An indirect immunofluorescence study of the distribution of fibronectin during the formation of the cushion tissue mesenchyme in the embryonic heart

Building an Artificial Cardiac Microenvironment: A Focus on the Extracellular Matrix

The increased knowledge in cell signals and stem cell differentiation, together with the development of new technologies, such as 3D bioprinting, has made the generation of artificial tissues more feasible for in vitro studies and in vivo applications. In the human body, cell fate, function, and survival are determined by the microenvironment, a rich and complex network composed of extracellular matrix (ECM), different cell types, and soluble factors. They all interconnect and communicate, receiving and sending signals, modulating and responding to cues. In the cardiovascular field, the culture of stem cells in vitro and their differentiation into cardiac phenotypes is well established, although differentiated cardiomyocytes often lack the functional maturation and structural organization typical of the adult myocardium. The recreation of an artificial microenvironment as similar as possible to the native tissue, though, has been shown to partly overcome these limitations, and can be obtained through the proper combination of ECM molecules, different cell types, bioavailability of growth factors (GFs), as well as appropriate mechanical and geometrical stimuli. This review will focus on the role of the ECM in the regulation of cardiac differentiation, will provide new insights on the role of supporting cells in the generation of 3D artificial tissues, and will also present a selection of the latest approaches to recreate a cardiac microenvironment in vitro through 3D bioprinting approaches.

Kaposi sarcoma herpesvirus promotes endothelial-to-mesenchymal transition through Notch-dependent signaling

Endothelial-to-mesenchymal transition (EndMT) is now widely considered a pivotal contributor to cancer progression. In this study, we show that the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is a sufficient cause of EndMT, potentially helping to explain the aggressiveness of KS that occurs commonly in AIDS patients. Upon KSHV infection, primary dermal microvascular endothelial cells lost expression of endothelial markers and acquired expression of mesenchymal markers, displaying new invasive and migratory properties along with increased survival. KSHV activated Notch-induced transcription factors Slug and ZEB1, and canonical Notch signaling was required for KSHV-induced EndMT. In contrast, KSHV did not utilize the TGFβ signaling pathway, which has also been linked to EndMT. Within KS lesions, KSHV-infected spindle cells displayed features compatible with KSHV-induced EndMT including a complex phenotype of endothelial and mesenchymal properties, Notch activity, and nuclear ZEB1 expression. Our results show that KSHV engages the EndMT program to increase the invasiveness and survival of infected endothelial cells, traits that likely contribute to viral persistence and malignant progression. One important implication of our findings is that therapeutic approaches to disrupt the Notch pathway may offer novel approaches for KS treatment.

Notch activation results in phenotypic and functional changes consistent with endothelial-to-mesenchymal transformation

Various studies have identified a critical role for Notch signaling in cardiovascular development. In this and other systems, Notch receptors and ligands are expressed in regions that undergo epithelial-to-mesenchymal transformation. However, there is no direct evidence that Notch activation can induce mesenchymal transdifferentiation. In this study we show that Notch activation in endothelial cells results in morphological, phenotypic, and functional changes consistent with mesenchymal transformation. These changes include downregulation of endothelial markers (vascular endothelial [VE]-cadherin, Tie1, Tie2, platelet-endothelial cell adhesion molecule-1, and endothelial NO synthase), upregulation of mesenchymal markers (alpha-smooth muscle actin, fibronectin, and platelet-derived growth factor receptors), and migration toward platelet-derived growth factor-BB. Notch-induced endothelial-to-mesenchymal transformation does not seem to require external regulation and is restricted to cells expressing activated Notch. Jagged1 stimulation of endothelial cells induces a similar mesenchymal transformation, and Jagged1, Notch1, and Notch4 are expressed in the ventricular outflow tract during stages of endocardial cushion formation. This is the first evidence that Jagged1-Notch interactions induce endothelial-to-mesenchymal transformation, and our findings suggest that Notch signaling may be required for proper endocardial cushion differentiation and/or vascular smooth muscle cell development.

Developmental expression patterns of Beta-ig (betaIG-H3) and its function as a cell adhesion protein

Beta-ig is a secretory protein embodied by fasciclin I-like repeats containing sequences that might bind integrins and glycosaminoglycans in vivo. Expression of Beta-ig is responsive to Transforming Growth Factor-beta and the protein is found to be associated with extracellular matrix (ECM) molecules, implicating Beta-ig as an ECM adhesive protein of developmental processes. The spatiotemporal distribution of Beta-ig during various stages of murine development was examined and its ability to support adhesion of various cell types assessed. In situ hybridization of mouse embryos (E12.5-E18.5) indicated a prominent, distinct expression pattern for Beta-ig message in connective tissue. Beta-ig transcripts were abundantly expressed during mesenchymal cell condensation in areas of axial, craniofacial and appendicular primordial cartilage from E12.5-E14.5. Beginning at E15.5, Beta-ig transcripts appeared in collagen-rich tissues, including dura mater and corneal stroma. During E16.5-E18.5, Beta-ig transcripts were observed in proliferating chondrocytes and areas of endochondral ossification in joint and articular cartilage formation. Connective tissues expressed Beta-ig transcripts within the nasal septum and surrounding cartilage primordia, and in the pericardium, optic cup, kidney, ovary, esophagus, diaphragm, bronchi, trachea and corneal epithelium, and during cardiac valve formation. These patterns of expression indicate that Beta-ig may be involved in tissue morphogenesis. Cells derived from mesenchyme attached onto a substratum comprised of purified recombinant Beta-ig. Taken together, the results indicate that Beta-ig is expressed principally in collagen-rich tissues where it may interact with cells and ECM molecules, perhaps playing a role in tissue morphogenesis.

Mechanisms involved in valvuloseptal endocardial cushion formation in early cardiogenesis: roles of transforming growth factor (TGF)-beta and bone morphogenetic protein (BMP)

Endothelial-mesenchymal transformation (EMT) is a critical event in the generation of the endocardial cushion, the primordia of the valves and septa of the adult heart. This embryonic phenomenon occurs in the outflow tract (OT) and atrioventricular (AV) canal of the embryonic heart in a spatiotemporally restricted manner, and is initiated by putative myocardially derived inductive signals (adherons) which are transferred to the endocardium across the cardiac jelly. Abnormal development of endocardial cushion tissue is linked to many congenital heart diseases. At the onset of EMT in chick cardiogenesis, transforming growth factor (TGFbeta)-3 is expressed in transforming endothelial and invading mesenchymal cells, while bone morphogenetic protein (BMP)-2 is expressed in the subjacent myocardium. Three-dimensional collagen gel culture experiments of the AV endocardium show that 1) myocardially derived inductive signals upregulate the expression of AV endothelial TGFbeta3 at the onset of EMT, 2) TGFbeta3 needs to be expressed by these endothelial cells to trigger the initial phenotypic changes of EMT, and 3) myocardial BMP2 acts synergistically with TGFbeta3 in the initiation of EMT.

Neural crest cells in outflow tract septation of the embryonic chicken heart: differentiation and apoptosis

The heart consists of cells deriving from the cardiogenic plate and also from extracardiac sources. One of the major extracardiac contributions is given by the neural crest. The differentiation pathway and fate of the neural crest cells in the outflow tract have been followed over a prolonged period during outflow tract septation. We studied the role of the neural crest in remodeling the outflow tract by long-term cell tracing, differentiation markers and apoptosis. The pattern of neural crest cells migrating to the heart was investigated by heterospecific chicken quail chimeras and by retroviral infection of the reporter gene LacZ to the stem cells. The tagged neural crest cells move to areas that are morphogenetically active, such as the outflow tract, the semilunar valves, the wall of the arteries and the cardiac ganglia. Two differentiated subpopulations are discerned on the basis of immunohistochemical characterization with antibodies against smooth muscle cells in the arterial vessel wall and against ganglionic cells that were scattered around the vessels of the arterial pole and the heart. A third subpopulation did not stain with these antibodies, but presented locally with the phenomenon of apoptosis as shown with the TUNEL approach. In a developmental series of chicken embryos the populations were followed until stage 40. It was evident that the outflow tract septum in the early phase of development consisted mainly of mesenchymal neural crest cells. In a later phase neural crest cells were still detected at semilunar valve level, but nearly absent in the outflow tract septum below valve level. The septum at that time had become myocardialized. It is evident that neural crest cells are actually removed from this part of the heart by apoptosis. We are pursuing the hypothesis that an important function of apoptotic cells in heart development might be to activate the cardiomyocytes to muscularize the outflow tract septum through mobilizing or delivering growth factors at the time and place that septum formation is initiated.

Distribution of fibronectin, type I collagen, type IV collagen, and laminin in the cardiac jelly of the mouse embryonic heart with retinoic acid-induced complete transposition of the great arteries

BACKGROUND

In the mouse model of complete transposition of the great arteries (TGA) produced by all-trans retinoic acid (RA), parietal and septal ridges in the outflow tract (OT) are hypoplastic. At first, these ridges are generated by an expanded cardiac jelly (mainly myocardial basement membrane). Thereafter, endothelial cells delaminate and invade into the adjacent cardiac jelly to form endocardial cushion tissue (formation of cushion ridge). During cushion tissue formation, basement membrane antigens play an important role in the regulation of this endothelial-mesenchymal transformation.

METHODS

To examine whether the myocardial basement membrane components are altered in the RA-treated heart OT, immunohistochemistry for fibronectin, type I collagen, type IV collagen, and laminin was carried out in mouse embryonic hearts at 9.5 and 10.5 ED (embryonic day; vaginal plug = day 0) with or without prior exposure to RA.

RESULTS

Particulate/fibrillar fibronectin and fibrillar type I collagen were observed in the thick cardiac jelly of the control heart at the onset of mesenchymal formation. In the RA-treated heart, an intermittent patchy staining for fibronectin and a sparse distribution of type I collagen were observed in the thin cardiac jelly. Laminin and type IV collagen were distributed continuously on the basal surface (layer adjacent to the basal plasma membrane) of endocardium and myocardium in both control and RA-treated hearts.

CONCLUSIONS

The alterations in the antigens of the myocardial basement membrane (cardiac jelly) may be responsible for the hypoplasticity of parietal and septal ridges that characterizes RA-induced TGA morphology. This may be one of the reasons why mesenchymal cell formation is inhibited in the RA-induced TGA.

Production of a monoclonal antibody by in vitro immunization that recognizes a native chondroitin sulfate epitope in the embryonic chick limb and heart

We report the production of a monoclonal antibody (d1C4) by in vitro immunization that has immunoreactivity with a native chondroitin sulfate epitope in embryonic chick limb and heart. Murine lymphocytes were stimulated by direct exposure to unfixed, unsolubilized precartilage mesenchymal aggregates in high-density micromass culture derived from Stage 22-23 chick limb buds. Specificity of d1C4 reactivity was demonstrated by sensitivity of immunohistochemical staining to pretreatment with chondroitinase ABC or AC, preferential immunoreactivity with chondroitin-6-sulfate glycosaminoglycan (CS-C GAG) in ELISA, and competition of immunohistochemical staining with CS-C GAG. Immunohistochemical analysis of the expression of the d1C4 epitope revealed a striking localization of immunoreactivity in the extracellular matrix (ECM) of precartilage aggregates of chick limb mesenchyme in high-density micromass culture by 16 hr and the prechondrogenic limb core at Stage 23 in vivo. Immunoreactivity in both cultured limb mesenchyme and the embryonic limb continued through differentiation of prechondrogenic condensations into cartilage tissue. In the developing chick heart, d1C4 staining was found throughout the ECM of atrioventricular cushion tissue by Stage 25, but was localized to mesenchyme adjacent to the myocardium in the outflow tract cushions. There was an abrupt demarcation between d1C4-reactive intracardiac mesenchyme and unreactive extracardiac mesenchyme of the dorsal mesocardium in the Stage 22 embryo. This study demonstrates the efficacy of in vitro immunization of lymphocytes for the production of MAbs to native ECM constituents, such as CS-GAGs. Immunohistochemical data utilizing d1C4 suggest that CS-GAGs bearing this epitope may be important in early morphogenetic events leading to cartilage differentiation in the limb and valvuloseptal morphogenesis in the heart.

Contribution of the primitive epicardium to the subepicardial mesenchyme in hamster and chick embryos

A study about the hypothetical contribution of the epicardial cells to the subepicardial mesenchyme was carried out in Syrian hamster embryos of 9-12 days post coitum (dpc) and chick embryos of 3-5 days of incubation. In the epicardium and subepicardium of these embryos we have immunolocated the proteins cytokeratin (CK), vimentin (VIM), fibronectin (FN), and two antigens related to the transformation of endocardial cells into valvuloseptal mesenchyme, ES/130 and JB3. In the hamster embryos, CK+ subepicardial mesenchymal cells (SEMC) were apparently migrating from the primitive epicardium from 9.5 dpc at the atrioventricular (AV) groove and proximal outflow tract (OFT). The morphological signs of delamination extended by 11 dpc to the epicardium of the interventricular groove and the dorsal part of the ventricle. The relative abundance of the CK+ SEMC decreased in embryos of 12 dpc. VIM colocalized with CK in most SEMC, and in some epicardial mesothelial cells, mainly at the areas of delamination. CK immunoreactivity was also found in some early subepicardial capillaries. Similar observations were made in the chick embryos studied. The immunoreactive patterns obtained at the subepicardium with anti-FN, ES/130, and JB3 antibodies were similar to those reported in the areas of endothelial transformation of the endocardial cushions. We suggest that these observations are compatible with an epithelial-mesenchymal transformation involving the epicardial mesothelium and originating at least a part of the SEMC.

Fibulin-1, vitronectin, and fibronectin expression during avian cardiac valve and septa development

BACKGROUND

Extracellular matrix (ECM) proteins have been implicated as mediators of events important to valvuloseptal development (reviewed by Little and Rongish, Experentia, 51:873-882, 1995). The aim of this study was to identify connective tissue ECM proteins present at sites of valvuloseptal morphogenesis, and to determine how their patterns of expression change during the developmental process.

METHODS

Immunofluorescence microscopy was used to examine the distribution of fibulin-1, vitronectin, and fibronectin in the embryonic chicken heart over a broad developmental time frame (Hamburger and Hamilton stages 14 to 44), emphasizing stages that illustrate endocardial cushion formation, growth, fusion, and development into valvuloseptal components.

RESULTS AND CONCLUSIONS

Fibulin-1 immunolabeling was concentrated in endocardial cushions, notably at boundaries with the myocardium, during stages when the cushions are differentiating into valvular and septal components. Fibulin-1 was detected in the endocardial cushions prior to their seeding with cushion cells, but became undetectable by early midgestation. Vitronectin expression was similar to fibulin-1, but less restricted in its distribution. Vitronectin was observed before endocardial cushion cell migration commenced and persisted until the formation of prevalvular structures (early midgestation) in the atrioventricular cushions. Vitronectin remained detectable in the semilunar valves until late midgestation. Fibronectin was present in the endocardial cushion region and in portions of the endocardium and myocardium throughout the stages presented. Our data suggests that the ECM of the endocardial cushions undergoes remodelling in a regionally and temporally specific manner which corresponds with morphogenetic changes during valvuloseptal development.

Cell surface glycoconjugates and the extracellular matrix of the developing mouse embryo epicardium

Cell surface glycoconjugates and the extracellular matrix (ECM) of the proepicardium and the developing epicardium were studied in early mouse embryos by light and electron microscopy with histochaemical and immunocytochaemical techniques. The extracardially located proepicardium consists of polarized mesothelial cells forming the proepicardial vesicles. These vesicles contain a fine proteoglycan network and an acellular ECM rich in hyaluronic acid. Membrane-bound glycoconjugates are shown with cuprolinic blue, alcian blue and ruthenium red on the apical (outer) cell surface, while fibronectin and laminin are present on the basal (luminal) cell surface. These membrane and matrix components of the proepicardium might be involved in specific attachment of proepicardial cells to the bare heart tube and might facilitate the initial migration of epicardial cells over the myocardial surface. In the cell coat of the cardiomyocytes of the bare heart tube the fibronectin and laminin are concentrated in patches. The formation of the epicardial covering is a rapid process, requiring only about 2 days (9-11 days) to ensheath the entire heart tube from the inflow to the outflow segment. The subepicardial matrix between the newly formed epicardial covering and myocardial layer is acellular at first, but contains a condensing proteoglycan network, membrane and matrix fibronectin, type IV collagen and laminin on the myocardial cell surface. The formation and the distribution of the subepicardial ECM show regional characteristics. The accumulating ECM forms wide subepicardial spaces and protuberances in the atrioventricular and interventricular sulci. The sulci of the heart seem to provide the optimum microenvironment for haematopoiesis and vasculogenesis. Haematopoietic islands and coronary vessel forerunners appear and concentrate in the regularly spaced surface protuberances. The vasculogenesis proceeds from the inflow to the outflow segment of the heart. The first blood capillaries appear in the sinoatrial sulcus of the 10-day embryo. By 11-13 days the subepicardial blood vessels form an interconnected network and establish the coronary artery orifices.

In vivo blockade of tumor necrosis factor-alpha in cholesterol-fed rabbits after cardiac transplant inhibits acute coronary artery neointimal formation

BACKGROUND

We previously identified in piglet cardiac allografts an immunoinflammatory response in coronary arteries in which increased fibronectin regulated by interleukin-1 beta was associated with early evidence of intimal thickening. In the present study, we used rabbits to assess whether acute neointimal formation after cardiac transplantation was reduced by blockade of tumor necrosis factor (TNF)-alpha, which modulates interleukin-1 beta, or by cyclosporine A.

METHODS AND RESULTS

Sixteen rabbits underwent heterotopic cardiac transplantation and were given saline, TNF-soluble receptor (sr), or cyclosporine A. In host hearts from saline- or TNFsr-treated groups, few coronary arteries (approximately 13% to 16%) had intimal thickening, whereas values were higher in the cyclosporine A-treated group (approximately 30%). In donor hearts from the saline-treated group, however, approximately 68% of vessels had intimal thickening versus approximately 32% in TNFsr- and approximately 30% in cyclosporine A-treated groups (P < .01 for both). Severity of intimal thickening assessed quantitatively as percent vessel area was approximately 38% in the saline-treated group but reduced in TNFsr- and cyclosporine A-treated groups to approximately 22% and 18%, respectively (P < .01 for each). Immunohistochemistry revealed increased staining for major histocompatibility complex II, T cells, interleukin-1 beta, TNF-alpha, and fibronectin in donor coronary arteries from saline-treated animals when compared with TNFsr- and cyclosporine A-treated animals. Grade 3 myocardial rejection was observed in both saline- and TNFsr-treated groups, but only grade 1 was apparent in the cyclosporine A-treated group.

CONCLUSIONS

In vivo blockade of TNF-alpha suppresses the acute development of neointimal formation by selectively reducing the vascular immunoinflammatory reaction and accumulation of fibronectin, whereas cyclosporine A suppresses both the myocardial and the vascular immune reaction.

Role of mesenchymal cells in the neovascularization of the rabbit phallus

The neovascularization of the rabbit phallus at ages between prenatal days 15 and 21 was investigated by light- and electron microscopy, computer-aided light microscopic reconstruction, and immunocytochemistry. The phalli are embedded by an abundance of mesenchymal cells, which are in contact with the neighboring ones or with the endothelial lining of growing capillaries. They often form solid cell cords that eventually make contact with the growing capillaries. The computer-aided reconstruction of the serial light micrographs reveals that these cell cords are involved in connecting the adjacent capillaries. The incorporation of such mesenchymal cell projections into the endothelial lining, occasionally conjugated with simple attachment devices, is frequently observed by transmission and scanning electron microscopy. The contact areas between the mesenchymal and endothelial cells show immunoreactions of fibronectin. These results indicate the successive transformation of mesenchymal cells to endothelial cells of the growing capillaries. As endothelial cells of the growing capillaries show mitotic proliferation, such vasoformative mesenchymal cells seem to be involved in the acceleration of the neocapillarization of the rabbit phallus. Fibronectin actively produced in the mesenchymal cells may participate in their migration and the mechanical linkage with the endothelial cells.

GATA-4 is a novel transcription factor expressed in endocardium of the developing heart

We have isolated and characterized Xenopus cDNA clones for a new transcription factor that represents an early marker for the developing heart. The cDNAs encode a protein that we have designated GATA-4; it contains the highly conserved DNA-binding domain that characterizes this family of cell-type restricted transcriptional activators. Whole-embryo in situ analysis of Xenopus embryos demonstrates that the GATA-4 gene is transcribed in presumptive cardiac ventral mesoderm at the time that bilateral progenitors fuse and form the cardiac tube. GATA-4 is therefore the earliest molecular marker of cardiogenesis yet characterized. By stage 30, the GATA-4 mRNA is expressed in the developing atria and ventricles; at stage 38, cross-sections reveal that the gene is active in the endocardial layer, but not in myocardium. By stage 40, GATA-4 message is detected in the great vessels. In the adult frog, the GATA-4 gene is highly transcribed in heart and gut; lower levels of message are detected in various endoderm-derived tissues and gonads. Expression in the stomach is largely confined to the epithelium. The GATA-4 gene is first activated at stage 11; mRNA is initially present throughout the marginal zone of explants and later partially localized to the ventral marginal zone. GATA-4 mRNA is also detected at high levels in cultured endodermal explants derived from the vegetal region of early embryos. In mesoderm induction experiments, GATA-4 transcription is not induced in animal caps treated with activin or bFGF. The GATA-4 gene may provide a new early marker for studying the inductive processes that lead to the formation of the cardiovascular system and for the specification of the endocardial lineage.

Expression of alpha-tropomyosin during cardiac development in the chick embryo

A new monoclonal antibody (mAb) that recognizes alpha-tropomyosin in cardiac muscle cells was used in a qualitative (polyacrylamide gel electrophoresis and indirect immunofluorescence) and quantitative (fluorescence-activated cell sorting) study of the expression of this protein during heart development. alpha-Tropomyosin expression was weak in early stages of chick embryo development (Hamburger and Hamilton stage 18), and increased steadily until Hamburger Hamilton stage 40. In early stages, the protein was found mainly in cytoplasm, whereas by the final stages, it was more abundant in the cytoskeletal compartment. The mAb cross-reacted with alpha-tropomyosin in smooth and striated muscle cells from chickens, mice, and humans, but did not cross-react with nonmuscle tropomyosin.

Role of vitronectin in embryonic rat endocardial cell migration in vitro

Vitronectin is one of the extracellular matrices that mediate cell spreading and attachment in vitro. In the present paper, we demonstrate the involvement of vitronectin in the migration of cushion mesenchymal cells of the embryonic rat heart. Immunohistochemistry established the localization of vitronectin in the myocardial cells and in some of the cushion mesenchymal cells of the truncus arteriosus and atrioventricular canal. In vitro, vitronectin, fibronectin, and collagen type-I revealed significant stimulating activity for cushion mesenchymal cell migration. The distance migrated by cushion mesenchymal cells cultured on vitronectin, collagen type-I, or both vitronectin and fibronectin was similar, but that on fibronectin was significantly shorter. Following the addition of anti-vitronectin IgG to the medium, the migration distance of cushion mesenchymal cells on fibronectin was remarkably increased. Most explants on vitronectin or on both vitronectin and fibronectin became detached from dishes after the addition of the antivitronectin antibody. Immunostaining revealed that cushion mesenchymal cells cultured on substrata other than vitronectin synthesized vitronectin. From these results, it is suggested that vitronectin is synthesized by myocardial cells and some cushion mesenchymal cells, and that vitronectin inhibits cell movement on fibronectin. This feature of vitronectin may be important in the regulation of the migration of cushion mesenchymal cell in vivo.

Effects of injecting fibronectin and antifibronectin antibodies on cushion mesenchyme formation in the chick. An in vivo study

During heart development in the chick some of the endocardial cells that cover the cushion areas leave the cushion endocardium, seed the underlying cardiac jelly, and are transformed into mesenchyme. Cushion mesenchymal (CM) cells migrate from the endocardium toward the myocardium using the cardiac jelly as substratum. Developing cushions have been microinjected with fibronectin (FN), antifibronectin antibodies (AbFN), and four synthetic peptide probes. Two of these peptides (P7 and P10) contained the sequence Arg-Gly-Asp-Ser (RGDS), while the other two (P15 and PColl) did not. Cushion area, individual cell area, cell density, cell orientation and a factor of form were evaluated in both experimental and control cushions. CM cell migration was inhibited by FN and AbFN, only partially inhibited by P10 and unaffected by P7. Cushions injected with P15 and PColl were unaffected. These results can be explained by steric modifications of the extracellular matrix, that may render cardiac jelly nonpermissive for CM cell migration, or by interaction of the substances injected at the endocardial cell surface. Migrating CM cells do not present any preferential orientation in any particular direction. CM cell migration seems to depend upon intrinsic migratory behaviour and the presence of FN at the CM cell surface. The enforcement of the direction of CM cell migration does not appear to rely upon matrix signals but be the result of randomly migrating cells becoming distributed more evenly in the matrix.

Reorganization of the ependyma during axolotl spinal cord regeneration: changes in intermediate filament and fibronectin expression

Changes in intermediate filament content and extracellular matrix material showed that the injury response of ependymal cells in lesioned axolotl spinal cord involves an epithelial-to-mesenchymal transformation, and that fibrous astrocytes are excluded from the remodeling lesion site. Antibody localization was used to visualize cytokeratin-, vimentin-, and glial fibrillary acidic protein- (GFAP-) containing intermediate filaments, as well as the adhesive glycoprotein fibronectin. In normal axolotl spinal cord cytokeratins were found near the apical surface of the ependymal cells. Transmission electron microscopic examination suggested that these cytokeratins were in tonofilaments. Cytokeratin expression was lost and vimentin production was initiated in ependymal cells 2-3 weeks following spinal cord injury. There was a period of approximately 1-2 weeks when cytokeratins and vimentin were co-expressed in vivo. This co-expression was maintained in vitro by culture on a fibronectin-coated substratum. As the central canal reformed, vimentin expression was lost. Ependymal cells lacked GFAP intermediate filaments, but GFAP was present in fibrous astrocytes of the neuropil and white matter. Following injury, GFAP localization showed that fibrous astrocytes disappeared from the remodeling lesion site and reappeared only after the ependymal epithelium reformed and newly myelinated axons were found. Fibronectin expression closely followed the expression of vimentin during mesenchymal ependymal cell outgrowth. These results suggest that the ependymal cell outgrowth requires changes in cell shape followed by changes in production of extracellular matrix.

Fibronectin, hyaluronan, and a hyaluronan binding protein contribute to increased ductus arteriosus smooth muscle cell migration

"Intimal cushions" which develop in the late gestation lamb ductus arteriosus (DA) are characterized by smooth muscle cells migrating into a large subendothelial space. Our previous in vitro studies, comparing DA cells with those from the aorta (Ao), have shown, even in early gestation, a 10-fold increase in DA endothelial incorporation of hyaluronan into the subendothelial matrix, a 2-fold increase in smooth muscle fibronectin synthesis and, in response to endothelial conditioned medium, a 2-fold increase in chondroitin sulfate. To determine whether these extracellular matrix components may be playing a role in inducing DA smooth muscle migration, we seeded Da or Ao smooth muscle cells onto three-dimensional collagen (2.0 mg/ml) gels and assessed migration 2, 5, and 8 days later. After 8 days, significantly greater numbers of DA compared to Ao cells were found invading the gels (23.1 +/- 3.1% vs 16.2 +/- 2.3%, P less than 0.01). Addition of GRGDS peptides (0.5 mM) or antibodies against fibronectin significantly decreased migration in the DA cells, but had no effect on migration in the Ao. Addition of endothelial conditioned medium to induce smooth muscle chondroitin sulfate production had no effect on DA cell migration. Inclusion of hyaluronan in the gel (0.5-1.5 mg), however, further enhanced DA cell migration, being greatest (31.9 +/- 3.1%) at a concentration of 1 mg/ml. Hyaluronan was without effect on Ao smooth muscle cell migration. The ability of hyaluronan to promote migration in cultures of DA smooth muscle cells was blocked completely by the addition of antibodies (1:100 dilution, 1 micrograms/ml) to a cell surface hyaluronan binding protein (HABP). As well, addition of anti-HABP to cells on gels containing collagen only significantly reduced migration in the DA but not the Ao. Immunofluorescent staining revealed that in DA cells, HABP was more concentrated in lamellipodia and leading edges than in Ao cells. As well, DA smooth muscle cells synthesized greater amounts of HABP as determined by Western immunoblotting and immunoprecipitation using polyclonal antisera to HABP. Thus, our studies indicate that both increased fibronectin and HABP contribute to the enhanced migration of DA smooth muscle cells. These results, together with our previous studies showing a 10-fold increase in hyaluronan accumulation in the DA endothelial matrix, would suggest a mechanism for increased DA smooth muscle migration into the subendothelial matrix observed in vivo.

An instructive role for the interstitial matrix in tissue patterning: tissue segregation and intercellular invasion

Intercellular invasion is the intrusion of the cells of one tissue into space occupied by a second tissue. The alternative situation to invasion, one characteristic of most coherent tissues, is segregation, with identifiable boundaries existing between contiguous tissues. The interfaces between mesenchymal and myocardial tissues in the developing avian heart show a profoundly different character in different regions of the heart: the interface between epicardial mesenchyme and heart wall myocardium is planar, without intermingling of the two cell types, whereas the interface between endocardial cushion mesenchyme and myocardium is diffuse, with extensive invasion of both tissue types across the border to produce intermingling of the two tissues. Thus, invasion and tissue segregation coexist in different regions of the mesenchyme-myocardium contact zone. Investigation of the involvement of the interstitial matrix in invasion and segregation has been conducted by maintaining the two tissues in mutual contact in organ culture. Investigation of the mechanisms by which the two cell types sort out in randomized chimeric tissue reaggregates has provided insight into the conditions for tissue segregation. We have modeled invasion in organ culture by fusing aggregates of myocardial cells with aggregates of cardiac mesenchymal cells. Cells of both tissues invaded the partner aggregate during a period of 1-3 d of coculture. Both invasion and segregation in the aggregates appear to depend on the presence or absence of a fibronectin-rich interstitial matrix elaborated by the cardiac mesenchyme. During sorting, the matrix appears selectively in regions occupied by the mesenchyme. Under conditions of culture that are nonpermissive for matrix deposition, sorting fails to occur. Stimulation of matrix deposition by addition of serum, transforming growth factor beta, or isolated matrix itself is accompanied by sorting out of the two tissues. Sorting out is blocked reversibly by inclusion of the fibronectin adhesion site peptide, GRGDSP. Invasion of fused aggregates is preceded by a redistribution of the fibronectin-containing matrix of the mesenchymal aggregate such that matrix-poor regions come to occupy the interface with the myocardial partner aggregate. The invasion that ensues involves mesenchymal cells emigrating from, and myocardial cells intruding into, matrix-poor regions of the mesenchymal aggregate.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytoplasmic stress fibers in the developing heart

Rhodamine-conjugated phalloidin staining was used to study the distribution of filamentous actin in the developing heart of embryonic chicks and rats during the morphogenetic period of cardiac septation. In the chick, intense fluorescence indicative of abundant filamentous actin was observed along the myocardium and in the mesenchymal condensations that formed within the aorticopulmonary septum at day 5. Such cellular condensations and concentration of filamentous actin were not seen in the atrioventricular cushions nor in the preseptation outflow tract. Similar results were found in the 14-day rat embryo. In electron micrographs, microfilament bundles with irregular dense bodies were seen in elongated mesenchymal cells between the valve sites of both species. Cell-cell contacts were observed between such elongated cells and myocyte processes protruding from the nearby myocardial sheath. These histochemical and ultrastructural observations suggest that such mesenchymal condensations serve a specialized mechanical tensile role during embryonic septation of cardiac outflow channels.

Role of transforming growth factor-beta in the development of the mouse embryo

Using immunohistochemical methods, we have investigated the role of transforming growth factor-beta (TGF-beta) in the development of the mouse embryo. For detection of TGF-beta in 11-18-d-old embryos, we have used a polyclonal antibody specific for TGF-beta type 1 and the peroxidase-antiperoxidase technique. Staining of TGF-beta is closely associated with mesenchyme per se or with tissues derived from mesenchyme, such as connective tissue, cartilage, and bone. TGF-beta is conspicuous in tissues derived from neural crest mesenchyme, such as the palate, larynx, facial mesenchyme, nasal sinuses, meninges, and teeth. Staining of all of these tissues is greatest during periods of morphogenesis. In many instances, intense staining is seen in mesenchyme when critical interactions with adjacent epithelium occur, as in the development of hair follicles, teeth, and the submandibular gland. Marked staining is also seen when remodeling of mesenchyme or mesoderm occurs, as during formation of digits from limb buds, formation of the palate, and formation of the heart valves. The presence of TGF-beta is often coupled with pronounced angiogenic activity. The histochemical results are discussed in terms of the known biochemical actions of TGF-beta, especially its ability to control both synthesis and degradation of both structural and adhesion molecules of the extracellular matrix.

Conditioning of native substrates by chondroitin sulfate proteoglycans during cardiac mesenchymal cell migration

It is generally proposed that embryonic mesenchymal cells use sulfated macromolecules during in situ migration. Attempts to resolve the molecular mechanisms for this hypothesis using planar substrates have been met with limited success. In the present study, we provide evidence that the functional significance of certain sulfated macromolecules during mesenchyme migration required the presence of the endogenous migratory template; i.e., native collagen fibrils. Using three-dimensional collagen gel lattices and whole embryo culture procedures to produce metabolically labeled sulfated macromolecules in embryonic chick cardiac tissue, we show that these molecules were primarily proteoglycan (PG) in nature and that their distribution was class specific; i.e., heparan sulfate PG, the minor labeled component (15%), remained pericellular while chondroitin sulfate (CS) PG, the predominately labeled PG (85%), was associated with collagen fibrils as "trails" of 50-60-nm particles when viewed by scanning electron microscopy. Progressive "conditioning" of collagen with CS-PG inhibited the capacity of the template to support subsequent cell migration. Lastly, metabolically labeled, PG-derived CS chains were compared with respect to degree of sulfation in either the C-6 or C-4 position by chromatographic separation of chondroitinase AC digestion products. Results from temporal and regional comparisons of in situ-labeled PGs indicated a positive correlation between the presence of mesenchyme and an enrichment of disaccharide-4S relative to that from regions lacking mesenchyme (i.e., principally myocardial tissue). The suggestion of a mesenchyme-specific CS-PG was substantiated by similarly examining the PGs synthesized solely by cardiac mesenchymal cells migrating within hydrated collagen lattice in culture. These data were incorporated into a model of "substratum conditioning" which provides a molecular mechanism by which secretion of mesenchyme-specific CS-PGs not only provides for directed and sustained cell movement, but ultimately inhibits migration of the cell population as a whole.

Pathways of avian neural crest cell migration in the developing gut

The NC-1 and E/C8 monoclonal antibodies recognize a similar population of neural crest cells as they migrate from vagal levels of the neural tube and colonize the branchial arch region of 2- to 3-day-old chicken embryos. Some of these immunoreactive cells then appear to enter the gut mesenchyme on the third day of incubation just caudal to the third branchial cleft. After entering the gut, these cells migrate in a rostral-caudal direction, using primarily the superficial splanchnic mesodermal epithelium of the gut as a substratum. The antigen-positive cells remain preferentially associated with the splanchnopleure. Few antigenic cells enter the mesenchyme surrounding the endoderm at anterior levels whereas they are found throughout the mesenchyme when nearing the umbilicus. At postumbilical levels, immunoreactive cells are distributed on both sides of the differentiating muscle layer but not within it. Although fibronectin immunoreactivity can be found throughout the wall of the gut, there is no apparent relationship between the distribution of fibronectin and the location of the immunoreactive cells. These results suggest that a mechanism more complex than a mere interaction with fibronectin may account for migration of crest-derived cells in the gut.

Distribution of fibronectin during the morphogenesis of the truncus

The distribution of fibronectin during morphogenesis of the truncus has been studied in chick embryo hearts between days 4 and 10 of incubation. Fibronectin appears as a natural marker for some truncal cells and permits the visualization of the different processes of cell rearrangement which result in truncal morphogenesis. The mesenchymal cells involved in formation of the aorto-pulmonary septum and in formation of the arterial tunica media are intensely fluorescent for fibronectin. These cells present fibrillar deposits of fibronectin associated with their surfaces. Very little staining for fibronectin is observed in association with the anlage of the semilunar valves. Prospective adventitial cells are negative for fibronectin. The close association between the fluorescence and the surface of the cells involved in formation of the aorto-pulmonary septum and the arterial tunica media suggests that fibronectin may be implicated in the formation of both structures. The intensity and amount of the fluorescence staining decreases as morphogenesis of the truncus is completed. The decrease in fibronectin staining is suggested to be related to changes in cell phenotypic expression.

Mechanisms of cell migration in the vertebrate embryo

In vertebrate embryos, many cells are involved in active and passive movements before they regroup into defined tissues. Oriented migration is controlled by different mechanisms, which may include chemotaxis, galvanotaxis, haptotaxis, contact guidance, contact inhibition of movement, and population pressure. A given cell type may utilize different mechanisms in different species and even in the same species when segregating into different lineages. Most of these processes are not yet understood at the molecular level. An even greater difficulty is faced by the molecular embryologist in attempting to unravel the mechanisms governing epithelium-mesenchyme interconversion, which can regulate the initiation and termination of migration. Cells migrating in the extracellular matrix interact directly with fibronectin, although this glycoprotein does not induce the egress of cells from epithelia. Recent studies on the molecular mechanism of intercellular adhesion have led to the identification and characterization of several surface molecules (CAM). Cell surface modulation of such cell adhesion molecules throughout development should contribute to the shaping of the embryo.

Biologically active synthetic peptides as probes of embryonic development: a competitive peptide inhibitor of fibronectin function inhibits gastrulation in amphibian embryos and neural crest cell migration in avian embryos

We describe a new method for analyzing embryonic events dependent on a specific peptide recognition signal. A short, specific amino acid sequence in fibronectin has been implicated as a recognition site in fibronectin-mediated interactions. Fibroblast adhesion to fibronectin is competitively inhibited by certain synthetic peptides, including the decapeptide Arg-Gly-Asp-Ser-Pro-Ala-Ser-Ser-Lys-Pro, which appears to contain the cell recognition sequence. We found that this peptide inhibited both amphibian gastrulation and avian neural crest cell migration in vivo, as well as the attachment and migration of neural crest cells in vitro. These processes are major cell migratory events previously suggested to involve fibronectin. Negative controls included another conserved fibronectin peptide from the collagen-binding region containing the sequence Cys-Gln-Asp-Ser-Glu-Thr-Arg-Thr-Phe-Tyr and another peptide. Our results demonstrate the feasibility of using synthetic peptides directed at recognition sites in extracellular proteins as probes of morphogenetic processes, and they provide further support for the hypothesis that fibronectin is involved in gastrulation and neural crest cell migration.