Tissue engineering of replacement skin: the crossroads of biomaterials, wound healing, embryonic development, stem cells and regeneration

Properties and biocompatibility of chitosan and silk fibroin blend films for application in skin tissue engineering

Chitosan/silk fibroin (CS/SF) blend films were prepared and evaluated for feasibility of using the films as biomaterial for skin tissue engineering application. Fourier transform infrared spectroscopy and differential scanning calorimetry analysis indicated chemical interaction between chitosan and fibroin. Chitosan enhanced β-sheet conformation of fibroin and resulted in shifting of thermal degradation of the films. Flexibility, swelling index, and enzyme degradation were also increased by the chitosan content of the blend films. Biocompatibility of the blend films was determined by cultivation with fibroblast cells. All films showed no cytotoxicity by XTT assay. Fibroblast cells spread on CS/SF films via dendritic extensions, and cell-cell interactions were noted. Cell proliferation on CS/SF films was also demonstrated, and their phenotype was examined by the expression of collagen type I gene. These results showed possibility of using the CS/SF films as a supporting material for further study on skin tissue engineering.

Tissue engineering and regenerative medicine: history, progress, and challenges

The past three decades have seen the emergence of an endeavor called tissue engineering and regenerative medicine in which scientists, engineers, and physicians apply tools from a variety of fields to construct biological substitutes that can mimic tissues for diagnostic and research purposes and can replace (or help regenerate) diseased and injured tissues. A significant portion of this effort has been translated to actual therapies, especially in the areas of skin replacement and, to a lesser extent, cartilage repair. A good amount of thoughtful work has also yielded prototypes of other tissue substitutes such as nerve conduits, blood vessels, liver, and even heart. Forward movement to clinical product, however, has been slow. Another offshoot of these efforts has been the incorporation of some new exciting technologies (e.g., microfabrication, 3D printing) that may enable future breakthroughs. In this review we highlight the modest beginnings of the field and then describe three application examples that are in various stages of development, ranging from relatively mature (skin) to ongoing proof-of-concept (cartilage) to early stage (liver). We then discuss some of the major issues that limit the development of complex tissues, some of which are fundamentals-based, whereas others stem from the needs of the end users.

Cloning and characterization of a trypsin-like serine protease gene, a novel regeneration-related gene from Apostichopus japonicus

Trypsin-like serine protease (TLS) plays an important role in many physiological processes including wound healing, phlogosis reaction, blood clotting, regeneration etc. In this paper, a 1216 bp full-length cDNA sequence of TLS including 39 bp 5' UTR and 355 bp 3'UTR coding for a theoretical 273 amino acids protein was cloned from Apostichopus japonicus by means of the RACE technique for the first time. Bioinformatic analysis revealed that the gene with a 20 residues N-terminal signal peptide and a conserved C-terminal domain belongs to the trypsin-like serine protease superfamily. His78, Asp130 and Ser223 are the principal residues of the catalytic center. In-situ hybridization (ISH) analysis revealed that the TLS gene was widely distributed in different tissues. The expression patterns during different regeneration stages of the TLS gene in the body wall, intestine and respiratory trees were investigated using real-time quantitative PCR. The results show that there was a remarkable and temporary up-regulation of TLS gene expression in the body wall within 1h and subsequent down-regulation of TLS similar to intestine and respiratory trees. With the recovery of tissues, the expression level of the TLS gene was gradually up-regulated and finally reached normal levels. TLS was regulated during different regeneration stages suggesting that TLS is important in the regeneration process of A. japonicus.

Tissue engineering of skin

Each one of us is a self-organizing mass of multiple cell types. From fertilization of the embryo our tissue structures develop until an adult morphology is achieved. At that point our capacity for self-organization is directed to maintaining that morphology in the face of the insults of our daily life and the processes of aging. When a given insult overwhelms our capacity to repair by regeneration the result is scar repair.

Nanomaterials and nanotechnology for skin tissue engineering

A recent literature review of the field shows that tissue-engineered skin has been in clinical use for the last several decades and that, over this time the technology has advanced rapidly. Despite this progress no synthetic skin yet produced has completely replicated normal, healthy skin. Therefore, researchers must continue to develop materials that successfully overcome the problems with current skin tissue substitutes. This paper is a comprehensive review of the prospects for nanotechnology and nanomaterials to close this gap by mimicking surface properties for reconstruction of a variety of skin tissues. In addition, a number of commercially available products that regenerate different layers of the burn-damaged or chronically wounded skin are reviewed.

Self-assembly of silk-collagen-like triblock copolymers resembles a supramolecular living polymerization

We produced several pH-responsive silk-collagen-like triblocks, one acidic and two alkaline. At pH values where the silk-like block is uncharged the triblocks self-assemble into filaments. The pH-induced self-assembly was examined by atomic force microscopy, light scattering, and circular dichroism. The populations of filaments were found to be very monodisperse, indicating that the filaments start to grow from already present nuclei in the sample. The growth then follows pseudo-first-order kinetics for all examined triblocks. When normalized to the initial concentration, the growth curves of each type of triblock overlap, showing that the self-assembly is a generic process for silk-collagen-silk triblocks, regardless of the nature of their chargeable groups. The elongation speed of the filaments is slow, due to the presence of repulsive collagen-like blocks and the limited number of possibilities for an approaching triblock to successfully attach to a growing end. The formation of filaments is fully reversible. Already present filaments can start growing again by addition of new triblocks. The structure of all filaments is very rich in β-turns, leading to β-rolls. The triblocks attain this structure only when attaching to a growing filament.

Spatial arrangement of polycaprolactone/collagen nanofiber scaffolds regulates the wound healing related behaviors of human adipose stromal cells

A sufficient cell source and minimal invasiveness in obtaining human adipose stromal cells (hASCs) hold great promise for their utilization in wound repair. However, little is known about how cell-residing microenvironments regulate the cellular response. In this study we explored the effects of polycaprolactone (PCL)/collagen nanofibers with distinct spatial arrangements (aligned and random) on phenotypic expression of hASCs in vitro. Elongated cell morphology, higher proliferation, and faster migration rate were observed for hASCs cultured on the aligned nanofibers, showing that hASCs could detect the nanofiber spatial arrangement and then distinctively respond. This study on the expression of extracellular matrix (ECM) related genes in hASCs revealed higher synthesis capacity for critical ECM molecules including tropoelastin, collagen I, and matrix metalloproteinase (MMP)-1 on the aligned nanofibers. Integrins α(5), β(1), β(3), β(6,) and transforming growth factor (TGF)-β(1) were differentially regulated by PCL/collagen nanofiber arrangements. Our results indicate that fiber orientation-induced phenotypic change of hASCs may be regulated by integrins and TGF-β signaling synergistically. These findings demonstrate the potential application of hASCs and aligned PCL/collagen nanofibers for accelerated wound repair.

Human CD34+ stem cells promote healing of diabetic foot ulcers in rats

OBJECTIVE

Diabetic patients with foot ulcers usually manifest with high amputation and mortality rates. Preliminary evidence supports the effectiveness of stem cell (St) therapy on diabetic foot ulcers. The objective of this study was to evaluate the efficacy of stem cells in the healing of wounds among streptozotocin-induced diabetic albino rats.

METHODS

Thirty male albino rats were divided into three groups each of 10 rats: control group, diabetic control (DC) group and St group. Diabetes was induced by intra-peritoneal injection of streptozotocin. A full thickness circular wound of ∼10 mm in diameter was performed on the front of right legs of all rats. In the diabetic St group, the wounds were treated by injection of umbilical cord blood-derived CD34+ stem cells into the wound bed. Half of each group rats were sacrificed after 1 week and the rest after 2 weeks. The wound areas were used for histopathology, immunohistochemistry and transmission electron microscope studies. Assessment of wound surface area, epidermal thickness, blood vessel proliferation and collagen deposition were performed.

RESULTS

There was a significant decrease in mean wound surface area, increase in mean epidermal thickness, blood vessel proliferation and collagen deposition in the St group compared with the DC group.

CONCLUSION

Treatment with CD34+-enriched cells decreased wound size, accelerated epidermal healing and dramatically accelerated revascularization of the wounds compared with the DC group.

Denervation affects regenerative responses in MRL/MpJ and repair in C57BL/6 ear wounds

The MRL/MpJ mouse displays the rare ability amongst mammals to heal injured ear tissue without scarring. Numerous studies have shown that the formation of a blastema-like structure leads to subsequent tissue regeneration in this model, indicating many parallels with amphibian limb regeneration and mammalian embryogenesis. We have recently shown that the MRL/MpJ mouse also possesses an enhanced capacity for peripheral nerve regeneration within the ear wound. Indeed, nerves are vital for the initial phase of blastema formation in the amphibian limb. In this study we investigated the capacity for wound regeneration in a denervated ear. The left ears of MRL/MpJ mice and C57BL/6 (a control strain known to have a poorer regenerative capacity) were surgically denervated at the base via an incision and nerve transection, immediately followed by a 2-mm ear punch wound. Immunohistochemical analysis showed a lack of neurofilament expression in the denervated ear wound. Histology revealed that denervation prevented blastema formation and chrondrogenesis, and also severely hindered normal healing, with disrupted re-epithelialisation, increasing wound size and progressive necrosis towards the ear tip. Denervation of the ear obliterated the regenerative capacity of the MRL/MpJ mouse, and also had a severe negative effect on the ear wound repair mechanisms of the C57BL/6 strain. These data suggest that innervation may be important not only for regeneration but also for normal wound repair processes.

Stem Cell Therapy: A New Treatment for Burns?

Stem cell therapy has emerged as a promising new approach in almost every medicine specialty. This vast, heterogeneous family of cells are now both naturally (embryonic and adult stem cells) or artificially obtained (induced pluripotent stem cells or iPSCs) and their fates have become increasingly controllable, thanks to ongoing research in this passionate new field. We are at the beginning of a new era in medicine, with multiple applications for stem cell therapy, not only as a monotherapy, but also as an adjunct to other strategies, such as organ transplantation or standard drug treatment. Regrettably, serious preclinical concerns remain and differentiation, cell fusion, senescence and signalling crosstalk with growth factors and biomaterials are still challenges for this promising multidisciplinary therapeutic modality. Severe burns have several indications for stem cell therapy, including enhancement of wound healing, replacement of damaged skin and perfect skin regeneration - incorporating skin appendages and reduced fibrosis -, as well as systemic effects, such as inflammation, hypermetabolism and immunosuppression. The aim of this review is to describe well established characteristics of stem cells and to delineate new advances in the stem cell field, in the context of burn injury and wound healing.

Hair follicle regeneration in skin grafts: current concepts and future perspectives

The repair and management of full-thickness skin defects resulting from burns and chronic wounds remain a significant unmet clinical challenge. For those skin defects exceeding 50%-60% of total body surface area, it is impractical to treat with autologous skin transplants because of the shortage of donor sites. The possibility of using tissue-engineered skin grafts for full-thickness wound repair is a promising approach. The primary goal of tissue-engineered skin grafts is to restore lost barrier function, but regeneration of appendages, such as hair follicles, has to be yet achieved. The successful regeneration of hair follicles in immunodeficient mice suggests that creating human hair follicles in tissue-engineered skin grafts is feasible. However, many limitations still need to be explored, particularly enriching isolated cells with trichogenic capacity, maintaining this ability during processing, and providing the cells with proper environmental cues. Current advances in hair follicle regeneration, in vitro and in vivo, are concisely summarized in this report, and key requirements to bioengineer a hair follicle are proposed, with emphasis on a three-dimensional approach.

Tissue-engineered skin: bottleneck or breakthrough

During the past few years, tissue-engineered skin constructs has offered great promise in the treatment of deep burns and various skin-related disorders. The overall impact of bioengineered skin research, as a multidisciplinary field, has also enhanced our understanding of the structure-function relations and the physiological processes of tissue regeneration within these constructed three-dimensional skin models. Despite of the fact that currently available bioengineered skin products have a range of problems such as patient safety, clinical efficacy and convenience of use, they are still one of the most advanced strategies because of their wide use and potential development in the biomedical field. Here, the challenges and developments in tissue-engineered skin research are discussed. Central to the discussion is the extensive application and future prospects of these bioengineered skin constructs.

The role of ECM proteins and protein fragments in guiding cell behavior in regenerative medicine

The promise of biomaterials design for regenerative medicine tissue engineering is predicated on the fundamental ability to direct or guide specific and highly coordinated cellular behaviors that culminate in the creation of physiologically functional tissues and organs. To date, our efforts have focused primarily on the grafting and presentation of short synthetic peptides with just cause. Short peptides are capable of high levels of control, can be manufactured relatively easily in a highly reproducible manner under GMP guidelines and are readily modified to enable their integration with numerous current and emerging chemistries for biomaterials grafting. However, while extracellular matrix (ECM)-derived peptides have demonstrated their initial purpose of promoting cell adhesion, their general lack of specificity and significantly decreased receptor binding affinities have proven detrimental in attempts to regulate highly specific and integrated processes necessary for tissue regeneration. Unlike adhesion peptides, the natural ECM displays a complex interplay with cells by supporting environmentally sensitive and cell dependent integrin specificity and binding affinity. Furthermore, the adhesion ligands on ECM proteins display a finely tuned and evolutionarily directed spatial periodicity, of which is dynamically controlled through both mechanical and chemical modifications. These and other emerging concepts from matrix biology require our attention if biomaterials design is to fulfill its promise. Here, we are charged with debating the statement 'The use of short synthetic adhesion peptides, like RGD, is the best approach in the design of biomaterials that guide cell behavior for regenerative medicine tissue engineering'. In this Leading Opinion Paper I will focus on aspects of natural ECM proteins and protein fragments that have proven difficult, if not impossible to date, to recapitulate in peptide-based systems. While this represents an argument against the use of peptides per se, it might also be viewed as outlining the challenges and opportunities for the biomaterials field.

Human TSC2-null fibroblast-like cells induce hair follicle neogenesis and hamartoma morphogenesis

Hamartomas are composed of cells native to an organ but abnormal in number, arrangement or maturity. In the tuberous sclerosis complex (TSC), hamartomas develop in multiple organs because of mutations in TSC1 or TSC2. Here we show that TSC2-null fibroblast-like cells grown from human TSC skin hamartomas induced normal human keratinocytes to form hair follicles and stimulated hamartomatous changes. Follicles were complete with sebaceous glands, hair shafts and inner and outer root sheaths. TSC2-null cells surrounding the hair bulb expressed markers of the dermal sheath and dermal papilla. Tumour xenografts recapitulated characteristics of TSC skin hamartomas with increased mammalian target of the rapamycin complex 1 (mTORC1) activity, angiogenesis, mononuclear phagocytes and epidermal proliferation. Treatment with an mTORC1 inhibitor normalized these parameters and reduced the number of tumour cells. These studies indicate that TSC2-null cells are the inciting cells for TSC skin hamartomas, and suggest that studies on hamartomas will provide insights into tissue morphogenesis and regeneration.

Stem cell differentiation to epidermal lineages on electrospun nanofibrous substrates for skin tissue engineering

Bone marrow (BM) mesenchymal stem cells (MSC) capable of differentiating along the epidermal lineage on engineered nanofibrous scaffolds have great potential for bionanomaterial-cell transplantation therapy of skin wounds. MSC have been the focus of many tissue engineering studies, mainly because of their multipotential properties. We investigated the potential of human BM-derived MSC for epidermal cell differentiation in vitro on electrospun collagen/poly(l-lactic acid)-co-poly(3-caprolactone) (Coll/PLLCL) nanofibrous scaffolds. PLLCL and Coll/PLLCL nanofibrous scaffolds were fabricated by an electrospinning process and their chemical and mechanical characterization carried out by scanning electron microscopy (SEM), water contact angle determination, Fourier transform infrared spectroscopy, and tensile testing. The differentiation of MSC was carried out using epidermis inducing factors, including epidermal growth factor (EGF) and 1,25-dihydroxyvitamin D(3), in culture medium. The proliferation of MSC evaluated by cell proliferation assay showed that the number of cells grown on Coll/PLLCL nanofibrous scaffolds was significantly higher than those on PLLCL scaffolds. The SEM results showed that MSC differentiated on Coll/PLLCL nanofibrous scaffolds showed a round keratinocyte morphology and expressed keratin 10, filaggrin and partial involucrin protein by immunofluorescent microscopic studies. The interaction of MSC and nanofibers was studied and we concluded that the electrospun Coll/PLLCL nanofibers could mimic the native skin extracellular matrix environment and are promising substrates for advanced skin tissue engineering. Our studies on the differentiation of MSC along the epidermal lineage on nanofibrous scaffolds suggest their potential application in skin regeneration without regional differentiation.

Artificial skin--culturing of different skin cell lines for generating an artificial skin substitute on cross-weaved spider silk fibres

Background

In the field of Plastic Reconstructive Surgery the development of new innovative matrices for skin repair is in urgent need. The ideal biomaterial should promote attachment, proliferation and growth of cells. Additionally, it should degrade in an appropriate time period without releasing harmful substances, but not exert a pathological immune response. Spider dragline silk from Nephila spp meets these demands to a large extent.

Methodology/Principal Findings

Native spider dragline silk, harvested directly out of Nephila spp spiders, was woven on steel frames. Constructs were sterilized and seeded with fibroblasts. After two weeks of cultivating single fibroblasts, keratinocytes were added to generate a bilayered skin model, consisting of dermis and epidermis equivalents. For the next three weeks, constructs in co-culture were lifted on an originally designed setup for air/liquid interface cultivation. After the culturing period, constructs were embedded in paraffin with an especially developed program for spidersilk to avoid supercontraction. Paraffin cross- sections were stained in Haematoxylin & Eosin (H&E) for microscopic analyses.

Conclusion/Significance

Native spider dragline silk woven on steel frames provides a suitable matrix for 3 dimensional skin cell culturing. Both fibroblasts and keratinocytes cell lines adhere to the spider silk fibres and proliferate. Guided by the spider silk fibres, they sprout into the meshes and reach confluence in at most one week. A well-balanced, bilayered cocultivation in two continuously separated strata can be achieved by serum reduction, changing the medium conditions and the cultivation period at the air/liquid interphase. Therefore spider silk appears to be a promising biomaterial for the enhancement of skin regeneration.

In vivo visualization of the origination of skin graft vasculature in a wild-type/GFP crossover model

INTRODUCTION

Skin substitutes are increasingly produced in tissue engineering, but still the understanding of the physiological skin revascularization process is lacking. To study in vivo conditions we recently introduced a mouse model, in which we already characterized the angiogenic changes within the wound bed and the skin graft. The aim of this study was to identify the origination of the vasculature during skin graft revascularization in vivo and to track vessel development over time.

METHODS

We created a crossover wild-type/GFP skin transplantation model, in which each animal carried the graft from the other strain. The preparation of the modified dorsal skin fold chamber including cross-over skin grafting was performed in male C57BL/6J wild-type mice (n=5) and C57BL/6-Tg(ACTB-EGFP)1O sb/J mice (n=5). Intravital microscopy in 12 areas of wild-type and GFP skin grafts was performed daily over a time period of 10 days.

RESULTS

Graft reperfusion started after 48-72 h. After reperfusion GFP-positive structures from the wound bed were visible in the graft capillaries with the highest density in the center of the graft. Overall, we observed a replacement of existing graft capillaries with vessels from the wound bed in 68% of the vessels. Of note, vessel replacement occurred in almost 100% of graft vessels in the periphery. Additionally, vessels within the graft showed a temporary angiogenic response between days 3-8, which originated predominantly from the autochthonous graft vasculature, but also contained already grown-in vessels from the wound bed.

CONCLUSIONS

These in vivo data indicate an early in-growth of angiogenic bed vessels into the existing vascular channels of the graft and subsequent centripetal replacement. Additionally we observed a temporary angiogenic response of the autochthonous capillaries of the skin graft with contribution from bed vessels. These findings further support the theory that sprouting angiogenesis from the wound bed in combination with the replacement of existing graft vessels are the key factors in skin graft taking. Thus, manufacturing of skin substitutes should be aimed at providing pre-formed vascular channels within the construct to improve vascularization.

A denatured collagen microfiber scaffold seeded with human fibroblasts and keratinocytes for skin grafting

Biomaterial scaffolds are categorized into artificial or natural polymers, or combinations of the two. Artificial polymers often undergo serum protein adsorption, elicit foreign body and encapsulation immune responses post-implantation. Large pore bovine electrospun collagen I was therefore screened as a candidate for human keratinocyte and fibroblast cell scaffolds. Human HaCaT keratinocyte and dermal fibroblasts were seeded on electrospun denatured collagen I microfiber (DCM) scaffolds and after 72 h Livedead(®) assays performed to determine adhesive cell, survival and scaffold penetration. Both keratinocytes and fibroblasts attached to and survived on DCM scaffolds, however only fibroblasts migrated over and into this biomaterial. HaCaT keratinocytes remained largely stationary on the scaffold surface in discrete islands of monolayered cells. For this reason, normal human epidermal keratinocyte (NHEK) scaffold interactions were assessed using scanning and transmission electron microscopy (EM) that demonstrated DCM scaffolds comprised networks of interlocking and protruding collagen fibers with a mean diameter of 2-5 μm, with a mean inter-fiber pore size of 6.7 μm (range 3-10 μm) and scaffold thickness 50-70 μm. After 72 h the keratinocytes and fibroblasts on DCM scaffolds had attached, flattened and spread over the entire scaffold with assembly of lamellapodia and focal adhesion (FA)-like junctions. Using transmission EM, NHEKs and HaCaT keratinocytes assembled desmosomes, lamellapodia and FA junctions, however, neither hemidesmosomes nor basal lamina were present. In long term (21 day) co-culture fibroblasts migrated throughout the scaffold and primary keratinocytes (and to a lesser extend HaCaTs) stratified on the scaffold surface forming a human skin equivalent (HSE). In vivo testing of these HSEs on immunocompetent (BalbC) and immunodeficient (SCID) excisionally wounded model mice demonstrated scaffold wound biocompatibility and ability to deliver human cells after scaffold biodegradation.

Delayed re-epithelialization in periostin-deficient mice during cutaneous wound healing

Background

Matricellular proteins, including periostin, are important for tissue regeneration.

Methods and Findings

Presently we investigated the function of periostin in cutaneous wound healing by using periostin-deficient (−/−) mice. Periostin mRNA was expressed in both the epidermis and hair follicles, and periostin protein was located at the basement membrane in the hair follicles together with fibronectin and laminin γ2. Periostin was associated with laminin γ2, and this association enhanced the proteolytic cleavage of the laminin γ2 long form to produce its short form. To address the role of periostin in wound healing, we employed a wound healing model using WT and periostin−/− mice and the scratch wound assay in vitro. We found that the wound closure was delayed in the periostin−/− mice coupled with a delay in re-epithelialization and with reduced proliferation of keratinocytes. Furthermore, keratinocyte proliferation was enhanced in periostin-overexpressing HaCaT cells along with up-regulation of phosphorylated NF-κB.

Conclusion

These results indicate that periostin was essential for keratinocyte proliferation for re-epithelialization during cutaneous wound healing.

The role of glucose, serum, and three-dimensional cell culture on the metabolism of bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) have become a critical addition to all facets of tissue engineering. While most in vitro research has focused on their behavior in two-dimensional culture, relatively little is known about the cells' behavior in three-dimensional culture, especially with regard to their metabolic state. To evaluate MSC metabolism during twodimensional culture, murine bone marrow-derived MSCs were cultured for one week using twelve different medium compositions, varying in both glucose and fetal bovine serum (FBS) concentrations. The results indicate that glucose concentration was the more important factor in sustaining cell growth and viability. To evaluate metabolic state during three-dimensional culture, MSCs were cultured for one week using two different medium compositions and two different concentrations of collagen gel matrix. The medium compositions only varied in glucose concentration. The results indicate that glucose and extracellular matrix were significant factors in the metabolic response of the cells. However, cells cultured in low density collagen exhibited considerable cell death, likely because of physical contraction of the collagen hydrogel which was not observed in the higher density collagen. These findings will be useful to the development of in vitro cell culture models that properly mimic in vivo physiological processes.

Effects of nanofiber/stem cell composite on wound healing in acute full-thickness skin wounds

Acute full-thickness skin wounds (FTSW) caused by extensive burns or high-energy trauma are not adequately addressed by current clinical treatments. This study hypothesized that biomimetic nanofiber scaffolds (NFSs) functionalized with rich attachment of bone-marrow-derived mesenchymal stem cells (BM-MSCs) can promote wound healing in acute FTSW. Results in a rat model showed that both NFS and BM-MSCs contributed to the wound healing. Wounds in NFS group with a higher density of BM-MSCs achieved complete closure 8 days earlier than the control group. Implanted BM-MSCs were found to promote epithelial edge ingrowth and collagen synthesis. The colocation of BM-MSCs (tagged with quantum-dots) with the expression of keratin 10 and filaggrin indicated the participation of BM-MSCs in epidermal differentiation at early and intermediate stages under the local wounding environment. Overall, this study suggests a great potential of using NFS/BM-MSC composites for the treatment of acute FTSW.

Overview on current and future materials for chest wall reconstruction

This article focuses on new materials available to thoracic surgeons for the reconstruction of chest wall defects. Each surgeon is called to select the best reconstructive strategy based on the disease for which the resection is needed, the possible extension to adjacent structures, the availability of professional colleagues for multidisciplinary involvement, and the preferred (or available) material for full or partial thickness reconstruction.

Combination of stromal cell-derived factor-1 and collagen-glycosaminoglycan scaffold delays contraction and accelerates reepithelialization of dermal wounds in wild-type mice

While dermal substitutes can mitigate scarring and wound contraction, a significant drawback of current dermal replacement technologies is the apparent delay in vascular ingrowth compared with conventional skin grafts. Herein, we examined the effect of the chemokine stromal cell-derived factor-1 (SDF-1) on the performance of a porous collagen-glycosaminoglycan dermal analog in excisional wounds in mice. C57BL/6 mice with 1 cm × 1 cm dorsal full-thickness wounds were covered with a collagen-glycosaminoglycan scaffold, followed by four daily topical applications of 1 μg SDF-1 or phosphate-buffered saline vehicle. Some animals were also pretreated with five daily doses of 300 mg/kg granulocyte colony-stimulating factor. Animals treated with SDF-1 and no granulocyte colony-stimulating factor reepithelialized 36% faster than vehicle controls (16 vs. 25 days), and exhibited less wound contraction on postwounding day 18 (∼ 35% greater wound area) plus three-fold longer neoepidermis formed than controls. Conversely, granulocyte colony-stimulating factor promoted contraction and no epidermal regeneration. Early (postwounding Day 3) inflammatory cell infiltration in the SDF-1-treated group was 86% less, while the fraction of proliferating cells (positive Ki67 staining) was 32% more, when compared with controls. These results suggest that SDF-1 simultaneously delays contraction and promotes reepithelialization and may improve the wound-healing performance of skin substitutes.

Potential applications of natural origin polymer-based systems in soft tissue regeneration

Despite the many advances in tissue engineering approaches, scientists still face significant challenges in trying to repair and replace soft tissues. Nature-inspired routes involving the creation of polymer-based systems of natural origins constitute an interesting alternative route to produce novel materials. The interest in these materials comes from the possibility of constructing multi-component systems that can be manipulated by composition allowing one to mimic the tissue environment required for the cellular regeneration of soft tissues. For this purpose, factors such as the design, choice, and compatibility of the polymers are considered to be key factors for successful strategies in soft tissue regeneration. More recently, polysaccharide-protein based systems have being increasingly studied and proposed for the treatment of soft tissues. The characteristics, properties, and compatibility of the resulting materials investigated in the last 10 years, as well as commercially available matrices or those currently under investigation are the subject matter of this review.

Nanofibers from blends of polyvinyl alcohol and polyhydroxy butyrate as potential scaffold material for tissue engineering of skin

Nanofibers were prepared by electrospinning from pure polyvinyl alcohol (PVA), polyhydroxybutyrate (PHB), and their blends. Miscibility and morphology of both polymers in the nanofiber blends were studied by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC), revealing that PVA and PHB were miscible with good compatibility. DSC also revealed suppression of crystallinity of PHB in the blend nanofibers with increasing proportion of PVA. The hydrolytic degradation of PHB was accelerated with increasing PVA fraction. Cell culture experiments with a human keratinocyte cell line (HaCaT) and dermal fibroblast on the electrospun PHB and PVA/PHB blend nanofibers showed maximum adhesion and proliferation on pure PHB. However, the addition of 5 wt % PVA to PHB inhibited growth of HaCaT cells but not of fibroblasts. On the contrary, adhesion and proliferation of HaCaT cells were promoted on PVA/PHB (50/50) fibers, which inhibited growth of fibroblasts.

Peripheral nerve regeneration in the MRL/MpJ ear wound model

The MRL/MpJ mouse displays an accelerated ability to heal ear punch wounds without scar formation (whereas wounds on the dorsal surface of the trunk heal with scar formation), offering a rare opportunity for studying tissue regeneration in adult mammals. A blastema-like structure develops and subsequently the structure of the wounded ear is restored, including cartilage, skin, hair follicles and adipose tissue. We sought to assess if the MRL/MpJ strain also possessed an enhanced capacity for peripheral nerve regeneration. Female MRL/MpJ and C57BL/6 mice were wounded with a 2-mm excisional biopsy punch to the centre of each ear and two 4-mm excisional biopsy punches to the dorsal skin. Immunohistochemical dual staining of pan-neurofilament and CD31 markers was used to investigate reinnervation and vascularisation of both the dorsal surface of the trunk and ear wounds. The MRL/MpJ mouse ear exhibited a significantly (P > 0.01) higher density of regenerated nerves than C57BL/6 between 10 and 21 days post-wounding when the blastema-like structure was forming. Unlike dorsal skin wounds, nerve regeneration in the ear wound preceded vascularisation, recapitulating early mammalian development. Immunohistochemical data suggest that factors within the blastemal mesenchyme, such as aggrecan, may direct nerve regrowth in the regenerating ear tissue.

Dense fibrillar collagen matrices for tissue repair

The preparation of dense fibrillar collagen matrices, through a sol/gel transition at variable concentrations, offers routes to produce a range of simple, non toxic materials. Concentrated hydrogels entrapping cells show enhanced properties in terms of reduced contraction and enhanced cell proliferation . Dense fibrillar matrices attain tissue like mechanical properties and show ultrastructures described in connective tissues, namely liquid crystalline cholesteric geometries. Their colonization by cells and possible association with a mineral phase in a tissue like manner validate their use as biomimetic materials for regenerative medicine.

Implications for collagen binding from the crystallographic structure of fibronectin 6FnI1-2FnII7FnI

Collagen and fibronectin (FN) are two abundant and essential components of the vertebrate extracellular matrix; they interact directly with cellular receptors and affect cell adhesion and migration. Past studies identified a FN fragment comprising six modules, ⁶FnI^1–2FnII^7–9FnI, and termed the gelatin binding domain (GBD) as responsible for collagen interaction. Recently, we showed that the GBD binds tightly to a specific site within type I collagen and determined the structure of domains ^8–9FnI in complex with a peptide from that site. Here, we present the crystallographic structure of domains ⁶FnI^1–2FnII⁷FnI, which form a compact, globular unit through interdomain interactions. Analysis of NMR titrations with single-stranded collagen peptides reveals a dominant collagen interaction surface on domains ²FnII and ⁷FnI; a similar surface appears involved in interactions with triple-helical peptides. Models of the complete GBD, based on the new structure and the ^8–9FnI·collagen complex show a continuous putative collagen binding surface. We explore the implications of this model using long collagen peptides and discuss our findings in the context of FN interactions with collagen fibrils.

Artificial human tissues from cord and cord blood stem cells for multi-organ regenerative medicine: viable alternatives to animal in vitro toxicology

New medicinal products and procedures must meet very strict safety criteria before being applied for use in humans. The laboratory procedures involved require the use of large numbers of animals each year. Furthermore, such investigations do not always give an accurate translation to the human setting. Here, we propose a viable alternative to animal testing, which uses novel technology featuring human cord and cord blood stem cells. With over 130 million children born each year, cord and cord blood remains the most widely available alternative to the use of animals or cadaveric human tissues for in vitro toxicology.

Enhanced neovascularization of dermis substitutes via low-pressure plasma-mediated surface activation

PURPOSE

The effect of cold low-pressure plasma treatment on neovascularization of a dermis substitute was evaluated in a mouse model.

MATERIAL AND METHODS

Collagen-elastin matrices (Matriderm(®)) were used as scaffolds. Low-pressure argon/hydrogene plasma-treated scaffolds were transplanted into the dorsal skinfold chambers of balb/c mice (group 1, n=10). Untreated scaffolds served as controls (group 2, n=10). Intravital fluorescence microscopy was performed within the border zone of the scaffolds on days 1, 5 and 10. Functional vessel density (FVD), vessel diameter, intervascular distance, microvascular permeability, and leukocyte-endothelium interaction were analyzed.

RESULTS

An increase of FVD associated with a reduction of the intervascular distance was observed. Statistical analysis revealed that the functional vessel density in the border zone of the scaffolds was significantly enhanced in the plasma-treated group compared to controls. For group 1, an increase of FVD from 282±8 cm/cm(2) on days 5 to 315±8 cm/cm(2) on day 10 was observed. Whereas values of 254±7 cm/cm(2) on day 5 and 275±13 cm/cm(2) on day 10 have resulted in group 2 (mean±S.E.M., Student's t-test, p<0.05).

CONCLUSION

The surface treatment by cold low-pressure plasma intensifies the angiogenesis and accelerates the neovascularization of collagen-elastin matrix.

Fusion of concentrically layered tubular tissue constructs increases burst strength

Tubular tissue constructs prepared from neonatal human dermal fibroblasts entrapped in fibrin gel were incubated on a mandrel for three weeks to allow for initial fibrin remodeling into tissue before being concentrically layered and incubated for an additional three weeks on the mandrel. Upon harvest, double layer constructs were not statistically different from single layer control constructs in terms of length, collagen density, cell density, tensile modulus, or ultimate tensile strength. However, the thickness and burst pressure were both approximately twice the single layer control values. Metabolically active cells were detected at the interface, and scanning electron microscopy revealed fiber structures bridging the two layers, co-localizing with the cells, which exhibited minimal migration across the layers. In contrast, double layer constructs where tissue fusion was prohibited by mechanical distraction of the layers showed no increase in burst pressure despite having increased thickness and the same collagen and cell densities of the single layer control constructs; moreover, the burst failure occurred sequentially in the layers in contrast to simultaneous failure for the fused double layer constructs. This study provides insight into the nature of the interface and the role of cell behavior when tissue fusion occurs between two layers of bioartificial tissue in vitro. It also suggests a method for improving the burst strength of fibrin-based tubular tissue constructs by increasing the construct thickness via concentrically layering and fusing two constructs.

Regeneration: Thomas Hunt Morgan's window into development

Early in his career Thomas Hunt Morgan was interested in embryology and dedicated his research to studying organisms that could regenerate. Widely regarded as a regeneration expert, Morgan was invited to deliver a series of lectures on the topic that he developed into a book, Regeneration (1901). In addition to presenting experimental work that he had conducted and supervised, Morgan also synthesized and critiqued a great deal of work by his peers and predecessors. This essay probes into the history of regeneration studies by looking in depth at Regeneration and evaluating Morgan's contribution. Although famous for his work with fruit fly genetics, studying Regeneration illuminates Morgan's earlier scientific approach which emphasized the importance of studying a diversity of organisms. Surveying a broad range of regenerative phenomena allowed Morgan to institute a standard scientific terminology that continues to inform regeneration studies today. Most importantly, Morgan argued that regeneration was a fundamental aspect of the growth process and therefore should be accounted for within developmental theory. Establishing important similarities between regeneration and development allowed Morgan to make the case that regeneration could act as a model of development. The nature of the relationship between embryogenesis and regeneration remains an active area of research.

Tissue engineering of skin

The engineering of skin substitutes and their application on human patients has become a reality. However, cell biologists, biochemists, technical engineers, and surgeons are still struggling with the generation of complex skin substitutes that can readily be transplanted in large quantities, possibly in only one surgical intervention and without significant scarring. Constructing a dermo-epidermal substitute that rapidly vascularizes, optimally supports a stratifying epidermal graft on a biodegradable matrix, and that can be conveniently handled by the surgeon, is now the ambitious goal. After all, this goal has to be reached coping with strict safety requirements and the harsh rules of the economic market.

Human embryonic stem-cell derivatives for full reconstruction of the pluristratified epidermis: a preclinical study

BACKGROUND

Cell therapy for large burns is dependent upon autologous epidermis reconstructed in vitro. However, the effectiveness of current procedures is limited by the delay needed to culture the patient's own keratinocytes. To assess whether the keratinocyte progeny of human embryonic stem cells (hESCs) could be used to form a temporary skin substitute for use in patients awaiting autologous grafts, we investigated the cells' capability of constructing a pluristratified epidermis.

METHODS

hESCs from lines H9 and SA01 were seeded at least in triplicate on fibroblast feeder cells for 40 days in a medium supplemented with bone morphogenetic protein 4 and ascorbic acid. Molecular characterisation of cell differentiation was done throughout the process by quantitative PCR, fluorescence-activated cell sorting, and immunocytochemical techniques. Keratinocyte molecular differentiation and functional capacity to construct a human epidermis were assessed in vitro and in vivo.

FINDINGS

From hESCs, we generated a homogeneous population of cells that showed phenotypic characteristics of basal keratinocytes. Expression levels of genes encoding keratin 14, keratin 5, integrin alpha6, integrin beta4, collagen VII, and laminin 5 in these cells were similar to those in basal keratinocytes. After seeding on an artificial matrix, keratinocytes derived from hESCs (K-hESCs) formed a pluristratified epidermis. Keratin-14 immunostaining was seen in the basal compartment, with keratin 10 present in layers overlying the basal layer. Involucrin and filaggrin, late markers of epidermal differentiation, were detected in the uppermost layers only. 12 weeks after grafting onto five immunodeficient mice, epidermis derived from K-hESCs had a structure consistent with that of mature human skin. Human involucrin was appropriately located in spinous and granular layers and few Ki67-positive cells were detected in the basal layer.

INTERPRETATION

hESCs can be differentiated into basal keratinocytes that are fully functional--ie, able to construct a pluristratified epidermis. This resource could be developed to provide temporary skin substitutes for patients awaiting autologous grafts.

FUNDING

Institut National de la Santé et de la Recherche Médicale, University Evry Val d'Essonne, Association Française contre les Myopathies, Fondation René Touraine, and Genopole.

De novo follicular regeneration of the skin by wingless int 3 and bone morphogenetic protein 2 genes introduced into dermal fibroblasts and fibroblast growth factor-2 protein

In this study, we regenerated skin and its appendages by transplanting cultured normal dermal fibroblasts, into which morphogen genes had been introduced. We cultured normal dermal fibroblasts obtained from Fisher 344 rats on the surface of hydroxyapatite beads, and then adsorbed them onto the surface of a collagen sponge, which was transplanted into a full-thickness skin defect prepared on the backs of rats. Before transplantation, genes were introduced into the dermal fibroblasts via adenovirus vector (ad)-bone morphogenetic protein 2 and ad-wingless int 3 genes in addition to fibroblast growth factor-2 protein. By Week 4, the appearance of follicle germs or primitive hair germs was observed only in the ad-bone morphogenetic protein 2+ad-wingless int 3 combined with the fibroblast growth factor-2 protein group. By Week 16, in that same group, hair follicles having mature pilosebaceous systems with equally spaced localization had formed in the ulcer wound.

Imaging engineered tissues using structural and functional optical coherence tomography

As the field of tissue engineering evolves, there will be an increasingly important need to visualize and track the complex dynamic changes that occur within three-dimensional constructs. Optical coherence tomography (OCT), as an emerging imaging technology applied to biological materials, offers a number of significant advantages to visualize these changes. Structural OCT has been used to investigate the longitudinal development of engineered tissues and cell dynamics such as migration, proliferation, detachment, and cell-material interactions. Optical techniques that image functional parameters or integrate multiple imaging modalities to provide complementary contrast mechanisms have been developed, such as the integration of optical coherence microscopy with multiphoton microscopy to image structural and functional information from cells in engineered tissue, optical coherence elastography to generate images or maps of strain to reflect the spatially-dependent biomechanical properties, and spectroscopic OCT to differentiate different cell types. From these results, OCT demonstrates great promise for imaging and visualizing engineered tissues, and the complex cellular dynamics that directly affect their practical and clinical use.