kidney, liver, and heart, and in neural crest cells. The expression patterns and protein distribution of ADAM 10 suggest it may play a significant role in the morphogenesis of several epithelial tissues. When a dominant-negative metalloprotease-mutant form of ADAM 10 is expressed in the ectoderm or ADAM 10 expression is knocked down with morpholinos, morphogenesis and tissue specification are altered.

Ephrin-B ligands play a dual role in the control of neural crest cell migration

Little is known about the mechanisms that direct neural crest cells to the appropriate migratory pathways. Our aim was to determine how neural crest cells that are specified as neurons and glial cells only migrate ventrally and are prevented from migrating dorsolaterally into the skin, whereas neural crest cells specified as melanoblasts are directed into the dorsolateral pathway. Eph receptors and their ephrin ligands have been shown to be essential for migration of many cell types during embryonic development. Consequently, we asked if ephrin-B proteins participate in the guidance of melanoblasts along the dorsolateral pathway, and prevent early migratory neural crest cells from invading the dorsolateral pathway. Using Fc fusion proteins, we detected the expression of ephrin-B ligands in the dorsolateral pathway at the stage when neural crest cells are migrating ventrally. Furthermore, we show that ephrins block dorsolateral migration of early-migrating neural crest cells because when we disrupt the Eph-ephrin interactions by addition of soluble ephrin-B ligand to trunk explants, early neural crest cells migrate inappropriately into the dorsolateral pathway. Surprisingly, we discovered the ephrin-B ligands continue to be expressed along the dorsolateral pathway during melanoblast migration. RT-PCR analysis, in situ hybridisation, and cell surface-labelling of neural crest cell cultures demonstrate that melanoblasts express several EphB receptors. In adhesion assays, engagement of ephrin-B ligands to EphB receptors increases melanoblast attachment to fibronectin. Cell migration assays demonstrate that ephrin-B ligands stimulate the migration of melanoblasts. Furthermore, when Eph signalling is disrupted in vivo, melanoblasts are prevented from migrating dorsolaterally, suggesting ephrin-B ligands promote the dorsolateral migration of melanoblasts. Thus, transmembrane ephrins act as bifunctional guidance cues: they first repel early migratory neural crest cells from the dorsolateral path, and then later stimulate the migration of melanoblasts into this pathway. The mechanisms by which ephrins regulate repulsion or attraction in neural crest cells are unknown. One possibility is that the cellular response involves signalling to the actin cytoskeleton, potentially involving the activation of Cdc42/Rac family of GTPases. In support of this hypothesis, we show that adhesion of early migratory cells to an ephrin-B-derivatized substratum results in cell rounding and disruption of the actin cytoskeleton, whereas plating of melanoblasts on an ephrin-B substratum induces the formation of microspikes filled with F-actin.

Wnt and BMP signaling govern lineage segregation of melanocytes in the avian embryo

Recent studies show that specification of some neural crest lineages occurs prior to or at the time of migration from the neural tube. We investigated what signaling events establish the melanocyte lineage, which has been shown to migrate from the trunk neural tube after the neuronal and glial lineages. Using in situ hybridization, we find that, although Wnts are expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, the Wnt inhibitor cfrzb-1 is expressed in the neuronal and glial precursors and not in melanoblasts. This expression pattern suggests that Wnt signaling may be involved in specifying the melanocyte lineage. We further report that Wnt-3a-conditioned medium dramatically increases the number of pigment cells in quail neural crest cultures while decreasing the number of neurons and glial cells, without affecting proliferation. Conversely, BMP-4 is expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, but is decreased coincident with the timing of melanoblast migration. This expression pattern suggests that BMP signaling may be involved in neural and glial cell differentiation or repression of melanogenesis. Purified BMP-4 reduces the number of pigment cells in culture while increasing the number of neurons and glial cells, also without affecting proliferation. Our data suggest that Wnt signaling specifies melanocytes at the expense of the neuronal and glial lineages, and further, that Wnt and BMP signaling have antagonistic functions in the specification of the trunk neural crest.

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.

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.

Sacral neural crest cell migration to the gut is dependent upon the migratory environment and not cell-autonomous migratory properties

Avian neural crest cells from the vagal (somite level 1-7) and the sacral (somite level 28 and posterior) axial levels migrate into the gut and differentiate into the neurons and glial cells of the enteric nervous system. Neural crest cells that emigrate from the cervical and thoracic levels stop short of the dorsal mesentery and do not enter the gut. In this study we tested the hypothesis that neural crest cells derived from the sacral level have cell-autonomous migratory properties that allow them to reach and invade the gut mesenchyme. We heterotopically grafted neural crest cells from the sacral axial level to the thoracic level and vice versa and observed that the neural crest cells behaved according to their new position, rather than their site of origin. Our results show that the environment at the sacral level is sufficient to allow neural crest cells from other axial levels to enter the mesentery and gut mesenchyme. Our study further suggests that at least two environmental conditions at the sacral level enhance ventral migration. First, sacral neural crest cells take a ventral rather than a medial-to-lateral path through the somites and consequently arrive near the gut mesenchyme many hours earlier than their counterparts at the thoracic level. Our experimental evidence reveals only a narrow window of opportunity to invade the mesenchyme of the mesentery and the gut, so that earlier arrival assures the sacral neural crest of gaining access to the gut. Second, the gut endoderm is more dorsally situated at the sacral level than at the thoracic level. Thus, sacral neural crest cells take a more direct path to the gut than the thoracic neural crest, and also their target is closer to the site from which they initiate migration. In addition, there appears to be a barrier to migration at the thoracic level that prevents neural crest cells at that axial level from migrating ventral to the dorsal aorta and into the mesentery, which is the portal to the gut.

Thrombospondin-1 and neural crest cell migration

Using a monoclonal antibody raised against human platelet thrombospondin, we found anti-thrombospondin immunoreactivity in the extracellular matrix of avian embryos, coincident with the ventral pathways followed by trunk neural crest cells. To confirm that the antibody recognized thrombospondin-1 and to determine the tissue of origin of the thrombospondin matrix, a thrombospondin-1 cRNA probe was used for whole mount in situ hybridization. This probe revealed thrombospondin-1 mRNAs in the developing myotome before and during neural crest cell migration. The effect of thrombospondin-1 on neural crest cell migration, morphology, and adhesion was assayed in vitro. Quail trunk neural crest cells cultured on 4 microg/ml of thrombospondin-1 migrate at 1.14 +/- 0.54 microm/min, which is significantly greater than the rate of cell migration on tissue culture plastic. Using a shaker-based adhesion assay, a significantly greater number of neural crest cells remain attached to dishes coated with 4 microg/ml of thrombospondin-1 than to tissue culture plastic alone. The number of neural crest cells that remain attached to 4 microg/ml of thrombospondin-1 is similar to the number that remain attached to dishes coated with 10 microg/ml of fibronectin. These observations indicate that neural crest cells migrate through a thrombospondin-filled extracellular matrix, and that thrombospondin-1 promotes neural crest cell migration and adhesion. Thus, thrombospondin-1 is the first somite-derived extracellular matrix molecule with properties consistent with a role in the promotion of migration into the anterior somite, as opposed to the repulsion of neural crest cells from the posterior half of the somite.

F-Spondin, expressed in somite regions avoided by neural crest cells, mediates inhibition of distinct somite domains to neural crest migration

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial-half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

Neural crest development: the interplay between morphogenesis and cell differentiation

The final pattern of tissues established during embryogenesis reflects the outcome of two developmental processes: differentiation and morphogenesis. Avian neural crest cells are an excellent system in which to study this interaction. In the first phase of neural crest cell migration, neural crest cells separate from the neural epithelium via an epithelial-mesenchymal transformation. We present three models to account for this process: (1) separation by asymmetric mitosis, (2) separation by generating tractional force in order to rupture cell adhesions and (3) loss of expression or function of cell-cell adhesion molecules that keep the presumptive neural crest cells tethered to the neural epithelium. Evidence is presented that the segregation of the neural crest lineage apart from the neural epithelium is caused by the epithelial-mesenchymal transformation. Once they have detached from the neural tube, neural crest cells take two pathways in the trunk of the chick embryo: (1) the ventral path between the neural tube and somite, where neural crest cells give rise to neurons and glial cells of the peripheral nervous systems, and (2) the dorsolateral path between the ectoderm and dermamyotome of the somite, where they differentiate into pigment cells of the skin. We present data to suggest that the migration and differentiation along the ventral path is controlled primarily by environmental cues, which we refer to as the environment-directed model of neural crest morphogenesis. Conversely, only melanoblasts can migrate into the dorsolateral space, and the ability to invade that path is dependent upon their early specification as melanoblasts. We call this the phenotype-directed model for neural crest cell migration and suggest that this latter model for the positioning of neural crest derivatives in the embryo may be more common than previously suspected. These observations invite a re-examination of patterning of other crest derivates, which previously were believed to be controlled by environmental cues.

Sequential expression and role of Hu RNA-binding proteins during neurogenesis

We have identified three avian (chicken) Hu/elav family RNA-binding protein genes. cHuD and cHuC are expressed specifically in neurons of both the central and peripheral nervous systems. Although cHuA is expressed in a wide variety of tissues, including neurogenic precursor cells, it is transiently down-regulated, and is then re-expressed in maturing neurons. Misexpression of cHuD in cultured neural crest cells results in a dramatic increase in the proportion of cells exhibiting neuronal morphology, molecular markers for neurons, and neurotrophin dependence. These data confirm that cHuD protein is involved in regulating neuronal differentiation.

Thyroid hormone decreases the number of adrenergic cells that develop in neural crest cultures and can inhibit the stimulatory action of retinoic acid

Environmental cues are known to be important in the migration, survival, and differentiation of neural crest cells and their derivatives. Retinoic acid (RA) can increase the number of adrenergic cells that develop in neural crest cultures in a dose dependent manner. These results with RA prompted us to investigate the effects of other retinoids and other related compounds on neural crest cultures. We have investigated the role of thyroid hormone (T3) in the development of adrenergic cells in quail neural crest cultures. T3 produced a significant decrease in the number of catecholamine-positive cells that developed in neural crest cultures after 7 days in vitro, as compared to untreated controls. The decrease in adrenergic cells produced by T3 was paralleled by a decrease in the number of tyrosine hydroxylase-positive cells, but T3 did not reduce either total or melanocyte cell number. Cultures were sensitive to T3 during the first 5 days in culture and T3 was not cytotoxic to adrenergic cells. The decrease in adrenergic cells seen with T3 was partially reversed by RA suggesting that these two compounds may be working through a common pathway.

The dorsal neural tube organizes the dermamyotome and induces axial myocytes in the avian embryo

Somites, like all axial structures, display dorsoventral polarity. The dorsal portion of the somite forms the dermamyotome, which gives rise to the dermis and axial musculature, whereas the ventromedial somite disperses to generate the sclerotome, which later comprises the vertebrae and intervertebral discs. Although the neural tube and notochord are known to regulate some aspects of this dorsoventral pattern, the precise tissues that initially specify the dermamyotome, and later the myotome from it, have been controversial. Indeed, dorsal and ventral neural tube, notochord, ectoderm and neural crest cells have all been proposed to influence dermamyotome formation or to regulate myocyte differentiation. In this report we describe a series of experimental manipulations in the chick embryo to show that dermamyotome formation is regulated by interactions with the dorsal neural tube. First, we demonstrate that when a neural tube is rotated 180 degrees around its dorsoventral axis, a secondary dermamyotome is induced from what would normally have developed as sclerotome. Second, if we ablate the dorsal neural tube, dermamyotomes are absent in the majority of embryos. Third, if we graft pieces of dorsal neural tube into a ventral position between the notochord and ventral somite, a dermamyotome develops from the sclerotome that is proximate to the graft, and myocytes differentiate. In addition, we also show that myogenesis can be regulated by the dorsal neural tube because when pieces of dorsal neural tube and unsegmented paraxial mesoderm are combined in tissue culture, myocytes differentiate, whereas mesoderm cultures alone do not produce myocytes autonomously. In all of the experimental perturbations in vivo, the dorsal neural tube induced dorsal structures from the mesoderm in the presence of notochord and floorplate, which have been reported previously to induce sclerotome. Thus, we have demonstrated that in the context of the embryonic environment, a dorsalizing signal from the dorsal neural tube can compete with the diffusible ventralizing signal from the notochord. In contrast to dorsal neural tube, pieces of ventral neural tube, dorsal ectoderm or neural crest cells, all of which have been postulated to control dermamyotome formation or to induce myogenesis, either fail to do so or provoke only minimal inductive responses in any of our assays. However, complicating the issue, we find consistent with previous studies that following ablation of the entire neural tube, dermamyotome formation still proceeds adjacent to the dorsal ectoderm. Together these results suggest that, although dorsal ectoderm may be less potent than the dorsal neural tube in inducing dermamyotome, it does nonetheless possess some dermamyotomal-inducing activity. Based on our data and that of others, we propose a model for somite dorsoventral patterning in which competing diffusible signals from the dorsal neural tube and from the notochord/floorplate specify dermamyotome and sclerotome, respectively. In our model, the positioning of the dermamyotome dorsally is due to the absence or reduced levels of the notochord-derived ventralizing signals, as well as to the presence of dominant dorsalizing signals. These dorsal signals are possibly localized and amplified by binding to the basal lamina of the ectoderm, where they can signal the underlying somite, and may also be produced by the ectoderm as well.

Regulated expression of neurofibromin in migrating neural crest cells of avian embryos

Neurofibromatosis type 1 (NF1) is a common human genetic disease involving various neural crest (NC)-derived cell types, in particular, Schwann cells and melanocytes. The gene responsible for NF1 encodes the protein neurofibromin, which contains a domain with amino acid sequence homology to the ras-guanosine triphosphatase activating protein, suggesting that neurofibromin may play a role in intracellular signaling pathways regulating cellular proliferation or differentiation, or both. To determine whether neurofibromin plays a role in NC cell development, we used antibodies raised against human neurofibromin fusion proteins in western blot and immunocytochemical studies of early avian embryos. These antibodies specifically recognized the 235 kD chicken neurofibromin protein, which was expressed in migrating trunk and cranial NC cells of early embryos (E1.5 to E2), as well as in endothelial and smooth muscle cells of blood vessels and in a subpopulation of non-NC-derived cells in the dermamyotome. At slightly later stages (E3 to E5), neurofibromin immunostaining was observed in various NC derivatives, including dorsal root ganglia and peripheral nerves, as well as non-NC-derived cell types, including heart, skeletal muscle, and kidney. At still later stages (E7 to E9), neurofibromin immunoreactivity was found in almost all tissues in vivo. To determine whether the levels of neurofibromin changed during melanocyte and Schwann cell development, tissue culture experiments were performed. Cultured NC cells were found to express neurofibromin at early time points in culture, but the levels of immunoreactivity decreased as the cells underwent pigmentation. Schwann cells, on the other hand, continued to express neurofibromin in culture. These data suggest, therefore, that neurofibromin may play a role in the development of both NC cells and a variety of non-NC-derived tissues.

Avian neural crest cells can migrate in the dorsolateral path only if they are specified as melanocytes

Neural crest cells are conventionally believed to migrate arbitrarily into various pathways and to differentiate according to the environmental cues that they encounter. We present data consistent with the notion that melanocytes are directed, by virtue of their phenotype, into the dorsolateral path, whereas other neural crest derivatives are excluded. In the avian embryo, trunk neural crest cells that migrate ventrally differentiate largely into neurons and glial cells of the peripheral nervous system. Neural crest cells that migrate into the dorsolateral path become melanocytes, the pigment cells of the skin. Neural crest cells destined for the dorsolateral path are delayed in their migration until at least 24 hours after migration commences ventrally. Previous studies have suggested that invasion into the dorsolateral path is dependent upon a change in the migratory environment. A complementary possibility is that as neural crest cells differentiate into melanocytes they acquire the ability to take this pathway. When quail neural crest cells that have been grown in culture for 12 hours are labeled with Fluoro-gold and then grafted into the early migratory pathway at the thoracic level, they migrate only ventrally and are coincident with the host neural crest. When fully differentiated melanocytes (96 hours old) are back-grafted under identical conditions, however, they enter the dorsolateral path and invade the ectoderm at least one day prior to the host neural crest. Likewise, neural crest cells that have been cultured for at least 20 hours and are enriched in melanoblasts immediately migrate in the dorsolateral path, in addition to the ventral path, when back-grafted into the thoracic level. A population of neural crest cells depleted of melanoblasts--crest cells derived from the branchial arches--are not able to invade the dorsolateral path, suggesting that only pigment cells or their precursors are able to take this migratory route. These results suggest that as neural crest cells differentiate into melanocytes they can exploit the dorsolateral path immediately. Even when 12-hour crest cells are grafted into stage 19–21 embryos at an axial level where host crest are invading the dorsolateral path, these young neural crest cells do not migrate dorsolaterally. Conversely, melanoblasts or melanocytes grafted under the same circumstances are found in the ectoderm. These latter results suggest that during normal development neural crest cells must be specified, if not already beginning to differentiate, as melanocytes in order to take this path.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of trkC receptor mRNA during development of the avian nervous system

Neurotrophin-3 (NT-3) has mitogenic and neurogenic activities on distinct central and peripheral nervous system (CNS and PNS) progenitors in avian embryos. It was therefore important to characterize in detail the expression pattern of TrkC, a high-affinity receptor for NT-3, during nervous system ontogeny. We report that trkC-encoding transcripts are expressed in the CNS primordium in several spatiotemporal distinct waves. trkC mRNA becomes evident in the dividing neuroepithelium where it is expressed homogeneously. A subsequent enhancement of the signal in dorsal areas of the neural tube occurs concomitant with the migration of neural crest cells from the CNS. Expression of trkC mRNA is then reduced in the germinal epithelium while progressively appearing on postmitotic neurons at the periphery of the neural tube. At a time preceeding the onset of normal motoneuron death, trkC signal is transiently undetectable in the ventral third of the neural tube. Diffuse expression in the spinal cord is resumed on embryonic day (E) 7. Subsets of premigratory and migrating neural crest progenitors also express the trkC receptor. Intense trkC signal is then evident throughout the newly organizing dorsal root ganglia (DRG), and becomes later restricted to defined postmitotic neuronal populations. Cranial ganglia also express the trkC gene from early stages of gangliogenesis. Furthermore, whereas the primary sympathetic ganglia show trkC mRNA, in the secondary ganglia a barely detectable signal could be observed. The dynamic up- and down-regulations of trkC reported here to occur both in the CNS and PNS primordia correspond to diverse, though only partially known, developmental processes. Taken together, these results support the notion that the NT-3-TrkC complex mediates diverse functions during neural development.

Hu neuronal proteins are expressed in proliferating neurogenic cells

We have utilized immunochemical techniques to investigate the developmental expression of the Hu proteins, a neuron-specific family of RNA binding proteins in vertebrates. Previous work suggests that these proteins may play an important role in neuronal development and maintenance. For the present study, we developed a monoclonal antibody (MAb 16A11) that binds specifically to an epitope present in gene products of all known Hu genes, including HuD, HuC, and Hel-N1. Using brief pulses (1-2 h) of the DNA precursor analog bromodeoxyuridine (BrdU) in conjunction with MAb 16A11, we observed Hu+/BrdU+ cells in nascent sensory and sympathetic ganglia in vivo, and in populations of cultured neural crest cells. In addition, a few Hu+ cells were ambiguously BrdU+ in the neural tube. We conclude that Hu+ cells first appear in avian neurogenic populations immediately before neuronal birthdays in the peripheral nervous system, and at the time of withdrawal from the mitotic cycle in the central nervous system. Consistent with these conclusions, we have also observed neural crest-derived cells that are both Hu+ and in metaphase of the cell cycle. We suggest that Hu proteins function early in neurogenic differentiation.

Neurotrophin-3 affects proliferation and differentiation of distinct neural crest cells and is present in the early neural tube of avian embryos

Neurotrophin-3 is mitogenic for cultured quail neural crest cells (Kalcheim et al., 1992, Proc. Natl. Acad. Sci. USA 89:1661-1665). We now report that neurotrophin-3 also influences the survival and/or differentiation of a subset of postmitotic neural crest precursors into neurons, provided these progenitors are grown on a cellular substrate. When cultured for 1 day on monolayers of NT-3-producing, chinese hamster ovary cells, 59% of the neural crest clusters growing on the transfected line revealed the presence of intense neuronal outgrowth, compared to 25% of that in controls. Moreover, dissociated neural crest cells grown for 20 h on top of mesodermal cells in the presence of various concentrations of purified recombinant neurotrophin-3 displayed a dose-dependent increase in neuronal number. Localization experiments using specific polyclonal antibodies, revealed that neurotrophin-3 is confined to neuroepithelial cells of quail neural tubes in situ on E2 and E3, and to E2 neural tubes grown in culture for 24 h. At this stage, neural crest cells and somites were negative. At later stages, staining was likewise apparent in peripheral nerves and dorsal root ganglia. We, therefore, propose that NT-3, a factor that is expressed in the early avian central nervous system, has multiple effects both on the proliferation and differentiation of distinct neural crest cells, which depend on the state of commitment of the responsive progenitors.

Restriction of neurogenic ability during neural crest cell differentiation

Multipotent neural crest cells undergo developmental restrictions during embryogenesis and eventually give rise to the neurons and glia of the peripheral nervous system, melanocytes, and pheochromocytes. To understand how neuronal potential is restricted to a subpopulation of crest-derived cells, we have utilized sensitive markers of early neuronal differentiation to assess neurogenesis in crest-derived cell populations subjected to defined experimental conditions in vitro and in vivo. We describe environmental conditions that either (a) result in the irreversible loss of neurogenic potential over a characteristic time course or (b) maintain neurogenic potential among neural crest cells.

Differentiation of neurogenic precursors within the neural crest cell lineage

It has been suggested that many, if not all crest-derived neurons develop from a limited subpopulation of neurogenic precursors. To develop cell-type specific markers that identify these precursors directly we have used differential screening of crest-derived cell populations known to have, or not to have, neurogenic ability. We have determined that the neuron-specific human auto-antibodies designated Anti-Hu bind to cytoplasmic and nuclear determinants not only in mature avian neurons and neuroendocrine cells but also in subpopulations of morphologically non-neuronal avian crest-derived cells. Significantly, these Anti-Hu+ non-neuronal crest-derived cells are present only in populations that have neurogenic ability and are absent from populations that lack neurogenic ability. Moreover, following additional development in vivo or in vitro, Anti-Hu+ non-neuronal crest-derived cells appear to express other neuronal traits. These results suggest that Anti-Hu-immunoreactivity is an early indicator of neurogenesis among crest-derived cells, and that Anti-Hu+ non-neuronal cells are either neurogenic precursors or immature neurons. Similarly, using the same differential screening paradigm, we have identified two monoclonal antibodies, designated 12E10 and 17F5, which also label both neurons and some apparently nonneuronal cells in neurogenic populations of neural crest cells. Anti-Hu-IR appears to precede expression of either of these two markers.

Gene transfer of lacZ into avian neural tube and neural crest cells by retroviral infection of grafted embryonic tissues

We describe here a new method for transferring genes into cells of the neural tube and neural crest of early avian embryos in vivo. Using the marker gene lacZ as an example, we infected dissected neural tubes from Hamburger-Hamilton stage 12-13 quail embryos with a replication-defective retrovirus carrying lacZ during a 2 hr period of exposure to the virus in culture. Infected neural tubes were then grafted into uninfected host chicken embryos in ovo and, after continued development for several days, the chimeric embryos were processed for beta-galactosidase histochemistry to identify the progeny of infected cells. We show that virus-infected neural tubes grafted isotopically into the trunk region of host embryos gave rise to cells of both the spinal cord and neural crest. Infected neural crest cells localized within dorsal root ganglia, sympathetic ganglia, peripheral nerves, and within the skin, where they were likely to give rise to melanocytes. These data are consistent with those using other cell marking techniques applied to the neural crest, indicating that retrovirus infection in culture, grafting, and beta-galactosidase expression has a neutral effect on neural crest cell migration and localization. These results indicate the heterospecific grafting of early avian tissues infected with retroviruses carrying foreign genes may be an effective strategy for testing the biological role of various gene products during development.

Protein kinases are required for embryonic neural crest cell galvanotaxis

Embryonic quail neural crest cells migrate towards the negative pole of an imposed dc electric field as small as 7 mV/mm (0.4 mV per average cell length). The involvement of protein kinases in the mechanism utilized by these cells to detect and respond to such imposed fields was tested through the use of several kinase inhibitors. Evidence for the involvement of protein kinase C (PKC) included: (1) inhibition of the directed motility by 1 microM sphingosine that was reversed by the addition of the phorbol ester, PMA; (2) stimulation of a faster response to the imposed field by PMA; and (3) inhibition of the directed translocation by 5 microM H-7. However, another PKC inhibitor, staurosporine, did not inhibit the directed translocation (1 nM-1 microM). We also found evidence for the involvement of either cAMP- or cGMP-dependent protein kinase. The galvanotactic response was partially inhibited by the addition of 10 microM H-9 and the response was enhanced in the presence of the phosphodiesterase inhibitor, IBMX. However, the adenylate cyclase stimulant, forskolin, had no significant influence on the directed motility, although it reduced the average cell velocity. While these experiments suggest that cAMP- or cGMP-dependent protein kinase or PKC may be involved in the galvanotaxis response, two other protein kinases appeared not to be required. The myosin light chain kinase inhibitor, ML-7, had no effect on the directed motility in an imposed field, so myosin light chain kinase may not be required for galvanotaxis. Similarly, 5 microM W-7 had no significant effect on the directed translocation, suggesting that calmodulin-dependent protein kinase is not involved. Interestingly, the continuous activity of a protein kinase is apparently not required for the directed translocation response. The addition of the PKC and cAMP-dependent protein kinase inhibitor, H-7, after the cells had been exposed to the field for 1 hour, had no effect on the subsequent directed translocation. Thus, for these inhibitors to block the directed translocation, they must be present at the same time as the initial field application. This implies that an integral step in the cellular response mechanism for galvanotaxis involves the stimulation of a protein kinase whose effect is long lasting.

BHK-21-derived cell lines that produce basic fibroblast growth factor, but not parental BHK-21 cells, initiate neuronal differentiation of neural crest progenitors

We present evidence that basic fibroblast growth factor (bFGF)-producing cells stimulate primary differentiation of neurons from neural crest progenitors. Baby hamster kidney (BHK-21) cells were stably cotransfected with plasmid pSV2/neo, which contains the gene conferring resistance to the neomycin analog G418 and expression vectors containing the human bFGF cDNA. Various clones, which differed in their bFGF production levels, were isolated. Homogeneous neural crest cells were cultured on monolayers of bFGF-producing, BHK-21-derived cell lines. While the parental BHK-21 cells, which do not produce detectable bFGF, had poor neurogenic ability, the various bFGF-producing clones promoted a 1.5- to 4-fold increase in neuronal cell number compared to the parental cells. This increase was correlated with the levels of bFGF produced by the different transfected clones, which ranged between 2.3 and 140 ng/mg protein. In contrast, no stimulation of neuronal differentiation was observed when neural crest cells were grown on monolayers of parental BHK cells transfected with plasmid pSV2/neo alone, or on a parental BHK-derived clone, which secretes high amounts of recombinant vascular endothelial growth factor (VEGF). Furthermore, the neuron-promoting ability of bFGF-producing cells could be mimicked by addition of exogenous bFGF to neural crest cells grown on the parental BHK line. A similar treatment of neural crest cells grown on laminin substrata, instead of BHK cells, resulted in increased survival of non-neuronal cells, but not of neurons (see also Kalcheim, C. 1989, Dev. Biol. 134, 1–10). Taken together, these results suggest that bFGF stimulates neuronal differentiation of neural crest cells by a cell-mediated signalling mechanism.

Differentiation of reptilian neural crest cells in vitro

An attempt was made to culture neural crest cells of the turtle embryo in vitro. Trunk neural tubes from the St. 9/10 embryos were explanted in culture dishes. The developmental potency of the turtle neural crest cells in vitro was shown to be essentially similar to that of avian neural crest cells, although they seem to be more sensitive to melanocyte-stimulating hormone (MSH) stimulation. We describe conditions under which explanted neural tube gives rise to neural crest cells that differentiate into neuronal cells and melanocytes. The potency of melanocyte differentiation was found to vary according to the concentration of fetal bovine serum (FBS, from 5 to 20%). Melanization of neural crest cells cultured in the medium containing FBS and alpha-MSH was more extensive than those cultured with FBS alone, combinations of FBS and chick embryo extract, or turtle embryo extract. These culture conditions seem to be useful for the study of the developmental potency of the neural crest cells as well as for investigating local environmental factors.

Neurotrophin 3 is a mitogen for cultured neural crest cells

Neurotrophin 3 (NT-3) promotes the survival and induces neurite outgrowth from a subset of neural crest (NC) and placode-derived neurons. We now report that this growth factor regulates the proliferation of cultured NC progenitor cells grown in a serum-free defined medium. In cultures of somites containing NC cells at migratory stages, NT-3 promotes a 2- to 8.4-fold increase in the number of NC cells incorporating [3H]thymidine into nuclei and a 1.8- to 4.8-fold increase in NC cell number compared to controls without added factor. NT-3 also promoted, to a lesser extent, the proliferation of NC cells in homogeneous cultures established from NC clusters. In addition to its effect on NC cells, NT-3 was mitogenic to somite cells in the mixed NC/somite cultures. These data demonstrate that NT-3 can act directly on the NC cells. They also indicate that the response of NC cells to NT-3 may be modulated by the presence of somitic cells. We suggest that NT-3 may be one of the central nervous system-derived factors that mediate NC cell proliferation in vivo.

Identification of early neurogenic cells in the neural crest lineage

Pluripotent neural crest cells are restricted progressively during development. The sequence of restrictions and the time(s) in early development at which such restrictions are imposed on crest-derived cells are largely unknown. We have used a human autoantibody (Anti-Hu) to characterize neurogenic populations of avian neural crest-derived cells. Anti-Hu binds specifically to neurons and neuroendocrine cells in older (greater than E4) quail embryos. Early in development, Anti-Hu also binds a subpopulation of neural crest-derived cells that lack neuronal morphology and do not express other neuronal traits. These cells may represent a putative neurogenic precursor subpopulation within the early crest cell lineage. To test this hypothesis, we have characterized Anti-Hu immunoreactivity within crest-derived populations known to have, or to lack, the ability to give rise to new neurons. We report that the presence of Anti-Hu+ nonneuronal cells is correlated with the neurogenic ability of a given cell population. Moreover, Anti-Hu+ nonneuronal cells are transient and appear to be replaced by Anti-Hu+ neuronal cells. We conclude that Anti-Hu is a very early indicator of neurogenesis among crest-derived cells and that Anti-Hu+ nonneuronal cells are either neurogenic precursors or immature neurons.

Immunohistochemical localization of fibronectin-like protein in the inner ear of the developing gerbil and rat

Immunohistochemistry was used to demonstrate the distribution of fibronectin-like protein within the developing inner ear of two species of altricial rodents: gerbils and rats. While there were temporal differences between the two species, the developmental sequence of immunostaining was virtually identical. Most notably, in rats from embryonic day 18 through day 1 postpartum, and in gerbils from birth through day 4 postpartum, intense, discrete fibronectin-like immunoreactivity was observed in the cochlea immediately beneath the inner and outer hair cells, sites of active auditory nerve fiber growth and nerve-hair cell synaptogenesis at these ages. The results suggest that fibronectin is appropriately positioned spatially and temporally to play a significant role in promoting, guiding and/or maintaining neural innervation within the developing organ of Corti. The temporo-spatial pattern of immunostaining in Schwann cells and auditory (VIIIth cranial) nerve neurons implies that fibronectin also plays a significant role in the early formation of myelin. In non-neural elements of the cochlea, fibronectin is a major structural component within the basilar membrane at all of the developmental stages investigated.

Partial restriction in the developmental potential of late emigrating avian neural crest cells

Trunk neural crest cells migrate along two major pathways: a ventral pathway through the somites whose cells form neuronal derivatives and dorsolateral pathway underneath the ectoderm whose cells become pigmented. In avian embryos, the latest emigrating neural crest cells move only along the dorsolateral pathway. To test whether late emigrating neural crest cells are more restricted in developmental potential than early migrating cells, cultures were prepared from the neural tubes of embryos at various stages of neural crest cell migration. "Early" and "middle" aged neural crest cells differentiated into many derivatives including pigmented cells, neurofilament-immunoreactive cells, and adrenergic cells. In contrast, "late" neural crest cells differentiated into pigment cells and neurofilament-immunoreactive cells, but not into adrenergic cells even after 10-14 days. To further challenge the developmental potential of early and late emigrating neural crest cells, they were transplanted into embryos during the early phases of neural crest cell migration, known to be permissive for adrenergic neuronal differentiation. The cells were labeled with the vital dye, DiI, and injected onto the ventral pathway at stages 14-17. Two and three days after injection, some early neural crest cells were found to express catecholamines, suggesting they were adrenergic neuroblasts. In contrast, DiI-labeled late neural crest cells never became catecholamine-positive. These results suggest that the late emigrating neural crest cell population has a more restricted developmental potential than the early migrating neural crest cell population.

The sympathoadrenal lineage in avian embryos. II. Effects of glucocorticoids on cultured neural crest cells

The neural crest-derived precursors of the sympathoadrenal lineage depend on environmental cues to differentiate as sympathetic neurons and pheochromocytes. We have used the monoclonal antibody A2B5 as a marker for neuronal differentiation and antisera against catecholamine synthesis enzymes to investigate the differentiation of catecholaminergic cells in cultures of quail neural crest cells. Cells corresponding phenotypically to sympathetic neurons and pheochromocytes can be identified in neural crest cell cultures after 5-6 days in vitro. Expression of the A2B5 antigen precedes expression of immunocytochemically detectable levels of tyrosine hydroxylase in cultured neural crest cells. Glucocorticoid treatment decreases the proportion of TH+ neural crest cells that express neuronal traits. We conclude that environmental cues normally encountered by sympathoadrenal precursors in vivo can influence the differentiation of a subpopulation of cultured neural crest cells in the sympathoadrenal lineage.

Neural-tube-derived melanocyte subsets undergo commitment to their distinct lineages in culture

Neural-crest-derived melanocytes populate two anatomical sites in the chicken, the epidermis of regenerating feathers and the uveal tract of the eyes. These two anatomical populations of melanocytes differ morphologically and functionally. Morphologically, feather and uveal melanocytes synthesize structurally different pigment granules (melanosomes). Feather melanosomes are rod-shaped, 0.2 x 0.8 micron, whereas uveal melanosomes are larger and more oval, 0.6 x 0.9 micron. Functionally, feather melanocytes continuously synthesize melanosomes during feather regeneration, and transfer these melanosomes to neighboring keratinocytes. Ocular melanocytes, on the other hand, synthesize melanosomes until their cytoplasm becomes congested with melanosomes, at which time the melanocytes become melanogenically dormant and do not transfer granules to neighboring cells. Cultures of melanocytes established from neural tubes of Light Brown Leghorn chick embryos produce two populations of melanocytes containing small (0.45 micron) or larger (0.90 micron) melanosomes which resemble the two types described in situ. Both types of melanocytes emigrate from along the entire length of the neural tube during several embryonic stages. Melanocyte cultures developed from neural tubes of the Recessive White breed of chicken, which has tyrosinase-negative, feather melanocytes and pigmented, functionally normal uveal melanocytes, also develop a mixture of amelanotic and pigmented melanocytes which maintain their respective characteristics even after separation by flow cytometry and reculture. These findings suggest that epidermal and uveal melanocytes are two distinct sub-populations of melanocytes whose commitment to separate lineages can occur in culture in the absence of their respective target tissue environment.

Avian model for 13-cis-retinoic acid embryopathy: demonstration of neural crest related defects

The effects of 13-cis-retinoic acid on the developing chick embryo were investigated. Fertilized eggs were injected via the yolk sac with single 50 microliters doses of either 1.5 micrograms, 15 micrograms, or 150 micrograms of 13-cis-retinoic acid in dimethyl sulfoxide on varying days of incubation (embryonic days 2, 3, 4, 5, or 6). Control embryos were given solvent alone or a mock injection. The embryos were examined on day 14 of incubation. The effects of retinoic acid on mortality and total malformations were both dose and developmental-stage responsive. The defects caused by 13-cis-retinoic acid occurred in mesenchymal tissues derived in part from the cranial neural crest ectomesenchyme. The craniofacial and cardiovascular malformations produced in the chick are analogous to those seen in animal models of retinoid teratogenesis and in human fetuses exposed to 13-cis-retinoic acid during maternal therapy for cystic acne. Following 13-cis-retinoic acid treatment, craniofacial and specific cardiovascular malformations were increased significantly compared to those in matched solvent and mock treated controls. The greatest number of malformations occurred when 13-cis-retinoic acid was given after cranial neural crest cell migration was complete. We propose that the primary effect of 13-cis-retinoic acid is on region-specific localization and differentiation of the mesenchymal subpopulation of cranial neural crest cells.

Plasminogen activator activity is associated with neural crest cell motility in tissue culture

We have examined the possibility that proteases such as plasminogen activator (PA) contribute to the extraordinary motile capability of neural crest cells. We show that trunk neural crest cells that migrate from isolated neural tubes in vitro produce PA and that the level of cell-associated PA increases dramatically after 8 days in culture. This increase is not the result of differentiation or time in culture, because neural crest cell clusters that form on top of the neural tube and differentiate into pigment cells but are immotile produce very low levels of PA. If these clusters are removed from the neural tube and replated on a plastic substratum where they migrate, the level of PA associated with the cells increases dramatically, suggesting that PA production is associated with motility. Inhibitors of PA/plasmin activity significantly reduce neural crest cell motility in vitro, further supporting the idea that proteases are important in neural crest cell migration.

Basic fibroblast growth factor stimulates survival of nonneuronal cells developing from trunk neural crest

The influence of basic fibroblast growth factor (bFGF), a central nervous system (CNS)-derived molecule, on survival of trunk neural crest cells was investigated. As previously shown (C. Kalcheim and N. M. Le Douarin, 1986, Dev. Biol. 116, 451-466), the interposition of untreated silastic membranes between neural tube and neural crest cells of the dorsal root ganglion (DRG) anlage led to selective death of neural crest cells that remained distally located with respect to the implants. Membranes were then treated with laminin and bFGF (100 ng/ml) and implanted. Under these conditions, rescued cells were observed for over 30 hr after grafting in 15 of 19 embryos. In contrast, no surviving cells could be found in any of 10 control embryos implanted with laminin-treated silastic membranes. We have also investigated the effects of bFGF on survival of identified subpopulations of trunk neural crest cells cultured with somite cells in a serum-free, chemically defined medium. bFGF promoted a dose-dependent increase in the number of HNK-1-positive nonneuronal cells in 1- to 4-day-old cultures (1.8- to 8.2-fold over controls using FGF at concentrations of 10 pg/ml to 1 ng/ml, respectively). FGF had no mitogenic effect on the neural crest-derived nonneuronal cells since the number of HNK-1-immunoreactive nonneuronal cells having incorporated [3H]thymidine into their nuclei remained unchanged in control as compared to treated cultures. However, the same concentrations of FGF were found to stimulate the incorporation of [3H]thymidine into acid-insoluble material in somite cultures devoid of neural crest. Moreover, bFGF significantly enhanced survival of nonneuronal cells in pure neural crest cultures established from neural crest clusters, thus demonstrating a direct effect of bFGF on survival and/or differentiation of neural crest-derived nonneuronal cells. These data support the hypothesis that CNS-derived molecules influence early development of selective subsets of neural crest cells developing into sensory ganglia.

Gene transfer and expression studies in cultured avian neural crest cells differentiating into melanocytes

Neural crest cells obtained from explanted neural tubes take up, express, and retain exogenous DNA applied by the CaPO4 co-precipitation method during their differentiation into melanocytes. High efficiencies of gene transfer were obtained with both supercoiled DNA and intact phage particles; linear DNA or DNA from the phage yielded very low efficiencies. There is some evidence that transferred gene expression is differentiation dependent. The system should be useful for studies concerned with the analysis of cell developmental genes and their regulatory elements.

Fibronectin receptor exhibits high lateral mobility in embryonic locomoting cells but is immobile in focal contacts and fibrillar streaks in stationary cells

The dynamic process of embryonic cell motility was investigated by analyzing the lateral mobility of the fibronectin receptor in various locomotory or stationary avian embryonic cells, using the technique of fluorescence recovery after photobleaching. The lateral mobility of fibronectin receptors, labeled by a monoclonal antibody, was defined by the diffusion coefficient and mobile fraction of these receptors. Even though the lateral diffusion coefficient did not vary appreciably (2 X 10(-10) cm2/S less than or equal to D less than or equal to 4 X 10(-10) cm2/S) with the locomotory state and the cell type, the mobile fraction was highly dependent on the degree of cell motility. In locomoting cells, the population of fibronectin receptors, which was uniformly distributed on the cell surface, displayed a high mobile fraction of 66 +/- 19% at 25 degrees C (82 +/- 14% at 37 degrees C). In contrast, in nonmotile cells, the population of receptors was concentrated in focal contacts and fibrillar streaks associated with microfilament bundles and, in these sites, the mobile fraction was small (16 +/- 8%). When cells were in a stage intermediate between highly motile and stationary, the population of fibronectin receptors was distributed both in focal contacts with a small mobile fraction and in a diffuse pattern with a reduced mobile fraction (33 +/- 9%) relative to the diffuse population in highly locomotory cells. The mobile fraction of the fibronectin receptor was found to be temperature dependent in locomoting but not in stationary cells. The mobile fraction could be modulated by affecting the interaction between the receptor and the substratum. The strength of this interaction could be increased by growing cells on a substratum coated with polyclonal antibodies to the receptor. This caused the mobile fraction to decrease. The interaction could be decreased by using a probe, monoclonal antibodies to the receptor known to perturb the adhesion of certain cell types which caused the mobile fraction to increase. From these results, we conclude that in locomoting embryonic cells, most fibronectin receptors can readily diffuse in the plane of the membrane. This degree of lateral mobility may be correlated to the labile adhesions to the substratum presumably required for high motility. In contrast, fibronectin receptors in stationary cells are immobilized in focal contacts and fibrillar streaks which are in close association with both extracellular and cytoskeletal structures; these stable complexes appear to provide firm anchorage to the substratum.

A subpopulation of cultured avian neural crest cells has transient neurogenic potential

Neural crest cells of vertebrate embryos produce neurons, glia, pigment cells, and connective tissue in vivo and in vitro. To test the developmental potential of apparently undifferentiated crest cells, we have used the monoclonal antibody A2B5, which recognizes a cell surface glycolipid characteristic of neurons, to identify and immunoablate a subpopulation of cultured avian neural crest cells with a neuronal phenotype. Our results indicate that a limited neurogenic precursor subpopulation is present in cultures of avian neural crest cells and that the fate of this subpopulation can be influenced by environmental conditions arising when dispersal of neural crest cells is delayed.