Early basement membrane formation following the grafting of cultured epidermal sheets detached with thermolysin or Dispase

Harnessing Human Placental Membrane-Derived Bioinks: Characterization and Applications in Bioprinting and Vasculogenesis

Bioprinting applications in the clinical field generate great interest, but developing suitable biomaterial inks for medical settings is a challenge. Placental tissues offer a promising solution due to their abundance, stability, and status as medical waste. They contain basement membrane components, have a clinical history, and support angiogenesis. This study formulates bioinks from two placental tissues, amnion (AM) and chorion (CHO), and compares their unique extracellular matrix (ECM) and growth factor compositions. Rheological properties of the bioinks are evaluated for bioprinting and maturation of human endothelial cells. Both AM and Cho-derived bioinks sustained human endothelial cell viability, proliferation, and maturation, promoting optimal vasculogenesis. These bioinks derived from human sources have significant potential for tissue engineering applications, particularly in supporting vasculogenesis. This research contributes to the advancement of tissue engineering and regenerative medicine, bringing everyone closer to clinically viable bioprinting solutions using placental tissues as valuable biomaterials.

Repair of peripheral nerve injuries using a prevascularized cell-based tissue-engineered nerve conduit

One of the major challenges in the development of a larger and longer nerve conduit for peripheral nerve repair is the limitation in oxygen and nutrient diffusion within the tissue after transplantation preventing Schwann cell and axonal migration. This restriction is due to the slow neovascularization process of the graft starting from both nerve endings. To overcome this limitation, we propose the design of a living tissue-engineered nerve conduit made of an internal tube with a three-dimensional structure supporting axonal migration, which is inserted inside a hollow external tube that plays the role of an epineurium and is strong enough to be stitched to the severed nerve stumps. The internal tube is made of a rolled living fibroblast sheet and can be seeded with endothelial cells to promote the formation of a network containing capillary-like structures which allow rapid inosculation with the host nerve microvasculature after grafting. Human nerve conduits were grafted in immunodeficient rats to bridge a 15 mm sciatic nerve gap. Human capillaries within the pre-vascularized nerve conduit successfully connected to the host circulation 2 weeks after grafting. Twenty-two weeks after surgery, rats transplanted with the nerve conduits had a similar motor function recovery compared to the autograft group. By promoting rapid vascularization of the internal nerve tube from both ends of the nerve stumps, this endothelialized nerve conduit model displays a favorable environment to enhance axonal migration in both larger caliber and longer nerve grafts.

In Vitro Prevascularization of Self-Assembled Human Bone-Like Tissues and Preclinical Assessment Using a Rat Calvarial Bone Defect Model

In vitro prevascularization has the potential to address the challenge of maintaining cell viability at the core of engineered constructs, such as bone substitutes, and to improve the survival of tissue grafts by allowing quicker anastomosis to the host microvasculature. The self-assembly approach of tissue engineering allows the production of biomimetic bone-like tissue constructs including extracellular matrix and living human adipose-derived stromal/stem cells (hASCs) induced towards osteogenic differentiation. We hypothesized that the addition of endothelial cells could improve osteogenesis and biomineralization during the production of self-assembled human bone-like tissues using hASCs. Additionally, we postulated that these prevascularized constructs would consequently improve graft survival and bone repair of rat calvarial bone defects. This study shows that a dense capillary network spontaneously formed in vitro during tissue biofabrication after two weeks of maturation. Despite reductions in osteocalcin levels and hydroxyapatite formation in vitro in prevascularized bone-like tissues (35 days of culture), in vivo imaging of prevascularized constructs showed an improvement in cell survival without impeding bone healing after 12 weeks of implantation in a calvarial bone defect model (immunocompromised male rats), compared to their stromal counterparts. Globally, these findings establish our ability to engineer prevascularized bone-like tissues with improved functional properties.

Transplantation of autologous cells and porous gelatin microcarriers to promote wound healing

Definitive treatment to achieve wound healing in major burns frequently include skin transplantation, where split-thickness skin grafts is considered gold standard. This method is associated with several drawbacks. To overcome these hurdles, efforts have been made to develop tissue engineered skin substitutes, often comprised of a combination of cells and biomaterials. In the present study, we aimed to investigate transplantation of autologous keratinocytes and fibroblasts seeded on porous gelatin microcarriers using a porcine wound model. Pre-seeded microcarriers were transplanted to a total of 168 surgical full-thickness wounds (2cm diameter) on eight adult female pigs and covered with occlusive dressings. The experimental groups included wounds transplanted with microcarriers seeded with the combination of keratinocytes and fibroblasts, microcarriers seeded with each cell type individually, microcarriers without cells, each cell type in suspension, and NaCl control. Wounds were allowed to heal for one, two, four or eight weeks before being excised and fixated for subsequent histological and immunohistochemical analysis. In vitro, we confirmed that viable cells populate the surface and the pores of the microcarriers. In vivo, the microcarriers were to a large extent degraded after two weeks. After one week, all treatment groups, with the exception of microcarriers alone, displayed significantly thicker neo-epidermis compared to controls. After two weeks, wounds transplanted with microcarriers seeded with cells displayed significantly thicker neo-epidermis compared to controls. After four weeks there was no difference in the thickness of neo-epidermis. In conclusion, the experiments performed illustrate that autologous cells seeded on porous gelatin microcarriers stimulates the re-epithelialization of wounds. This method could be a promising candidate for skin transplantation. Future studies will focus on additional outcome parameters to evaluate long-term quality of healing following transplantation.

Interferon regulatory factor 6 is required for proper wound healing in vivo

BACKGROUND

Van der Woude syndrome (VWS) is the most common form of syndromic orofacial cleft caused predominantly by mutations in Interferon Regulatory Factor 6 (IRF6). We previously reported that individuals with VWS have increased risk of wound healing complications following cleft repair compared with individuals with nonsyndromic orofacial clefts (nonsyndromic cleft lip and palate-NSCLP). In vitro, absence of IRF6 leads to impaired keratinocyte migration and embryonic wound healing. However, there is currently no data on tissue repair in adult animals and cells with reduced levels of IRF6 like in VWS.

RESULTS

Excisional wounds of Irf6^+/- and wild-type animals were analyzed 4 and 7 days post-wounding. Although all wounds were reepithelialized after 7 days, the epidermal and wound volume of repaired wounds was larger in Irf6^+/- . These data were supported by increased keratinocyte proliferation in the neoformed epidermis and a less mature granulation tissue with increased cytokine levels. This effect was not cell autonomous, as Irf6^+/- neonatal keratinocytes in vitro did not exhibit defects in scratch wound closure or proliferation. Keratinocytes from individuals with VWS also migrated similarly to keratinocytes from NSCLP individuals.

CONCLUSIONS

These data support a role for IRF6 in wound healing by regulating keratinocyte proliferation, granulation tissue maturation, and cytokine levels.

The enzymatic de-epithelialization technique determines denuded amniotic membrane integrity and viability of harvested epithelial cells

The human amniotic membrane (HAM) is widely used for its wound healing effect in clinical practice, as a feeder for the cell cultivation, or a source of cells to be used in cell therapy. The aim of this study was to find effective and safe enzymatic HAM de-epithelialization method leading to harvesting of both denuded undamaged HAM and viable human amniotic epithelial cells (hAECs). The efficiency of de-epithelialization using TrypLE Express, trypsin/ ethylenediaminetetraacetic (EDTA), and thermolysin was monitored by hematoxylin and eosin staining and by the measurement of DNA concentration. The cell viability was determined by trypan blue staining. Scanning electron microscopy and immunodetection of collagen type IV and laminin α5 chain were used to check the basement membrane integrity. De-epithelialized hAECs were cultured and their stemness properties and proliferation potential was assessed after each passage. The HAM was successfully de-epithelialized using all three types of reagents, but morphological changes in basement membrane and stroma were observed after the thermolysin application. About 60% of cells remained viable using trypsin/EDTA, approximately 6% using TrypLE Express, and all cells were lethally damaged after thermolysin application. The hAECs isolated using trypsin/EDTA were successfully cultured up to the 5^th passage with increasing proliferation potential and decreased stem cell markers expression (NANOG, SOX2) in prolonged cell culture. Trypsin/EDTA technique was the most efficient for obtaining both undamaged denuded HAM and viable hAECs for consequent culture.

A Cell-Based Self-Assembly Approach for the Production of Human Osseous Tissues from Adipose-Derived Stromal/Stem Cells

Achieving optimal bone defect repair is a clinical challenge driving intensive research in the field of bone tissue engineering. Many strategies focus on seeding graft materials with progenitor cells prior to in vivo implantation. Given the benefits of closely mimicking tissue structure and function with natural materials, the authors hypothesize that under specific culture conditions, human adipose-derived stem/stromal cells (hASCs) can solely be used to engineer human reconstructed osseous tissues (hROTs) by undergoing osteoblastic differentiation with concomitant extracellular matrix production and mineralization. Therefore, the authors are developing a self-assembly methodology allowing the production of such osseous tissues. Three-dimensional (3D) tissues reconstructed from osteogenically-induced cell sheets contain abundant collagen type I and are 2.7-fold less contractile compared to non-osteogenically induced tissues. In particular, hROT differentiation and mineralization is reflected by a greater amount of homogenously distributed alkaline phosphatase, as well as higher calcium-containing hydroxyapatite (P < 0.0001) and osteocalcin (P < 0.0001) levels compared to non-induced tissues. Taken together, these findings show that hASC-driven tissue engineering leads to hROTs that demonstrate structural and functional characteristics similar to native osseous tissue. These highly biomimetic human osseous tissues will advantageously serve as a platform for molecular studies as well as for future therapeutic in vivo translation.

Cell-based approach for 3D reconstruction of lymphatic capillaries in vitro reveals distinct functions of HGF and VEGF-C in lymphangiogenesis

Regeneration of lymphatic vessels is important for treatment of various disorders of lymphatic system and for restoration of lymphatic function after surgery. We have developed a method for generating a human 3D lymphatic vascular construct. In this system, human lymphatic endothelial cells, co-cultured with fibroblasts, spontaneously organized into a stable 3D lymphatic capillary network without the use of any exogenous factors. In vitro-generated lymphatic capillaries exhibited the major molecular and ultra-structural features of native, human lymphatic microvasculature: branches in the three dimensions, wide lumen, blind ends, overlapping borders, adherens and tight junctions, anchoring filaments, lack of mural cells, and poorly developed basement membrane. Furthermore, we show that fibroblast-derived VEGF-C and HGF cooperate in the formation of lymphatic vasculature by activating ERK1/2 signaling, and demonstrate distinct functions of HGF/c-Met and VEGF-C/VEGFR-3 in lymphangiogenesis. This lymphatic vascular construct is expected to facilitate studies of lymphangiogenesis in vitro and it holds promise as a strategy for regeneration of lymphatic vessels and treatment of lymphatic disorders in various conditions.

Adipose-derived stem cells improve efficacy of melanocyte transplantation in animal skin

Vitiligo is a pigmentary disorder induced by a loss of melanocytes. In addition to replacement of pure melanocytes, cocultures of melanocytes with keratinocytes have been used to improve the repigmentation outcome in vitiligo treatment. We previously identified by in vitro studies, that adipose-derived stem cells (ADSCs) could be a potential substitute for keratinocytes in cocultures with melanocytes. In this study, the efficacy of pigmentation including durability of grafted melanocytes and short-term safety was examined in the nude mouse and Sprague-Dawley rat after grafting of primary cultured human melanocytes, with or without different ratios of primary cultured human ADSCs. Simultaneous grafting of melanocytes and ADSCs, which were separately cultured and mixed on grafting at the ratios of 1:1, 1:2, or 1:3, showed better efficacy than that of pure melanocytes. Grafting of melanocytes cocultured with ADSCs resulted in a similar outcome as the grafting of cell mixtures. Skin pigmentation by melanocytes : ADSCs at the ratios of 1:1 and 1:2 was better than at 1:3. No significant difference was observed between the 1-week and 2-week durations in coculturing. Time-course microscopic examination showed that the grafted melanocytes remained a little longer than 6-week post-grafting. No inflammatory cell infiltration was observed in the grafted skin and no melanocytes were detectable in other organs. Collectively, grafting of melanocytes and ADSCs was equally safe and more effective than grafting of melanocytes alone. Despite the absence of significant differences in efficacy between the group of 1:1 and that of 1:2 ratio, 1:2 ratio for 1-week coculturing may be better for clinical use from the cost-benefit viewpoint.

Progress in developing a living human tissue-engineered tri-leaflet heart valve assembled from tissue produced by the self-assembly approach

The aortic heart valve is constantly subjected to pulsatile flow and pressure gradients which, associated with cardiovascular risk factors and abnormal hemodynamics (i.e. altered wall shear stress), can cause stenosis and calcification of the leaflets and result in valve malfunction and impaired circulation. Available options for valve replacement include homograft, allogenic or xenogenic graft as well as the implantation of a mechanical valve. A tissue-engineered heart valve containing living autologous cells would represent an alternative option, particularly for pediatric patients, but still needs to be developed. The present study was designed to demonstrate the feasibility of using a living tissue sheet produced by the self-assembly method, to replace the bovine pericardium currently used for the reconstruction of a stented human heart valve. In this study, human fibroblasts were cultured in the presence of sodium ascorbate to produce tissue sheets. These sheets were superimposed to create a thick construct. Tissue pieces were cut from these constructs and assembled together on a stent, based on techniques used for commercially available replacement valves. Histology and transmission electron microscopy analysis showed that the fibroblasts were embedded in a dense extracellular matrix produced in vitro. The mechanical properties measured were consistent with the fact that the engineered tissue was resistant and could be cut, sutured and assembled on a wire frame typically used in bioprosthetic valve assembly. After a culture period in vitro, the construct was cohesive and did not disrupt or disassemble. The tissue engineered heart valve was stimulated in a pulsatile flow bioreactor and was able to sustain multiple duty cycles. This prototype of a tissue-engineered heart valve containing cells embedded in their own extracellular matrix and sewn on a wire frame has the potential to be strong enough to support physiological stress. The next step will be to test this valve extensively in a bioreactor and at a later date, in a large animal model in order to assess in vivo patency of the graft.

Comparison of different cooling rates for fibroblast and keratinocyte cryopreservation

Easy, cost-effective and reliable cryopreservation protocols are crucial for the successful and effective application of tissue engineering. Several different protocols are in use, but no comprehensive comparisons across different machine-based and manual methods have been made. Here, we compare the effects of different cooling rates on the post-thaw survival and proliferative capacity of two basic cell lines for skin tissue engineering fibroblasts and keratinocytes, cultured and frozen in suspension or as a monolayer. We demonstrate that effectiveness of cryopreservation cannot be reliably determined immediately after thawing: the results at this stage were not indicative of cell growth in culture 3 days post-thaw. Cryopreservation of fibroblasts in an adherent state greatly diminishes their subsequent growth potential. This was not observed when freezing in suspension. In keratinocytes, however, adherent freezing is as effective as freezing in suspension, which could lead to significant cost and labour savings in a tissue-engineering environment. The 'optimal' cryopreservation protocol depends on cell type and intended use. Where time, ease and cost are dominant factors, the direct freezing into a nitrogen tank (straight freeze) approach remains a viable method. The most effective solution across the board, as measured by viability 3 days post-thaw, was the commonly used, freezing container method. Where machine-controlled cryopreservation is deemed important for tissue-engineering Good Manufacturing Practice, we present results using a portfolio of different cooling rates, identifying the 'optimal' protocol depending on cell type and culture method. Copyright © 2013 John Wiley & Sons, Ltd.

Human epithelial stem cells persist within tissue-engineered skin produced by the self-assembly approach

To adequately and permanently restore organ function after grafting, human tissue-engineered skin substitutes (TESs) must ultimately contain and preserve functional epithelial stem cells (SCs). It is therefore essential that a maximum of SCs be preserved during each in vitro step leading to the production of TESs such as the culture process and the elaboration of a skin cell bank by cryopreservation. To investigate the presence and functionality of epithelial SCs within the human TESs made by the self-assembly approach, slow-cycling cells were identified using 5'-bromo-2'-deoxyuridine (BrdU) in the three-dimensional construct. A subset of basal epithelial cells retained the BrdU label and was positive for the SC-associated marker keratin 19 within TESs after a chase of 21 days in culture post-BrdU labeling. Moreover, keratinocytes harvested from TESs gave rise to SC-like colonies in secondary monolayer subcultures, indicating that SCs were preserved within TESs. To evaluate the effect of cryopreservation with dimethyl sulfoxide and storage in liquid nitrogen on SCs, human epithelial cells were extracted from skin samples, amplified in culture, and used to produce TESs, before cryopreservation as well as after thawing. We found that the proportion and the growth potential of epithelial SCs in monolayer culture and in TESs remained constant before and after cryopreservation. Further, the functionality of these substitutes was demonstrated by successfully grafting human TESs on athymic mice for 6 months. We conclude that human epithelial skin SCs are adequately preserved upon human tissue reconstruction. Thus, these TESs produced by the self-assembly approach are suitable for clinical applications.

Epidermal growth factor variations in amniotic membrane used for ex vivo tissue constructs

INTRODUCTION

The amniotic membrane (AM) is used for engineering ex vivo tissue constructs used in ocular surface reconstruction. Epidermal growth factor (EGF) content of the AM is believed to play a key role in supporting corneal epithelial cell expansion on AM. This study investigated EGF content in AM in relation to intra- and inter-donor variations and the effect of processing and preservation (handling).

METHODS

Fifteen human AM, both fresh and handled, were analyzed for EGF gene and protein expression by real-time polymerase chain reaction and ELISA, respectively.

RESULTS

EGF gene expression was predominantly seen in the AM epithelium (p<0.01). Similarly, EGF protein too was predominantly seen in the epithelial layer (p<0.01) for fresh and handled samples. EGF protein content varied between membranes (inter-donor) and at different sites within the same membrane (intra-donor). The highest EGF protein concentration was noted in the AM apical and mid-region epithelium. Significant EGF protein loss (p<0.01) was observed after handling.

CONCLUSION

There is a considerable variation in EGF content between and within donors. This is further affected by handling of the AM. Such variations could affect the clinical efficacy of tissue constructs. Current use of AM for ex vivo expansion of epithelial cells is not standardized and remains an area of concern.

Preservation, sterilization and de-epithelialization of human amniotic membrane for use in ocular surface reconstruction

In the past 20 years, human amniotic membrane (AM) has become widely used as an ophthalmic surgical patch as well as a substrate for stem cell tissue equivalents for ocular surface reconstruction. AM reduces ocular surface scarring and inflammation, and enhances epithelialization. In addition, it shows limited immunogenicity and some anti-microbial properties. Before being applied clinically, the donor of AM is required to undergo a thorough health screening and the membrane has to undergo an accepted processing routine, which includes preservation, sterilization and de-epithelialization. There have been various articles describing methods in preserving, sterilizing and de-epithelializing AM. Each preparation technique has been reported to have differential effects on the physical and biological properties of the AM. Therefore, it is difficult to establish a standardized procedure. In this review, we discuss the present techniques and several novel, new approaches in the preparation of AM for use in ocular surface reconstruction, and their impact on AM structure and biological activity.

Skin tissue engineering

The major applications of tissue-engineered skin substitutes are in promoting the healing of acute and chronic wounds. Several approaches have been taken by commercial companies to develop products to address these conditions. Skin substitutes include both acellular and cellular devices. While acellular skin substitutes act as a template for dermal formation, this discussion mainly covers cellular devices. In addressing therapeutic applications in tissue engineering generally, a valuable precursor is an understanding of the mechanism of the underlying pathology. While this is straightforward in many cases, it has not been available for wound healing. Investigation of the mode of action of the tissue-engineered skin substitutes has led to considerable insight into the mechanism of formation, maintenance and treatment of chronic wounds. Four aspects mediating healing are considered here for their mechanism of action: (i) colonization of the wound bed by live fibroblasts in the implant, (ii) the secretion of growth factors, (iii) provision of a suitable substrate for cell migration, particularly keratinocytes and immune cells, and (iv) modification of the immune system by secretion of neutrophil recruiting chemokines. An early event in acute wound healing is an influx of neutrophils that destroy planktonic bacteria. However, if the bacteria are able to form biofilm, they become resistant to neutrophil action and prevent reepithelialization. In this situation the wound becomes chronic. In chronic wounds, fibroblasts show a senescence-like phenotype with decreased secretion of neutrophil chemoattractants that make it more likely that biofilms become established. Treatment of the chronic wounds involves debridement to eliminate biofilm, and the use of antimicrobials. A role of skin substitutes is to provide non-senescent fibroblasts that attract and activate neutrophils to prevent biofilm re-establishment. The emphasis of the conclusion is the importance of preventing contaminating bacteria becoming established and forming biofilms.

IFATS collection: Using human adipose-derived stem/stromal cells for the production of new skin substitutes

The ability to harvest and culture stem cell populations from various human postnatal tissues is central to regenerative medicine applications, including tissue engineering. The discovery of multipotent mesenchymal stem cells within the stromal fraction of adipose tissue prompted their use for the healing and reconstruction of many tissues. Here, we examined the influence of adipose-derived stem/stromal cells (ASCs) on skin's regenerative processes, from a tissue engineering perspective. Using a self-assembly approach, human skin substitutes were produced. They featured a stromal compartment containing human extracellular matrix endogenously produced from either dermal fibroblasts or adipose-derived stem/stromal cells differentiated or not toward the adipogenic lineage. Human keratinocytes were seeded on each stroma and cultured at the air-liquid interface to reconstruct a bilayered skin substitute. These new skin substitutes, containing an epidermis and a distinctive stroma devoid of synthetic biomaterial, displayed characteristics similar to human skin. The influence of the type of stromal compartment on epidermal morphogenesis was assessed by the evaluation of tissue histology, the expression of key protein markers of the epidermal differentiation program (keratin [K] 14, K10, transglutaminase), the expression of dermo-epidermal junction components (laminins, collagen VII), and the presence of basement membrane and hemidesmosomes. Our findings suggest that adipose-derived stem/stromal cells could usefully substitute dermal fibroblasts for skin reconstruction using the self-assembly method. Finally, by exploiting the adipogenic potential of ASCs, we also produced a more complete trilayered skin substitute consisting of the epidermis, the dermis, and the adipocyte-containing hypodermis, the skin's deepest layer. Disclosure of potential conflicts of interest is found at the end of this article.

Multifunctional implantable particles for skin tissue regeneration: preparation, characterization, in vitro and in vivo studies

The transplantation of cell-polymer constructs has been developed as a novel approach to curing tissue defects. However, a number of methodological problems remain to be solved, including the loss of a proper cellular milieu, the relatively long period of culture time and the complexity of the application. The aim of the present article is to evaluate the feasibility of porous gelatin-based implantable particles as a novel strategy for delivery of cultured cells and bioactive molecules to correct dermal defects. For this purpose, implantable porous gelatin particles (100-230 microm) encapsulating proliferative growth factors were prepared and characterized, and their influence on fibroblasts was assessed. In vivo examinations were undertaken to observe guided dermal tissue regeneration after the transplantation of the implantable particles. Our results indicate the feasibility of transplanting multifunctional implantable particles as a culture substrate, as a protein transplantation vehicle or as a biodegradable implant for skin regeneration, thus giving an indication of the possible applications in tissue engineering.

Employing human keratinocytes cultured on macroporous gelatin spheres to treat full thickness-wounds: An in vivo study on athymic rats

Providing cutaneous wounds with sufficient epidermis to prevent infections and fluid loss is one of the most challenging tasks associated with surgical treatment of burns. Recently, application of cultured keratinocytes in this context has allowed this challenge to be met without several of the limitations connected with the use of split-thickness skin grafts. The continuous development of this novel approach has now revealed that transplantation of cultured autologous keratinocytes as single-cell suspensions exhibits several advantages over the use of cultured epidermal grafts. However, a number of methodological problems remain to be solved, primarily with regards to the complexity of culturing these cells; loss of viability and other negative effects during their preparation and transportation; the relatively long period of time required following transplantation to obtain a sufficiently protective epidermis. In the present investigation we attempted to eliminate these limitations by culturing the keratinocytes on macroporous gelatin spheres. Accordingly, the efficacies of normal human keratinocytes in single-cell suspension or growing on macroporous gelatin spheres, as well as of split-thickness skin grafts in healing wounds on athymic rats were compared. Human keratinocytes were found to adhere and proliferate efficiently both on the surface and within the pores of such spheres. Transplantation of such cells adherent to the spheres resulted in significantly more rapid formation of a stratified epidermis than did transplantation of single-cell suspensions or spheres alone. Twenty-three days after transplantation, the epidermis formed from the cells bound to the spheres was not as thick as the epidermis on wounds covered with split-thickness skin grafts, but significantly thicker than on wounds to which single-cell suspensions, spheres alone or no transplant at all was applied. Furthermore, fluorescence in situ hybridisation revealed that the transplanted keratinocytes, both those adherent to gelatin spheres and those in single-cell suspension, were components of the newly formed epidermis. These findings indicate that application of biodegradable macroporous spheres may prove to be of considerable value in designing cell-based therapies for the treatment of acute and persistent wounds.

An active artificial cornea with the function of inducing new corneal tissue generation in vivo-a new approach to corneal tissue engineering

An active artificial cornea which can perform the function of inducing new cornea generation in vivo but does not need culture cells in vitro and which has similar optical and mechanical properties to those of the human cornea was constructed. An animal keratoplasty experiment using the artificial cornea as the implant showed that the animals' corneas could keep smooth surface and clear stroma postoperatively, and that the repopulation of the host's keratocytes, the degradation of the implant and new corneal tissue generation were completed at 5-6 months after surgery. Such an artificial cornea has several advantages over other corneal equivalents constructed in the typical way of tissue engineering: in having similar mechanical and optical properties to those of the human cornea and with no exogenetic cells, it can be used universally in different implantation surgeries without immunoreaction; it is easy to prepare and process into different shapes and sizes on a large scale, and suitable for long-distance transportation and long-term storage. All these characteristics make it a new approach to cornea tissue engineering having potential in many clinical applications.

The effect of gelatin–chondroitin sulfate–hyaluronic acid skin substitute on wound healing in SCID mice

Tissue-engineered skin substitutes provided a feasibility to overcome the shortage of skin autograft by culturing keratinocytes and dermal fibroblasts in vitro. In this study, we applied bi-layer gelatin-chondrointin-6-sulfate-hyaluronic acid (gelatin-C6S-HA) biomatrices onto the severe combined immunodeficiency (SCID) mice to evaluate its effect on promoting wound healing. Human foreskin keratinocytes and dermal fibroblasts were cultured with reconstructed skin equivalent (rSE) for 7 days. The rSE was then grafted to the dorsum of SCID mice to evaluate its biocompatibility by histologic and immunohistochemistry analysis. The results showed that human epidermis were well-developed with the expression of differentiated markers and basement membrane-specific proteins at 4 weeks. After implantation, the percentages of skin graft take were satisfactory, while cell-seeded group was better than non-cell-seeded one. The basement membrane proteins including laminin, type IV collagen, type VII collagen, integrin alpha6, and integrin beta4 were all detected at the dermal-epidermal junction, which showed a continuous structure in the 4 weeks after grafting. This bi-layer gelatin-C6S-HA skin substitute not only has positive effect on promoting wound healing, but also has high rate of graft take. This rSE would have the potential to be applied on the extensively and deeply burned patients who suffer from severe skin defect in the near future.

Reconstructed human skin produced in vitro and grafted on athymic mice

BACKGROUND

The best alternative to a split-thickness graft for the wound coverage of patients with extensive burns should be in vitro reconstructed autologous skin made of both dermis and epidermis and devoid of exogenous extracellular matrix proteins and synthetic material. We have designed such a reconstructed human skin (rHS) and present here its first in vivo grafting on athymic mice.

METHODS

The rHS was made by culturing newborn or adult keratinocytes on superimposed fibrous sheets obtained after culturing human fibroblasts with ascorbic acid. Ten days after keratinocyte seeding, reconstructed skins were either cultured at the air-liquid interface or grafted on athymic mice. We present the macroscopic, histologic, and phenotypic properties of such tissues in vitro and in vivo after grafting on nude mice.

RESULTS

After maturation in vitro, the reconstructed skin exhibited a well-developed human epidermis that expressed differentiated markers and basement membrane proteins. Four days after grafting, a complete take of all grafts was obtained. Histological analysis revealed that the newly generated epidermis of newborn rHS was thicker than that of adult rHS after 4 days but similar 21 days after grafting. The basement membrane components (bullous pemphigoid antigens, laminin, and type IV and VII collagens) were detected at the dermo-epidermal junction, showing a continuous line 4 days after grafting. Ultrastructural studies revealed that the basement membrane was continuous and well organized 21 days after transplantation. The macroscopic aspect of the reconstructed skin revealed a resistant, supple, and elastic tissue. Elastin staining and elastic fibers were detected as a complex network in the rHS that contributes to the good elasticity of this new reconstructed tissue.

CONCLUSIONS

This new rHS model gives supple and easy to handle skins while demonstrating an adequate wound healing on mice. These results are promising for the development of this skin substitute for permanent coverage of burn wounds.

Mechanisms of wound reepithelialization: hints from a tissue-engineered reconstructed skin to long-standing questions

Wound closure of epithelial tissues must occur efficiently to restore rapidly their barrier function. We have developed a tissue-engineered wound-healing model composed of human skin keratinocytes and fibroblasts to better understand the mechanisms of reepithelialization. It allowed us to quantify the reepithelialization rate, which was significantly accelerated in the presence of fibrin or platelet-rich plasma. The reepithelialization of these 6 mm excisional wounds required the contribution of keratinocyte proliferation, migration, stratification, and differentiation. The epidermis regenerated progressively from the surrounding wound margins. After 3 days, the neoepidermis showed a complete spectrum of changes. Near the wound margin, the differentiation of the neoepidermis (keratins 1/10, filaggrin, and loricrin) and regeneration of the dermoepidermal junction (laminin 5 and collagen IV) were more advanced than toward the wound center, where the proliferative index was significantly increased. The spatial distribution of keratinocytes distinguished by particular features suggests two complementary mechanisms of reepithelialization: 1) the passive displacement of the superficial layers near the wound margin that would rapidly regenerate a barrier function and 2) the crawling of keratinocytes over each other at the tip of the progressing neoepidermis. Therefore, this study brings a new perspective to long-standing questions concerning wound reepithelialization.

The mixed lineage kinase leucine-zipper protein kinase exhibits a differentiation-associated localization in normal human skin and induces keratinocyte differentiation upon overexpression

Leucine-zipper protein kinase/dual leucine zipper bearing kinase/mitogen-activated protein kinase-upstream kinase is a recently described protein serine/threonine kinase which belongs to the mixed lineage kinase family. The overall pattern of expression of the leucine-zipper protein kinase/dual leucine zipper bearing kinase/mitogen-activated protein kinase-upstream kinase gene in embryonic and adult mouse tissues suggested that this kinase could be involved in the regulation of epithelial cell proliferation and differentiation. In order to get more insights into the potential role of leucine-zipper protein kinase in these cellular processes, we characterized its expression in normal human skin, both at the mRNA and protein levels. In situ hybridization, western blotting, and indirect immunofluorescence studies were conducted to localize leucine-zipper protein kinase on various human skin tissues. This is one of the first reports that leucine-zipper protein kinase has a very precise pattern of expression in human skin epithelia, as both mRNA and protein are restricted to the granular layer of the epidermis and inner root sheath of hair follicles. Detection of leucine-zipper protein kinase protein on skin from various body sites, donors of different ages as well as on reconstructed human skin always reveals that leucine-zipper protein kinase is present only in the very differentiated keratinocytes of epidermis and hair follicles. To determine directly whether leucine-zipper protein kinase exhibits any effect on cell growth and differentiation, keratinocytes were transfected with an expression vector harboring the leucine-zipper protein kinase cDNA. The presence of this construct in keratinocytes results in growth arrest together with a concomitant increase in filaggrin expression. Collectively, our results indicate that leucine-zipper protein kinase plays an active part in cellular processes related to terminal differentiation of epidermal keratinocytes.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Role of wound healing myofibroblasts on re-epithelialization of human skin

In human skin, large burned surfaces heal using two concomitant phenomena: re-epithelialization and dermal neoformation. Numerous studies report the role of interactions between keratinocytes and fibroblasts, but the relationship between wound healing myofibroblasts and keratinocytes is not clear, even though these two cell types coexist during healing. We investigated the influence of myofibroblasts on keratinocyte growth and differentiation using an in vitro skin model. A histological study was performed to determine the speed and quality of epithelialization. When the dermis was populated with fibroblasts, a continuous epidermis was formed in 7-10 days. In contrast, with wound healing myofibroblasts or without cell in dermis, the complete reepithelialization never occurred over the 10-day period studied. After 7 further days of epidermal differentiation, histology showed an epidermis more disorganized and expression of basement membrane constituents was reduced when wound healing myofibroblasts or no cells were added in the dermis instead of fibroblasts. These results suggest that wound healing myofibroblasts are not efficient to stimulate keratinocyte growth and differentiation. Treatment of fibroblasts with TGFbeta1 induced an increase of epidermal cell differentiation as seen when myofibroblasts were present. However, this cytokine did not change re-epithelialization rate and induced an increase of basement membrane matrix deposition in opposition to myofibroblasts. Thus, TGFbeta1 action is not sufficient to explain all the different keratinocyte reactions towards fibroblasts and wound healing myofibroblasts. Our conclusion is that myofibroblasts seem to have a limited role in the re-epithelialization process and might be more associated with the increased extracellular matrix secretion.

Characterization of a new tissue-engineered human skin equivalent with hair

We designed a new tissue-engineered skin equivalent in which complete pilosebaceous units were integrated. This model was produced exclusively from human fibroblasts and keratinocytes and did not contain any synthetic material. Fibroblasts were cultured for 35 d with ascorbic acid and formed a thick fibrous sheet in the culture dish. The dermal equivalent was composed of stacked fibroblast sheets and exhibited some ultrastructural organization found in normal connective tissues. Keratinocytes seeded on this tissue formed a stratified and cornified epidermis and expressed typical markers of differentiation (keratin 10, filaggrin, and transglutaminase). After 4 wk of culture, a continuous and ultrastructurally organized basement membrane was observed and associated with the expression of laminin and collagen IV and VII. Complete pilosebaceous units were obtained by thermolysin digestion and inserted in this skin equivalent in order to assess the role of the transfollicular route in percutaneous absorption. The presence of hair follicles abolished the lag-time observed during hydrocortisone diffusion and increased significantly its rate of penetration in comparison to the control (skin equivalent with sham hair insertion). Therefore, this new hairy human skin equivalent model allowed an experimental design in which the only variable was the presence of pilosebaceous units and provided new data confirming the importance of hair follicles in percutaneous absorption.

Tissue-engineered human skin substitutes developed from collagen-populated hydrated gels: clinical and fundamental applications

The field of tissue engineering has opened several avenues in biomedical sciences, through ongoing progress. Skin substitutes are currently optimised for clinical as well as fundamental applications. The paper reviews the development of collagen-populated hydrated gels for their eventual use as a therapeutic option for the treatment of burn patients or chronic wounds: tools for pharmacological and toxicological studies, and cutaneous models for in vitro studies. These skin substitutes are produced by culturing keratinocytes on a matured dermal equivalent composed of fibroblasts included in a collagen gel. New biotechnological approaches have been developed to prevent contraction (anchoring devices) and promote epithelial cell differentiation. The impact of dermo-epidermal interactions on the differentiation and organisation of bio-engineered skin tissues has been demonstrated with human skin cells. Human skin substitutes have been adapted for percutaneous absorption studies and toxicity assessment. The evolution of these human skin substitutes has been monitored in vivo in preclinical studies showing promising results. These substitutes could also serve as in vitro models for better understanding of the immunological response and healing mechanism in human skin. Thus, such human skin substitutes present various advantages and are leading to the development of other bio-engineered tissues, such as blood vessels, ligaments and bronchi.

Hypertrophic scarring is associated with epidermal abnormalities: an immunohistochemical study

The role of epidermal keratinocytes in the early phases of normal unimpaired wound healing has been studied extensively. However, little is known about the cell biological processes in the epidermis and the basal membrane zone during the later phases of dermal matrix formation and remodelling of the scar tissue. This study investigated epidermal growth and differentiation and maturation of the basal membrane zone. Biopsies were taken from (clinically) hypertrophic and non-hypertrophic scars at 3 and 12 months after a breast-reduction operation. Tissues were analysed using immunohistochemical techniques. The data showed that epidermal abnormalities with respect to differentiation persist up to 3 months, as witnessed by the expression of cytokeratin 16. Remarkably, hypertrophic scars that remained hypertrophic throughout the period of analysis (up to 12 months) showed significantly more cytokeratin 16 expression at 3 months, when compared either with normal scars or with hypertrophic scars that became normal after 12 months. Staining for Ki-67 antigen, a marker for cell proliferation, revealed an increase in basal keratinocyte proliferation rate in 3-month-old hypertrophic scars compared with non-hypertrophic scars. After 12 months, this difference had disappeared completely and the number of cycling basal cells had returned to normal values. Three-month-old hypertrophic scars showed more acanthosis than non-hypertrophic scars of the same age, irrespective of whether they remained hypertrophic or became normal scars. After 12 months, this difference was no longer present. Staining for various heparan sulphate proteoglycan epitopes revealed that restoration of the basal membrane was incomplete at 3 months, but was complete at 12 months with respect to this component. No differences in the expression of several components of the basal membrane zone (heparan sulphate proteoglycan, laminin, tenascin) were noted between hypertrophic and non-hypertrophic scars. These data show that in the early phase of hypertrophic scarring, epidermal abnormalities are found compared with normal wound healing. In addition, early (3 months) epidermal abnormalities are associated with the clinical outcome at 12 months. These findings raise the possibility that the epidermal compartment is involved in the pathogenic process.

Equine laminitis basement membrane pathology: loss of type IV collagen, type VII collagen and laminin immunostaining

Disintegration of the basement membrane (BM) of the equine hoof lamellae and failure of the BM to remain attached to the basal cells of the secondary epidermal lamellae (SEL) is one of the earliest pathological events to occur in acute laminitis. Changes in the lamellar basement membrane were investigated by immunolabelling the key structural components of the BM, type IV collagen, type VII collagen and laminin in the lamellar BM of horses 48 h after the induction of laminitis. Lamellar tissues were harvested from 2 normal horses and 2 horses with acute laminitis. Immunostaining with antibody raised against human epitopes for type IV collagen, type VII collagen and laminin successfully stained the basement membranes of horse hoof lamellar tissues. Vascular tissue did not immunostain with type VII collagen antibody. Normal BM stained as a fine dark brown line and the lamellar BM was adhered to the basal cells of the SELs with no evidence of lamellar separation. At least 2 changes to the lamellar BM occurred in acute laminitis: loss of attachment of lamellar epidermal basal cells to their underlying BM and disintegration of the lamellar BM. In some sections from feet affected by acute laminitis, there was widespread separation of the SELs from their BM without loss of BM immunostaining and in others there was extensive loss of BM immunostaining. In lesions characterised by lamellar separation, the epidermal basal cells at the tips of the primary epidermal lamellae appeared to have slipped away from their BM and were an amorphous clump of epidermal cells devoid of immunostained BM. The BM from which they had separated remained in its original position in the dermis and was clearly outlined by all 3 antibodies. In other areas, however, virtually all the BM immunoreactivity at the PEL tips was absent. Only the occasional distorted SEL tip and fragments of BM retained sufficient immunostaining to allow anatomical identification. Numerous polymorphonuclear leucocytes (PMNs) invariably surrounded the tips of lamellae showing large scale loss of immunoreactivity and many PMNs had penetrated the lamellar BM and were within the epidermal compartments. PMNs were less frequent in the midlamellar region. Immunostaining of the BM of many SELs was absent in the midlamellar region. In some lamellae loss of BM immunostaining had occurred only at the bases of the SELs and fragments of immunostained BM were present in the zones of lysed BM suggesting that BM lysis was incomplete at the time of tissue fixation. In other lamellae, lysis of the BM was complete; there was no immunostained BM between SELs and the bulk of the epidermal cells of each PEL were an amorphous column of cells on either side of the central keratinised axis of the PEL. The lamellar BM which remained appeared as immunostained strands of unattached BM along the edges of the PDLs. Activation of BM degrading metalloproteinases (MMPs) occurs in acute laminitis and it seems likely that uncontrolled MMP activity is responsible for the loss and disorganisation of lamellar BM demonstrated in this study.

Cutaneous gene therapy

Gene therapy efforts in a variety of tissues have foundered on fundamental technologic barriers, such as difficulties in achieving high-efficiency gene transfer to diseased tissues and in sustaining delivered transgene production. The skin offers an attractive tissue for development of approaches to therapeutic gene delivery by virtue of its accessibility for regulation by topical agents, the ease of gene transfer into cutaneous tissues, and the ready ability to monitor the impact of somatic gene transfer. With the ability of the skin to deliver therapeutic polypeptides to the systemic circulation and the recent molecular characterization of monogenic skin diseases, efforts to target genes to the skin are expected to accelerate. The current status of gene therapy efforts is reviewed, with a special focus on the skin.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Cultured epidermal sheet grafting with Hemaseel HMN fibrin sealant on nude mice

Grafting of cultured epidermal sheets is a promising technique for skin restoration in extensive burns, but the technique has some limitations, resulting in variable graft takes. These experiments were designed to evaluate the innocuity of Hemaseel HMN fibrin sealant in the grafting process and in vivo evolution of cultured epidermis. A total of 30 mice were grafted, 15 were controls, 15 received tissue sealant application before the deposition of the cultured human epidermal sheets. Seven days after transplantation, compared to controls, the percentage of graft take over the total surface area grafted was greater in animals that had received the tissue sealant application. No difference was found 14 and 21 days postgrafting. In contrast, the percentage of graft take over the bony area (spinal) was significantly increased in animals grafted with previous application of sealant compared to controls at 7, 14 and 21 days postgrafting. Immunohistological and ultrastructural analysis showed that the evolution of the cultured human epidermis after transplantation was similar in both groups. The basement membrane was well structured 21 days after transplantation. The sealant was present at 4 days but not at 21 days postgrafting. Therefore, we conclude that the application of fibrin sealant before cultured epidermal sheet deposition on nude mouse graft bed is innocuous and enhances their mechanical stability. Since in this nude mouse system Hemaseel HMN fibrin sealant increased the percentage of graft take over areas difficult to engraft, we think that it may be advantageous in cultured epidermal sheet grafting on burn patients.