Matrix-driven translocation of cells and nonliving particles

The extracellular matrix in tissue morphogenesis: No longer a backseat driver

The forces driving tissue morphogenesis are thought to originate from cellular activities. While it is appreciated that extracellular matrix (ECM) may also be involved, ECM function is assumed to be simply instructive in modulating the cellular behaviors that drive changes to tissue shape. However, there is increasing evidence that the ECM may not be the passive player portrayed in developmental biology textbooks. In this review we highlight examples of embryonic ECM dynamics that suggest cell-independent activity, along with developmental processes during which localized ECM alterations and ECM-autonomous forces are directing changes to tissue shape. Additionally, we discuss experimental approaches to unveil active ECM roles during tissue morphogenesis. We propose that it may be time to rethink our general definition of morphogenesis as a cellular-driven phenomenon and incorporate an underappreciated, and surprisingly dynamic ECM.

Extracellular matrix assembly stress initiates Drosophila central nervous system morphogenesis

Forces controlling tissue morphogenesis are attributed to cellular-driven activities, and any role for extracellular matrix (ECM) is assumed to be passive. However, all polymer networks, including ECM, can develop autonomous stresses during their assembly. Here, we examine the morphogenetic function of an ECM before reaching homeostatic equilibrium by analyzing de novo ECM assembly during Drosophila ventral nerve cord (VNC) condensation. Asymmetric VNC shortening and a rapid decrease in surface area correlate with the exponential assembly of collagen IV (Col4) surrounding the tissue. Concomitantly, a transient developmentally induced Col4 gradient leads to coherent long-range flow of ECM, which equilibrates the Col4 network. Finite element analysis and perturbation of Col4 network formation through the generation of dominant Col4 mutations that affect assembly reveal that VNC morphodynamics is partially driven by a sudden increase in ECM-driven surface tension. These data suggest that ECM assembly stress and associated network instabilities can actively participate in tissue morphogenesis.

Dynamic movement and turnover of extracellular matrices during tissue development and maintenance

Extracellular matrices (ECMs) are essential for the architecture and function of animal tissues. ECMs have been thought to be highly stable structures; however, too much stability of ECMs would hamper tissue remodelling required for organ development and maintenance. Regarding this conundrum, this article reviews multiple lines of evidence that ECMs are in fact rapidly moving and replacing components in diverse organisms including hydra, worms, flies, and vertebrates. Also discussed are how cells behave on/in such dynamic ECMs, how ECM dynamics contributes to embryogenesis and adult tissue homoeostasis, and what molecular mechanisms exist behind the dynamics. In addition, it is highlighted how cutting-edge technologies such as genome engineering, live imaging, and mathematical modelling have contributed to reveal the previously invisible dynamics of ECMs. The idea that ECMs are unchanging is to be changed, and ECM dynamics is emerging as a hitherto unrecognized critical factor for tissue development and maintenance.

Living tissues are more than cell clusters: The extracellular matrix as a driving force in morphogenesis

In the study of morphogenesis, there is a general tendency to look at the extracellular matrix (ECM) as a mechanically passive agent that simply gives support to cells, and consequently, to place all the explanatory burden on cellular behaviors. Here we aimed to show that not only cells, but also the ECM may be an important force of morphogenesis. Understanding the mechanical role of the ECM broadens our view of morphogenesis and stresses the importance of considering embryonic tissues as a composite of cells and ECM.

A versatile 3D tissue matrix scaffold system for tumor modeling and drug screening

Most of the anticancer drug candidates entering preclinical trials fail to be approved for clinical applications. The following are among the main causes of these failures: studying molecular mechanisms of cancer development, identifying therapeutic targets, and testing drug candidates using inappropriate tissue culture models, which do not recapitulate the native microenvironment where the cancer cells originate. It has become clear that three-dimensional (3D) cell cultures are more biologically and clinically relevant than 2D models. The spatial and mechanical conditions of 3D cultures enable the cancer cells to display heterogeneous growth, assume diverse phenotypes, express distinct gene and protein products, and attain metastatic potential and resistance to drugs that are reminiscent of tumors in humans. However, the current 3D culture systems using synthetic polymers or selected components of the extracellular matrix (ECM) are defective (particularly the biophysical and biochemical properties of the native ECM) and remain distant to optimally support the signaling cue-oriented cell survival and growth. We introduce a reconstitutable tissue matrix scaffold (TMS) system fabricated using native tissue ECM, with tissue-like architecture and resilience. The structural and compositional properties of TMS favor robust cell survival, proliferation, migration, and invasion in culture and vascularized tumor formation in animals. The combination of porous and hydrogel TMS allows compartmental culture of cancerous and stromal cells, which are distinguishable by biomarkers. The response of the cancer cells grown on TMS to drugs well reflects animal and clinical observations. TMS enables more biologically relevant studies and is suitable for preclinical drug screening.

Extracellular matrix motion and early morphogenesis

For over a century, embryologists who studied cellular motion in early amniotes generally assumed that morphogenetic movement reflected migration relative to a static extracellular matrix (ECM) scaffold. However, as we discuss in this Review, recent investigations reveal that the ECM is also moving during morphogenesis. Time-lapse studies show how convective tissue displacement patterns, as visualized by ECM markers, contribute to morphogenesis and organogenesis. Computational image analysis distinguishes between cell-autonomous (active) displacements and convection caused by large-scale (composite) tissue movements. Modern quantification of large-scale 'total' cellular motion and the accompanying ECM motion in the embryo demonstrates that a dynamic ECM is required for generation of the emergent motion patterns that drive amniote morphogenesis.

From Skeletal Development to Tissue Engineering: Lessons from the Micromass Assay

Damage and degeneration of the skeletal elements due to disease, trauma, and aging lead to a significant health and economical burden. To reduce this burden, skeletal tissue engineering strategies aim to regenerate functional bone and cartilage in the adult body. However, challenges still exist. Such challenges involve the identification of the external cues that determine differentiation, how to control chondrocyte hypertrophy, and how to achieve specific tissue patterns and boundaries. To address these issues, it could be insightful to look at skeletal development, a robust morphogenetic process that takes place during embryonic development and is commonly modeled in vitro by the micromass assay. In this review, we investigate what the tissue engineering field can learn from this assay. By comparing embryonic skeletal precursor cells from different anatomic locations and developmental stages in micromass, the external cues that guide lineage commitment can be identified. The signaling pathways regulating chondrocyte hypertrophy, and the cues required for tissue patterning, can be elucidated by combining the micromass assay with genetic, molecular, and engineering tools. The lessons from the micromass assay are limited by two major differences between developmental and regenerative skeletogenesis: cell type and scale. We highlight an important difference between embryonic and adult skeletal progenitor cells, in that adult progenitors are not able to form mesenchymal condensations spontaneously. Also, the mechanisms of tissue patterning need to be adjusted to the larger tissue engineering constructs. In conclusion, mechanistic insights of skeletal tissue generation gained from the micromass model could lead to improved tissue engineering strategies and constructs.

The effect of growth-mimicking continuous strain on the early stages of skeletal development in micromass culture

Embryonic skeletogenesis involves proliferation, condensation and subsequent chondrogenic differentiation of mesenchymal precursor cells, and the strains and stresses inherent to these processes have been hypothesized to influence skeletal development. The aim of this study was to determine the effect of growth-mimicking strain on the process of early skeletal development in vitro. To this end, we applied continuous uniaxial strain to embryonic skeletal precursor cells in micromass culture. Strain was applied at different times of culture to specifically address the effect of mechanical loading on the sequential stages of cellular proliferation, condensation and differentiation. We found that growth-mimicking strain at all three times did not affect proliferation or chondrogenic differentiation under the tested conditions. However, the timing of the applied strain did play a role in the density of mesenchymal condensations. This finding suggests that a mechanically dynamic environment, and specifically strain, can influence skeletal patterning. The growth-mimicking micromass model presented here may be a useful tool for further studies into the role of mechanical loading in early skeletal development.

Linear patterning of mesenchymal condensations is modulated by geometric constraints

The development of the vertebral column starts with the formation of a linear array of mesenchymal condensations, forming the blueprint for the eventual alternating pattern of bone and cartilage. Despite growing insight into the molecular mechanisms of morphogenesis, the impact of the physical aspects of the environment is not well understood. We hypothesized that geometric boundary conditions may play a pivotal role in the linear patterning of condensations, as neighbouring tissues provide physical constraints to the cell population. To study the process of condensation and the patterning thereof under tightly controlled geometric constraints, we developed a novel in vitro model that combines micropatterning with the established micromass assay. The spacing and alignment of condensations changed with the width of the cell adhesive patterns, a phenomenon that could not be explained by cell availability alone. Moreover, the extent of chondrogenic commitment was increased on substrates with tighter geometric constraints. When the in vivo pattern of condensations was investigated in the developing vertebral column of chicken embryos, the measurements closely fit into the quantitative relation between geometric constraints and inter-condensation distance found in vitro. Together, these findings suggest a potential role of geometric constraints in skeletal patterning in a cellular process of self-organization.

Extracellular matrix degradation and remodeling in development and disease

The extracellular matrix (ECM) serves diverse functions and is a major component of the cellular microenvironment. The ECM is a highly dynamic structure, constantly undergoing a remodeling process where ECM components are deposited, degraded, or otherwise modified. ECM dynamics are indispensible during restructuring of tissue architecture. ECM remodeling is an important mechanism whereby cell differentiation can be regulated, including processes such as the establishment and maintenance of stem cell niches, branching morphogenesis, angiogenesis, bone remodeling, and wound repair. In contrast, abnormal ECM dynamics lead to deregulated cell proliferation and invasion, failure of cell death, and loss of cell differentiation, resulting in congenital defects and pathological processes including tissue fibrosis and cancer. Understanding the mechanisms of ECM remodeling and its regulation, therefore, is essential for developing new therapeutic interventions for diseases and novel strategies for tissue engineering and regenerative medicine.

Alpha5beta1 integrin-fibronectin interactions specify liquid to solid phase transition of 3D cellular aggregates

BACKGROUND

Tissue organization during embryonic development and wound healing depends on the ability of cells on the one hand to exchange adhesive bonds during active rearrangement and on the other to become fixed in place as tissue homeostasis is reached. Cells achieve these contradictory tasks by regulating either cell-cell adhesive bonds, mediated by cadherins, or cell-extracellular matrix (ECM) connections, regulated by integrins. Integrin alpha5beta1 and soluble fibronectin (sFN) are key players in cell-ECM force generation and in ECM polymerization. Here, we explore the interplay between integrin alpha5beta1 and sFN and its influence on tissue mechanical properties and cell sorting behavior.

METHODOLOGY/PRINCIPAL FINDINGS

We generated a series of cell lines varying in alpha5beta1 receptor density. We then systematically explored the effects of different sFN concentrations on aggregate biomechanical properties using tissue surface tensiometry. We found previously unreported complex behaviors including the observation that interactions between fibronectin and integrin alpha5beta1 generates biphasic tissue cohesion profiles. Specifically, we show that at constant sFn concentration, aggregate cohesion increases linearly as alpha5beta1 receptor density is increased from low to moderate levels, producing a transition from viscoelastic-liquid to pseudo viscoelastic-solid behavior. However, further increase in receptor density causes an abrupt drop in tissue cohesion and a transition back to viscoelastic-liquid properties. We propose that this may be due to depletion of sFn below a critical value in the aggregate microenvironment at high alpha5beta1 levels. We also show that differential expression of alpha5beta1 integrin can promote phase-separation between cells.

CONCLUSIONS/SIGNIFICANCE

The interplay between alpha5-integrin and sFn contributes significantly to tissue cohesion and, depending on their level of expression, can mediate a shift from liquid to elastic behavior. This interplay represents a tunable level of control between integrins and the ECM that can influence tissue cohesion and other mechanical properties, which may translate to the specification of tissue structure and function. These studies provide insights into important biological processes such as embryonic development, wound healing, and for tissue engineering applications.

Heparan sulfate proteoglycans: a GAGgle of skeletal-hematopoietic regulators

This review summarizes our current understanding of the presence and function of heparan sulfate proteoglycans (HSPGs) in skeletal development and hematopoiesis. Although proteoglycans (PGs) comprise a large and diverse group of cell surface and matrix molecules, we chose to focus on HSPGs owing to their many proposed functions in skeletogenesis and hematopoiesis. Specifically, we discuss how HSPGs play predominant roles in establishing and regulating niches during skeleto-hematopoietic development by participating in distinct developmental processes such as patterning, compartmentalization, growth, differentiation, and maintenance of tissues. Special emphasis is placed on our novel hypothesis that mechanistically links endochondral skeletogenesis to the establishment of the hematopoietic stem cell (HSC) niche in the marrow. HSPGs may contribute to these developmental processes through their unique abilities to establish and mediate morphogen, growth factor, and cytokine gradients; facilitate signaling; provide structural stability to tissues; and act as molecular filters and barriers.

Collagen fibril flow and tissue translocation coupled to fibroblast migration in 3D collagen matrices

In nested collagen matrices, human fibroblasts migrate from cell-containing dermal equivalents into surrounding cell-free outer matrices. Time-lapse microscopy showed that in addition to cell migration, collagen fibril flow occurred in the outer matrix toward the interface with the dermal equivalent. Features of this flow suggested that it depends on the same cell motile machinery that normally results in cell migration. Collagen fibril flow was capable of producing large-scale tissue translocation as shown by closure of a approximately 1-mm gap between paired dermal equivalents in floating, nested collagen matrices. Our findings demonstrate that when fibroblasts interact with collagen matrices, tractional force exerted by the cells can couple to matrix translocation as well as to cell migration.

Expression of the endocannabinoid system in fibroblasts and myofascial tissues

The endocannabinoid (eCB) system, like the better-known endorphin system, consists of cell membrane receptors, endogenous ligands and ligand-metabolizing enzymes. Two cannabinoid receptors are known: CB(1) is principally located in the nervous system, whereas CB(2) is primarily associated with the immune system. Two eCB ligands, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), are mimicked by cannabis plant compounds. The first purpose of this paper was to review the eCB system in detail, highlighting aspects of interest to bodyworkers, especially eCB modulation of pain and inflammation. Evidence suggests the eCB system may help resolve myofascial trigger points and relieve symptoms of fibromyalgia. However, expression of the eCB system in myofascial tissues has not been established. The second purpose of this paper was to investigate the eCB system in fibroblasts and other fascia-related cells. The investigation used a bioinformatics approach, obtaining microarray data via the GEO database (www.ncbi.nlm.nih.gov/geo/). GEO data mining revealed that fibroblasts, myofibroblasts, chondrocytes and synoviocytes expressed CB(1), CB(2) and eCB ligand-metabolizing enzymes. Fibroblast CB(1) levels nearly equalled levels expressed by adipocytes. CB(1) levels upregulated after exposure to inflammatory cytokines and equiaxial stretching of fibroblasts. The eCB system affects fibroblast remodeling through lipid rafts associated with focal adhesions and dampens cartilage destruction by decreasing fibroblast-secreted metalloproteinase enzymes. In conclusion, the eCB system helps shape biodynamic embryological development, diminishes nociception and pain, reduces inflammation in myofascial tissues and plays a role in fascial reorganization. Practitioners wield several tools that upregulate eCB activity, including myofascial manipulation, diet and lifestyle modifications, and pharmaceutical approaches.

Biology of developmental and regenerative skeletogenesis

Embryonic skeletal development involves the recruitment, commitment, differentiation, and maturation of mesenchymal cells into those in the skeletal tissue lineage, specifically cartilage and bone along the intramembranous and endochondral ossification pathways. The exquisite control of skeletal development is regulated at the level of gene transcription, cellular signaling, cell-cell and cell-matrix interactions, as well as systemic modulation. Mediators include transcription factors, growth factors, cytokines, metabolites, hormones, and environmentally derived influences. Understanding the mechanisms underlying developmental skeletogenesis is crucial to harnessing the inherent regenerative potential of skeletal tissues for wound healing and repair, as well as for functional skeletal tissue engineering. In this review, a number of key issues are discussed concerning the current and future challenges of the scientific investigation of developmental skeletogenesis in the embryo, specifically limb cartilage development, and how these challenges relate to regenerative or reparative skeletogenesis in the adult. Specifically, a more complete understanding the biology of skeletogenic progenitor cells and the cellular and molecular mechanisms governing tissue patterning and morphogenesis should greatly facilitate the development of regenerative approaches to cartilage repair.

Phase transformations in a model mesenchymal tissue

Connective tissues, the most abundant tissue type of the mature mammalian body, consist of cells suspended in complex microenvironments known as extracellular matrices (ECMs). In the immature connective tissues (mesenchymes) encountered in developmental biology and tissue engineering applications, the ECMs contain varying amounts of randomly arranged fibers, and the physical state of the ECM changes as the fibers secreted by the cells undergo fibril and fiber assembly and organize into networks. In vitro composites consisting of assembling solutions of type I collagen, containing suspended polystyrene latex beads ( approximately 6 microm in diameter) with collagen-binding surface properties, provide a simplified model for certain physical aspects of developing mesenchymes. In particular, assembly-dependent topological (i.e., connectivity) transitions within the ECM could change a tissue from one in which cell-sized particles (e.g., latex beads or cells) are mechanically unlinked to one in which the particles are part of a mechanical continuum. Any particle-induced alterations in fiber organization would imply that cells could similarly establish physically distinct microdomains within tissues. Here we show that the presence of beads above a critical number density accelerates the sol-gel transition that takes place during the assembly of collagen into a globally interconnected network of fibers. The presence of this suprathreshold number of beads also dramatically changes the viscoelastic properties of the collagen matrix, but only when the initial concentration of soluble collagen is itself above a critical value. Our studies provide a starting point for the analysis of phase transformations of more complex biomaterials including developing and healing tissues as well as tissue substitutes containing living cells.

Borrelia burgdorferi binds to, invades, and colonizes native type I collagen lattices

Borrelia burgdorferi binds strongly to the extracellular matrix and cells of the connective tissue, a binding apparently mediated by specific proteins and proteoglycans. We investigated the interactions between B. burgdorferi cells and intact type I collagen using hydrated lattices that reproduce features of in vivo collagen matrices. B. burgdorferi cells of several strains adhered avidly to these acellular matrices by a mechanism that was not mediated by decorin or other proteoglycans. Moreover, following adhesion to these matrices, B. burgdorferi grew and formed microcolonies. The collagen used in these studies was confirmed to lack decorin by immunoblot analysis; B. burgdorferi cells lacking the decorin adhesin bound readily to intact collagen matrices. B. burgdorferi also bound to collagen lattices that incorporated enzymes that degraded glycosaminoglycan chains in any residual proteoglycans. Binding of the bacteria to intact collagen was nonetheless specific, as bacteria did not bind agar and showed only minimal binding to bovine serum albumin, gelatin, pepsinized type I collagen, and intact collagen that had been misassembled under nonphysiological pH and ionic-strength conditions. Proteinase K treatment of B. burgdorferi cells decreased the binding, as did a lack of flagella, suggesting that surface-exposed proteins and motility may be involved in the ability of B. burgdorferi to interact with intact collagen matrices. The high efficiency of binding of B. burgdorferi strains to intact collagen matrices permits replacement of the commonly used isotopic binding assay with visual fluorescent microscopic assays and will facilitate future studies of these interactions.

Assembly of collagen matrices as a phase transition revealed by structural and rheologic studies

We have studied the structural and viscoelastic properties of assembling networks of the extracellular matrix protein type-I collagen by means of phase contrast microscopy and rotating disk rheometry. The initial stage of the assembly is a nucleation process of collagen monomers associating to randomly distributed branched clusters with extensions of several microns. Eventually a sol-gel transition takes place, which is due to the interconnection of these clusters. We analyzed this transition in terms of percolation theory. The viscoelastic parameters (storage modulus G' and loss modulus G") were measured as a function of time for five different frequencies ranging from omega = 0.2 rad/s to 6.9 rad/s. We found that at the gel point both G' and G" obey a scaling law, with the critical exponent Delta = 0.7 and a critical loss angle being independent of frequency as predicted by percolation theory. Gelation of collagen thus represents a second order phase transition.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

Type X collagen and other up-regulated components of the avian hypertrophic cartilage program

Elucidating the cellular and molecular processes involved in growth and remodeling of skeletal elements is important for our understanding of congenital limb deformities. These processes can be advantageously studied in the epiphyseal growth zone, the region in which all of the increase in length of a developing long bone is achieved. Here, young chondrocytes divide, mature, become hypertrophic, and ultimately are removed. During cartilage hypertrophy, a number of changes occur, including the acquisition of synthesis of new components, the most studied being type X collagen. In this review, which is based largely on our own work, we will first examine the structure and properties of the type X collagen molecule. We then will describe the supramolecular forms into which the molecule becomes assembled within tissues, and how this changes its physical properties, such as thermal stability. Certain of these studies involve a novel, immunohistochemical approach that utilizes an antitype X collagen monoclonal antibody that detects the native conformation of the molecule. We describe the developmental acquisition of the molecule, and its transcriptional regulation as deduced by in vivo footprinting, transient transfection, and gel-shift assays. We provide evidence that the molecule has unique diffusion and regulatory properties that combine to alter the hypertrophic cartilage matrix. These conclusions are derived from an in vitro system in which exogenously added type X collagen moves rapidly through the cartilage matrix and subsequently produces certain changes mimicking ones that have been shown normally to occur in vivo. These include altering the cartilage collagen fibrils and effecting changes in proteoglycans. Last, we describe the subtractive hybridization, isolation, and characterization of other genes up-regulated during cartilage hypertrophy, with specific emphasis on one of these--transglutaminase.

The region encoded by the alternatively spliced exon IIIA in mesenchymal fibronectin appears essential for chondrogenesis at the level of cellular condensation

Fibronectin in the extracellular matrix of tissues acts as a substrate for cell adhesion and migration during development. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. Fibronectin mRNA alternative splicing patterns change from B+A+V+ to B+A-V+ during chondrogenesis. In this report, immunohistochemical analysis demonstrates that while fibronectin protein containing the region encoded by exon IIIB is present throughout the limb at all stages of development, fibronectin protein containing the region encoded by exon IIIA disappears from cartilaginous regions just after condensation in vivo and in high-density mesenchymal micromass cultures in vitro. Treatment of mesenchymal micromass cultures prior to condensation with an antibody specific for the region encoded by exon IIIA disrupts the formation of cellular condensations and inhibits subsequent chondrogenesis in a dose- and time-dependent manner. Furthermore, microinjection of the exon IIIA antibody into embryonic chick limb primordia in vivo results in malformations characterized by smaller limbs and loss of limb skeletal elements. These results strongly suggest that the presence of the region encoded by exon IIIA in mesenchymal fibronectin is necessary for the condensation event that occurs during chondrogenesis.

Interaction of the NH2-terminal domain of fibronectin with heparin. Role of the omega-loops of the type I modules

Determinants of the interaction of the 29-kDa NH2-terminal domain of fibronectin with heparin were explored by analysis of normal and mutant recombinant NH2-terminal fibronectin fragments produced in an insect cell Baculovirus host vector system. A genomic/cDNA clone was constructed that specified a secretable human fibronectin NH2 fragment. With the use of site-directed mutagenesis a set of 29 kDa fragments was obtained that contained glycine or glutamic acid residues in place of basic residues at various candidate sites for heparin binding in the five type I modules that make up the domain. The recombinant fragment containing the wild type sequence had a nearly normal circular dichroic spectra and a melting profile, as assayed by loss of ellipticity at 228 nm, that was indistinguishable from that of the native fragment obtained by trypsinization of plasma fibronectin. A substantial proportion of the wild type recombinant fragment bound to heparin-Sepharose, where it was eluted at the same NaCl concentration as the native fragment. The wild type fragment was capable of promoting matrix-driven translocation, a morphogenetic effect in artificial extracellular matrices that depends on the interaction of the fibronectin NH2 terminus with heparin-like molecules on the surfaces of particles. Mutant fragments in which arginines predicted to be most exposed in the folded fragment were converted to glycines retained the same affinity for heparin as the wild type fragment. In contrast, a mutant fragment in which the single basic residue (Arg99) in the minor loop ("Omega-loop") of the second type I module was converted to a glycine had an essentially normal melting profile but exhibited no binding to heparin and failed to promote matrix-driven translocation. A mutant fragment in which the single basic residue (Arg52) of the first type I module was converted to a glycine also completely lacked heparin binding activity, but one in which the single basic residue (Arg191) the fourth type I module was converted to a glycine retained the ability to bind heparin. A mutant fragment in which the single basic residue (Lys143) in the Omega-loop of the third type I module was converted to a glutamic acid lacked heparin binding activity but had a CD spectrum similar to the heparin-liganded native protein and was capable of promoting matrix-driven translocation. The results indicate that multiple residues in the Omega-loops of the fibronectin NH2-terminal domain participate in its interactions with heparin. In addition, the conformation of one of the nonbinding mutants may mimic the heparin-induced structural alteration in this fibronectin domain required for certain morphogenetic events.

Viscosity and elasticity during collagen assembly in vitro: relevance to matrix-driven translocation

In order to better understand the gelation process associated with collagen assembly, and the mechanism of the in vitro morphogenetic phenomenon of "matrix-driven translocation" [S.A. Newman et al. (1985) Science, 228, 885-889], the viscosity and elastic modulus of assembling collagen matrices in the presence and absence of polystyrene latex beads was investigated. Viscosity measurements at very low shear rates (0.016-0.0549 s(-1)) were performed over a range of temperatures (6.9-11.5 degrees C) in a Couette viscometer. A magnetic levitation sphere rheometer was used to measure the shear elastic modulus of the assembling matrices during the late phase of the gelation process. Gelation was detected by the rapid increase in viscosity that occurred after a lag time tL that varied between O and approximately 500 s. After a rise in viscosity that occurred over an additional approximately 500 s, the collagen matrix was characterized by an elastic modulus of the order of several Pa. The lag time of the assembly process was relatively insensitive to differences in shear rate within the variability of the sample preparation, but was inversely proportional to the time the sample spent on ice before being raised to the test temperature, for test temperatures > 9 degrees C. This suggests that structures important for fibrillogenesis are capable of forming at 0 degrees C. The time dependence of the gelation process is well-described by an exponential law with a rate constant K approximately 0.1 s(-1). Significantly, K was consistently larger in collagen preparations that contained cell-sized polystyrene beads. From these results, along with prior information on effective surface tension differences of bead-containing and bead-lacking collagen matrices, we conclude that changes in matrix organization contributing to matrix-driven translocation are initiated during the lag phase of fibrillogenesis when the viscosity is < or = 0.1 Poise. The phenomenon may make use of small differentials in viscosity and/or elasticity, resulting from the interaction of the beads with the assembling matrix. These properties are well described by standard models of concentrated solutions.

Epithelial morphogenesis in mouse embryonic submandibular gland: its relationships to the tissue organization of epithelium and mesenchyme

Epithelial tissues in various organ rudiments undergo extensive shape changes during their development. The processes of epithelial shape change are controlled by tissue interactions with the surrounding mesenchyme which is kept in direct contact with the epithelium. One of the organs which has been extensively studied is the mouse embryonic submandibular gland, whose epithelium shows the characteristic branching morphogenesis beginning with the formation of narrow and deep clefts as well as changes in tissue organization. Various molecules in the mesenchyme, including growth factors and extracellular matrix components, affect changes of epithelial shape and tissue organization. Also, mesenchymal tissue exhibits dynamic properties such as directional movements in groups and rearrangement of collagen fibers coupled with force-generation by mesenchymal cells. The epithelium, during early branching morphogenesis, makes a cell mass where cell-cell adhesion systems are less developed. Such properties of both the mesenchyme and epithelium are significant for considering how clefts, which first appear as unstable tiny indentations on epithelial surfaces, are formed and stabilized.

Fibronectin mRNA alternative splicing is temporally and spatially regulated during chondrogenesis in vivo and in vitro

Fibronectin, a component of the extracellular matrix in a variety of tissues, participates in many critical cellular processes, including differentiation, adhesion, and migration. A positive correlation exists between the presence of fibronectin and the onset of chondrogenesis, the differentiation of mesenchyme into cartilage. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. We have previously shown that the fibronectin mRNA splicing patterns change during chondrogenesis (Bennett et al. [1991] J. Biol. Chem, 266:5918-5924). All of the fibronectin mRNAs from prechondrogenic chick limb mesenchyme contain exons IIIB, IIIA, and V (B + A + V +), whereas all of the fibronectin mRNAs from chick cartilage contain exons IIIB and V but do not contain exon IIIA (B + A - V +). In this study, we show that fibronectin mRNAs containing exon IIIA (FN-A) and/or the mRNAs containing exon IIIB (FN-B) are expressed in a specific and different spatiotemporal manner in the developing chick limb in vivo, as well as in limb mesenchymal cells undergoing chondrogenesis in vitro. Specifically, in situ hybridization reveals that FN-B mRNAs are present throughout the various stages (HH 20-30) of limb cartilage development in vivo, whereas FN-A mRNAs disappear following the condensation phase of chondrogenesis and absent from the resulting cartilage, Chick limb cartilage fibronectin mRNAs are therefore B + A-, as in other embryonic cartilage tissues. Furthermore, limb mesenchymal cells undergoing chondrogenesis in vitro lose FN-A mRNAs immediately following condensation, recapitulating the events that occur during chondrogenesis in vivo. These results suggest an important role for fibronectin mRNA alternative splicing during chondrogenic differentiation.

Migration of myogenic cells from the somites to the fore-limb buds of developing mouse embryos

In this study, we have isolated newly formed somites from the caudal regions of 8.5 day mouse embryos and transplanted them orthotopically into correspondingly staged hosts at the level of the prospective limb-forming region. The experimental embryos were then cultured intact for 32-36 hr. The donor somites used were pre-labelled with DiI, a fluorescent lipophilic dye, or were obtained from transgenic embryos that carried a 1 kb 5' regulatory sequence of the desmin gene linked to the gene encoding Escherichia coli beta-galactosidase. The transgene is specifically expressed in skeletal muscles (Li et al. [1993] Development 117:947-959). The aim of these experiments was to show definitively that the musculature of the mammalian limb is derived from the somites. The results demonstrated that DiI-labelled cells from the implanted somites were able to invade the proximal region of the fore-limb bud during the course of development. The use of transgenic somites as grafts confirmed that some of the somitic cells found in the limbs were myogenic cells. To determine whether the displacement of somitic cells is an active or passive process, somatopleure obtained from the prospective limb-forming regions of day 8.5 day embryos was implanted into 8.5 day hosts. We did not detect the presence of DiI-labelled somatopleural cells in the fore-limb after 32-36 hr of culture. This suggests that somitic cells reached the limb bud via active locomotion rather than as a result of being passively dragged there, as the limb elongates during development. In addition, we injected latex beads into the somites, as probes, to determine whether extracellular matrix-driven translocation plays a role in driving the somitic cells to the limb bud. In a majority of the specimens examined, we could not detect the presence of these beads in the limb bud. However, in the trunk of these embryos, the beads were found dispersed throughout the ventral neural crest pathway.

Spatiotemporal profile of N-cadherin expression in the developing limb mesenchyme

During embryonic limb development, chondrogenesis of the mesenchyme is preceded by a crucial cellular condensation phase. Because condensation is likely to result from specific cell-cell interactions, we have examined the possible involvement of N-cadherin, a Ca(2+)-dependent cell adhesion molecule, in this condensation event. Previously, we have reported that N-cadherin is expressed in chick embryonic limb bud, and that perturbation of N-cadherin-mediated cell adhesion significantly inhibits limb mesenchymal cellular aggregation and chondrogenesis both in vivo and in vitro. To further examine the relationship between N-cadherin expression and chondrogenesis, we have quantified and characterized the spatiotemporal expression of N-cadherin mRNA and protein in the developing chick embryonic limb, and analyzed the mechanism whereby exogenous Ca2+ stimulates chondrogenesis in vitro. Immunohistochemistry revealed that N-cadherin is expressed on mesenchymal cell surface in a developmentally specific manner during limb bud maturation. N-Cadherin was detected in the mesenchyme of the central core of the early limb bud, and was maximally expressed between stages 24 to 26, corresponding to the period of cellular condensation in vivo. Mature cartilage did not express N-cadherin. In chick limb mesenchyme micromass cultures in vitro, N-cadherin protein expression was seen associated with distinct cellular aggregates of condensing mesenchyme, but not in mature cartilaginous nodules or the mesenchyme situated between the condensing aggregates. In situ hybridization localized N-cadherin mRNA expression in condensing regions of limb mesenchyme in a stage 25/26 limb bud; at later stages, N-cadherin mRNA expression was seen in the perichondrium and the dense mesenchyme, but not in mature cartilage. The agreement between the mRNA and protein data thus suggests that N-cadherin expression in the developing limb bud is regulated at the transcriptional level. Western immunoblot analysis further confirmed that, from stages 19 to 36, the level of N-cadherin in the limb bud increased 5-fold through stage 25/26, then decreased during the progression of chondrogenesis and osteogenesis. A parallel profile was also seen in micromass limb mesenchyme cultures in vitro. The importance of N-cadherin for Ca2+ mediated mesenchymal cell aggregation and chondrogenesis was examined by selective, proteolytic dissociation of the mesenchymal cells with retention or removal of N-cadherin. Cells with N-cadherin were shown to exhibit Ca(2+)-dependent aggregation and were consistently more chondrogenic than those without N-cadherin. These results provide strong support for the functional role of N-cadherin in chondrogenesis.

Expression and functional involvement of N-cadherin in embryonic limb chondrogenesis

Cell adhesion molecules have been shown to be important mediators of morphogenesis and pattern formation. In this study, we have shown that N-cadherin is expressed in a specific spatiotemporal manner in the developing limb bud during chondrogenesis in vivo and in cultured limb mesenchyme in vitro. The time period of maximal expression of N-cadherin corresponds to the period of active cellular condensation, an event believed to be a necessary prerequisite for chondrogenic differentiation. To directly assess the functional involvement of N-cadherin in cellular condensation, we have examined the effects of perturbing N-cadherin activity on both cell aggregation and chondrogenesis using NCD-2, a rat monoclonal antibody directed against the binding region of N-cadherin. Non-immune rat IgG was used as a control. Our results show that functional N-cadherin is necessary for chondrogenesis to proceed both in vivo and in vitro. Limb mesenchymal cells exhibited characteristic Ca(2+)-dependent cell aggregation in suspension, which was inhibited in the presence of exogenous NCD-2. In micromass cultures of limb mesenchymal cells, NCD-2 inhibited overt chondrogenesis in a dose-dependent manner. Furthermore, NCD-2 inhibition of chondrogenesis in micromass cultures was time-dependent, suggesting that N-cadherin is crucially involved during the latter half of the first 24 hours of culture, a time period most likely corresponding to active cellular condensation. NCD-2 also significantly influenced limb development when injected into embryonic limb buds in vivo. In addition to significant inhibition of chondrogenesis and developmental delays, gross developmental deformities and perturbation of overall pattern formation were also observed. Taken together, these results demonstrate that N-cadherin is functionally required in mediating the cell-cell interactions among mesenchymal cells important for chondrogenesis in micromass culture in vitro and in the intact limb bud in vivo.

Proteoglycans and the modulation of cell adhesion by steric exclusion

The hypothesis that cell aggregation may be driven by linear polymers in the matrix, particularly glycosaminoglycans, is revisited in light of more recent evidence. A model is proposed that extends the concept of steric exclusion to include a role in determining the directionality of cell migration and neurite extension. Recent literature is reviewed to support the conclusion that in living tissues the theoretical conditions for driving aggregation and migration by steric exclusion are met. The ability of a linear polymer to exclude cells is a function of its viscosity, which is optimum with glycosaminoglycans similar to chondroitin sulfate. It is ineffective with low viscosity glycosaminoglycans such as most heparin or heparan sulfate. Hyaluronic acid, a massive polymer, excludes cells poorly when present as an open matrix gel but forms an effective exclusion barrier when attached to the cell surface. According to a model for steric exclusion in organogenesis, when cells have a glycocalyx of linear polymer, they should disperse and migrate down a viscosity gradient of excluding matrix polymer; when they shed or internalize their surface coat in the continued presence of matrix, they should be excluded into a smaller volume and thus stimulated to aggregate.

Effect of wound healing and tissue transplantation on the navigation of axons in organ-cultured embryonic chick eyes

Wound closure and repair of embryonic neuroepithelium were studied in organ-cultured embryonic retinae. Eyes from 3 to 4-day-old embryos were cultured after removing pieces of retinal tissue. During the subsequent 24 hours of incubation, the 150 to 200 microns wide holes in the retina closed completely. Histological studies showed that the wound closure was not accomplished by cell migration or cell proliferation, but by an approximation of the wound edges mediated by extracellular matrix fibrils of the vitreous body. The wound contraction facilitated the integration of transplants into the retinal neuroepithelium with a perfect alignment of the implants with the host at the vitreal surface. Within 24 hours, a continuous inner limiting membrane between transplant and host retina was established. The effect of wound healing and tissue transplantation on the navigation of optic axons in the retina was investigated. The wound contraction in the retina caused the optic axons near the lesion site to grow to the wound center, where the axons traversed the retina and formed a neuroma at the ventricular side, resembling the organization of axons at the optic disc. In the transplantation paradigm, axons from the host retina migrated into the transplant and vice versa. However, due to the wound contraction around the transplant, most axons grew into the interface between the transplant and host tissue.

Human ovarian surface epithelial cells are capable of physically restructuring extracellular matrix

OBJECTIVE

After ovulation the human ovarian surface epithelium proliferates at the wound edges, migrates over the ovulatory defect, and contributes to its repair primarily by the action of proteolytic enzymes and by the deposition of new matrix material. We examined the potential for human ovarian surface epithelial cells to physically remodel extracellular matrix in culture, similar to collagen gel lattice contraction by fibroblasts, a well-known culture model for wound repair, as an additional role of human ovarian surface epithelium in wound repair.

STUDY DESIGN

Human ovarian surface epithelium cells from ovarian biopsies of 11 patients were grown in culture and plated onto a combination of collagen gel and rat ovarian surface epithelial-derived extracellular matrix. The degree of matrix contraction was measured as the percentage of the original culture diameter.

RESULTS

Human ovarian surface epithelial cells surrounded and contracted the combination of matrices into a dense matrix organoid. The degree of organoid contraction was related to the number of human ovarian surface epithelial cells plated per organoid and to the inclusion of fibroblasts within the collagen gel but was not affected either by adding epidermal growth factor and hydrocortisone to the culture medium or by reducing the serum component of the medium.

CONCLUSION

Human ovarian surface epithelial organoids may be useful for the study of normal and abnormal ovarian events such as ovulatory wound repair and cyst formation.

Roles for the extracellular matrix in plant development and pollination: a special case of cell movement in plants

Pattern formation in plants is now thought to be primarily dependent on positional information during development. We discuss the prevalent theories on how position is deciphered by cells in an organism and highlight the recent advances implicating molecules of the cell wall or extracellular matrix (ECM) in this process. We compare the functions of the ECM in plants and animals and describe the various cell and substrate adhesion molecules of the animal ECM which play a role in morphogenesis and cell movement. We propose that analogous molecules may occur in plants and provide evidence for the presence of a substrate adhesion molecule like vitronectin in plants and algae. We provide a model for how substrate adhesion molecules may be involved in a special case of cell movement in plants, pollination.

In vitro particulate adherence to fibronectin: correlation with in vivo particulate adherence to sites of bladder injury

This study examined the role of fibronectin in promoting particulate attachment to sites of urothelial injury. Variables influencing adherence of the rat transitional carcinoma cell line 4909 and "non-cellular" styrene-divinylbenzene microspheres to fibronectin were studied in an in vitro system. A soluble synthetic peptide fragment (Gly-Arg-Gly-Asp-Ser [GRGDS]) duplicating the receptor binding domain of fibronectin (RGD) was used to determine whether cell adherence could be inhibited by fibronectin receptor blockade. In vitro findings were correlated with an in vivo assay of both cellular and non-cellular particulate adherence to injured urothelium. Time, plated cell density, substrate concentration, GRGDS concentration, and cell viability, were all found to be significant independent variables influencing in vitro cellular adherence (p less than 0.0001). Receptor blockade with GRGDS significantly decreased in vitro tumor cell adherence to fibronectin. In vitro microsphere binding increased as a direct function of fibronectin concentration but was not time dependent (p less than 0.0001 and p = 0.14 for fibronectin concentration and time respectively). The in vivo adherence of both tumor cells and microspheres was significantly increased in injured bladders compared to controls (p less than 0.01). Receptor blockade with GRGDS failed to inhibit in vivo cell adherence to sites of urothelial injury. Microspheres proved to be competitive inhibitors of cellular adherence in competitive binding assays. In vitro microsphere binding demonstrated a pH dependence with maximal binding at pH 7.2. These data suggest that in vitro tumor cell adherence to fibronectin differs from in vivo tumor cell adherence to sites of urothelial injury. Manipulations which inhibit in vitro adherence, specifically fibronectin receptor blockade and cell death, fail to effect in vivo binding to the extreme that non-cellular particulate appears to bind to the same site, and with similar affinity, as cellular particles.

Organelle rearrangement and cell volume changes during squeezing invasion of peritoneal elastic lamina by targeted murine breast carcinoma cells

Murine breast cancer cell lines were developed to selectively invade the peritoneum while they proliferated in ascites form in the abdominal cavity. In a dominant form of invasion, tumor cells showed special affinity for elastin fibers and squeezed through narrow gaps in the elastic fiber meshwork of the stroma. Even in fixed tissue, such cells could be recognized as being in the process of invasive migration because of their dumbbell shape. This appearance was similar to that of diapedetic blood cells traversing bone marrow sinus endothelium. Three-dimensional STERECON graphics reconstruction from serial thick sections of 44 such cells was carried out. The reconstructions showed that, in mid-penetration, the cells spread extensively over the exterior surface of the elastic fiber meshwork. The cell surface contact of these forward projections was mainly with the elastic fiber outer coat of microfibrils, but small areas of the cell surface also fused directly to inner-core elastin. The morphological rearrangement of the cytoskeleton was minimal in both types of attachment areas. The location of these forward facing attachments is consistent with mechanisms for pulling the invasive cell through the gap. Lamellopodia formation and clustering of cytoplasmic organelles occurred more commonly at the forward-facing part of the cell. Morphometry of the reconstructions showed that a contraction of the whole cell occurred during the squeezing/migration process suggestive of an additional pushing process. However, our invasive cell lines showed marked differences in the degree of cell shrinkage. The process of adhesion and squeezing of tumor cells through elastin meshworks in vivo is clearly a complex phenomenon. Changes in cell surface activity appear to play a significant role in establishing the necessary 'foothold' component of invasion and, possibly, in the generation of tractive force as well.