Human wound healing fibroblasts have greater contractile properties than dermal fibroblasts

Microvesicles derived from dermal myofibroblasts modify the integrity of the blood and lymphatic barriers using distinct endocytosis pathways

Microvesicles (MVs) are a subtype of extracellular vesicles that can transfer biological information from their producer cells to target cells. This communication can in turn affect both normal and pathological processes. Mounting evidence has revealed that dermal wound myofibroblasts (Wmyo) produce MVs, which can transfer biomolecules impacting receptor cells such as human dermal microvascular endothelial cells (HDMECs). While the effects of MVs on HDMECs are generally well described in the literature, little is known about the transport of MVs across the HDMEC barrier, and their potential effect on the barrier integrity remains unknown. Here, we investigated these roles of Wmyo-derived MVs on two sub-populations of HDMECs, blood endothelial cells (BECs) and lymphatic endothelial cells (LECs). Using an in vitro model to mimic the endothelial barrier, we showed that MVs crossed the LEC barrier but not the BEC barrier. In addition, we demonstrated that MVs were able to influence the cell-cell junctions of HDMECs. Specifically, we observed that after internalization via the predominantly caveolin-dependent pathway, MVs induced the opening of junctions in BECs. Conversely, in LECs, MVs mainly use the macropinocytosis pathway and induce closure of these junctions. Moreover, proteins in the MV membrane were responsible for this effect, but not specifically those belonging to the VEGF family. Finally, we found that once the LEC barrier permeability was reduced by MV stimuli, MVs ceased to cross the barrier. Conversely, when the BEC barrier was rendered permeable following stimulation with MVs, they were subsequently able to cross the barrier via the paracellular pathway. Taken together, these results suggest that the study of Wmyo-derived MVs offers valuable insights into their interaction with the HDMEC barrier in the context of wound healing. They highlight the potential significance of these MVs in the overall process.

The diffusion of normal skin wound myofibroblast-derived microvesicles differs according to matrix composition

Microvesicles (MVs) are a subtype of extracellular vesicles that can transfer biological information over long distances, affecting normal and pathological processes including skin wound healing. However, the diffusion of MVs into tissues can be impeded by the extracellular matrix (ECM). We investigated the diffusion of dermal wound myofibroblast-derived MVs into the ECM by using hydrogels composed of different ECM molecules such as fibrin, type III collagen and type I collagen that are present during the healing process. Fluorescent MVs mixed with hydrogels were employed to detect MV diffusion using fluorometric methods. Our results showed that MVs specifically bound type I collagen and diffused freely out of fibrin and type III collagen. Further analysis using flow cytometry and specific inhibitors revealed that MVs bind to type I collagen via the α2β1 integrin. These data demonstrate that MV transport depends on the composition of the wound environment.

Dermal Fibroblast Heterogeneity and Its Contribution to the Skin Repair and Regeneration

Significance: Dermal fibroblasts are the major cell type in the skin's dermal layer. These cells originate from distinct locations of the embryo and reside in unique niches in the dermis. Different dermal fibroblasts exhibit distinct roles in skin development, homeostasis, and wound healing. Therefore, these cells are becoming attractive candidates for cell-based therapies in wound healing. Recent Advances: Human skin dermis comprises multiple fibroblast subtypes, including papillary, reticular, and hair follicle-associated fibroblasts, and myofibroblasts after wounding. Recent studies reveal that these cells play distinct roles in wound healing and contribute to diverse healing outcomes, including nonhealing chronic wound or excessive scar formation, such as hypertrophic scars (HTS) and keloids, with papillary fibroblasts having antiscarring and reticular fibroblast scar-forming properties. Critical Issues: The identities and functions of dermal fibroblast subpopulations in many respects remain unknown. In this review, we summarize the current understanding of dermal fibroblast heterogeneity, including their defined cell markers and dermal niches, dynamic changes, and contributions to skin wound healing, with the emphasis on scarless healing, healing with excessive scars (HTS and keloids), chronic wounds, and the potential application of this heterogeneity for developing cell-based therapies that allow wounds to heal faster with less scarring. Future Directions: Heterogeneous dermal fibroblast populations and their functions are poorly characterized. Refining and advancing our understanding of dermal fibroblast heterogeneity and their participation in skin homeostasis and wound healing may create potential therapeutic applications for nonhealing chronic wounds or wounds that heal with excessive scarring.

GPR43 regulates sodium butyrate-induced angiogenesis and matrix remodeling

Butyrate is a short-chain fatty acid (SCFA) derived from microbiota and is involved in a range of cell processes in a concentration-dependent manner. Low concentrations of sodium butyrate (NaBu) were shown to be proangiogenic. However, the mechanisms associated with these effects are not yet fully known. Here, we investigated the contribution of the SCFA receptor GPR43 in the proangiogenic effects of local treatment with NaBu and its effects on matrix remodeling using the sponge-induced fibrovascular tissue model in mice lacking the Gpr43 gene (Gpr43-KO) and the wild-type (WT) mice. We demonstrated that NaBu (0.2 mM intraimplant) treatment enhanced the neovascularization process, blood flow, and VEGF levels in a GPR43-dependent manner in the implants. Moreover, NaBu was able to modulate matrix remodeling aspects of the granulation tissue such as proteoglycan production, collagen deposition, and α-smooth muscle actin (α-SMA) expression in vivo, besides increasing transforming growth factor (TGF)-β1 levels in the fibrovascular tissue, in a GPR43-dependent manner. Interestingly, NaBu directly stimulated L929 murine fibroblast migration and TGF-β1 and collagen production in vitro. GPR43 was found to be expressed in human dermal fibroblasts, myofibroblasts, and endothelial cells. Overall, our findings evidence that the metabolite-sensing receptor GPR43 contributes to the effects of low dose of NaBu in inducing angiogenesis and matrix remodeling during granulation tissue formation. These data provide important insights for the proposition of new therapeutic approaches based on NaBu, beyond the highly explored intestinal, anti-inflammatory, and anticancer purposes, as a local treatment to improve tissue repair, particularly, by modulating granulation tissue components.NEW & NOTEWORTHY Our data show the contribution of the metabolite-sensing receptor GPR43 in the effects of low dose of sodium butyrate (NaBu) on stimulating angiogenesis and extracellular matrix remodeling in a model of granulation tissue formation in mice. We also show that human dermal fibroblasts, myofibroblasts, and endothelial cells express the receptor GPR43. These data provide important insights for the use of NaBu in local therapeutic approaches applicable to tissue repair in sites other than the intestine.

Wound healing efficacy of Jamun honey in diabetic mice model through reepithelialization, collagen deposition and angiogenesis

Diabetic patients are frequently afflicted with impaired wound healing where linear progression of molecular and cellular events compromised. Despite of meaningful progress in diabetic treatment, management of diabetic chronic wounds is still challenging. Jamun (Syzygium cumini) honey may be a promising candidate for diabetic wound healing and need to explore in detail. So present study was designed to evaluate the efficacy of Jamun honey (JH) for diabetic wound healing in in vitro wound (primary fibroblasts) model and in in vivo of diabetic mice (Streptozotocin induced) model. The fibroblast cell model was studied for migratory behaviour and myofibrolasts infiltration under honey interventions via scratch/migration assay, immuno-cytochemistry and western blot. We applied FDA approved Manuka honey (MH) as positive control and JH as test honey to evaluate wound re-epithelialization, sub-epithelial connective tissue modification and angiogenesis via histo-pathological and immuno-histochemical analysis. JH (0.1% v/v) dilution has notably improved wound closure, migration with concomitant α-SMA expressions in vitro. Topical application of JH in diabetic mice model showed significant (*p ≤ 0.05) wound closure, reepithelialization, collagen deposition (I/III) and balanced the myofibroblasts formation. It also modulated vital angiogenic markers (viz HIF-1α, VEGF, VEGF R-II) significantly (*p ≤ 0.05). All these observations depicted that JH promotes sequential stages of wound healing in diabetic mice model. The results of the present study established Jamun honey as good as Manuka honey considering wound closure, re-epithelialization, collagen deposition and pro-angiogenic potential.

Shedding of proangiogenic microvesicles from hypertrophic scar myofibroblasts

Hypertrophic scars are a common complication of burn injuries and represent a major challenge in terms of prevention and treatment. These scars are characterized by a supraphysiological vascular density and by the presence of pathological myofibroblasts (Hmyos) displaying a low apoptosis propensity. However, the nature of the association between these two hallmarks of hypertrophic scarring remains largely unexplored. Here, we show that Hmyos produce signalling entities known as microvesicles that significantly increase the three cellular processes underlying blood vessel formation: endothelial cell proliferation, migration and assembly into capillary-like structures. The release of microvesicles from Hmyos was dose-dependently induced by the serum protein α-2-macroglobulin. Using flow cytometry, we revealed the presence of the α-2-macroglobulin receptor-low-density lipoprotein receptor-related protein 1-on the surface of Hmyos. The inhibition of the binding of α-2-macroglobulin to its receptor abolished the shedding of proangiogenic microvesicles from Hmyos. These findings suggest that the production of microvesicles by Hmyos contributes to the excessive vascularization of hypertrophic scars. α-2-Macroglobulin modulates the release of these microvesicles through interaction with low-density lipoprotein receptor-related protein 1.

PLGF-1 contained in normal wound myofibroblast-derived microvesicles stimulated collagen production by dermal fibroblasts

During the last stages of wound healing, myofibroblasts differentiate mainly from fibroblasts. Myofibroblasts from normal skin wounds (Wmyo) can communicate with its surrounding using secreted factors. They also have the capacity to produce microvesicles (MVs), a type of extracellular vesicles, as mediators of intercellular communication. MVs cargo are potentially capable of regulating the behavior of targeted cells and tissues. The aim of this study is to evaluate the effect of Wmyo-derived MVs on dermal fibroblasts and to determine the responsible signaling molecule. Microvesicles were obtained from culture media of myofibroblasts and characterized using protein quantification, dynamic light scattering and transmission electron microscopy. Uptake of fluorescent MVs in fibroblasts was assessed by flow cytometry. Cytokines concentrations were quantified in MV samples by a multiplex ELISA. Different concentration of MVs or a selected cytokine were used as treatments over fibroblasts culture for 5 days. Following the treatments, parameters linked to the extracellular matrix were studied. Lastly, the selected cytokine was neutralized within MVs before evaluating collagen production. We showed that Wmyo derived-MVs were internalized by dermal fibroblasts. Cytokine array analysis revealed that a large amount of placental growth factor 1 (PLGF-1) (0.88 ± 0.63 pg/μg proteins in MVs) could be detected in MVs samples. Cutaneous fibroblasts treated with MVs or PLGF-1 showed significantly stimulated procollagen I level production (Fold change of 1.80 ± 0.18 and 2.07 ± 0.18, respectively). Finally, the neutralization of PLGF-1 in MVs significantly inhibited the production of procollagen I by fibroblasts. Our study shows that Wmyo derived-MVs are involved in intercellular communication by stimulating collagen production by fibroblasts during wound healing. This effect is possibly attained through PLGF-1 signalling. These findings represent a promising opportunity to gain insight into how MVs and Wmyo may mediate the healing of a skin wound.

Mesenchymal stromal cells contract collagen more efficiently than dermal fibroblasts: Implications for cytotherapy

Background

Stem cell therapy is the next generation a well-established technique. Cell therapy with mesenchymal stem cells (MSC) has been demonstrated to enhance wound healing in diabetic mice, at least partly due to improved growth factor production. However, it is unclear whether MSC can biomechanically affect wound closure. Utilizing the well-established cell-populated collagen gel contraction model we investigated the interactions between MSC and the extracellular matrix.

Methods

Murine fetal liver-derived Mesenchymal Stem Cells (MSCs) or fetal Dermal Fibroblasts (DFs) were cultured in cell–populated collagen gels (CPCGs). The effect of cell density, conditioned media, growth factors (TGF-B1, FGF, PDGF-BB), cytoskeletal disruptors (colchicine, cytochalasin-D), and relative hypoxia on gel contraction were evaluated. Finally, we also measured the expression of integrin receptors and some growth factors by MSCs within the contracting gels.

Results

Our results show that at different densities, MSCs induced a higher gel contraction compared to DFs. Higher cell density resulted in faster and more complete contraction of CPCGs. Cytoskeletal inhibitors either inhibited or prevented MSC-mediated contraction in a dose dependent fashion. Growth factors, conditioned media from both MSC and DF, and hypoxia all influenced CPCG contraction.

Discussion

The results suggest that MSCs are capable of directly contributing to wound closure through matrix contraction, and they are more effective than DF. In addition, this study demonstrates the importance of how other factors such as cell concentration, cytokines, and oxygen tension can provide potential modulation of therapies to correct wound healing impairments.

Extracellular matrix/stromal vascular fraction gel conditioned medium accelerates wound healing in a murine model

Conditioned medium (CM) is a new treatment modality in regenerative medicine and has shown a successful outcome in wound healing. We recently introduced extracellular matrix/stromal vascular fraction gel (ECM/SVF-gel), an adipose-derived stem cell and adipose native extracellular matrix-enriched product for cytotherapy. This study aimed to evaluate the effect of CM from ECM/SVF-gel (Gel-CM) on wound healing compared with the conventional CM from adipose tissue (Adi-CM) and stem cell (SVF-CM). In vitro wound healing effect of three CMs on keratinocytes and fibroblasts was evaluated in terms of proliferation property, migratory property, and extracellular matrix production. In vivo, two full-thickness wounds were created on the back of each mice. The wounds were randomly divided to receive Gel-CM, Adi-CM, SVF-CM, and PBS injection. Histologic observations and collagen content of wound skin were made. Growth factors concentration in three CMs was further quantified. In vitro, Gel-CM promoted the proliferation and migration of keratinocytes and fibroblasts and enhanced collagen I synthesis in fibroblasts compared to Adi-CM and SVF-CM. In vivo, wound closure was faster, and dermal and epidermal regeneration was improved in the Gel-CM-treated mice compared to that in Adi-CM and SVF-CM-treated mice. Moreover, The growth factors concentration (i.e., vascular endothelial growth factor, basic fibroblast growth factor, hepatocyte growth factor, and transforming growth factor-β) in Gel-CM were significantly higher than those in Adi-CM and SVF-CM. Gel-CM generated under serum free conditions significantly enhanced wound healing effect compared to Adi-CM and SVF-CM by accelerating cell proliferation, migration, and production of ECM. This improved trophic effect may be attributed to the higher growth factors concentration in Gel-CM. Gel-CM shows potential as a novel and promising alternative to skin wound healing treatment. But limitations include the safety and immunogenicity studies of Gel-CM still remain to be clearly clarified and more data on mechanism study are needed.

Fibroblast heterogeneity: implications for human disease

Fibroblasts synthesize the extracellular matrix of connective tissue and play an essential role in maintaining the structural integrity of most tissues. Researchers have long suspected that fibroblasts exhibit functional specialization according to their organ of origin, body site, and spatial location. In recent years, a number of approaches have revealed the existence of fibroblast subtypes in mice. Here, we discuss fibroblast heterogeneity with a focus on the mammalian dermis, which has proven an accessible and tractable system for the dissection of these relationships. We begin by considering differences in fibroblast identity according to anatomical site of origin. Subsequently, we discuss new results relating to the existence of multiple fibroblast subtypes within the mouse dermis. We consider the developmental origin of fibroblasts and how this influences heterogeneity and lineage restriction. We discuss the mechanisms by which fibroblast heterogeneity arises, including intrinsic specification by transcriptional regulatory networks and epigenetic factors in combination with extrinsic effects of the spatial context within tissue. Finally, we discuss how fibroblast heterogeneity may provide insights into pathological states including wound healing, fibrotic diseases, and aging. Our evolving understanding suggests that ex vivo expansion or in vivo inhibition of specific fibroblast subtypes may have important therapeutic applications.

Evaluation of dermal wound healing activity of synthetic peptide SVVYGLR

SVVYGLR peptide (SV peptide) is a 7-amino-acid sequence with angiogenic properties that is derived from osteopontin in the extracellular matrix and promotes differentiation of fibroblasts to myofibroblast-like cells and the production of collagen type Ⅲ by cardiac fibroblasts. However, the effects of SV peptide on dermal cells and tissue are unknown. In this study, we evaluated the effects of this peptide in a rat model of dermal wound healing. The synthetic SV peptide was added to dermal fibroblasts or keratinocytes, and their cellular motility was evaluated. In an in vivo wound healing exeriment, male rats aged 8 weeks were randomly assigned to the SV peptide treatment, non-treated control, or phosphate-buffered saline (PBS) groups. Wound healing was assessed by its repair rate and histological features. Scratch assay and cell migration assays using the Chemotaxicell method showed that SV peptide significantly promoted the cell migration in both fibroblasts and keratinocytes. In contrast the proliferation potency of these cells was not affected by SV peptide. In the rat model, wound healing progressed faster in the SV peptide-treated group than in the control and PBS groups. The histopathological analyses showed that the SV peptide treatment stimulated the migration of fibroblasts to the wound area and increased the number of myofibroblasts. Immunohistochemical staining showed a marked increase of von Willebland factor-positive neomicrovessels in the SV peptide-treated group. In conclusion, SV peptide has a beneficial function to promote wound healing by stimulating granulation via stimulating angiogenesis, cell migration, and the myofibroblastic differentiation of fibroblasts.

Kynurenine Modulates MMP-1 and Type-I Collagen Expression Via Aryl Hydrocarbon Receptor Activation in Dermal Fibroblasts

Dermal fibrosis is characterized by a high deposition of extracellular matrix (ECM) and tissue cellularity. Unfortunately all means of treating this condition are unsatisfactory. We have previously reported the anti-fibrotic effects of Kynurenine (Kyn), a tryptophan metabolite, in fibrotic rabbit ear model. Here, we report the mechanism by which Kyn modulates the expression of key ECM components in dermal fibroblasts. The results showed that Kyn activates aryl hydrocarbon receptor (AHR) nuclear translocation and up-regulates cytochrome-P450 (CYP1A-1) expression, the AHR target gene. A specific AHR antagonist, 6,2',4'-trimethoxyflavone, inhibited the Kyn-dependent modulation of CYP1A-1, MMP-1, and type-I collagen expression. Establishing the anti-fibrogenic effect of Kyn and its mechanism of action, we then developed nano-fibrous Kyn slow-releasing dressings and examined their anti-fibrotic efficacy in vitro and in a rat model. Our results showed the feasibility of incorporating Kyn into PVA/PLGA nanofibers, prolonging the Kyn release up to 4 days tested. Application of medicated-dressings significantly improved the dermal fibrosis indicated by MMP-1 induction, alpha-smooth muscle actin and type-I collagen suppression, and reduced tissue cellularity, T-cells and myofibroblasts. This study clarifies the mechanism by which Kyn modulates ECM expression and reports the development of a new slow-releasing anti-fibrogenic dressing. J. Cell. Physiol. 231: 2749-2760, 2016. © 2016 Wiley Periodicals, Inc.

Keratinocyte Induced Differentiation of Mesenchymal Stem Cells into Dermal Myofibroblasts: A Role in Effective Wound Healing

We have previously demonstrated that human mesenchymal stem cells (hMSCs) migrate toward human keratinocytes as well as toward conditioned medium from cultured human keratinocytes (KCM) indicating that the hMSCs respond to signals from keratinocytes [1]. Using fluorescently labeled cells we now show that in vitro hMSCs appear to surround keratinocytes, and this organization is recapitulated in vivo. Incubation of hMSCs with KCM induced dermal myofibroblast like differentiation characterized by expression of cytoskeletal markers and increased expression of cytokines including SDF-1, IL-8, IL-6 and CXCL5. Interaction of keratinocytes with hMSCs appears to be important in the wound healing process. Therapeutic efficacy of hMSCs in wound healing was examined in two animal models representing normal and chronic wound healing. Accelerated wound healing was observed when hMSCs and KCM exposed hMSCs (KCMSCs) were injected near wound site in nude and NOD/SCID mice. Long term follow up of wound healing revealed that in the hMSC treated wounds there was little evidence of residual scarring. These dermal myofibroblast like hMSCs add to the wound healing process. Together, the keratinocyte and hMSCs morphed dermal myofibroblast like cells as well as the factors secreted by these cells support wound healing with minimal scarring. The ability of hMSCs to support wound healing process represents another striking example of the importance of keratinocyte and hMSCs interplay in the wound microenvironment resulting in effective wound healing with minimal scarring.

Human Keratoconus Cell Contractility is Mediated by Transforming Growth Factor-Beta Isoforms

Keratoconus (KC) is a progressive disease linked to defects in the structural components of the corneal stroma. The extracellular matrix (ECM) is secreted and assembled by corneal keratocytes and regulated by transforming growth factor-β (TGF-β). We have previously identified alterations in the TGF-β pathway in human keratoconus cells (HKCs) compared to normal corneal fibroblasts (HCFs). In our current study, we seeded HKCs and HCFs in 3D-collagen gels to identify variations in contractility, and expression of matrix metalloproteases (MMPs) by HKCs in response the TGF-β isoforms. HKCs showed delayed contractility with decreased Collagen I:Collagen V ratios. TGF-β1 significantly increased ECM contraction, Collagen I, and Collagen V expression by HKCs. We also found that HKCs have significantly decreased Collagen I:Collagen III ratios suggesting a potential link to altered collagen isoform expression in KC. Our findings show that HKCs have significant variations in collagen secretion in a 3D collagen gel and have delayed contraction of the matrix compared to HCFs. For the first time, we utilize a collagen gel model to characterize the contractility and MMP expression by HKCs that may contribute to the pathobiology of KC.

A myofibroblast-mast cell-neuropeptide axis of fibrosis in post-traumatic joint contractures: an in vitro analysis of mechanistic components

Previous studies have implicated a myofibroblast-mast cell-neuropeptide axis of fibrosis in pathologic joint capsules from post-traumatic contractures. The hypothesis to be tested is that joint capsule cells (JC) from human elbows with post-traumatic contractures and their interactions with mast cells (MC) and neuropeptides in the microenvironment underlie the pathogenesis of contractures. The hypothesis was tested using an in vitro collagen gel contraction model. The JC were isolated from human elbow capsules and mixed with neutralized PureCol collagen I. The gels were treated in various ways, including addition of MC (HMC-1), the neuropeptide substance P (SP), an NK1 receptor (SP receptor) antagonist RP67580 and the mast cell stabilizer ketotifen fumarate (KF). The collagen gels were released from the wells and gel size (contraction) was measured optically at multiple time points. The JC contracted collagen gels in a dose-dependent manner. This was enhanced in the presence of MC and increased further with SP. Increasing concentrations of the SP receptor antagonist, RP67580 or the mast cell stabilizer, KF decreased the magnitude of contraction. These observations identify putative mechanistic components of a myofibroblast-mast cell-neuropeptide axis of fibrosis in the joint capsules in post-traumatic contractures and potential prophylactic or therapeutic interventions.

Automatic and Quantitative Measurement of Collagen Gel Contraction Using Model-Guided Segmentation

Quantitative measurement of collagen gel contraction plays a critical role in the field of tissue engineering because it provides spatial-temporal assessment (e.g., changes of gel area and diameter during the contraction process) reflecting the cell behaviors and tissue material properties. So far the assessment of collagen gels relies on manual segmentation, which is time-consuming and suffers from serious intra- and inter-observer variability. In this study, we propose an automatic method combining various image processing techniques to resolve these problems. The proposed method first detects the maximal feasible contraction range of circular references (e.g., culture dish) and avoids the interference of irrelevant objects in the given image. Then, a three-step color conversion strategy is applied to normalize and enhance the contrast between the gel and background. We subsequently introduce a deformable circular model (DCM) which utilizes regional intensity contrast and circular shape constraint to locate the gel boundary. An adaptive weighting scheme was employed to coordinate the model behavior, so that the proposed system can overcome variations of gel boundary appearances at different contraction stages. Two measurements of collagen gels (i.e., area and diameter) can readily be obtained based on the segmentation results. Experimental results, including 120 gel images for accuracy validation, showed high agreement between the proposed method and manual segmentation with an average dice similarity coefficient larger than 0.95. The results also demonstrated obvious improvement in gel contours obtained by the proposed method over two popular, generic segmentation methods.

Regulating tension in three-dimensional culture environments

The processes of development, repair, and remodeling of virtually all tissues and organs, are dependent upon mechanical signals including external loading, cell-generated tension, and tissue stiffness. Over the past few decades, much has been learned about mechanotransduction pathways in specialized two-dimensional culture systems; however, it has also become clear that cells behave very differently in two- and three-dimensional (3D) environments. Three-dimensional in vitro models bring the ability to simulate the in vivo matrix environment and the complexity of cell-matrix interactions together. In this review, we describe the role of tension in regulating cell behavior in three-dimensional collagen and fibrin matrices with a focus on the effective use of global boundary conditions to modulate the tension generated by populations of cells acting in concert. The ability to control and measure the tension in these 3D culture systems has the potential to increase our understanding of mechanobiology and facilitate development of new ways to treat diseased tissues and to direct cell fate in regenerative medicine and tissue engineering applications.

Protective effects of Chlorella-derived peptide against UVC-induced cytotoxicity through inhibition of caspase-3 activity and reduction of the expression of phosphorylated FADD and cleaved PARP-1 in skin fibroblasts

UVC irradiation induces oxidative stress and leads to cell death through an apoptotic pathway. This apoptosis is caused by activation of caspase-3 and formation of poly(ADP-ribose) polymerase-1 (PARP-1). In this study, the underlying mechanisms of Chlorella derived peptide (CDP) activity against UVC-induced cytotoxicity were investigated. Human skin fibroblasts were treated with CDP, vitamin C, or vitamin E after UVC irradiation for a total energy of 15 J/cm². After the UVC exposure, cell proliferation and caspase-3 activity were measured at 12, 24, 48, and 72 h later. Expression of phosphorylated FADD and cleaved PARP-1 were measured 16 h later. DNA damage (expressed as pyrimidine (6-4) pyrimidone photoproducts DNA concentration) and fragmentation assay were performed 24 h after the UVC exposure. Results showed that UVC irradiation induced cytotoxicity in all groups except those treated with CDP. The caspase-3 activity in CDP-treated cells was inhibited from 12 h onward. Expression of phosphorylated FADD and cleaved PARP-1 were also reduced in CDP-treated cells. Moreover, UVC-induced DNA damage and fragmentation were also prevented by the CDP treatment. This study shows that treatment of CDP provides protective effects against UVC-induced cytotoxicity through the inhibition of caspase-3 activity and the reduction of phosphorylated FADD and cleaved PARP-1 expression.

Angiogenic properties of myofibroblasts isolated from normal human skin wounds

During wound healing, angiogenesis plays a crucial role in inducing adequate perfusion of the new tissue, thereby allowing its survival. This angiogenic process contributes to the formation of granulation tissue, alongside myofibroblasts. Myofibroblasts are cells specialized in wound contraction and synthesis of new extracellular matrix. Fibroblasts, considered by some to be at the origin of myofibroblasts, have already been shown to promote neovascularization. Thus, we hypothesized that myofibroblasts play a key role during angiogenic development in wound healing. We isolated myofibroblasts from normal human skin wounds and dermal microvascular endothelial cells (HDMVEC) and fibroblasts from skin. Using an in vitro fibrin-based model, we compared the proangiogenic activity of wound myofibroblasts to that of fibroblasts in the presence of HDMVEC. By immunostaining with collagen IV antibodies, we observed the formation of a capillary network significantly more developed when HDMVEC were cultured with myofibroblasts compared to the network formed in the presence of fibroblasts. The differences between these cell types did not result from a differential secretion of Vascular Endothelial Growth Factor or basic Fibroblast Growth Factor. However, in the presence of myofibroblasts, a significant decrease in matrix metalloproteinase activity was observed. This finding was correlated with a significant increase in Tissue Inhibitor of MetalloProteinase (TIMP)-1 and TIMP-3. Furthermore, inhibition of TIMP-1 secretion using shRNA significantly decreased myofibroblasts induced angiogenesis. These results led to the hypothesis that normal wound myofibroblasts contribute to the vascular network development during wound healing. Our data emphasize the critical role of wound myofibroblasts during healing.

Cell motility and mechanics in three-dimensional collagen matrices

Fibrous connective tissues provide mechanical support and frameworks for other tissues of the body and play an integral role in normal tissue physiology and pathology. Three-dimensional collagen matrices exhibit mechanical and structural features that resemble fibrous connective tissue and have become an important model system to study cell behavior in a tissue-like environment. This review focuses on motile and mechanical interactions between cells—especially fibroblasts—and collagen matrices. We describe several matrix contraction models, the interactions between fibroblasts and collagen fibrils at global and subcellular levels, unique features of mechanical feedback between cells and the matrix, and the impact of the cell-matrix tension state on cell morphology and mechanical behavior. We develop a conceptual framework to explain the balance between cell migration and collagen translocation including the concept of promigratory and procontractile growth factor environments. Finally, we review the significance of these concepts for the physiology of wound repair.

Fibroblasts-a diverse population at the center of it all

The capacity of fibroblasts to produce and organize the extracellular matrix and to communicate with other cells makes them a central component of tissue biology. Even so, fibroblasts remain a somewhat enigmatic population. Our inability to fully comprehend these cells is in large part due to the paucity of unique cellular markers and to their pervasive diversity. Much of our understanding of fibroblast diversity has evolved from studies where subpopulations of these cells have been produced without resorting to cell surface markers. In this regard, cloning and mechanical separation of tissues prior to establishing cultures has provided multiple subpopulations. Nonetheless, in isolated situations, the expression or lack of expression of Thy-1/CD90 has been used to separate fibroblast subsets. The role of fibroblasts in intercellular communication is emerging through the implementation of organotypic studies in which three-dimensional fibroblast culture are combined with other populations of cells. Such studies have revealed critical paracrine loops that are essential for organ development and for wound repair. These studies also provide a backdrop for the emerging field of tissue engineering. The participation of fibroblasts in the regulation of tissue homeostasis and their contribution to the aging process are emerging issues that require better understanding. In short, fibroblasts represent a multifaceted, complex group of cells.

Wound healing effect of acellular artificial dermis containing extracellular matrix secreted by human skin fibroblasts

In this study, an acellular artificial dermis, composed of human collagen and glycosaminoglycan (GAG) secreted by cultured human fibroblasts on a bovine collagen sponge, was developed. Much of the newly secreted extracellular matrix (ECM) remained after the cell removal process. The main theme of this study focused on the matrix, rather than the viable cell components of the skin, as the major dermal deficit in the wound. Both the acellular artificial and bioartificial dermises, containing viable cells with ECM, were significantly less soluble than the collagen sponge, and the relative GAG content in the bioartificial and acellular artificial dermises was approximately 115-120% of the chondroitin-6-sulfate (CS) content found in the collagen sponge. In the group receiving the collagen sponge, the wound area gradually decreased to approximately 10% of its original area, while in the groups receiving the bioartificial and acellular artificial dermises, the wound area also gradually decreased to approximately 60 and 50%, respectively, of the original size over the 5 weeks after grafting. Both the bioartificial and acellular artificial dermises formed thicker, denser collagen fibers; more new blood vessel formation was observed in both cases. The basement membrane of the regenerated epidermal-dermal junction was thicker and more linear in the acellular artificial dermis graft than in the collagen sponge graft. In conclusion, the wound healing effects of acellular artificial dermis are no less than those of the bioartificial dermis, and much better than the collagen sponge graft with respect to wound contraction, angiogenesis, collagen formation, and basement membrane repair.

Clonal characterization of fibroblasts in the superficial layer of the adult human dermis

The dermis of adult human skin contains a physiologically heterogeneous population of fibroblasts that interact to produce its unique architecture and that participate in inflammatory and wound repair functions in vivo. This heterogeneity has been well documented for fibroblasts located in the superficial papillary dermis and the deep reticular dermis. However, the existence of diverse fibroblast subpopulations within a given region of the dermis has not been explored. In this study, fibroblast cultures have been established from the superficial dermis following enzymatic dissociation of the tissue. These fibroblasts have been cloned by limiting dilution and initially selected on the basis of morphology and proliferation kinetics. Fibroblasts in some of the clones selected for study express alpha-smooth muscle actin, a myofibroblast characteristic. Significant differences for fibroblast clones obtained from the same piece of skin have been observed with regard to their rate of collagen lattice contraction, their ability to organize a fibronectin matrix, their release of specific growth factors/cytokines into culture medium, and their response to interleukin-1alpha. These differences in both morphological and physiological characteristics indicate that the superficial papillary dermis contains a heterogeneous population of fibroblasts. This heterogeneity might indicate that diverse subpopulations of fibroblasts are required to interact in both homeostatic and pathological situations in skin.

The role of hyaluronic acid in wound healing: assessment of clinical evidence

Hyaluronic acid (hyaluronan), a naturally occurring polymer within the skin, has been extensively studied since its discovery in 1934. It has been used in a wide range of medical fields as diverse as orthopedics and cosmetic surgery, but it is in tissue engineering that it has been primarily advanced for treatment. The breakdown products of this large macromolecule have a range of properties that lend it specifically to this setting and also to the field of wound healing. It is non-antigenic and may be manufactured in a number of forms, ranging from gels to sheets of solid material through to lightly woven meshes. Epidermal engraftment is superior to most of the available biotechnologies and, as such, the material shows great promise in both animal and clinical studies of tissue engineering. Ongoing work centers around the ability of the molecule to enhance angiogenesis and the conversion of chronic wounds into acute wounds.

Healing and Normal Fibroblasts Exhibit Differential Proliferation, Collagen Production, α-SMA Expression, and Contraction

This study determines the differences in proliferation, collagen production, alpha-smooth muscle actin (alpha-SMA) expression, and contraction between healing and normal fibroblasts. Transected and sham-operated rat medial collateral ligaments (MCL) were used to obtain healing and normal fibroblasts, respectively. It was found that healing fibroblasts in monolayer culture proliferated 1.4-fold faster at 48 h and had 1.7-fold greater protein expression of alpha-SMA than normal fibroblasts. In addition, it was noted that the proliferation of healing fibroblasts in collagen gels was not significantly different from that of normal fibroblasts at 24 h, but it was at 48 h. Furthermore, in collagen gels, healing fibroblasts produced more type I collagen than normal fibroblasts and generated 1.6- and 1.7-fold larger contractile forces at 15 and 20 h, respectively, than their normal counterparts. Taken together, the results of this study show that healing fibroblasts possess a differential proliferation, alpha-SMA protein expression, and contraction than normal fibroblasts.

Growth kinetics and integrin expression of fibroblasts infiltrating devitalised patellar tendons are different from those of intrinsic fibroblasts

We compared the biological characteristics of extrinsic fibroblasts infiltrating the patellar tendon with those of normal, intrinsic fibroblasts in the normal tendon in vitro. Infiltrative fibroblasts were isolated from the patellar tendons of rabbits six weeks after an in situ freeze-thaw treatment which killed the intrinsic fibroblasts. These intrinsic cells were also isolated from the patellar tendons of rabbits which had not been so treated.

Proliferation and invasive migration into the patellar tendon was significantly slower for infiltrative fibroblasts than for normal tendon fibroblasts. Flow-cytometric analysis indicated that expression of α5β1 integrin at the cell surface was significantly lower in infiltrative fibroblasts than in normal tendon fibroblasts. The findings suggest that cellular proliferation and invasive migration of fibroblasts into the patellar tendon after necrosis are inferior to those of the normal fibroblasts. The inferior intrinsic properties of infiltrative fibroblasts may contribute to a slow remodelling process in the grafted tendon after ligament reconstruction.

Cell forces in tissues

This articles reviews the measurement, the effects and the biological importance of forces that cells exert on each other. It does not review the effects of forces originating from movement of tissues, muscular activity, movement and gravity.

Glutathione enhances fibroblast collagen contraction and protects keratinocytes from apoptosis in hyperglycaemic culture

BACKGROUND

Cutaneous wound healing is relatively slow in patients with diabetes.

OBJECTIVES

To test the hypothesis that this defect in healing of wounds in patients with diabetes results from dysfunction of skin fibroblasts and epidermal keratinocytes and that this dysfunction is related to disrupted intracellular glutathione (GSH) homeostasis.

METHODS

We investigated the effects of esterified GSH on the contraction of fibroblasts in a fibroblast-populated collagen lattice and on keratinocyte apoptosis.

RESULTS

High glucose medium (hyperglycaemia) reduced the contraction ability of fibroblasts (P < 0.05). The normalization of glucose medium concentrations for hyperglycaemic fibroblasts did not restore the contraction capacity. The percentage of apoptotic keratinocytes was statistically higher in hyperglycaemic cells (P < 0.05). GSH media concentrations ranging from 0.1 to 100 micromol L(-1) restored the ability of hyperglycaemic fibroblasts to contract the gels in a concentration-dependent manner. Primary human keratinocytes grown in hyperglycaemic medium were more susceptible to apoptosis, and treatment with esterified GSH rescued the keratinocytes from apoptosis.

CONCLUSIONS

These data suggest that intracellular GSH can normalize skin cell functions disrupted by in vitro cell growth under hyperglycaemic conditions.

Epidermis promotes dermal fibrosis: role in the pathogenesis of hypertrophic scars

Hypertrophic scarring is a pathological process characterized by fibroblastic hyperproliferation and by excessive deposition of extracellular matrix components. It has been hypothesized that abnormalities in epidermal-dermal crosstalk explain this pathology. To test this hypothesis, a tissue-engineered model of self-assembled reconstructed skin was used in this study to mimic interactions between dermal and epidermal cells in normal or pathological skin. These skin equivalents were constructed using three dermal cell types: normal wound (Wmyo) or hypertrophic wound (Hmyo) myofibroblasts and normal skin fibroblasts (Fb). Epidermis was reconstructed with normal skin keratinocytes (NK) or hypertrophic scar keratinocytes (HK). In the absence of keratinocytes, Hmyo formed a thicker dermis than Wmyo. When seeded with NK, the dermal thickness of Hmyo (121.2 +/- 31.4 microm vs 196.2 +/- 27.8 microm) and Fb (43.7 +/- 7.1 microm vs 83.6 +/- 16.3 microm) dermis was significantly (p < 0.05) reduced, while that of Wmyo (201.5 +/- 15.7 microm vs 160.7 +/- 21.1 microm) was increased. However, the presence of HK always induced significantly thicker dermis formation than observed with NK (Wmyo: 238.8 +/- 25.9 microm; Hmyo: 145.5 +/- 22.4 microm; Fb: 74.2 +/- 11.2 microm). These results correlated with collagen and MMP-1 secretion and with cell proliferation, which were increased when keratinocytes were added, except for the collagen secretion of Hmyo and Fb in the presence of NK. The level of dermal apoptosis was not different when epidermis was added to the dermis (<1% in each category). These observations strongly suggest that hypertrophic scar keratinocytes play a role in the development of pathological fibrosis by influencing the behaviour of dermal cells.

Wound fluid inhibits wound fibroblast nitric oxide synthesis

BACKGROUND

Fibroblast-derived nitric oxide (NO) is an autocrine stimulator of collagen synthesis by wound fibroblasts. Little is known about the in vivo regulation of wound fibroblast NO synthesis. We investigated the net effect of wound environment on wound fibroblast NO production and characterized a soluble factor mediating this effect.

MATERIALS AND METHODS

Wound fibroblasts and acellular wound fluid (pool of 100 Lewis rats) were isolated from subcutaneously implanted polyvinyl alcohol sponges harvested 10 days post-wounding. Fibroblasts were incubated in the presence of 10% (v/v) wound fluid. Nitrite, an index of NO synthesis, was measured in supernatants by Griess reagent.

RESULTS

Wound fibroblasts spontaneously synthesize large amounts of NO. Spontaneous NO synthesis was further increased by LPS + IFN-gamma (P < 0.001). Wound fluid significantly inhibited both spontaneous and LPS plus IFN-gamma-stimulated NO synthesis (by 88 and 55%, respectively; P < 0.01). Wound fluid from 5- to 35-day-old wounds equally suppressed NO synthesis. Separation by Sephadex G-100 gel filtration identified the active factor in wound fluid to have a molecular weight of about 100 kDa. Characterization of this factor showed it to be a heat-resistant (56 degrees C, 30 min), trypsin-sensitive, and neuraminidase-resistant protein (ammonium sulfate precipitation). The isoelectric point appeared to be 7.0, as determined by ion exchange chromatography. Addition of high arginine did not restore the effect of wound fluid on fibroblast NO synthesis, suggesting that substrate is not a limiting factor.

CONCLUSION

Our data demonstrate that following postoperative day 5 the wound environment contains a high molecular weight protein that inhibits NO synthesis by wound fibroblasts.

Comparing the contractile properties of human fibroblasts in leg ulcers with normal fibroblasts

OBJECTIVE

The tissue contraction phenomenon associated with wound healing is of prime importance for wound closure. Contractile properties of human fibroblasts from chronic venous leg ulcers were compared with those of normal fibroblasts using in vitro models.

METHOD

Biopsies were taken from the uninvolved skin of the thigh, the epithelialised ulcer edge and the non-epithelialised ulcer centre in four patients (average age: 78 years). Fibroblasts were obtained by an explant technique and expanded in vitro in Dulbecco's Modified Eagle's Medium supplemented with 10% foetal calf serum and used for the assays at their fourth passage. Intracellular alpha-smooth muscle actin expression (alphaSM-actin) was studied by immunofluorescence labelling of cells cultured in monolayer. Contractile properties were evaluated using three-dimensional collagen lattices.

RESULTS

Fibroblasts from the ulcer centre were the richest cells in actin filaments. Both populations of venous ulcer fibroblasts contracted more rapidly and to a greater extent than normal fibroblasts. The peak contractile forces developed by fibroblasts from the ulcer centre and the ulcer edge were 30% and 18% greater than normal fibroblasts respectively.

CONCLUSION

Some functions of fibroblasts, in particular the generation of contractile forces and the formation of cytoplasmic actin filaments, seem not to be affected in chronic venous ulcers.

DECLARATION OF INTEREST

This study was supported by the Fondation Coloplast pour la Qualite de la Vie of France.

An exploration of two opposing theories of wound contraction

There are two schools of thought of how wound contraction occurs. Some researchers argue that myofibroblasts have contractile properties, whereas others propose that fibroblasts exert the force needed to achieve contraction.

Normal skin wound and hypertrophic scar myofibroblasts have differential responses to apoptotic inductors

During wound healing, myofibroblasts play a central role in matrix formation and wound contraction. At the end of healing, there is evidence that myofibroblasts disappear via apoptotic pathways. Hypertrophic scars are a fibroproliferative disorder that leads to considerable morbidity. It has been postulated that a defect in myofibroblast apoptosis could be responsible for the pathological scar formation, but no evidence exists. We have isolated and cultured human normal wound (Wmyo) and hypertrophic scar (Hmyo) myofibroblasts and compared their basal apoptotic rates and their sensitivity to serum starvation and Fas antibody-induced apoptosis to that obtained for dermal fibroblasts (Fb). A higher rate of apoptosis as evidenced by morphological criteria and a propidium iodide assay was observed for Wmyo in comparison to Fb and Hmyo. These results came along with a low level of the anti-apoptotic proteins Bcl-2 and Bclx(L) in Wmyo, whereas there was an increase in the level of the pro-apoptotic molecule Bax when compared to the results obtained for Fb and Hmyo. Hmyo showed a higher level of Bcl-2 compared to Fb but no difference in the Bax or Bclx(L) level. After serum starvation, Wmyo revealed an increased apoptotic rate, whereas Hmyo and Fb did not show any difference. Anti-Fas treatment did not modify the levels of apoptosis but strongly increased the cell growth of Hmyo as compared to Wmyo. This is the first study presenting a broad vision of the apoptotic sensitivity of normal and pathological myofibroblasts. These results confirmed the hypothesis of defects in apoptosis and growth during pathological scar formation impeding myofibroblast disappearance at the end of healing.

Sensitivity of myofibroblasts to H2O2-mediated apoptosis and their antioxidant cell network

During wound healing, the transition from granulation to scar tissue shows a decrease in myofibroblast cellularity. Previous results have correlated the disappearance of these cells with the induction of apoptotic cell death by some unknown stimuli. In contrast, hypertrophic scar appearance after wound healing is thought to be linked to a disorder of apoptotic function which induces myofibroblast persistence in granulation tissue. Oxidative stress being an important mediator of apoptosis, we have evaluated the apoptotic response of normal and pathological wound myofibroblasts (WMyo and HMyo respectively) in their interaction with two oxidative stress inducers: hydrogen peroxide, using a high concentration as a single dose, and sodium ascorbate which induced a continuous release of H2O2 at a low concentration. Our results showed that, according to the H2O2 treatment type, HMyo were more sensitive (after ascorbate treatment) or less sensitive (after H2O2 treatment) when compared to WMyo and Fb. We next assessed the presence of several molecules known to be involved in the antioxidant network protecting cells against H2O2 injury and found HMyo to have a higher level of activity of glutathione peroxidase and a lower level of activity of catalase than WMyo. These results can help explain the contradictory responses of myofibroblasts according to the oxidative stress treatment. This is the first study linking refractory oxidative stress mediated cell death to cellular phenotype in hypertrophic myofibroblasts, and indicates a pivotal role for the antioxidant enzyme system in this type of resistance.

TNF-? in vivo stimulates apoptosis in fibroblasts through caspase-8 activation and modulates the expression of pro-apoptotic genes

Apoptosis of matrix producing cells is common among many inflammatory diseases. The goal of the present study was to examine the apoptotic effects of tumor necrosis factor-alpha (TNF-alpha) on fibroblastic cells in vivo and to investigate the role of different caspases in this process. This was accomplished in vivo by subcutaneous injection of TNF-alpha in mice. The direct effects of TNF-alpha on fibroblast apoptosis were studied in vitro with normal diploid human fibroblasts. The results indicate that TNF-alpha in vivo induces apoptosis of fibroblasts. By RNase protection assay, we demonstrated that TNF-alpha stimulates expression of 12 apoptotic genes. Fluorometric studies demonstrated that TNF-alpha in vivo predominantly increased caspase-8 and -3 activity and by use of specific inhibitors, the activation of caspase-3 was shown to be initiated by caspase-8 with only a minor contribution from caspase-9. Thus, TNF-alpha acts to modulate the expression of many genes that favors apoptosis of fibroblastic cells, which is dependent mostly upon signaling through caspase-8.

Lipopolysaccharides Indirectly Stimulate Apoptosis and Global Induction of Apoptotic Genes in Fibroblasts

Following Gram-negative bacterial infection there is a reduction in matrix-producing cells. The goal of the present study was to examine the apoptotic effects of lipopolysaccharide (LPS) on fibroblastic cells and to investigate the role that the host response plays in this reaction. This was accomplished in vivo by subcutaneous inoculation of LPS in wild type and TNFR1(-/-)R2(-/-) mice. The direct effects of LPS on fibroblast apoptosis was studied in vitro with normal diploid human fibroblasts. The results indicate that LPS in vivo induces apoptosis of fibroblasts. By RNA profiling we demonstrated that LPS stimulates global expression of apoptotic genes and down-regulates anti-apoptotic genes. Fluorometric studies demonstrated that LPS in vivo significantly increased caspase-8 and caspase-3 activity and by use of specific inhibitors, the activation of caspase-3 was shown to be initiated by caspase-8 with no contribution from caspase-9. In vitro studies demonstrated that LPS did not induce apoptosis of fibroblasts, whereas tumor necrosis factor (TNF) did. In addition, the pattern of apoptotic gene expression induced by TNF in vitro was nearly identical to that induced by LPS in vivo, as measured by RNase protection assay. Moreover, pre-treatment of cells with TNF greatly enhanced apoptosis induced by a second stimulation with TNF 24 h later, suggesting that the global induction of pro-apoptotic genes was functionally significant. Thus, LPS acts to modulate the expression of a large number of genes that favor apoptosis of fibroblastic cells that is dependent upon activation of caspase-8 and is largely mediated by TNF.

Skeletal myosin heavy chain function in cultured lung myofibroblasts

Myofibroblasts are unique contractile cells with both muscle and nonmuscle properties. Typically myofibroblasts are identified by the expression of alpha smooth muscle actin (ASMA); however some myofibroblasts also express sarcomeric proteins. In this study, we show that pulmonary myofibroblasts express three of the eight known sarcomeric myosin heavy chains (MyHCs) (IIa, IId, and embryonic) and that skeletal muscle myosin enzymatic activity is required for pulmonary myofibroblast contractility. Furthermore, inhibition of skeletal myosin activity and myofibroblast contraction results in a decrease in both ASMA and skeletal MyHC promoter activity and ASMA protein expression, suggesting a potential coupling of skeletal myosin activity and ASMA expression in myofibroblast differentiation. To understand the molecular mechanisms whereby skeletal muscle genes are regulated in myofibroblasts, we have found that members of the myogenic regulatory factor family of transcription factors and Ca(2+) - regulated pathways are involved in skeletal MyHC promoter activity. Interestingly, the regulation of skeletal myosin expression in myofibroblasts is distinct from that observed in muscle cells and suggests that cell context is important in its control.

Evidence for sequential utilization of fibronectin, vitronectin, and collagen during fibroblast-mediated collagen contraction

Contraction plays a major role in wound healing and is inevitably mediated through the mechanical interaction of fibroblast cytoskeleton and integrins with their extracellular matrix ligands. Cell-matrix attachment is critical for such events. In human dermal fibroblasts most such interactions are mediated by the beta1-type integrins. This study investigated the role played by key components in this system, notably fibronectin, vitronectin, and integrin subcomponents alpha2 and alpha5, which recognize collagen and fibronectin. Inhibition of adhesion through these ligands was studied either by antibody blocking or with fibronectin and/or vitronectin depletion. Functional effects of inhibition were monitored as force generation in collagen-glycosaminoglycan (IntegraTM) sponges, over 20 hours using a culture force monitor. Dose and time-course inhibition studies indicated that initial attachment and force generation (approx. 0-5 hours postseeding) was through fibronectin receptors and this was followed by vitronectin ligand and receptor utilization (4 hours onward). Utilization of the collagen integrin subcomponent alpha2 appeared to be increasingly important between 6 and 16 hours and dominant thereafter. Additionally, there was evidence for functional interdependence between the three ligand systems fibronectin, vitronectin, and collagen. We propose that there is a short cascade of sequential integrin-ligand interactions as cells attach to, extend through, and eventually contract their matrix. (WOUND REP REG 2002;10:-408)