The distribution and spatial organization of the extracellular matrix encountered by mesencephalic neural crest cells

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Male-biased aganglionic megacolon in the TashT mouse line due to perturbation of silencer elements in a large gene desert of chromosome 10

Neural crest cells (NCC) are a transient migratory cell population that generates diverse cell types such as neurons and glia of the enteric nervous system (ENS). Via an insertional mutation screen for loci affecting NCC development in mice, we identified one line—named TashT—that displays a partially penetrant aganglionic megacolon phenotype in a strong male-biased manner. Interestingly, this phenotype is highly reminiscent of human Hirschsprung’s disease, a neurocristopathy with a still unexplained male sex bias. In contrast to the megacolon phenotype, colonic aganglionosis is almost fully penetrant in homozygous TashT animals. The sex bias in megacolon expressivity can be explained by the fact that the male ENS ends, on average, around a “tipping point” of minimal colonic ganglionosis while the female ENS ends, on average, just beyond it. Detailed analysis of embryonic intestines revealed that aganglionosis in homozygous TashT animals is due to slower migration of enteric NCC. The TashT insertional mutation is localized in a gene desert containing multiple highly conserved elements that exhibit repressive activity in reporter assays. RNAseq analyses and 3C assays revealed that the TashT insertion results, at least in part, in NCC-specific relief of repression of the uncharacterized gene Fam162b; an outcome independently confirmed via transient transgenesis. The transcriptional signature of enteric NCC from homozygous TashT embryos is also characterized by the deregulation of genes encoding members of the most important signaling pathways for ENS formation—Gdnf/Ret and Edn3/Ednrb—and, intriguingly, the downregulation of specific subsets of X-linked genes. In conclusion, this study not only allowed the identification of Fam162b coding and regulatory sequences as novel candidate loci for Hirschsprung’s disease but also provides important new insights into its male sex bias.

Hirschsprung’s disease (also known as aganglionic megacolon) is a severe congenital defect of the enteric nervous system (ENS) resulting in complete failure to pass stools. It is characterized by the absence of neural ganglia (aganglionosis) in the distal gut due to incomplete colonization of the embryonic intestines by neural crest cells (NCC), the ENS precursors. Hirschsprung’s disease has an incidence of 1 in 5000 newborns and a 4:1 male sex bias. Although many genes have been associated with this complex genetic disease, most of its heritability as well as its male sex bias remain unexplained. Here, we describe an insertional mutant mouse line (“TashT”) in which virtually all homozygotes display colonic aganglionosis due to defective migration of enteric NCC, but in which only a subset of homozygotes develops megacolon. Surprisingly, this group is almost exclusively male. The TashT ENS defect stems, at least in part, from the disruption of long-range interactions between evolutionarily conserved elements with silencer activity and Fam162b, resulting in NCC-specific upregulation of this uncharacterized protein coding gene. Global analysis of gene expression further revealed that several hundreds of genes are significantly deregulated in TashT enteric NCC. Interestingly, this dataset includes multiple X-linked candidate genes potentially underlying the male sex bias. Taken together, our data pave the way for a clearer understanding of the intriguing male sex bias of Hirschsprung’s disease.

Simple rules for a "simple" nervous system? Molecular and biomathematical approaches to enteric nervous system formation and malformation

We review morphogenesis of the enteric nervous system from migratory neural crest cells, and defects of this process such as Hirschsprung disease, centering on cell motility and assembly, and cell adhesion and extracellular matrix molecules, along with cell proliferation and growth factors. We then review continuum and agent-based (cellular automata) models with rules of cell movement and logistical proliferation. Both movement and proliferation at the individual cell level are modeled with stochastic components from which stereotyped outcomes emerge at the population level. These models reproduced the wave-like colonization of the intestine by enteric neural crest cells, and several new properties emerged, such as colonization by frontal expansion, which were later confirmed biologically. These models predict a surprising level of clonal heterogeneity both in terms of number and distribution of daughter cells. Biologically, migrating cells form stable chains made up of unstable cells, but this is not seen in the initial model. We outline additional rules for cell differentiation into neurons, axon extension, cell-axon and cell-cell adhesions, chemotaxis and repulsion which can reproduce chain migration. After the migration stage, the cells re-arrange as a network of ganglia. Changes in cell adhesion molecules parallel this, and we describe additional rules based on Steinberg's Differential Adhesion Hypothesis, reflecting changing levels of adhesion in neural crest cells and neurons. This was able to reproduce enteric ganglionation in a model. Mouse mutants with disturbances of enteric nervous system morphogenesis are discussed, and these suggest future refinement of the models. The modeling suggests a relatively simple set of cell behavioral rules could account for complex patterns of morphogenesis. The model has allowed the proposal that Hirschsprung disease is mostly an enteric neural crest cell proliferation defect, not a defect of cell migration. In addition, the model suggests an explanations for zonal and skip segment variants of Hirschsprung disease, and also gives a novel stochastic explanation for the observed discordancy of Hirschsprung disease in identical twins.

Building stable chains with motile agents: Insights into the morphology of enteric neural crest cell migration

A defining characteristic of the normal development of the enteric nervous system (ENS) is the existence of an enteric neural crest (ENC) cell colonization wave, where the ENC cells form stable chains often associated with axons and near the vascular network. However, within this evolving neural network, the individual ENC cell elements constantly move, change direction and appear to act independently of neighbors. Three possible hypotheses are investigated. The simplest of these postulates that the ENS follows the vascular network as a template. We present evidence which does not support this hypothesis. Two viable alternatives are either that (i) the axons muster the ENC cells, providing the pattern for the chain migration or (ii) ENC cells form chains and the axons follow these paths. These two hypotheses are explored by developing a stochastic cellular automata model, where ENC agents follow simple rules, which reflect the underlying biology of movement, proliferation and differentiation. By simulating ENC precursors and the associated neurons and axons, two models with different fundamental mechanisms are developed. From local rules, a mesoscale network pattern with lacunae emerges, which can be analyzed quantitatively. Simulation and analysis establishes the parameters that affect the morphology of the resulting network. This investigation into the axon/ENC and ENC/ENC interplay suggests possible explanations for observations in mouse and avian embryos in normal and abnormal ENS development, as well as further experimentation.

Homocysteine inhibits cardiac neural crest cell formation and morphogenesis in vivo

Elevated homocysteine increases the risk of neurocristopathies. Here, we determined whether elevating homocysteine altered the proliferation or number of chick neural crest cells that form between the midotic and third somite in vivo. Homocysteine increased the number of neural tube cells but decreased neural crest cell number. However, the sum total of cells was not different from controls. In controls, the 5-bromo-2'-deoxyuridine-labeling index was higher in newly formed neural crest cells than in their progenitors, paralleling reports showing these progenitors must pass the restriction point before undergoing epithelial-mesenchymal transition. Homocysteine decreased the labeling index of newly formed neural crest cells, suggesting that it inhibited cell cycle progression of neural crest progenitors or the S-phase entry of newly formed neural crest cells. Homocysteine also inhibited neural crest dispersal and decreased the distance they migrated from the neural tube. These results show neural crest morphogenesis is directly altered by elevated homocysteine in vivo. Developmental Dynamics 229:63-73, 2004.

Synthetic matrix metalloproteinase inhibitor decreases early cardiac neural crest migration in chicken embryos

During early embryonic development, cardiac neural crest (NC) cells emerge from the forming neural tube, migrate beneath the ectoderm, enter the pharyngeal arches, and subsequently participate in the septation of the heart. Like tumor cells, NC cells penetrate through basement membranes and invade extracellular matrix during their emigration and migration and, therefore, are liable to use similar invasive mechanisms. Matrix metalloproteinases (MMPs) are a family of zinc proteolytic enzymes known to be important in cell migration and invasion of normal and metastatic cells. In an earlier study, we found that the spatial and temporal distribution pattern of MMP-2 positively correlates with cardiac NC migration, suggesting MMP enzymatic activity may be important in mediating cardiac cell NC migration. To test this hypothesis, a synthetic MMP inhibitor, KB8301, was used to block MMP enzymatic activity during in vitro and in vivo cardiac NC cell migration in chick embryos. Injection of KB8301 into the cell-free space adjacent to the neural tube at the level of the second somite before the NC cells emigrated caused major morphologic anomalies in embryos and disrupted cardiac NC morphogenesis. Unilateral injection of KB8301 at lower concentrations, significantly decreased cardiac NC migration on the injected side compared with the noninjected side and compared with that of the injected controls. This decrease correlated with a decrease in MMP activity in the embryos and was not attributable to differences in embryo size or rate of embryonic development after injection. KB8301 also significantly decreased the rate of NC cell motility and distance NC cells migrated from explanted neural tubes and increased cell area and perimeter. These data suggest that MMP enzymatic activity is an important mediator of early cardiac NC migration and that perturbation of endogenous MMP activity may lead to NC-related congenital defects.

MMP-2 expression during early avian cardiac and neural crest morphogenesis

Matrix metalloproteinase-type 2 (MMP-2) degrades extracellular matrix, mediates cell migration and tissue remodeling, and is implicated in mediating neural crest (NC) and cardiac development. However, there is little information regarding the expression and distribution of MMP-2 during cardiogenesis and NC morphogenesis. To elucidate the role of MMP-2, we performed a comprehensive study on the temporal and spatial distribution of MMP-2 mRNA and protein during critical stages of early avian NC and cardiac development. We found that ectodermally derived NC cells did not express MMP-2 mRNA during their initial formation and early emigration but encountered MMP-2 protein in basement membranes deposited by mesodermal cells. While NC cells did not synthesize MMP-2 mRNA early in migration, MMP-2 expression was seen in NC cells within the cranial paraxial and pharyngeal arch mesenchyme at later stages but was never detected in NC-derived neural structures. This suggested NC MMP-2 expression was temporally and spatially dependent on tissue interactions or differed within the various NC subpopulations. MMP-2 was first expressed within cardiogenic splanchnic mesoderm before and during the formation of the early heart tube, at sites of active pharyngeal arch and cardiac remodeling, and during cardiac cushion cell migration. Collectively, these results support the postulate that MMP-2 has an important functional role in early cardiogenesis, NC cell and cardiac cushion migration, and remodeling of the pharyngeal arches and cardiac heart tube.

Urokinase-type plasminogen activator regulates cranial neural crest cell migration in vitro

Proper migration and differentiation of neural crest (NC) cells are required for normal development of craniofacial structures, heart and great vessels, sensory and autonomic nervous systems, and other organs with vertebrate embryos. Serine-protease inhibitors reduce NC cell migration in vitro, suggesting the extracellular proteases are important mediators of NC cell migration. While plasminogen activator activity levels are high in NC cells relative to other embryonic tissue, its ability to regulate NC cell migration has not been specifically tested in vivo or in vitro through its ability to convert plasminogen to plasmin. Using a transfilter migration assay, NC cell migration was measured in the presence or absence of plasminogen. Our results showed that plasminogen significantly enhanced NC cell migration. This increase could not be attributed to differences in initial NC cell attachment or cytotoxicity and did not require a chemotactic gradient. The plasminogen-enhanced NC cell migration was blocked by aprotinin (a plasmin inhibitor) and was mimicked by the direct addition of plasmin to the NC cells, indicating that the plasminogen effect was mediated through plasmin generation. Furthermore, anticatalytic-uPA antibody blocked the plasminogen-enhanced NC cell migration showing that NC cell-associated uPA activity was required for this effect. Finally, decreasing NC-uPA activity by treating cells with transforming growth factor-Beta, also blocked the plasminogen-dependent increase in cell migration. These data show that in vitro, NC cell migration is regulated by NC-associated uPA activity suggesting that growth factor-regulation of this activity may play a major role in regulating NC cell migratory capacity in vivo.

The incorporation and dispersion of cells and latex beads on microinjection into the amniotic cavity of the mouse embryo at the early-somite stage

The ability of cells and latex beads to become incorporated into the cranial region of embryos after microinjection into the amniotic cavity was studied. Premigratory neural crest cells isolated from the lateral margins of the neuroepithelium, 3T3 fibroblast cells or H35 hepatoma cells were labelled with WGA-gold conjugates, and were then microinjected into the amniotic cavity of embryos with two to three somites in vitro. Latex beads were similarly microinjected into different groups of embryos. Incorporation of injected cells or latex beads was found in the neural crest of the midbrain and the hindbrain of 5-20% of the recipients 4 h after microinjection. At 6 and 12 h, increasingly more embryos (20-77%) were observed with labelled cells or latex beads in the crest region. While hepatoma cells and latex beads were restricted to the crest region, injected neural crest cells and fibroblasts were also found in the lateral mesenchyme, bounded laterally by the surface ectoderm and medially by the closing neural tube. By 24 h after microinjection, the injected cells or latex beads were found in 50-80% of the recipients. Neural crest cells and fibroblasts, which showed similar patterns of distribution in the embryos, were located on the dorsal aspect of the neural tube, the lateral mesenchyme, the pharyngeal arches and the regions for ganglia. Hepatoma cells and latex beads were limited to the dorsal regions of the neural tube. When microinjection was carried out in embryos with seven to eight somites, incorporation of cells or latex beads was found in 44-75% of embryos, but no dispersion of the incorporated cells or latex beads into the mesenchyme was found 24 h after microinjection. Incorporation and dispersion of cells and latex beads were not observed when embryos with 18-20 somites were used as recipients. The present study showed that neural crest or fibroblast cells when injected into the amniotic cavity could be incorporated into the neural crest, and then undergo migration along the neural crest pathways, whereas hepatoma cells and latex beads could only be incorporated. The incorporation and migration of the exogenous tissues are related to the formation and the accessibility of the neural crest in the recipients.

Cell adhesion receptor expression during melanoma progression and metastasis

Many steps in melanoma metastasis involve cell-cell or cell-matrix adhesive interactions. The surface molecules which mediate these processes therefore play an important role in regulating melanoma dissemination and their level of expression may alter during the course of tumor progression. Human melanocyte strains and melanoma cell lines have been characterised with regard to levels of cell surface receptors of the integrin family. Increased amounts of at least two integrins, VLA-4 (alpha 4 beta 1) and VnR (alpha v beta 3), appeared to correlate with progression in this tumor, type. A novel VnR composed of an alpha v beta 1 association has been observed in one melanoma cell line and there is the possibility that heterogeneity of integrin composition could affect biological behavior of these tumors. CD44, a cell surface glycoprotein which functions as the major receptor for hyaluronate, is another molecule whose expression increases in transformed cells of the melanocytic lineage. Iterative sorting on the FACS for stable variants, of both human and murine melanomas, expressing low and high levels of CD44 established that lack of expression of this molecule correlated with impaired ability to form pulmonary tumor nodules subsequent to i.v. injection into appropriate recipient mice. These findings illustrate that an understanding of the regulation of melanoma adhesion receptors could provide insights into the process of tumor spread.

Immunohistochemical studies of Peters' anomaly

Peters' anomaly is a congenital abnormality that affects mainly the posterior cornea. The pathogenesis is unclear. By immunohistochemical methods, the authors determined the presence of extracellular matrix proteins, including fibronectin, laminin, and collagen types I, III, IV, V, and VI, in seven corneal buttons from five patients with Peters' anomaly. Type III collagen was the only protein absent in either the pathologic or normal human corneas. Compared with normal controls, the stromal lamellae in Peters' anomaly corneas were more irregular and more closely packed. In addition, the fibronectin staining was markedly enhanced in all seven Peters' anomaly corneas. Staining with collagen type VI was also mildly increased. These results suggest that extracellular matrix elements such as fibronectin may be important factors in this developmental disorder.

Physical influences on neural crest cell migration in avian embryos: contact guidance and spatial restriction

Several ideas on how neural crest (NC) cell migration in bird embryos might be dependent on the physical qualities of the internal embryonic environment were studied. Contact guidance has been suggested to direct NC cells ventrally in the trunk, but this has been subject to doubt (see Newgreen and Erickson, 1986, Int. Rev. Cytol. 103, 118-119). On reexamination, in situ extracellular matrix (ECM) and cell processes on the medial face of the somites were found appropriately oriented for this function. In addition, tissue culture models of oriented ECM could induce orientation of NC cells which mimicked that observed in the embryo. It is concluded that in this situation, oriented structures contribute to directed migration of NC cells in vivo, but the mechanism of contact guidance (i.e., steric or adhesive guidance) could not be ascertained. Contact guidance, in the form of steric guidance, has also been suggested as limiting ventrad NC cell movement at the midbrain level due to an insurmountable ridge on the side of the midbrain. The presence of this ridge was confirmed but it is unlikely to be responsible for prevention of ventrad migration, because, although it subsides very rapidly, the cells still refuse to move ventrad, and because models of this ridge in vitro proved to be no obstacle to NC cells. NC cell migration is also described as being limited by gross space between other organs or tissues. In vitro, NC cells could penetrate Nucleopore filters with pore diameters of 0.86 micron or greater. Observation of cell-free spaces in embryos showed that these were almost all much larger than the minimum pore size established experimentally. It is therefore concluded that in general the dimensions of gross tissue spaces probably do not set important limits for NC cell migration, but that the dimensions of transiently distensible microspaces between ECM fibrils may be a critical physical parameter.

Migration-related changes in the cytoskeleton of cultured neural crest cells visualized by the monoclonal antibody I-5G9

An epitope recognized by the monoclonal antibody I-5G9 was expressed by all neural crest cells shortly after explantation into culture. At this time all neural crest cells actively migrated away from the neural tube. Immunoreactivity was localized intracellularly and organized into stress fiber-like filaments. Often, immunofluorescence was particularly high in short fibers in the lamellipodia of the leading edge of migrating cells. Two-week-old cultures had a diameter of 8-10 mm. At that stage a ring of immunoreactive cells was present at the periphery of each culture, an area where cells were still migratory. An inner concentric circle had reduced and more granular staining. In this area cells had ceased to migrate. In the center of the culture cells were multilayered, nonmigratory, and did not bind I-5G9. After creating a lesion in the nonreactive central region, some cells resumed migration into the lesioned area and reexpressed the epitope. I-5G9 staining and phalloidin fluorescence colocalized partially in some cells and completely in others. It is concluded that the epitope recognized by I-5G9 is expressed in a migration-dependent manner. The partial colocalization of I-5G9 and phalloidin fluorescence supports the notion that the epitope recognized by I-5G9 is specifically expressed in stress fibers of migratory cells, possibly in one of the actin-associated proteins or an F actin-associated protein complex.

Specific configurations of fibronectin-containing particles correlate with pathways taken by neural crest cells at two axial levels

Although neural crest (NC) cells can potentially enter a number of intertissue spaces, they select a particular pathway that varies depending on the axial level. In the cranial region, NC cells enter the dorsal-lateral pathway (i.e., immediately subjacent to the ectoderm) and avoid the ventral pathway (i.e., pathway between the mesoderm and neural tube and within the mesodermal cell population), whereas in the trunk region, the majority of the NC cells enter the ventral pathway (i.e., between the somite and neural tube) and not the dorsal-lateral pathway. Our working hypothesis is that one determining factor in directing NC cell migration is the composition and/or intermolecular associations of the extracellular matrix (ECM) in these pathways. Histochemical staining, immunostaining, and lectin-binding studies on cryofixed and conventionally fixed tissue were conducted to initially characterize the ECM found in potential NC cell pathways prior to and during initial NC cell migration at two different axial levels. We found that, regardless of the axial level, the pathways into which NC cells eventually enter possessed a characteristic ECM arrangement. This arrangement included: 1) the presence of multicomponent, glycoprotein-containing spherical particles (0.1-0.5 micron in diameter); and 2) a low-sulfated ECM content. Although all particles contained fibronectin, only those in specific regions were able to bind to a monoclonal antibody directed to the cell-binding domain of fibronectin, suggesting that the conformation of fibronectin may be important in the expression of any in situ function of the molecule.

The extracellular matrix of the central and peripheral nervous systems: structure and function

The extracellular matrix (ECM) is the naturally occurring substrate upon which cells migrate, proliferate, and differentiate. The ECM functions as a biological adhesive that maintains the normal cytoarchitecture of different tissues and defines the key spatial relationships among dissimilar cell types. A loss of coordination and an alteration in the interactions between mesenchymal cells and epithelial cells separated by an ECM are thought to be fundamental steps in the development and progression of cancer. Although a substantial body of knowledge has been accumulated concerning the role of the ECM in most other tissues, much less is known of the structure and function of the ECM in the nervous system. Recent experiments in mammalian systems have shown that an increased knowledge of the ECM in the nervous system can lead to a better understanding of complex neurobiological processes under developmental, normal, and pathological conditions. This review focuses on the structure and function of the ECM in the peripheral and central nervous systems and on the importance of ECM macromolecules in axonal regeneration, cerebral edema, and cerebral neoplasia.

Stimulation of collagen production in vitro by ascorbic acid released from explants of migrating avian neural crest

Embryonic neuronal tissues contain a collagen-stimulating factor, shown to enhance the hydroxylation and secretion of proline-containing macromolecules by cultured muscle cells. Here we report on a similar activity found during avian embryonic development in explants of migrating mesencephalic neural crest. The degree of proline hydroxylation of proteins secreted into the medium was stimulated 2.5-6-fold in neural crest-muscle and neural crest-somite cocultures, as compared with control cultures devoid of crest explants. No such stimulation occurred when cocultures were treated with the enzyme ascorbate oxidase (EC 1.10.3.3), suggesting that the active factor in neural crest explants was ascorbic acid or an ascorbate-like molecule. Further characterization of this molecule was performed in crest explants and other embryonic tissues by using HPLC with amperometric detection: this study revealed that migrating cephalic neural crest contains 1.5 micrograms ascorbic acid per mg protein. Our results suggest that ascorbic acid and/or related molecule(s) could act during development of the nervous system as a trigger for collagen production and subsequent assembly of an extracellular matrix.

Amphibian neural crest cell migration on purified extracellular matrix components: a chondroitin sulfate proteoglycan inhibits locomotion on fibronectin substrates

The ability of purified extracellular matrix components to promote the initial migration of amphibian neural crest (NC) cells was quantitatively investigated in vitro. NC cells migrated avidly on fibronectin (FN), displaying progressively more extensive dispersion at increasing amounts of material incorporated in the substrate. In contrast, dispersion on laminin substrates was optimal at low protein concentrations but strongly reduced at high concentrations. NC cells were unable to migrate on substrates containing a high molecular mass chondroitin sulfate proteoglycan (ChSP). When proteolytic peptides, representing isolated functional domains of the FN molecule, were tested as potential migration substrates, the cell binding region of the molecule (105 kD) was found to be as active as the intact FN. A 31-kD heparin-binding fragment also stimulated NC cell migration, whereas NC cells dispersed to a markedly lower extent on the isolated collagen-binding domain (40 kD), or the latter domain linked to the NH2-terminal part of the FN molecule. Migration on the intact FN was partially inhibited by antibodies directed against the 105- and 31-kD fragments, respectively; dispersion was further decreased when the antibodies were used in combination. Addition of the ChSP to the culture medium dramatically perturbed NC cell migration on substrates of FN, as well as of 105- or 31-kD fragments. However, preincubation of isolated cells or substrates with ChSP followed by washing did not affect NC cell movement. The use of substrates consisting of different relative amounts of ChSP and the 105-kD peptide revealed that ChSP counteracted the motility-promoting activity of the 105-kD FN fragment in a concentration-dependent manner also when bound to the substrate. Our results indicate that NC cell migration on FN involves two separate domains of the molecule, and that ChSP can modulate the migratory behavior of NC cells moving along FN-rich pathways and may therefore influence directionally and subsequent localization of NC cells in the embryo.

Attachment of neural crest cells to endogenous extracellular matrices

Newly emerging neural crest (NC) cells will enter either the lateral pathway under the surface ectoderm or the vental pathway along the neural tube depending on the axial level (Pratt et al.: Dev. Biol., 44:298-305, 1975; Thiery et al.: Dev. Biol., 93:324-343, 1982; Newgreen et al.: Cell Tissue Res., 221:521-549, 1982; LeDouarin et al.: In: The Role of Extracellular Matrix in Development. Alan R. Liss, Inc., New York, pp. 373-398, 1984; Brauer et al.: Anat. Rec., 211:57-68, 1985). A number of studies have shown a correlation between the type of extracellular matrix (ECM) associated with adjacent tissues (e.g., ectoderm, neural tube, and mesoderm) and the initial pathway taken by NC cells. Our working hypothesis is that the direction of NC cell migration (ventral vs. lateral pathway) depends on the composition of the ECM associated with the surface ectoderm and its ability to support NC cell attachment. In this study, we tested this hypothesis by isolating endogenous ECM associated with the ectoderm of each region and examining the ability of each endogenous ECM to support cranial and trunk NC cell attachment in vitro. Results indicated that both cranial and trunk NC cells preferentially attached to cranial ectodermal ECM as compared to trunk ectodermal ECM. The differences in NC cell attachment were not due to a preferential adsorption of cranial ectodermal ECM onto the ECM-conditioned plastic substrate over trunk ectodermal since approximately equal amounts of ECM bound to the plastic. These results supported the hypothesis and provide evidence that endogenous ectodermal ECM may be one factor potentially responsible for directing the NC cells along a ventral or a lateral pathway.

Distribution of basement membrane antigens in cryopreserved early embryonic hearts

The early embryonic heart is composed of two cylindrical epithelial layers, an inner endothelium and an outer myocardium. The cardiac jelly (CJ), an acellular accumulation of extracellular matrix (ECM), fills the space between the two epithelia. During development of the heart, a portion of the endothelial cells of the atrioventricular (AV) region differentiate into mesenchyme cells in a temporally and spacially specific manner. Although contiguous with those in the AV region, endothelial cells lining the ventricle never form mesenchyme in situ. At present, the mechanisms controlling the biphasic differentiation of the endothelium and the subsequent migration of cardiac mesenchymal cells are poorly understood. Although the CJ lies between two epithelial and is spatially equivalent to a basement membrane (BM), it has not traditionally been considered to be organized into a BM-like structure. The potential significance of this observation to developmental biology lies in the possibility that BM or their individual components (i.e., fibronectin (FN), laminin (LM), type IV collagen, and heparin sulfate proteoglycan (HSPG] may function as the regulatory site of epithelial differentiation and morphogenesis. A cryofixation technique was developed in order to determine the in situ immunohistochemical distribution of the BM components in the CJ. Results indicated that the CJ exists as the fusion between a larger myocardially derived BM having a lamina densa and an extended reticular lamina and an attenuated, endothelial-associated BM composed only of a lamina densa. Except for FN, the individual BM components were not all present during early stages, but instead appeared in a sequential manner, suggesting that all components of an adult-type BM are not required to initiate the assembly of a structural and functional BM during development. In the AV canal and outflow tract (OT), FN appeared as a progressively expanding gradient of material with the greatest density nearer the myocardium.

Neural tube defects. Some remarks on the possible role of glycosaminoglycans in the genesis of the dysraphic state, the anomaly in the configuration of the posterior cranial fossa, and hydrocephalus

Recent developments in the field of experimentally induced neural tube defects (NTD) indicate that specific substances, namely the glycosaminoglycans (GAGs) may play a role in the genesis of spinal malformations. The authors report the results obtained by evaluating the GAGs in rat fetuses with NTD, secondary to the administration of Trypan Blue during pregnancy. A characteristic decrease in GAGs formation in the spinal and cranial structures as well as in the subependymal regions of the brain was found in the malformed fetuses. The authors hypothesize that this anomaly in GAGs formation is responsible for both the NTD and the associated malformations, namely hydrocephalus and hypoplasia of the posterior cranial fossa.

Evidence for a novel enzymatic mechanism of neural crest cell migration on extracellular glycoconjugate matrices

Migrating embryonic cells have high levels of cell surface galactosyltransferase (GalTase) activity. It has been proposed that GalTase participates during migration by recognizing and binding to terminal N-acetylglucosamine (GlcNAc) residues on glycoconjugates within the extracellular matrix (Shur, B. D., 1982, Dev. Biol. 91:149-162). We tested this hypothesis using migrating neural crest cells as an in vitro model system. Cell surface GalTase activity was perturbed using three independent sets of reagents, and the effects on cell migration were analyzed by time-lapse microphotography. The GalTase modifier protein, alpha-lactalbumin (alpha-LA), was used to inhibit surface GalTase binding to terminal GlcNAc residues in the underlying substrate. alpha-LA inhibited neural crest cell migration on basal lamina-like matrices in a dose-dependent manner, while under identical conditions, alpha-LA had no effect on cell migration on fibronectin. Control proteins, such as lysozyme (structurally homologous to alpha-LA) and bovine serum albumin, did not effect migration on either matrix. Second, the addition of competitive GalTase substrates significantly inhibited neural crest cell migration on basal lamina-like matrices, but as above, had no effect on migration on fibronectin. Comparable concentrations of inappropriate sugars also had no effect on cell migration. Third, addition of the GalTase catalytic substrate, UDPgalactose, produced a dose-dependent increase in the rate of cell migration. Under identical conditions, the inappropriate sugar nucleotide, UDPglucose, had no effect. Quantitative enzyme assays confirmed the presence of GalTase substrates in basal lamina matrices, their absence in fibronectin matrices, and the ability of alpha-LA to inhibit GalTase activity towards basal lamina substrates. Laminin was found to be a principle GalTase substrate in the basal lamina, and when tested in vitro, alpha-LA inhibited cell migration on laminin. Together, these experiments show that neural crest cells have at least two distinct mechanisms for interacting with the substrate during migration, one that is fibronectin-dependent and one that uses GalTase recognition of basal lamina glycoconjugates.