Contractility, transforming growth factor-beta, and plasmin in fetal skin fibroblasts: role in scarless wound healing

Complications and treatment errors in root coverage procedures

Root coverage procedures have become very common in clinical dental practice. Even though these techniques are considered safe, the clinician may face several issues during the therapy due to their surgical nature. Some of these issues can be defined strictly as complications inherent to the procedure, whereas others are medical errors or treatment errors. This review will focus on describing treatment errors and complications that may arise during different phases of the root coverage therapeutic process and on how to prevent and manage them.

Cell morphology and mechanosensing can be decoupled in fibrous microenvironments and identified using artificial neural networks

Cells interpret cues from and interact with fibrous microenvironments through the body based on the mechanics and organization of these environments and the phenotypic state of the cell. This in turn regulates mechanoactive pathways, such as the localization of mechanosensitive factors. Here, we leverage the microscale heterogeneity inherent to engineered fiber microenvironments to produce a large morphologic data set, across multiple cells types, while simultaneously measuring mechanobiological response (YAP/TAZ nuclear localization) at the single cell level. This dataset describing a large dynamic range of cell morphologies and responses was coupled with a machine learning approach to predict the mechanobiological state of individual cells from multiple lineages. We also noted that certain cells (e.g., invasive cancer cells) or biochemical perturbations (e.g., modulating contractility) can limit the predictability of cells in a universal context. Leveraging this finding, we developed further models that incorporate biochemical cues for single cell prediction or identify individual cells that do not follow the established rules. The models developed here provide a tool for connecting cell morphology and signaling, incorporating biochemical cues in predictive models, and identifying aberrant cell behavior at the single cell level.

The mechanobiology of tendon fibroblasts under static and uniaxial cyclic load in a 3D tissue engineered model mimicking native extracellular matrix

Tendon mechanobiology plays a vital role in tendon repair and regeneration; however, this mechanism is currently poorly understood. We tested the role of different mechanical loads on extracellular matrix (ECM) remodelling gene expression and the morphology of tendon fibroblasts in collagen hydrogels, designed to mimic native tissue. Hydrogels were subjected to precise static or uniaxial loading patterns of known magnitudes and sampled to analyse gene expression of known mechano-responsive ECM-associated genes (Collagen I, Collagen III, Tenomodulin, and TGF-β). Tendon fibroblast cytomechanics was studied under load by using a tension culture force monitor, with immunofluorescence and immunohistological staining used to examine cell morphology. Tendon fibroblasts subjected to cyclic load showed that endogenous matrix tension was maintained, with significant concomitant upregulation of ECM remodelling genes, Collagen I, Collagen III, Tenomodulin, and TGF-β when compared with static load and control samples. These data indicate that tendon fibroblasts acutely adapt to the mechanical forces placed upon them, transmitting forces across the ECM without losing mechanical dynamism. This model demonstrates cell-material (ECM) interaction and remodelling in preclinical a platform, which can be used as a screening tool to understand tendon regeneration.

Scarless wound healing: From development to senescence

An essential element of tissue homeostasis is the response to injuries, cutaneous wound healing being the most studied example. In the adults, wound healing aims at quickly restoring the barrier function of the skin, leading however to scar, a dysfunctional fibrotic tissue. On the other hand, in fetuses a scarless tissue regeneration takes place. During ageing, the wound healing capacity declines; however, in the absence of comorbidities a higher quality in tissue repair is observed. Senescent cells have been found to accumulate in chronic unhealed wounds, but more recent reports indicate that their transient presence may be beneficial for tissue repair. In this review data on skin wound healing and scarring are presented, covering the whole spectrum from early embryonic development to adulthood, and furthermore until ageing of the organism.

The Contractile Phenotype of Dermal Fetal Fibroblasts in Scarless Wound Healing

PURPOSE OF REVIEW

Injured skin in the mammalian fetus can heal regeneratively due to the ability of fetal fibroblasts to effectively reorganize the extracellular matrix (ECM). This process occurs without fetal fibroblasts differentiating into highly contractile myofibroblasts which cause scarring and fibrosis in adult wounds. Here, we provide a brief review of fetal wound healing and the evidence supporting a unique contractile phenotype in fetal fibroblasts. Furthermore, we discuss the biomechanical role of the ECM in driving myofibroblast differentiation in wound healing and the implications for new clinical modalities based on the biophysical properties of fetal fibroblasts.

RECENT FINDINGS

We and others have found that fetal fibroblasts are refractory to the environmental stimuli necessary for myofibroblast differentiation in adult wound healing including mechanical stress.

SUMMARY

Understanding the biomechanical mechanisms that regulate the contractile phenotype of fetal fibroblasts may unlock new avenues for anti-scarring therapies that target myofibroblast differentiation of adult fibroblasts.

Comparison of the Application of Allogeneic Fibroblast and Autologous Mesh Grafting With the Conventional Method in the Treatment of Third-Degree Burns

Wound healing is a multipart process involving different cell types and growth factors. Third-degree burns are usually treated by early excision and skin grafting. Tissue engineering has been developed in this field in response to limitations associated with autografts. Allogeneic fibroblasts on meshed split thickness skin grafts (STSGs) are known to have useful properties in wound healing and can be used to construct a new model of living skin substitute. Fourteen patients were chosen from June 2009 until December 2010 as the sample for this study. After debridement and wound excision, meshed STSG was used to cover the entire wound. Alloskin (allofibroblasts cultured on a combination of silicone and glycosaminoglycan) was applied on one side and petroleum jelly-impregnated gauze (Iran Polymer and Petrochemical Institute) was applied on the other. The healing time, scar formation, and pigmentation score were assessed for the patients. All analyses were undertaken with SPSS 17 software. Alloskin demonstrated good properties compared to petroleum jelly-impregnated gauze. The average healing time and hypertrophic scar formation were significantly different between the two groups. In addition, the skin pigmentation score in the alloskin group was closer to normal. Alloskin grafting, including fibroblasts on meshed STSG, may be a useful method to reduce healing time and scar size and may require less autologous STSG in extensive burns where a high percentage of skin is burned and there is a lack of available donor sites.

Binding of anti-SSA antibodies to apoptotic fetal cardiocytes stimulates urokinase plasminogen activator (uPA)/uPA receptor-dependent activation of TGF-β and potentiates fibrosis

In congenital heart block (CHB), binding of maternal anti-SSA/Ro Abs to fetal apoptotic cardiocytes impairs their removal by healthy cardiocytes and increases urokinase plasminogen activator (uPA)/uPA receptor (uPAR)-dependent plasmin activation. Because the uPA/uPAR system plays a role in TGF-β activation, we evaluated whether anti-Ro binding to apoptotic cardiocytes enhances plasmin-mediated activation of TGF-β, thereby promoting a profibrosing phenotype. Supernatants from cocultures of healthy cardiocytes and apoptotic cardiocytes bound by IgG from a mother whose child had CHB (apoptotic-CHB-IgG [apo-CHB-IgG]) exhibited significantly increased levels of active TGF-β compared with supernatants from cocultures of healthy cardiocytes and apoptotic cardiocytes preincubated with IgG from a healthy donor. Treatment of the culture medium with anti-TGF-β Ab or TGF-β inhibitor (SB431542) abrogated the luciferase response, thereby confirming TGF-β dependency. Increased uPA levels and activity were present in supernatants generated from cocultures of healthy cardiocytes and apo-CHB-IgG cardiocytes compared with healthy cardiocytes and apoptotic cardiocytes preincubated with IgG from a healthy donor, respectively. Treatment of apo-CHB-IgG cardiocytes with anti-uPAR or anti-uPA Abs or plasmin inhibitor aprotinin prior to coculturing with healthy cardiocytes attenuated TGF-β activation. Supernatants derived from cocultures of healthy cardiocytes and apo-CHB-IgG cardiocytes promoted Smad2 phosphorylation and fibroblast transdifferentiation, as evidenced by increased smooth muscle actin and collagen expression, which decreased when fibroblasts were treated with supernatants from cocultures pretreated with uPAR Abs. These data suggested that binding of anti-Ro Abs to apoptotic cardiocytes triggers TGF-β activation, by virtue of increasing uPAR-dependent uPA activity, thus initiating and amplifying a cascade of events that promotes myofibroblast transdifferentiation and scar.

Does plasmin have anticoagulant activity?

The coagulation and fibrinolytic pathways regulate hemostasis and thrombosis, and an imbalance in these pathways may result in pathologic hemophilia or thrombosis. The plasminogen system is the primary proteolytic pathway for fibrinolysis, but also has important proteolytic functions in cell migration, extracellular matrix degradation, metalloproteinase activation, and hormone processing. Several studies have demonstrated plasmin cleavage and inactivation of several coagulation factors, suggesting plasmin may be not only be the primary fibrinolytic enzyme, but may have anticoagulant properties as well. The objective of this review is to examine both in vitro and in vivo evidence for plasmin inactivation of coagulation, and to consider whether plasmin may act as a physiological regulator of coagulation. While several studies have demonstrated strong evidence for plasmin cleavage and inactivation of coagulation factors FV, FVIII, FIX, and FX in vitro, in vivo evidence is lacking for a physiologic role for plasmin as an anticoagulant. However, inactivation of coagulation factors by plasmin may be useful as a localized anticoagulant therapy or as a combined thrombolytic and anticoagulant therapy.

Ontogeny of expression of transforming growth factor-beta and its receptors and their possible relationship with scarless healing in human fetal skin

Fetal cutaneous wounds that occur in early gestation heal without scar formation. Although much work has been done to characterize the role of transforming growth factor-beta (TGF-beta) isoforms and their receptors in the wound healing process, their roles in scarless wound repair observed in early gestation and their functions in human fetal skin development, and structural and functional maintenance are still not well understood. In this study, we explore the expression and distribution characteristics of three TGF-beta isoforms and their receptors, TGF-betaRI (TBRI) and TGF-betaRII (TBRII), in fetal and postnatal skins to understand the relevance of these five proteins to skin development and elucidate the mechanism(s) underlying the phenotypic transition from scarless to scar-forming healing observed during fetal gestation. Fetal skin biopsies of human embryo were obtained from spontaneous abortions at different gestational ages from 13 to 32 weeks and postnatal skin specimens were collected from patients undergoing plastic surgery. Gene expression and positive immunohistochemical signals of TGF-beta(1), TGF-beta(2), TGF-beta(3), TBRI, and TBRII could all be detected in fetal and postnatal skins. In early gestation, gene expression of TGF-beta(1), TBRI, and TBRII was weaker and protein contents were less compared with postnatal skins (p < 0.05). In contrast, more TGF-beta(2) mRNA transcript was found in early gestation than in late gestation and in postnatal skins, whereas protein content of this growth factor increased during gestation. Lastly, mRNA transcript and protein contents of TGF-beta(3) were apparently higher in early gestation compared to postnatal skin (p < 0.05). In postnatal skin, granules containing the three TGF-beta isoforms were mainly distributed in the cytoplasm and extracellular matrix of epidermal cells, interfollicular keratinocytes, and some fibroblasts. TBRI and TBRII were chiefly located in the cellular membrane of epidermal keratinocytes and some fibroblasts. The endogenous three TGF-beta isoforms and their receptors may be involved in the development of embryonic skin and in the maintenance of cutaneous structure and function, and also in postnatal wound healing. The differential levels of TGF-beta isoforms may provide either a predominantly antiscarring or profibrotic signal upon wounding depending on the gestational period. Lower expression of their receptors in early gestational skins may be a reason for the reduced ability to perceive ligands, ultimately leading to scar-free healing.

Mechanobiology of tendon

Tendons are able to respond to mechanical forces by altering their structure, composition, and mechanical properties--a process called tissue mechanical adaptation. The fact that mechanical adaptation is effected by cells in tendons is clearly understood; however, how cells sense mechanical forces and convert them into biochemical signals that ultimately lead to tendon adaptive physiological or pathological changes is not well understood. Mechanobiology is an interdisciplinary study that can enhance our understanding of mechanotransduction mechanisms at the tissue, cellular, and molecular levels. The purpose of this article is to provide an overview of tendon mechanobiology. The discussion begins with the mechanical forces acting on tendons in vivo, tendon structure and composition, and its mechanical properties. Then the tendon's response to exercise, disuse, and overuse are presented, followed by a discussion of tendon healing and the role of mechanical loading and fibroblast contraction in tissue healing. Next, mechanobiological responses of tendon fibroblasts to repetitive mechanical loading conditions are presented, and major cellular mechanotransduction mechanisms are briefly reviewed. Finally, future research directions in tendon mechanobiology research are discussed.

The role of allogenic fibroblasts in an acute wound healing model

Skin is the first tissue-engineered organ to have been successfully developed in the laboratory, and it has been clinically available for use as epidermal sheets for some time. As refinements in this field of tissue engineering continue, several key issues give cause for concern. One issue is the need to form a more complete dermal analogue before grafting. To this end, fibroblasts may be used in vitro to deposit extracellular matrix components within a basic scaffold, laying down those molecules not endogenous to the material and thereby improving the quality of the skin replacement. Many studies have shown the benefits of in vitro seeding with fibroblasts, but there has been some debate regarding the longevity of such cells after allotransplantation into an immunocompetent host. In this study, the authors set out to determine the longevity of transplanted cells in an immunocompetent porcine model. A total of 24 wounds were made on four female animals, 12 of which were covered with acellular hyaluronic acid dermal matrices, and the remainder of which were covered with matrices seeded with allogenic (male) fibroblasts. After a week in vivo, the wounds were grafted with either split-thickness skin grafts or cultured epithelial autograft. Biopsy specimens were obtained from wounds at varying time intervals and assessed using genetic analysis to determine the survival of allotransplanted cells. No cells were detectable by polymerase chain reaction analysis (sensitivity, 1:100,000) after 7 days in vivo. Subsequent histologic examination demonstrated little difference in wound morphology. The authors conclude that allogenic fibroblasts do not survive transplantation in a porcine wound model.

A multi-station dynamic-culture force monitor system to study cell mechanobiology

To study mechanobiological responses of cells, a dynamic-culture force monitor (D-CFM) system has been developed. The D-CFM extends our previous work to measure contractile forces of a cell-populated collagen gel (CPCG) using a cantilever beam with semiconductor strain gauges. Linear actuators are used in the system and are computer controlled using a LabVIEW interface to independently apply precise motion waveforms to multiple CPCGs. The feasibility tests showed that the new system can detect the differences in force patterns resulting from different motion waveforms imparted to the CPCG. This new system will facilitate the study of the effects of dynamic mechanical loading on cells, remodeling of extracellular matrix, and cell-matrix interactions in vitro.

Serine protease inhibitors modulate smoke-induced chemokine release from human lung fibroblasts

Smoking is associated with lung inflammation and a protease-antiprotease imbalance. We previously reported that cigarette smoke extract (CSE) stimulates human lung fibroblasts to release chemotactic cytokines. We hypothesized that serine protease inhibitors might modulate lung fibroblast release of chemotactic cytokines in response to CSE. To test this hypothesis, serine protease inhibitors (FK706, alpha1-antitrypsin, methoxysuccinyl-Ala-Ala-Pro-Val chloromethyl ketone, or Nalpha-p-tosyl-L-lysine chloromethyl ketone) were evaluated for their capacity to attenuate the release of neutrophil chemotactic activity (NCA) and monocyte chemotactic activity (MCA) from human fetal lung fibroblasts by the blind-well chemotactic chamber. Metalloproteinases and cysteine proteinases were not examined in this study. Similarly, the release and gene expression of chemokines and nuclear factor-kappaB (NF-kappaB) activation were measured by means of enzyme-linked immunosorbent assay and reverse transcriptase-polymerase chain reaction. Release of NCA, MCA, chemotactic chemokines including interleukin-8, granulocyte colony-stimulating factor, monocyte chemoattractant protein-1, and granulocyte-macrophage colony-stimulating factor, and the expression of interleukin-8 and monocyte chemoattractant protein-1 mRNA were attenuated by FK706. Furthermore, FK706 suppressed NF-kappaB activation. These data suggest that serine protease inhibitors attenuate the CSE-induced release of NCA and MCA from human fetal lung fibroblasts and that the inhibitory action of antiproteases might depend on NF-kappaB signaling pathway.

Synergistic effect of bismuth subgallate and borneol, the major components of Sulbogin, on the healing of skin wound

Most skin lesions heal delay and even heal efficiently within 1-2 weeks, the healed tissue is neither aesthetically nor functionally perfect. Therefore, facilitating skin healing rate and controlling healed skin quality are major aims of drug treatment for a wound event. Bismuth subgallate (BS) and Borneol (BO) are the two components of Sulbogin, a new Vaseline-based wound healing ointment, one for treating skin wounds. Although BO has antibiotic function, while BS is widely used clinically, neither has been used specifically for wound healing. The experiment described here aimed to study the effect of BS and BO on the healing of skin wounds. This study also compared the effects of BS and BO with Flamazine cream, which is currently the most popular drug for wound healing in hospitals. Full-thickness wounds (3 cm x 3 cm x 0.2 cm) were created on the back of adult male Sprague-Dawley rats. BS, BO, BS+BO, and Flamazine were then evenly applied to cheesecloth and placed over the lesion areas. The drug patches were replaced every 2-3 days until the wound areas were completely covered by epidermis in any kinds of drug treatment. The combined BS and BO treatment had the best effect on healing by decreasing lesion area, while increasing granulation tissue formation, re-epithelialization, eating behavior and reconstitution of skin appendages. This investigation showed that BS and BO have a synergistic effect on the skin wound restoration.

Serine protease inhibitors modulate chemotactic cytokine production by human lung fibroblasts in vitro

Chemotactic chemokines can be released from lung fibroblasts in response to interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha. An imbalance between proteases and antiproteases has been observed at inflammatory sites, and, therefore, protease inhibitors might modulate fibroblast release of chemotactic cytokines. To test this hypothesis, serine protease inhibitors (FK-706, alpha(1)-antitrypsin, or N(alpha)-p-tosyl-L-lysine chloromethyl ketone) were evaluated for their capacity to attenuate the release of neutrophil chemotactic activity (NCA) or monocyte chemotactic activity (MCA) from human fetal lung fibroblasts (HFL-1). Similarly, the release of the chemoattractants IL-8, granulocyte colony-stimulating factor, monocyte chemoattractant protein-1, macrophage colony-stimulating factor, and granulocyte/macrophage colony-stimulating factor, from HFL-1, were evaluated in response to IL-1beta and TNF-alpha. NCA, MCA, and chemotactic cytokines were attenuated by FK-706. However, matrix metalloproteinase inhibitors were without effect, and cysteine protease inhibitors only slightly attenuated chemotactic or cytokine release. These data suggest that IL-1beta and TNF-alpha may stimulate lung fibroblasts to release NCA and MCA by a protease-dependent mechanism and that serine protease inhibitors may attenuate the release.

A multi-station culture force monitor system to study cellular contractility

Cellular contraction contributes to the formation of scar tissue, which is characterized by an over-produced, disorganized collagen matrix. To study the contractility of cells in vitro and its potential contribution to scar tissue formation, we have developed a multi-station culture force monitor (CFM) system. This system consists of four vertical cantilever beams with semiconductor strain gages and a computerized data acquisition unit to monitor contractile forces of the cells in a collagen gel. Calibration showed that this system has a highly linear voltage-force relationship (R(2) > 0.99). Further, to demonstrate the applicability of this system, contractile forces of human skin fibroblasts in a collagen gel were measured. These fibroblasts were found to produce an average force of 0.2 nN/cell, which is consistent with the data in literature. The significant advantage of this CFM system is its ability to test multiple samples simultaneously. Therefore, the system can facilitate statistical design and analysis of experiments to study the effects of growth factors (e.g., TGF-betas) on cellular contraction and their potential role in scar tissue formation.

Differential expression of collagen integrin receptor on fetal vs. adult skin fibroblasts: implication in wound contraction during healing

BACKGROUND

Fetal skin wound healing is characterized by an absence of contraction and scar formation, two important observations associated with adult healing often leading to pathological problems.

OBJECTIVES

We have studied the capacity of adult and fetal human skin fibroblasts to contract collagen gels, collagen being the major structural component of dermal matrix.

METHODS

In parallel with collagen gel contraction studies, we have used fluorescence-activated cell sorter analysis to study the levels of collagen receptors expressed at the surface of fibroblasts derived from fetal or adult skin samples.

RESULTS

Strong differences were detected between freshly isolated fetal and adult fibroblasts. Fetal fibroblasts had a very low capacity to contract collagen gel, whereas adult cells significantly contracted gels in the same conditions. The expression of alpha1, alpha2 and alpha3 integrin subunits was also significantly different depending of the donor age: alpha1 and alpha3 integrin subunit expression was lower in fetal cells compared with adult cells, whereas alpha2 integrin subunit expression was higher. When grown in monolayers, adult cells showed rapid changes in their contractile capacity and integrin expression while fetal cells were only affected after several passages.

CONCLUSIONS

These observations indicate that intrinsic differences between fetal and adult fibroblasts can strongly influence the quality of wound repair.

Differences in mesenchymal tissue repair

Variations in certain mesenchymal tissue healing processes are not widely recognized. The current review summarizes key differences in healing mechanisms and healing potential after injury to soft tissues having different healing outcomes.

Effects of collagen matrix containing transforming growth factor (TGF)-beta(1) on wound contraction

We evaluated the effectiveness of transforming growth factor (TGF)-beta(1) on wound contraction, both alone and in combination with collagen matrix, using an in vivo delayed wound healing type model. To clarify the mechanisms involved in the effectiveness of TGF-beta(1), we also used a fibroblast-populated collagen gel contraction in vitro model. Although we found that TGF-beta(1) significantly accelerated contraction of the fibroblast-populated collagen gel in vitro, we demonstrated that both collagen matrix alone and 1.0 microg of TGF-beta(1) alone significantly inhibited wound contraction in the in vivo model. In addition, the combination of TGF-beta(1) and collagen matrix was much more effective than TGF-beta(1) alone, a finding which was supported by histopathological examination. Wounds treated with collagen matrix containing TGF-beta(1) showed horizontal rearrangement of collagen fibers in the dermal part as well as evidence of active fibroblast proliferation, which was not observed in the scar regions of controls. These results show that the application of TGF-beta(1) treated collagen matrix is effective for preventing contraction producing so called "neodermis" in treating a delayed healing type model and may be highly beneficial for treating chronic wounds.

Understanding the final stages of wound contraction

Much research has been undertaken to improve our understanding of the processes of wound contraction. This article, the second in a two-part series, focuses on granulation tissue modulation.

Secretory leukoprotease inhibitor augments hepatocyte growth factor production in human lung fibroblasts

Secretory leukoprotease inhibitor (SLPI), an 11.7-kD nonglycosylated serine protease inhibitor, is produced and released into the fluids of mucosal surfaces including human lung. It comprises two domains with homologous amino acid sequences: the N-terminal domain possessing antibacterial activity, and the C-terminal domain with antiprotease activity. Here we report the positive regulation of hepatocyte growth factor (HGF) production in human lung fibroblasts exerted by SLPI or its C-terminal domain under physiologic concentrations (1 to 10 microM). This HGF production by SLPI was unaffected by the addition of interleukin (IL)-1 receptor antagonist. In contrast, human skin fibroblasts exerted no SLPI-stimulated increase in HGF production, despite the fact that IL-1beta increased HGF production with an intensity similar to that of human lung fibroblasts. Both the time-course and dose-response studies in human lung fibroblasts revealed that the induction of HGF messenger RNA (mRNA) and protein occurred in parallel, indicating that the mechanism existed at the steady-state mRNA level. A synthetic elastase inhibitor failed to induce HGF, but alpha(1)-antitrypsin also stimulated HGF production in lung fibroblasts. Inactivation of the antiprotease activity of SLPI or its C-terminal domain by an oxidizing agent (N-chlorosuccinimide) abolished their stimulatory effect on HGF production. These findings demonstrate that SLPI exerts a novel HGF induction and functions as an anti-inflammatory and regenerative factor in addition to its role in protease inhibition.

Mathematical modelling of nitric oxide activity in wound healing can explain keloid and hypertrophic scarring

Keloid and hypertrophic lesions are both types of scarring pathologies which arise as a consequence of excess collagen deposition during the wound healing process. The exact mechanism by which this occurs is not understood and currently no effective treatment exists. In this paper, we study the possible role of nitric oxide in excess scar formation. In recent years, the physiological role of this free radical in mammalian tissue has been extensively studied; in particular numerous groups have studied its role in wound healing. We describe a mathematical model which offers a possible explanation for keloid scarring in terms of the presence of higher than normal nitric oxide concentrations related to the fact that nitric oxide stimulates synthesis of collagen by fibroblasts. As a consequence of this, we put forward a suggestion for a treatment strategy involving the surgical excision of the keloid lesion combined with the application of a low-dose nitric oxide inhibitor. In addition, we show that a quasi-steady-state analysis of our model reveals a possible approach to distinguishing between hypertrophic and keloid lesions, a task which has to date proven to be clinically difficult. We also present an extended model which confirms these results in the context of a more complicated and biologically more realistic system. The fuller model demonstrates additional features of keloid and hypertrophic scarring which we were not able to consider in the basic model, and as a consequence further supports our hypothesis that nitric oxide activity could play a key role in keloid scarring.

TGF-beta and fibrosis

Transforming growth factor-beta (TGF-beta) isoforms are multifunctional cytokines that play a central role in wound healing and in tissue repair. TGF-beta is found in all tissues, but is particularly abundant in bone, lung, kidney and placental tissue. TGF-beta is produced by many but not all parenchymal cell types, and is also produced or released by infiltrating cells such as lymphocytes, monocytes/macrophages, and platelets. Following wounding or inflammation, all these cells are potential sources of TGF-beta. In general, the release and activation of TGF-beta stimulates the production of various extracellular matrix proteins and inhibits the degradation of these matrix proteins, although exceptions to these principles abound. These actions of TGF-beta contribute to tissue repair, which under ideal circumstances leads to the restoration of normal tissue architecture and may involve a component of tissue fibrosis. In many diseases, excessive TGF-beta contributes to a pathologic excess of tissue fibrosis that compromises normal organ function, a topic that has been the subject of numerous reviews [1-3]. In the following chapter, we will discuss the role of TGF-beta in tissue fibrosis, with particular emphasis on renal fibrosis.

Developmental differences in the expression and modulation of extracellular matrix proteases and inhibitors in mouse skin fibroblasts

To investigate developmental differences in the wound repair process between fetal and adult skin fibroblasts, we studied the expression of plasminogen activator, plasminogen activator inhibitor, matrix metalloproteinase, and tissue inhibitor of metalloproteinase in E-15, E-17, newborn and adult mouse skin fibroblasts cultured within three dimensional matrices of either collagen or fibrin. Fibrin overlay and reverse overlay analyses revealed that mouse skin fibroblasts secreted tissue plasminogen activator and type1 plasminogen activator inhibitor. However, only E-15 and E-17 fibroblasts secreted the active form of tissue plasminogen activator, while in newborn and adult fibroblasts tissue plasminogen activator was conjugated to type1 plasminogen activator inhibitor. Only adult fibroblasts expressed a high level of active type1 plasminogen activator inhibitor. Gelatin zymography revealed that the predominant matrix metalloproteinase secreted by all the mouse fibroblasts was gelatinase A (matrix metalloproteinase -2). Matrix metalloproteinase -2 was partially activated in the adult fibroblasts cultured within a collagen matrix. The tissue inhibitor of metalloproteinase-2 was expressed by all fibroblasts, but levels were highest in the newborn and adult fibroblasts. When E-15 fibroblasts were cultured within a fibrin matrix, tissue plasminogen activator was downregulated. Transforming growth factor-betadownregulated tissue plasminogen activator while upregulating type1 plasminogen activator inhibitor, and platelet-derived growth factor enhanced tissue plasminogen activator expression in E-15 fibroblasts. Therefore, plasminogen activator and its inhibitor, and matrix metalloproteinase and its associated tissue inhibitor are differentially expressed in fetal and adult fibroblasts, and their expression is controlled by extracellular matrix components and growth factors present in wounds.

Aberrant wound healing and TGF-beta production in the autoimmune-prone MRL/+ mouse

Wound healing is a complex process that involves inflammation, apoptosis, growth, and tissue remodeling. The autoimmune-prone inbred mouse strain MRL/+ manifests accelerated and extensive healing to ear punch wounds, suggesting a link between immune defects and wound healing. Prior studies with lupus-prone mice have shown that hematopoietic cells of lupus-prone strains can transfer disease to otherwise non-autoimmune-prone recipients. In this study we performed reciprocal bone marrow transfers between MRL and the control strain B10.BR and found that radioresistant MRL/+ host cells, rather than hematopoietic cells, are required for the healing response. We have also made the novel observations that, compared to normal controls, MRL/+ hematopoietic cells overproduce TGF-beta1 and manifest impaired inflammatory responses to lipopolysaccharide challenge. These features suggest that the aberrant wound healing phenotype of MRL mice is independent of their propensity to develop autoimmunity.

Fetal wound repair: where do we go from here?

In contrast to adult wound healing, early-gestation fetal skin wound healing occurs rapidly, in a regenerative fashion, and without scar formation. The accelerated rate of healing, relative lack of an acute inflammatory response, and an absence of neovascularization distinguishes fetal from adult wound healing. However, this remarkable ability of the fetus to heal without scarring still remains poorly understood. The uncertainties include the role of cytokines, extracellular matrix components, homeobox genes, and certain cell types in the scarless wound repair process. Nevertheless, some strides have been made within the last two decades. This report, discusses the current knowledge of the mechanisms and characteristics of scarless fetal wound healing. Furthermore, to shy away from being just another all inclusive review, the authors point out deficiencies in the knowledge base on this important topic. Last, the future direction of research is discussed that may elucidate the mechanisms regulating the scarless repair phenomena.