Modes of cell migration in the vertebrate embryo

Cultures of skin fragments of Salamandra salamandra salamandra (L.) larvae

As part of a study on the pigmentary system of Salamandra salamandra salamandra (L.), we cultured skin fragments of 7-10-day-old larvae in order to examine the expression of molecules implicated in cellular adhesion and migration and in regulating cell-cell relationships. Keratinocytes, fibroblasts, Leydig cells, xanthophores, and melanophores migrated from the fragments and were observed in the outgrowth. Keratinocytes and fibroblasts organized into an epidermal layer and an underlying "dermal portion." The chromatophores were always located below the epithelial cells, often with fibroblasts. We examined by immunocytochemistry the expression of fibronectin, beta1-integrin, L-CAM, and A-CAM in the cultures. Many keratinocytes, fibroblasts, and Leydig cells expressed all the signal molecules tested. Xanthophores and melanophores were only immunoreactive to the anti-adhesion molecules antisera. Since the molecules tested are known to play a role in cell adhesion, growth, and spreading, as well as in regulating tissue differentiation and in maintaining normal tissue morphology, we may hypothesize that in Salamandra salamandra salamandra fibronectin, beta1-integrin, L-, and A-CAMs concertedly act to stabilize the architecture of the outgrowth and regulate the relationships between chromatophores and those between chromatophores and the other elements of the skin culture.

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

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

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

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

Biology of hypothalamic neurons and pituitary cells

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

Clonal dominance between subpopulations of mixed small cell lung cancer xenografts implanted ectopically in nude mice

Clonal evolution of neoplastic cells during solid tumour growth leads to the emergence of new tumour cell subpopulations with diverging phenotypic characteristics which may alter the behaviour of a malignant disease. Cellular interaction was studied in mixed xenografts in nude mice and during in vitro growth of two sets of small cell lung cancer (SCLC) subpopulations (54A, 54B and NYH, NYH2). The tumour cell lines differed in cellular DNA content enabling flow cytometric DNA analysis (FCM) to be used to monitor changes in the fractional composition of the mixed cell populations. The progeny clone 54B was found to dominate the parent 54A clone when grown as mixed subcutaneous xenografts in nude mice, whereas no dominance was exerted during in vitro growth. The in vivo dominance could not be explained by differences in growth kinetics between the two tumour cell lines, and the interaction was not dependent on 54B being in excess in mixed tumours. The dominance was dependent on close in vivo contact as no remote effect on the growth of 54A was found when the dominating 54B cells were growing in the opposite flank of tumour-bearing mice. Irradiation inactivated 54B cells were unable to exert the dominating effect, indicating that the interaction required viable and proliferating cells. Clonal dominance was not found in mixed NYH-NYH2 tumours indicating that the dominance mechanism(s) may not always be operational between subpopulations in heterogeneous tumours. Recognition of interaction between tumour cell populations may result in a better understanding of the behaviour of heterogeneous human malignancies.

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

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

Epithelium-mesenchyme interconversion as example of epithelial plasticity

This review focuses on epithelium-mesenchymal transitions (EMT), defined as dynamic cell restructurations changing the epithelial state of differentiation into a mesenchymal phenotype. These transitions, known to occur during embryogenesis are also involved during some pathological events of adult life, such as wound repair and metastasis of cancer cells. Numerous studies of embryonic EMTs, found during some morphogenetic processes, have stressed the importance of intercellular and cell-matrix adhesive interactions as key elements regulating cell dissociation and acquisition of cell motility. On the other hand, in vitro studies indicate that growth factors, growth-factor related molecules and extracellular matrix components are involved in initiation of EMT. Therefore, the cellular targets of EMT-inducing molecules are likely to include molecules participating in cell adhesion systems.

NCAM immunoreactivity during major developmental events in the rat maxillary nerve-whisker system

The distribution of immunoreactivity for neural cell adhesion molecule (NCAM) has been characterized during the formation of the trigeminal ganglion and during the process of axon outgrowth and target differentiation in the maxillary nerve-whisker system, in rat fetuses of known gestational age. Proliferating cells within the trigeminal placode are NCAM immunoreactive when first observed on embryonic day (E) 10. NCAM immunoreactivity is lost from placode-derived cells as they migrate to the trigeminal ganglion. It re-appears on ganglion cell somata and on centrally and peripherally projecting axons at the time of neurite outgrowth. NCAM-immunoreactive centrally projecting axons reach the developing brain stem two days before peripheral axons encounter the presumptive whisker pad. NCAM immunoreactivity on axons and somata is down regulated after P0, following target contact and whisker follicle differentiation. The presumptive dermis of the whisker pad at E13 appears as a sheet-like condensation of intensely NCAM immunostained cells. Discrete infraorbital row nerves can be identified on E13. These form in the subdermal region which contains only low levels of NCAM immunoreactivity. Condensations of NCAM immunostained mesenchyme replace the dermal sheet on E14 and each condensation is associated with a plexus of infraorbital nerve fibers. The epithelium overlying each condensation grows downward on E15. Focal epithelial regions become NCAM immunoreactive by E18. NCAM immunostaining within epithelial components of the whisker follicle is temporally correlated with contact by NCAM-immunoreactive infraorbital nerve fibers. The site restricted expression of NCAM immunoreactivity during trigeminal embryogenesis is consistent with the idea that NCAM plays an integral role in critical aspects of pattern formation in the maxillary nerve-whisker system, particularly in the organization of placode and non-placode derived trigeminal neuroblasts, axon outgrowth and in the differentiation of the vibrissae follicles.

Neuronal cell cultures: a tool for investigations in developmental neurobiology

The aim of this review is to describe environmental requirements for survival of neuronal cells in culture, and secondly to survey the complex interplay between hormones, neurotrophic factors, transport- and extracellular matrix- proteins, which characterize the developmental program of differentiating neurons. An overall reconsideration of the literature in this vast field is above the limits of the present paper; since progress and refinement in the techniques of neuronal cell cultures have paralleled the advancement in Developmental Neurobiology, we will run instead through the main steps which form the conceptual framework of neuronal cell cultures.

On the generation of form by the continuous interactions between cells and their extracellular matrix

The central issue of this essay is the problem of how multicellular organisms develop and maintain the complex architecture and intricate shape of tissues and organs. The concepts pattern formation, morphogenesis and differentiation are defined and discussed suggesting a distinction between processes that underlie uniformity (e.g. basic body plans) and those underlying inter- and intra-species variation. The initial stage of limb bone development--the formation of the mesenchymal condensation--is described in detail. On the basis of these data and many additional example from other developmental systems, the central role of continuous cell-ECM interactions in the generation of form is deduced. Evidence is provided as to the leading role of the mesenchymal-fibroblast-like cells in sculpturing tissue and organ architecture. It is proposed that a group of cells within their ECM, rather than the single cell, is the functional unit relevant to the generation of form. The continuous cell-ECM interactions lead to the generation of form not by a detailed obligate pathway, but rather by a process of 'selective stabilization' (Kirschner & Mitchison, 1986), i.e. a gradual organization into more stable structures, where existing structural configuration serve to increase the likelihood of certain configurations and reduce that of others. Data are quoted to support the notion that even cell division does not erase all the structural information imprinted in the cell. The role of the metazoan genome in morphogenesis is discussed in the light of the process of selective stabilization.

Transforming growth factor-beta control of cell-substratum adhesion during avian neural crest cell emigration in vitro

It has been proposed that, in higher vertebrates, the onset of neural crest cell migration from the neural tube involves spatially and temporally coordinated changes in cellular adhesiveness that are under the control of external signals released in the extracellular milieu by neighboring tissues. In the present study, we have analyzed the dynamics of changes in cell-substratum adhesiveness during crest cell emigration and searched for regulatory cues using an in vitro model system. This model is based on the fact that, in vivo, crest cell dispersion occurs gradually along a rostrocaudal wave, allowing us to explant portions of the neural axis, termed migratory and premigratory levels, that differ in the time in culture at which neural crest cells initiate migration and in the locomotory behavior of the cells. We found that neural crest cell emigration is not triggered by the main extracellular matrix molecules present in the migratory pathways, as none of these molecules could abolish the intrinsic difference in the timing of emigration between the different axial levels. Using an in vitro adhesion assay, we found that presumptive neural crest cells from premigratory level explants gradually acquired the ability to respond to extracellular matrix material with time in culture, suggesting that acquisition of appropriate, functional integrin receptors was a necessary step for migration. Finally, we showed that members of the transforming growth factor-beta family reduced in a dose-dependent manner the delay of neural crest cell emigration from premigratory level explants and were able to increase significantly the substratum-adhesion properties of crest cells. Our results suggest that acquisition of substratum adhesion by presumptive neural crest cells is a key event during their dispersion from the neural tube in vitro, and that members of the transforming growth factor-beta family may act as potent inducers of crest cell emigration, possibly by increasing the substratum adhesion of the cells.

Development of the neural crest

Mutations that affect the morphogenetic behaviour and differentiation of neural crest-derived cells in mouse embryos have been shown to alter genes that code for growth factors or growth factor receptors. Identification of these and other gene products provide opportunities to understand when and how developmentally distinct embryonic cell populations arise, and how interactions between localized developmental cues and responsive cell subpopulations can be modulated during development.