The culture of chick embryo mesoderm cells in hydrated collagen gels

Engineered Biomaterial Platforms to Study Fibrosis

Many pathologic conditions lead to the development of tissue scarring and fibrosis, which are characterized by the accumulation of abnormal extracellular matrix (ECM) and changes in tissue mechanical properties. Cells within fibrotic tissues are exposed to dynamic microenvironments that may promote or prolong fibrosis, which makes it difficult to treat. Biomaterials have proved indispensable to better understand how cells sense their extracellular environment and are now being employed to study fibrosis in many tissues. As mechanical testing of tissues becomes more routine and biomaterial tools become more advanced, the impact of biophysical factors in fibrosis are beginning to be understood. Herein, fibrosis from a materials perspective is reviewed, including the role and mechanical properties of ECM components, the spatiotemporal mechanical changes that occur during fibrosis, current biomaterial systems to study fibrosis, and emerging biomaterial systems and tools that can further the understanding of fibrosis initiation and progression. This review concludes by highlighting considerations in promoting wide-spread use of biomaterials for fibrosis investigations and by suggesting future in vivo studies that it is hoped will inspire the development of even more advanced biomaterial systems.

Collagen gel analysis of epithelial-mesenchymal transition in the embryo heart: an in vitro model system for the analysis of tissue interaction, signal transduction, and environmental effects

The cellular process of epithelial-mesenchymal cell transition (EMT) is a critical event in development that is reiterated in adult pathologies of metastasis and organ fibrosis. An initial understanding of the cellular and molecular events of this process emerged from an in vitro examination of heart valve development. Explants of the chick atrioventricular valve-forming region were placed on collagen gels and removed to show that EMT was regulated by a tissue interaction. Subsequent studies showed that specific TGFβ isoforms and receptors were required and steps of activation and invasion could be distinguished. The assay was modified for mouse hearts and has been used to explore signal transduction and gene expression in both species. The principle advantages of the system are a defined temporal window, when EMT takes place and the ability to isolate cells at various stages of the EMT process. These advantages are largely unavailable in other developmental or adult models. As the mesenchymal cells produced by EMT in the heart are involved in defects found in congenital heart disease, there is also a direct relevance of cardiac EMT to human birth defects. This relationship has been explored in relation to environmental exposures and in a number of genetic models. This review provides both an overview of the findings developed from the assay and protocols to enable the use of the assay by other laboratories. The assay provides a versatile platform to explore roles of specific gene products, drugs, and environmental agents on a critical cellular process.

Biocompatibility and biodegradation of cross-linked gelatin/hyaluronic acid sponge in rat subcutaneous tissue

A gelatin/hyaluronic acid (GH) sponge has been fabricated by freeze-drying and cross-linking. The GH sponge was insoluble when cross-linked with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. The morphologies of sponges were investigated using a field emission scanning electron microscope. The porosity of the GH sponge increased with hyaluronic acid content. The GH sponge was biodegradable, as evidenced by implantation in Wistar rat subcutaneous connective tissue. Fibroblasts infiltrated into the sponge matrix, and regenerated collagen in the matrix to a level of 25% by 15 days after surgery. The GH73 sponge induced an acute inflammatory response compared with the GH91 sponge. This inflammatory response could have been stimulated by the presence of hyaluronic acid up to Day 10, as it decreased afterwards. The C-reactive protein of blood samples also indicated the same result. The blood tests and histological results show that GH sponges have good biocompatibility and low antigenicity for tissue engineering scaffolds.

Retinal growth hormone is an anti-apoptotic factor in embryonic retinal ganglion cell differentiation

Cells of the neural retina in the chick embryo undergo several waves of apoptosis during development, including peaks at approximately embryonic day (ED) 7 and 12. Prominent among the cells involved in these phases of cell death are the retinal ganglion cells (RGCs). We have previously shown that growth hormone (GH) is expressed in the neural retina, and particularly, in the RGCs. Here we study the ability of GH to rescue retinal cells from apoptosis, both in vitro and in vivo. When retinas from embryos at ED 6-8 are explanted on collagen gels, the application of recombinant GH, at 10(-6)m, significantly reduced the incidence of apoptotic cells in the cultures as judged by terminal deoxynucleotide transferase-mediated dUTP-biotin nick end labelling (TUNEL). GH was delivered to neural retinas in ovo, by microinjection into the eye cup at ED 2. When these embryos were examined at ED 6-8, no reduction in cell death was observed below the normal low control levels. However, when antibodies to GH were microinjected, the incidence of cell death increased significantly at ED 6, providing evidence that in vivo immunoneutralization of endogenous GH results in triggering of apoptotic signaling pathways. Evidence that RGCs are a particular target of this neuroprotective effect of GH was provided by examination of cultures enriched for RGCs by immunopanning. In serum-free culture, RGCs, identified by anti-Islet 1 immunolabelling, were found to be susceptible to the effect of GH immunoneutralization, which approximately quadrupled the incidence of apoptosis in the cultures. We propose that GH is a naturally occurring autocrine and/or paracrine neuroprotective agent in the developing retina which is involved in the regulation of retinal cell numbers during early embryogenesis.

Microinjection of antifibronectin antibodies in the chicken blastoderm: inhibition of mesoblast cell migration but not of cell ingression at the primitive streak

The involvement of fibronectin in adhesion and migration of individual mesoblast cells during chicken gastrulation was examined after microinjection of functional and nonfunctional antifibronectin antibodies in the blastoderm during the period of rapid migration of mesoblast cells. The injection of affinity-purified polyclonal antihuman fibronectin antibody (total IgG or Fab fragment) or of monoclonal antichicken cellular fibronectin caused a thickening of the primitive streak, which was composed of loosely connected cells. This effect was most evident at the level of Hensen's node, and very few mesoblast cells were observed migrating in the space between upper layer and deep layer. The obvious explanation of this effect was that the de-epithelialization of upper layer cells persisted in the presence of antibodies, but ingressed cells failed to emigrate from the primitive streak. Immunostaining of microinjected antibodies showed binding to the basement membrane, to the cell surface of mesoblast cells that had migrated before microinjection occurred, and to the cell surface of deep layer cells. Cells that ingressed and detached in the course of reincubation of the embryo possessed little immunolabelling along their cell surface. The results suggest that the failure of ingressed cells to emigrate from the primitive streak and to form mesoblast was due (1) to alterations in adhesion between newly ingressed primitive streak cells, which had the ability to detach but possessed relatively little fibronectin along their cell surfaces and a small number of cell protrusions, and (2) probably to a lack of adhesion of detached cells to the basement membrane, which was blocked by the presence of antifibronectin antibodies. We conclude that the presence of fibronectin in the basement membrane is required for emigration of ingressed cells and migration of mesoblast cells to occur. Once migration has commenced, fibronectin is also deposited along the cell surface of migrating cells, a factor that may increase their mutual adhesion.

Extracellular matrix synthesis is required for the movement of sclerotome and neural crest cells on collagen

During early embryogenesis cells of several different populations disperse by active cell movement from one location to another. Preexisting extracellular materials are major determinants of these dispersal patterns, but the cells are also able to modify their substrata by synthesizing and secreting extracellular matrix molecules as they move. In order to determine the contribution made by these deposited materials, several tissues from the early chick embryo have been cultured in the presence of inhibitors of extracellular matrix synthesis and secretion. The tissues examined were sclerotome cells from differentiated somites and neural crest cells. For comparison, undifferentiated somites were also cultured. The movement of these cells was compared in type I collagen gel culture and in conventional culture on artificial substrata. Inhibitors of collagen synthesis were used (cis-hydroxy proline and L-azetidine-2-carboxylic acid) in addition to a proteoglycan inhibitor (p-nitrophenyl-xylopyranoside) and a secretion inhibitor (monensin). Results indicate that sclerotome cells require collagen synthesis for movement in a collagen matrix. Reversal of the effects of collagen inhibitors, by proline and type II collagen, suggest that sclerotome cells normally condition the type I matrix in order to move in it. Inhibition of proteoglycan synthesis produced the greatest effect on the movement of neural crest cells regardless of the substratum, confirming an important role for these molecules in the crest migratory routes. The attachment of all cells to collagen was highly sensitive to the presence of monensin, which is known to reduce the deposition of glycosaminoglycans and fibronectin. These results suggest that conditioning of the extracellular matrix by newly synthesized material is required for cell attachment and movement during early development.

Transferrin and iron requirements of embryonic mesoderm cells cultured in hydrated collagen matrices

Very early embryonic mesoderm cells were taken from the primitive streak-stage chick embryo and cultured in a matrix of type I collagen in the presence of serum. Previous work has shown that under these conditions cells do not leave the explant and move in the collagen in the absence of supplemented avian transferrin. Cells explanted onto tissue culture plastic in the presence of serum do not require this transferrin supplement. These observations were investigated further by culturing cells in collagen in the presence of the lipophilic iron chelator, ferric pyridoxal isonicotinoyl hydrazone (FePIH), which can replace transferrin as an iron-delivery agent. Under conditions in which FePIH could effectively stimulate chick embryo myoblast growth, no such long-term stimulation was obtained with the early mesoderm cells in collagen. This suggested that for mesoderm cells, FePIH could not replace transferrin. Antibody to the transferrin receptor and to transferrin itself inhibited growth of myoblasts in collagen and on plastic, and of mesoderm cells in collagen. Mesoderm cells on plastic, however, were refractory to the presence of the antibody directed to the receptor and seemed to show a low dependency on transferrin-delivered iron under these conditions, inasmuch as antiserum to transferrin itself only caused a partial inhibition of outgrowth. The results suggest that mesoderm cells in collagen require transferrin for both iron uptake and for another unspecified function. It is consistent with the results to propose that transferrin binding might modulate the cells' attachment to collagen, thus influencing outgrowth. The distribution of the actin cytoskeleton in mesoderm cells actively migrating in collagen, such as in the presence of transferrin, suggests a stronger attachment to the collagen than nonmigrating cells.

Fibroblast growth on a porous collagen sponge containing hyaluronic acid and fibronectin

We have previously shown that the presence of fibronectin (FN) and/or hyaluronic acid (HA) in a 3-dimensional type I collagen sponge enhances wound healing in vivo. In the present study the same material was used as a support for growth of fibroblasts in vitro. Using radiochemical techniques, scanning electron and light microscopy, the properties of fibroblasts cultured on the collagen sponge or on the sponge containing HA or FN have been compared with cultures grown on plastic dishes. Fibroblast replication and collagen synthesis were higher on plastic than on the collagen sponge. In the presence of HA or FN the entire thickness of the sponge was infiltrated by fibroblasts which rapidly replicated. The presence of HA or FN increased synthesis of collagen which was largely deposited around cells.

Tension in the culture dish: microfilament organization and migratory behavior of quail neural crest cells

We have investigated one aspect of the migratory behavior of quail neural crest (NC) cells by comparing the organization of microfilament bundles and the ability to distort migratory substrata by NC, somite, and notochord cells in vitro. In contrast to the numerous cytoplasmic stress fibers in somite-derived fibroblasts and notochord cells revealed by rhodamine-phalloidin staining and thin-section electron microscopy, microfilaments in NC cells are restricted to the cell cortex. To test the relative degrees of tension generated by these cell types on the underlying substratum, cells were cultured in collagen gels and on distortable silicone rubber sheets. Explanted somites and notochords produced dramatic radial alignment of 750 micrograms/ml collagen gels, whereas neural crest cells only aligned gels of lower concentrations. Fibroblasts did not migrate individually from explanted somites and notochords into 250 micrograms/ml collagen gels as readily as into higher concentration collagen lattices. In contrast, neural crest cells migrated into matrices of low concentration as well as into higher concentration collagen gels. Neural crest cells and their pigmented derivatives did not distort silicone rubber sheets, whereas somite and notochord-derived fibroblasts wrinkle this substratum after 4 days in culture. Thus, the differences in organization of the actin cytoskeleton reflect the tractional force exerted by these cells on their substratum. We hypothesize that the migratory behavior of NC cells in vivo may be related to their ability to translocate through embryonic extracellular matrices while generating relatively weak adhesions with the substratum, whereas the stronger forces generated by other embryonic cell types upon the delicate extracellular matrix may restrict their migration and may be associated with other morphogenetic events.

Analysis of the role of microfilaments and microtubules in acquisition of bipolarity and elongation of fibroblasts in hydrated collagen gels

Fibroblasts in situ reside within a collagenous stroma and are elongate and bipolar in shape. If isolated and grown on glass, they change from elongate to flat shape, lose filopodia, and acquire ruffles. This shape change can be reversed to resemble that in situ by suspending the cells in hydrated collagen gels. In this study of embryonic avian corneal fibroblasts grown in collagen gels, we describe for the first time the steps in the acquisition of the elongate shape and analyze the effect of cytoskeleton-disrupting drugs on filopodial activity, assumption of bipolarity, and cell elongation within extracellular matrix. We have previously shown by immunofluorescence that filopodia contain actin but not myosin and are free of organelles. The cell cortex is rich in actin and the cytosol, in myosin. By using antitubulin, we show in the present study that microtubules are aligned along the long axis of the bipolar cell body. The first step in assumption of the elongate shape is extension of filopodia by the round cells suspended in collagen, and this is not significantly affected by the drugs we used: taxol to stabilize microtubules; nocodazole to disassemble microtubules; and cytochalasin D to disrupt microfilaments. The second step, movement of filopodia to opposite ends of the cell, is disrupted by cytochalasin, but not by taxol or nocodazole. The third step, extension of pseudopodia and acquisition of bipolarity similarly requires intact actin, but not microtubules. If fibroblasts are allowed to become bipolar before drug treatment, moreover, they remain so in the presence of the drugs. To complete the fourth step, extensive elongation of the cell, both intact actin and microtubules are required. Retraction of the already elongated cell occurs on microtubule disruption, but retraction requires an intact actin cytoskeleton. We suggest that the cell interacts with surrounding collagen fibrils via its actin cytoskeleton to become bipolar in shape, and that microtubules interact with the actin cortex to bring about the final elongation of the fibroblast.

Morphology and behavior of quail neural crest cells in artificial three-dimensional extracellular matrices

Neural crest cells migrate extensively through a complex extracellular matrix (ECM) to sites of terminal differentiation. To determine what role the various components of the ECM may play in crest morphogenesis, quail (Coturnix coturnix japonica) neural crest cells have been cultured in three-dimensional hydrated collagen lattices containing various combinations of macromolecules known to be present in the crest migratory pathways. Neural crest cells migrate readily in native collagen gels whereas the cells are unable to use denatured collagen as a migratory substratum. The speed of movement decreases linearly as the concentration of collagen in the gel increases. Speed of movement of crest cells is stimulated in gels containing 10% fetal calf serum and chick embryo extract, 33 micrograms/ml fibronectin cell-binding fragments, 3 mg/ml chondroitin sulfate, or 3 mg/ml chondroitin sulfate proteoglycan when compared to rates of movement through collagen lattices alone. Low concentrations of hyaluronate (250-500 micrograms/ml) in a 750 micrograms/ml collagen gel do not alter rates of movement over collagen alone, but higher concentrations (4 mg/ml) greatly inhibit migration. Conversely, hyaluronate (250 micrograms/ml) significantly increases speed of movement if the crest cells are cultured in high concentration collagen gels (2.5 mg/ml), suggesting that hyaluronate is expanding spaces and consequently enhancing migration. The morphology and mode of movement of neural crest cells vary with the matrix in which they are grown and can be correlated with their speed of movement. Light and scanning electron microscopy reveal rounded, blebbing cells in matrices associated with slower translocation, whereas rounded cells with branching filopodia or lamellipodia are associated with rapid translocation. Bipolar cells with long processes are observed in cultures of rapidly moving cells that appear to be adhering strongly, as well as in cultures of cells that are stationary for long periods. These data, considered with the known distribution of macromolecules in the early embryo, suggest the following: (1) Both collagen and fibronectin can act as preferred substrata for migration. (2) Chondroitin sulfate and chondroitin sulfate proteoglycan increase speed of movement, but probably do so by decreasing adhesiveness and thereby producing more frequent detachment. In the embryo, crest cells would most likely avoid regions containing high concentrations of chondroitin sulfate. (3) Hyaluronate cannot act as a substratum for migration, but in low concentrations it can open spaces in the matrix and consequently may stimulate movement. The complex interactions of combined matr

Spreading of explants of embryonic chick mesenchymes and epithelia on fibronectin and laminin

Tissues from 2.5-day chick embryos were explanted onto glass coated with adsorbed fibronectin or laminin, or extracellular matrices (ECM) of deoxycholate-extracted chick embryo cells. Spreading of somitic and trunk neural-crest mesenchyme cells was equally rapid and extensive on fibronectin, laminin and on the fibronectin-rich, laminin-poor ECM produced by mesenchymal cells. No preference for fibronectin over laminin was displayed by these two mesenchymes when a choice of mutually exclusive alternating tracks was provided. Epithelial cells did not spread from explants of the neural tube on any substrate tested up to 24 h in vitro, but adhesion of the explant and outgrowth of axons was greatest on laminin. Explants of endodermal epithelium spread rapidly on or near ECM formed by endoderm cells. This ECM was deficient in laminin but contained dense fibronectin fibers. Spreading was less rapid on fibronectin, and even more retarded on laminin. Ectodermal epithelium explants spread rapidly on and near fibronectin-rich, laminin-poor ECM produced by ectoderm cells, and almost as rapidly on laminin, but spreading was strongly delayed and reduced on fibronectin. The observations suggest that the mesenchymal nature of somite and neural crest cells does not correspond to a lowered responsiveness to laminin relative to fibronectin, while the relationship between laminin and superior epithelial cell spreading should not be generalized. The spreading of the epithelia on complex ECM also indicates the presence of a component(s) other than fibronectin or laminin, which strongly promote(s) spreading. In addition, the methods used indicate that plasma fibronectin and laminin do not specifically bind to each other, and that bovine serum albumen may be inadequate in preventing the attachment of proteins, especially laminin, to cell culture substrates.