In Vivo Model of Wound Healing Based on Transplanted Tissue-Engineered Skin

Corneal fibrosis: From in vitro models to current and upcoming drug and gene medicines

Fibrotic diseases are characterised by myofibroblast differentiation, uncontrolled pathological extracellular matrix accumulation, tissue contraction, scar formation and, ultimately tissue / organ dysfunction. The cornea, the transparent tissue located on the anterior chamber of the eye, is extremely susceptible to fibrotic diseases, which cause loss of corneal transparency and are often associated with blindness. Although topical corticosteroids and antimetabolites are extensively used in the management of corneal fibrosis, they are associated with glaucoma, cataract formation, corneoscleral melting and infection, imposing the need of far more effective therapies. Herein, we summarise and discuss shortfalls and recent advances in in vitro models (e.g. transforming growth factor-β (TGF-β) / ascorbic acid / interleukin (IL) induced) and drug (e.g. TGF-β inhibitors, epigenetic modulators) and gene (e.g. gene editing, gene silencing) therapeutic strategies in the corneal fibrosis context. Emerging therapeutical agents (e.g. neutralising antibodies, ligand traps, receptor kinase inhibitors, antisense oligonucleotides) that have shown promise in clinical setting but have not yet assessed in corneal fibrosis context are also discussed.

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

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

The remarkable effect of menstrual blood stem cells seeded on bilayer scaffold composed of amniotic membrane and silk fibroin aiming to promote wound healing in diabetic mice

INTRODUCTION

Impaired chronic wound healing frequently occurs in diabetic patients. We hypothesized that menstrual blood-derived mesenchymal stem cells (MenSCs) in combination with bilayer scaffold consisted of human amniotic membrane (AM) and electrospun silk fibroin nanofibers could potentially promote wound healing in diabetic mice.

METHODS & METHODS

Two bilateral full-thickness wounds were created on dorsal skin of type-1 diabetic mice model and animals were equally divided in four groups including: no-treatment group (NT), amniotic membrane treated group (AM), bilayer scaffold treated group (bSC), and MenSCs-seeded bilayer scaffold treated group (bSC + MenSCs). Wound healing evaluations were performed at 3, 7, and 14 days after their treatment. The wound healing was analyzed by macroscopic and microscopic evaluations, and immunofluorescence staining of involucrin (IVL), type III collagen, CD31/ von Willebrand factor (vWF), and PGP9.5 were performed. Furthermore, number of neutrophils and macrophages and subpopulation of macrophages were assessed. In addition, the expression of Egr2, Mmp9, CXCL12, IDO1, Ptgs2 and VEGFA transcripts involved in wound repair were also analyzed.

RESULTS

After 14 days, the best epidermal and dermal regeneration belonged to the cases received bSC + MenSCs as wound dressing. Moreover, the wound healing was typically faster in this group compared to other groups. Immunofluorescence evaluation represented higher levels of CD31 and VWF, higher ratio of M2/M1 macrophages, greater expression of IVL, and higher levels of the PGP9.5 in the bSC + MenSCs group in comparison with other groups. Expression analysis of assessed genes also supported assumption of more regeneration and healing in the bSC + MenSCs group versus other groups.

CONCLUSION

These results indicate that enhanced immunomodulatory and reparative properties of MenSCs in conjunction with bilayer scaffold specified this cellular skin substitute for modulating wound chronicity and contribution to resolution of wound healing process in diabetic ulcer.

Preclinical models of diabetic wound healing: A critical review

The treatment of diabetic wounds (DWs) is always challenging for the medical community because of its multifaceted pathophysiology. Due to practical and ethical considerations, direct studies of therapeutic interventions on human subjects are limited. Thus, it is ideal for performing studies on animals having less genetic and biological variability. An ideal DW model should progress toward reproducibility, quantifiable interpretation, therapeutic significance, and effective translation into clinical use. In the last couple of decades, various animal models were developed to examine the complex cellular and biochemical process of skin restoration in DW healing. Also, these models were used to assess the potency of developed active pharmaceutical ingredients and formulations. However, many animal models lack studying mechanisms that can appropriately restate human DW, stay a huge translational challenge. This review discusses the available animal models with their significance in DW experiments and their limitations, focusing on levels of proof of effectiveness in selecting appropriate models to restate the human DW to improve clinical outcomes. Although numerous newer entities and combinatory formulations are very well appreciated preclinically for DW management, they fail in clinical trials, which may be due to improper selection of the appropriate model. The major future challenge could be developing a model that resembles the human DW environment, can potentiate translational research in DW care.

Investigation of the Potential of Nebivolol Hydrochloride-Loaded Chitosomal Systems for Tissue Regeneration: In Vitro Characterization and In Vivo Assessment

In this study, we evaluated the synergistic effect of nebivolol hydrochloride (NVH), a third-generation beta-blocker and NO donor drug, and chitosan on the tissue regeneration. Ionic gelation method was selected for the preparation of NVH-loaded chitosomes using chitosan lactate and sodium tripolyphosphate. The effect of different formulation variables was studied using a full factorial design, and NVH entrapment efficiency percentages and particle size were selected as the responses. The chosen system demonstrated high entrapment efficiency (73.68 ± 3.61%), small particle size (404.05 ± 11.2 nm), and good zeta potential value (35.6 ± 0.25 mV). The best-achieved formula demonstrated spherical morphology in transmission electron microscopy and amorphization of the crystalline drug in differential scanning calorimetry and X-ray diffraction. Cell culture studies revealed a significantly higher proliferation of the fibroblasts in comparison with the drug suspensions and the blank formula. An in vivo study was conducted to compare the efficacy of the proposed formula on wound healing. The histopathological examination showed the superiority of NVH-loaded chitosomes on the wound proliferation and the non-significant difference in the collagen deposition after 15 days of the injury to that of intact skin. In conclusion, NVH-loaded chitosomes exhibited promising results in enhancing skin healing and tissue regeneration.

Collagen-Derived Di-Peptide, Prolylhydroxyproline (Pro-Hyp): A New Low Molecular Weight Growth-Initiating Factor for Specific Fibroblasts Associated With Wound Healing

Many cells and soluble factors are involved in the wound healing process, which can be divided into inflammatory, proliferative, and remodeling phases. Fibroblasts play a crucial role in wound healing, especially during the proliferative phase, and show heterogeneity depending on lineage, tissue distribution, and extent of differentiation. Fibroblasts from tissue stem cells rather than from healthy tissues infiltrate wounds and proliferate. Some fibroblasts in the wound healing site express the mesenchymal stem cell marker, p75NTR. In the cell culture system, fibroblasts attached to collagen fibrils stop growing, even in the presence of protein growth factors, thus mimicking the quiescent nature of fibroblasts in healthy tissues. Fibroblasts in wound healing sites proliferate and are surrounded by collagen fibrils. These facts indicate presence of new growth-initiating factor for fibroblasts attached to collagen fibrils at the wound healing site, where the collagen-derived peptide, prolyl-hydroxyproline (Pro-Hyp), is generated. Pro-Hyp triggers the growth of p75NTR-positive fibroblasts cultured on collagen gel but not p75NTR-negative fibroblasts. Thus, Pro-Hyp is a low molecular weight growth-initiating factor for specific fibroblasts that is involved in the wound healing process. Pro-Hyp is also supplied to tissues by oral administration of gelatin or collagen hydrolysate. Thus, supplementation of gelatin or collagen hydrolysate has therapeutic potential for chronic wounds. Animal studies and human clinical trials have demonstrated that the ingestion of gelatin or collagen hydrolysate enhances the healing of pressure ulcers in animals and humans and improves delayed wound healing in diabetic animals. Therefore, the low molecular weight fibroblast growth-initiating factor, Pro-Hyp, plays a significant role in wound healing and has therapeutic potential for chronic wounds.

Cryopreserved human skin allografts promote angiogenesis and dermal regeneration in a murine model

Cryopreserved human skin allografts (CHSAs) are used for the coverage of major burns when donor sites for autografts are insufficiently available and have clinically shown beneficial effects on chronic non-healing wounds. However, the biologic mechanisms behind the regenerative properties of CHSA remain elusive. Furthermore, the impact of cryopreservation on the immunogenicity of CHSA has not been thoroughly investigated and raised concerns with regard to their clinical application. To investigate the importance and fate of living cells, we compared cryopreserved CHSA with human acellular dermal matrix (ADM) grafts in which living cells had been removed by chemical processing. Both grafts were subcutaneously implanted into C57BL/6 mice and explanted after 1, 3, 7, and 28 days (n = 5 per group). A sham surgery where no graft was implanted served as a control. Transmission electron microscopy (TEM) and flow cytometry were used to characterise the ultrastructure and cells within CHSA before implantation. Immunofluorescent staining of tissue sections was used to determine the immune reaction against the implanted grafts, the rate of apoptotic cells, and vascularisation as well as collagen content of the overlaying murine dermis. Digital quantification of collagen fibre alignment on tissue sections was used to quantify the degree of fibrosis within the murine dermis. A substantial population of live human cells with intact organelles was identified in CHSA prior to implantation. Subcutaneous pockets with implanted xenografts or ADMs healed without clinically apparent rejection and with a similar cellular immune response. CHSA implantation largely preserved the cellularity of the overlying murine dermis, whereas ADM was associated with a significantly higher rate of cellular apoptosis, identified by cleaved caspase-3 staining, and a stronger dendritic cell infiltration of the murine dermis. CHSA was found to induce a local angiogenic response, leading to significantly more vascularisation of the murine dermis compared with ADM and sham surgery on day 7. By day 28, aggregate collagen-1 content within the murine dermis was greater following CHSA implantation compared with ADM. Collagen fibre alignment of the murine dermis, correlating with the degree of fibrosis, was significantly greater in the ADM group, whereas CHSA maintained the characteristic basket weave pattern of the native murine dermis. Our data indicate that CHSAs promote angiogenesis and collagen-1 production without eliciting a significant fibrotic response in a xenograft model. These findings may provide insight into the beneficial effects clinically observed after treatment of chronic wounds and burns with CHSA.

Experimental models and methods for cutaneous wound healing assessment

Wound healing studies are intricate, mainly because of the multifaceted nature of the wound environment and the complexity of the healing process, which integrates a variety of cells and repair phases, including inflammation, proliferation, reepithelialization and remodelling. There are a variety of possible preclinical models, such as in mice, rabbits and pigs, which can be used to mimic acute or impaired for example, diabetic and nutrition-related wounds. These can be induced by many different techniques, with excision or incision being the most common. After determining a suitable model for a study, investigators need to select appropriate and reproducible methods that will allow the monitoring of the wound progression over time. The assessment can be performed by non-invasive protocols such as wound tracing, photographic documentation (including image analysis), biophysical techniques and/or by invasive protocols that will require wound biopsies. In this article, we provide an overview of some of the most often needed and used: (a) preclinical/animal models including incisional, excisional, burn and impaired wounds; (b) methods to evaluate the healing progression such as wound healing rate, wound analysis by image, biophysical assessment, histopathological, immunological and biochemical assays. The aim is to help researchers during the design and execution of their wound healing studies.

Evaluation of Cilia Function in Rat Trachea Reconstructed Using Collagen Sponge Scaffold Seeded with Adipose Tissue-Derived Stem Cells

The tracheal lumen is essential for conducting air to the lung alveoli and for voice production. However, patients with severe tracheal stenosis and malignant tumors invading the trachea often require tracheal resection. Recently, various reported tissue engineering methods for tracheal reconstruction show that regeneration of ciliated epithelium in the reconstructed areas, as well as preservation of the luminal structure is possible. However, only few studies report on the mucociliary transport function in reconstructed tracheae. We investigated mucociliary transport function within rat tracheal epithelium, reorganized after autologous adipose tissue-derived stem cell (ASC) transplantation. Rat ASCs were expanded in culture, and then seeded in a collagen sponge, which was physically supported with a polypropylene framework. The ASC-seeded collagen sponge was transplanted into the rat tracheal defect. We then examined the motility and transport function of cilia generated in the transplanted area using ciliary beat frequency (CBF) and microsphere movement analyses. Our data suggested that autologous ASC transplantation promoted ciliogenesis, consistent with previous reports. The CBF analysis revealed that motility of the cilia generated in the ASC group was comparable to that observed in the normal rat tracheal epithelium. Transport function in the ASC group was higher than that in the control group. These data suggested that autologous ASC transplantation increased ciliated cells in the reconstructed area without significantly disrupting cilia motility, thereby promoting transport function regeneration. Autologous ASC transplantation is expected to be beneficial in morphological and functional regeneration of tracheal epithelium. Anat Rec, 303:471-477, 2020. © 2019 American Association for Anatomy.

Human Reconstructed Skin in a Mouse Model

Currently, no ideal in vivo skin model, to exactly mimic the native human skin, has been utilized for laboratory and clinical application. Here, we describe a method to in vivo reconstitute a human skin model, so-called hRSK, by using culture-expanded skin cells. We grafted a mixture of dissociated human epidermal and dermal cells onto an excision wound on the back of immunodeficient mouse to generate the hRSK, and the hRSK, containing epidermis, dermis, and subcutis and also appendages such as hair follicles, histologically mirrors in situ human skin.

A single-centre, retrospective study of cryopreserved umbilical cord/amniotic membrane tissue for the treatment of diabetic foot ulcers

OBJECTIVE

Histopathological studies have shown a prolonged inflammatory phase in wounds of patients with diabetes, delaying formation of mature granulation tissue and reducing wound tensile strength, making these wounds difficult for physicians to heal. Cryopreserved human umbilical cord (cUC) tissues possess unique anti-inflammatory and anti-scarring properties and have been found to help improve closure of these chronic wounds.

METHOD

A retrospective chart review was performed to assess the efficacy of cUC as an advanced treatment modality to help promote the closure of chronic DFUs. Overall healing rate, duration to wound closure, and number of cUC applications used to achieve closure were used to assess cUC treatment efficacy.

RESULTS

A total of 32 wounds in 29 patients treated at a single health-care centre were included in the study population The average initial wound area for all wounds was 10.6 ± 2.15cm². Of the 32 wounds 28 achieved complete epithelialisation for an overall healing rate of 87.5%. Average time to wound closure was 13.8 ± 1.95 weeks with a median of 9 weeks and an average of 1.68 ± 0.18 cUC applications.

CONCLUSION

The results suggest cUC allograft may be effective in improving the healing of DFUs ulcers as well as potentially reducing the medical costs associated with chronic DFUs due to the low number of applications needed to achieve complete wound closure. Prospective, randomised controlled trials are suggested to better understand the efficacy of cUC in chronic wound healing.

DECLARATION OF INTEREST

Dr Raphael is a paid speaker for Amniox Medical, Inc.

Skin Grafting on 3D Bioprinted Cartilage Constructs In Vivo

Background:

Three-dimensional (3D) bioprinting of cartilage is a promising new technique. To produce, for example, an auricle with good shape, the printed cartilage needs to be covered with skin that can grow on the surface of the construct. Our primary question was to analyze if an integrated 3D bioprinted cartilage structure is a tissue that can serve as a bed for a full-thickness skin graft.

Methods:

3D bioprinted constructs (10 × 10 × 1.2 mm) were printed using nanofibrillated cellulose/alginate bioink mixed with mesenchymal stem cells and adult chondrocytes and implanted subcutaneously in 21 nude mice.

Results:

After 45 days, a full-thickness skin allograft was transplanted onto the constructs and the grafted construct again enclosed subcutaneously. Group 1 was sacrificed on day 60, whereas group 2, instead, had their skin-bearing construct uncovered on day 60 and were sacrificed on day 75 and the explants were analyzed morphologically. The skin transplants integrated well with the 3D bioprinted constructs. A tight connection between the fibrous, vascularized capsule surrounding the 3D bioprinted constructs and the skin graft were observed. The skin grafts survived the uncovering and exposure to the environment.

Conclusions:

A 3D bioprinted cartilage that has been allowed to integrate in vivo is a sufficient base for a full-thickness skin graft. This finding accentuates the clinical potential of 3D bioprinting for reconstructive purposes.

Culture of Keratinocytes for Transplantation without the Need of Feeder Layer Cells

Patients with large burn wounds have a limited amount of healthy donor skin. An alternative for the autologous skin graft is transplantation with autologous keratinocytes. Conventionally, the keratinocytes are cultured with mouse feeder layer cells in medium containing fetal calf serum (FCS) to obtain sufficient numbers of cells. These xenobiotic materials can be a potential risk for the patient. The aim of the present study was to investigate if keratinocytes could be expanded in culture without the need of a feeder layer and FCS. Keratinocytes were cultured on tissue culture plastic with or without collagen type IV coating in medium containing Ultroser G (serum substitute) and keratinocyte growth factor (KGF). An in vitro skin equivalent model was used to examine the capacity of these cells to form an epidermis. Keratinocytes in different passages (P2, P4, and P6) and freshly isolated cells were studied. Keratinocytes grown on collagen type IV were able to form an epidermis at higher passage numbers than cells grown in the absence of collagen type IV (P4 and P2, respectively). In both cases the reconstructed epidermis showed an increased expression of Ki-67, SKALP, involucrin, and keratin 17 compared to normal skin. Only 50,000 keratinocytes grown on collagen type IV in P4 were needed to form 1 cm2 epidermis, whereas 150,000 of freshly isolated keratinocytes were necessary. Using this culture technique sufficient numbers of keratinocytes, isolated from 1 cm2 skin, were obtained to cover 400 cm2 of wound surface in 2 weeks. The results show that keratinocytes can be cultured without the need of a fibroblast feeder layer and FCS and that these cells are still able to create a fully differentiated epidermis. This culture technique can be a valuable tool for the treatment of burn wounds and further development of tissue engineered skin.

An advanced mouse model for human skin wound healing

Here, we report a model for studying wound repair based on skin regenerated from human tissue culture-expanded cells. The reconstituted skin (hRSK) responds to injury similar to that of intact human skin, and its constituent cells contribute to the healing process. As we have demonstrated that hRSK composed of GFP-labelled cells also heals "normally," we believe this model will be useful in analysing the wound repair process using genetically modified human cells.

Hybrid Biomaterial with Conjugated Growth Factors and Mesenchymal Stem Cells for Ectopic Bone Formation

Bone is a highly vascularized tissue and efficient bone regeneration requires neovascularization, especially for critical-sized bone defects. We developed a novel hybrid biomaterial comprising nanocalcium sulfate (nCS) and fibrin hydrogel to deliver mesenchymal stem cells (MSCs) and angiogenic factors, vascular endothelial growth factor (VEGF) and fibroblast growth factor 9 (FGF9), to promote neovascularization and bone formation. MSC and growth factor(s)-loaded scaffolds were implanted subcutaneously into mice to examine their angiogenic and osteogenic potential. Micro CT, alkaline phosphatase activity assay, and histological analysis were used to evaluate bone formation, while immunohistochemistry was employed to assess neovessel formation. The presence of fibrin preserved the nCS scaffold structure and promoted de novo bone formation. In addition, the presence of bone morphogenic protein 2-expressing MSC in nCS and fibrin hydrogels improved bone regeneration significantly. While FGF9 alone had no significant effect, the combination FGF9 and VEGF conjugated in fibrin enhanced neovascularization and bone formation more than 4-fold compared to nCS with MSC. Overall, our results suggested that the combination of nCS (to support bone formation) with a fibrin-based VEGF/FGF9 release system (support vascular formation) is an innovative and effective strategy that significantly enhanced ectopic bone formation in vivo.

Effect of Structural Differences in Collagen Sponge Scaffolds on Tracheal Epithelium Regeneration

OBJECTIVE

We developed an in situ regeneration-inducible artificial trachea composed of a porcine collagen sponge and polypropylene framework and used it for tracheal reconstruction. In the present study, collagen sponges with different structures were prepared from various concentrations of collagen solutions, and their effect on the regeneration of tracheal epithelium was examined.

METHODS

Collagen sponges were prepared from type I and III collagen solutions. The structures of the sponges were analyzed using scanning electron microscopy (SEM). Artificial tracheae, which were formed using the collagen sponges with different structures, were implanted into rabbits, and regeneration of the tracheal epithelium on the artificial tracheae was evaluated by SEM analysis and histological examination.

RESULTS

The SEM analysis showed that collagen sponges prepared from 0.5% and 1.0% collagen solutions had a porous structure. However, the sponges prepared from a 1.5% collagen solution had a nonporous structure. After implantation of artificial tracheae prepared from 0.5% and 1.0% collagen solutions, their luminal surfaces were mostly covered with epithelium within 14 days. However, epithelial reorganization occurred later on artificial tracheae prepared from the 1.5% collagen solution.

CONCLUSION

Collagen sponges with a porous structure are suitable for regeneration of the tracheal epithelium in our artificial trachea.

Generation of a three-dimensional full thickness skin equivalent and automated wounding

In vitro models are a cost effective and ethical alternative to study cutaneous wound healing processes. Moreover, by using human cells, these models reflect the human wound situation better than animal models. Although two-dimensional models are widely used to investigate processes such as cellular migration and proliferation, models that are more complex are required to gain a deeper knowledge about wound healing. Besides a suitable model system, the generation of precise and reproducible wounds is crucial to ensure comparable results between different test runs. In this study, the generation of a three-dimensional full thickness skin equivalent to study wound healing is shown. The dermal part of the models is comprised of human dermal fibroblast embedded in a rat-tail collagen type I hydrogel. Following the inoculation with human epidermal keratinocytes and consequent culture at the air-liquid interface, a multilayered epidermis is formed on top of the models. To study the wound healing process, we additionally developed an automated wounding device, which generates standardized wounds in a sterile atmosphere.

The regenerative potential of fibroblasts in a new diabetes-induced delayed humanised wound healing model

Cutaneous diabetic wounds greatly affect the quality of life of patients, causing a substantial economic impact on the healthcare system. The limited clinical success of conventional treatments is mainly attributed to the lack of knowledge of the pathogenic mechanisms related to chronic ulceration. Therefore, management of diabetic ulcers remains a challenging clinical issue. Within this context, reliable animal models that recapitulate situations of impaired wound healing have become essential. In this study, we established a new in vivo humanised model of delayed wound healing in a diabetic context that reproduces the main features of the human disease. Diabetes was induced by multiple low doses of streptozotocin in bioengineered human-skin-engrafted immunodeficient mice. The significant delay in wound closure exhibited in diabetic wounds was mainly attributed to alterations in the granulation tissue formation and resolution, involving defects in wound bed maturation, vascularisation, inflammatory response and collagen deposition. In the new model, a cell-based wound therapy consisting of the application of plasma-derived fibrin dermal scaffolds containing fibroblasts consistently improved the healing response by triggering granulation tissue maturation and further providing a suitable matrix for migrating keratinocytes during wound re-epithelialisation. The present preclinical wound healing model was able to shed light on the biological processes responsible for the improvement achieved, and these findings can be extended for designing new therapeutic approaches with clinical relevance.

The pivotal role of vascularization in tissue engineering

Vascularization is one of the great challenges that tissue engineering faces in order to achieve sizeable tissue and organ substitutes that contain living cells. There are instances, such as skin replacement, in which a tissue-engineered substitute does not absolutely need a preexisting vascularization. However, tissue or organ substitutes in which any dimension, such as thickness, exceeds 400 μm need to be vascularized to ensure cellular survival. Consistent with the wide spectrum of approaches to tissue engineering itself, which vary from acellular synthetic biomaterials to purely biological living constructs, approaches to tissue-engineered vascularization cover numerous techniques. Those techniques range from micropatterns engineered in biomaterials to microvascular networks created by endothelial cells. In this review, we strive to provide a critical overview of the elements that must be considered in the pursuit of this goal and the major approaches that are investigated in hopes of achieving it.

Propolis modulates vitronectin, laminin, and heparan sulfate/heparin expression during experimental burn healing

OBJECTIVE

This study was aimed at assessing the dynamics of vitronectin (VN), laminin (LN), and heparan sulfate/heparin (HS/HP) content changes during experimental burn healing.

METHODS

VN, LN, and HS/HP were isolated and purified from normal and injured skin of domestic pigs, on the 3rd, 5th, 10th, 15th, and 21st days following thermal damage. The wounds were treated with apitherapeutic agent (propolis), silver sulfadiazine (SSD), physiological salt solution, and propolis vehicle. VN and LN were quantified using an immunoenzymatic assay and HS/HP was estimated by densitometric analysis.

RESULTS

Propolis treatment stimulated significant increases in VN, LN, and HS/HP contents during the initial phase of study, followed by a reduction in the estimated extracellular matrix molecules. Similar patterns, although less extreme, were observed after treatment with SSD.

CONCLUSIONS

The beneficial effects of propolis on experimental wounds make it a potential apitherapeutic agent in topical burn management.

The role of skin substitutes in the management of chronic cutaneous wounds

Chronic wounds, including diabetic and venous ulcers, represent disruption of normal healing processes resulting in a pathological state of nonhealing cutaneous inflammation. They place an increasingly significant economic burden on healthcare providers as their prevalence is rising in keeping with an aging population. Current treatment modalities are slow acting and resource intensive. Bioengineered skin substitutes from autogenic, allogenic, or xenogenic sources have emerged as a new and alternative therapeutic option. A range of such products is licensed for clinical use, which differ in terms of structure and cellular content. Placed directly onto a prepared wound bed, skin substitutes may stimulate or accelerate healing by promoting revascularization, cellular migration, and repopulation of wound fields through provision of an appropriate scaffold material to facilitate these processes. Products containing fetal or autologous cells also benefit from early release of bioactive molecules including growth factors and cytokines. To date, limited numbers of randomized controlled trials studying skin substitutes have been published but evidence from case series and case-control studies is encouraging. This review discusses chronic wound biology, the influence that skin substitutes can exert on this environment, the products currently available, and examines the evidence for their use in chronic wound management.

Bioengineered skin substitutes

Bioengineered skin has great potential for use in regenerative medicine for treatment of severe wounds such as burns or chronic ulcers. Genetically modified skin substitutes have also been used as cell-based devices or "live bioreactors" to deliver therapeutics locally or systemically. Finally, these tissue constructs are used as realistic models of human skin for toxicological testing, to speed drug development and replace traditional animal-based tests in a variety of industries. Here we describe a method of generating bioengineered skin based on a natural scaffold, namely, decellularized human dermis and epidermal stem cells.

Cell-type-specific differentiation and molecular profiles in skin transplantation: implication of medical approach for genetic skin diseases

Skin is highly accessible and valuable organ, which holds promise to accelerate the understanding of future medical innovation in association with skin transplantation, engineering, and wound healing. In skin transplantation biology, multistage and multifocal damages occur in both grafted donor and perilesional host skin and need to be repaired properly for the engraftment and maintenance of characteristic skin architecture. These local events are more unlikely to be regulated by the host immunity, because human skin transplantation has accomplished the donor skin engraftment onto the immunocompromised or immunosuppressive animals. Recent studies have emerged the importance of α-smooth muscle actin- (SMA-) positive myofibroblasts, via stage- and cell-specific contribution of TGFβ, PDGF, ET-1, CCN-2 signalling pathways, and mastocyte-derived mediators (e.g., histamine and tryptase), for the functional reorganisation of the grafted skin. Moreover, particular cell lineages from bone marrow (BM) cells have been shown to harbour the diferentiation capacity into multiple skin cell phenotypes, including epidermal keratinocytes and dermal endothelial cells and pericytes, undercontrolled by chemokines or cytokines. From a dermatological viewpoint, we review the recent update of cell-type- and molecular-specific action associated with reconstitution of the grafted skin and also focus on the novel application of BM transplantation medicine in genetic skin diseases.

A novel ovine ex vivo arteriovenous shunt model to test vascular implantability

The major objective of successful development of tissue-engineered vascular grafts is long-term in vivo patency. Optimization of matrix, cell source, surface modifications, and physical preconditioning are all elements of attaining a compatible, durable, and functional vascular construct. In vitro model systems are inadequate to test elements of thrombogenicity and vascular dynamic functional properties while in vivo implantation is complicated, labor-intensive, and cost-ineffective. We proposed an ex vivo ovine arteriovenous shunt model in which we can test the patency and physical properties of vascular grafts under physiologic conditions. The pressure, flow rate, and vascular diameter were monitored in real-time in order to evaluate the pulse wave velocity, augmentation index, and dynamic elastic modulus, all indicators of graft stiffness. Carotid arteries, jugular veins, and small intestinal submucosa-based grafts were tested. SIS grafts demonstrated physical properties between those of carotid arteries and jugular veins. Anticoagulation properties of grafts were assessed via scanning electron microscopy imaging, en face immunostaining, and histology. Luminal seeding with endothelial cells greatly decreased the attachment of thrombotic components. This model is also suture free, allowing for multiple samples to be stably processed within one animal. This tunable (pressure, flow, shear) ex vivo shunt model can be used to optimize the implantability and long-term patency of tissue-engineered vascular constructs.

Differential keratin expression during epiboly in a wound model of bioengineered skin and in human chronic wounds

Epiboly represents the process by which keratinocytes migrate to envelop a surface. The authors have been investigating a living bilayered skin construct (BSC) that is used in the treatment of lower extremity wounds due to venous insufficiency and diabetes. The construct demonstrates epiboly after injury and incubation in vitro, and this model may be useful for studying epidermal migration and the process of skin maturation. Punch biopsies of the construct in vitro were cultured and immunostained for specific keratins at baseline and at 24 to 72 hours. For comparison, skin biopsy specimens from human chronic venous ulcers and acute healing wounds were similarly processed. The authors found that K1 and K10 were fully expressed in the epidermis of the fully epibolized surface on BSC. K1 was also present in the migrating edge of specimens, whereas K10 was not detectable. K16 and K6 were evident in normal skin and the epibolized area of the construct; K6 expression was very prominent in the migrating edge. Importantly, K17 was distinctly limited to the epibolized surface and the migrating edge, and its expression was very similar to that observed in healing human wounds. In conclusion, differential expression of keratins in this epiboly model closely reflects in vivo studies and supports keratin specificity in the processes of migration and differentiation of new epidermis. Therefore, these findings provide further and important validity for the study of epithelialization and the hope of developing prognostic markers for venous ulcer healing.

Development of a three-dimensional human skin equivalent wound model for investigating novel wound healing therapies

Numerous difficulties are associated with the conduct of preclinical studies related to skin and wound repair. Use of small animal models such as rodents is not optimal because of their physiological differences to human skin and mode of wound healing. Although pigs have previously been used because of their human-like mode of healing, the expense and logistics related to their use also renders them suboptimal. In view of this, alternatives are urgently required to advance the field. The experiments reported herein were aimed at developing and validating a simple, reproducible, three-dimensional ex vivo de-epidermised dermis human skin equivalent wound model for the preclinical evaluation of novel wound therapies. Having established that the human skin equivalent wound model does in fact “heal," we tested the effect of two novel wound healing therapies. We also examined the utility of the model for studies exploring the mechanisms underpinning these therapies. Taken together the data demonstrate that these new models will have wide-spread application for the generation of fundamental new information on wound healing processes and also hold potential in facilitating preclinical optimization of dosage, duration of therapies, and treatment strategies prior to clinical trials.

Vascularization of the dermal support enhances wound re-epithelialization by in situ delivery of epidermal keratinocytes

Despite significant advances in management of severe wounds such as burns and chronic ulcers, autologous split-thickness skin grafts are still the gold standard of care. The main problems with this approach include pain and discomfort associated with harvesting autologous tissue, limited availability of donor sites, and the need for multiple surgeries. Although tissue engineering has great potential to provide alternative approaches for tissue regeneration, several problems have hampered progress in translating technological advances to clinical reality. Specifically, engineering of skin substitutes requires long culture times and delayed vascularization after implantation compromises graft survival. To address these issues we developed a novel two-prong strategy for tissue regeneration in vivo: (1) vascularization of acellular dermal scaffolds by infiltration of angiogenic factors; and (2) generation of stratified epidermis by in situ delivery of epidermal keratinocytes onto the prevascularized dermal support. Using athymic mouse as a model system, we found that incorporation of angiogenic factors within acellular human dermis enhanced the density and diameter of infiltrating host blood vessels. Increased vascularization correlated with enhanced proliferation and stratification of the neoepidermis originating from the fibrin-keratinocyte cell suspension. This strategy promoted tissue regeneration in vivo with no need for engineering skin substitutes; therefore, it may be useful for treatment of major wounds when skin donor sites are scarce and rapid wound coverage is required.

Surgical approaches to create murine models of human wound healing

Wound repair is a complex biologic process which becomes abnormal in numerous disease states. Although in vitro models have been important in identifying critical repair pathways in specific cell populations, in vivo models are necessary to obtain a more comprehensive and pertinent understanding of human wound healing. The laboratory mouse has long been the most common animal research tool and numerous transgenic strains and models have been developed to help researchers study the molecular pathways involved in wound repair and regeneration. This paper aims to highlight common surgical mouse models of cutaneous disease and to provide investigators with a better understanding of the benefits and limitations of these models for translational applications.

A minimally invasive human in vivo cutaneous wound model for the evaluation of innate skin reactivity and healing status

Individual variability in skin reactivity and healing capacity after trauma are important clinical issues. The aims were to develop an in vivo, human wound model based on a standardised minimal skin injury and to demonstrate therapeutic effect of simple wound therapies in terms of morphological wound outcome with changes in skin blood perfusion as a quantified indicator of wound healing. In a series of experiments, wounds were induced on the normal forearm skin of volunteers using a blood collection lancet. This was well tolerated. Wounds were assessed by naked eye examination or laser Doppler perfusion imaging (LDPI) at baseline and at up to 6 further time points up to 96 h in control wounds and wounds treated by commonly used occlusive dressing options. Assessment by clinical observation with 10x magnification showed over 96 h a progression of erythema, surface crust, a new keratinisation layer and finally healed areas. LDPI quantifying wound erythema showed a peak at 24 h and near normal levels at 96 h. Inter-individual variability was evident but intra-individual variability was much less pronounced. Wounds treated with occlusion showed a statistically significant more rapid return to baseline blood perfusion as measured by LDPI compared to controls supported by favourable healing parameters in the clinical assessment. The paper exemplifies use of non-invasive, bioengineering technique for quantification of individual innate variability in skin reactivity, wound healing capacity and therapeutic effect in a well-tolerated in vivo, human, minimal skin trauma model.

Ex vivo expanded haematopoietic progenitor cells improve dermal wound healing by paracrine mechanisms

BACKGROUND

Although dermal wounds are common, treatment remains limited and largely ineffective. Recent studies suggest that therapeutic application of progenitor cells is useful for tissue regeneration.

OBJECTIVE

We here investigated the effects exerted by the recently characterized immortalized haematopoietic progenitor cell line DKmix and their conditioned medium in a murine wound healing model.

METHODS AND RESULTS

Injection of both DKmix cells and their conditioned medium accelerated wound repair between days 3 and 10 compared with PBS-injected control mice (n = 8, P < 0.01 DKmix cells vs control, P < 0.01 conditioned medium vs control at day 6). The treated groups exhibited more CD31(+)-capillaries at day 6 after injury compared with the control group (n = 4, P < 0.01 DKmix cells vs control, P < 0.001 conditioned medium vs control), whereas there was no change in infiltrated CD68(+) macrophages. Conditioned medium of DKmix cells induced tube formation of human endothelial cells in Matrigel assays (n = 4-6, P < 0.05 conditioned medium vs control) as well as migration (n = 4, P < 0.01 conditioned medium vs control) and proliferation of murine 3T3 fibroblasts (n = 5, P < 0.05 conditioned medium vs control). Abundant levels of matrix metalloproteinase -2 and -9 in the supernatants were detected. Protein arrays of the supernatants revealed a strong secretion of cytokines and growth factors, such as monocyte chemoatractant protein-1 and GM-CSF from DKmix cells.

CONCLUSION

DKmix cells improve skin-substitute wound healing by promoting angiogenesis as well as migration and proliferation of fibroblasts. These data suggest that immortalized haematopoietic progenitor cells significantly improve dermal wound healing by paracrine effects.

Wound healing on athymic mice with engineered skin substitutes fabricated with keratinocytes harvested from an automated bioreactor

The Kerator is a computer controlled bioreactor for the automated culture and harvest of keratinocytes that can reduce labor and materials involved in the fabrication of engineered skin substitutes (ESS). Previous studies have shown that the Kerator is comparable to tissue culture flasks by keratinocyte confluence during culture, clonogenic potential of harvested keratinocytes and microanatomy, cell viability, and surface hydration of ESS fabricated with the harvested keratinocytes. In this study, the Kerator and tissue culture flasks were further compared by keratinocyte proliferation in vitro and wound healing after transplantation of ESS to athymic mice. The number of bromodeoxyuridine-positive keratinocytes in ESS fabricated with keratinocytes harvested from Kerator after 2 wk of in vitro maturation was 34 +/- 3 per high power field (hpf) (mean +/- SEM), which was not significantly different from that fabricated with keratinocytes harvested from flasks (34 +/- 1.5 per hpf). Percentage original wound area 6 wk after surgery of ESS fabricated with keratinocytes from the Kerator was 36% +/- 3.3%, which was not significantly different from that of ESS fabricated with keratinocytes from flasks (30% +/- 4.3%). In both cases, 78% (7 of 9) mice transplanted were positive for engraftment of human keratinocytes by direct immunofluorescence for HLA-ABC antigens. These results further confirm that the ESS fabricated with keratinocytes harvested from Kerator and flasks are equivalent in vitro and in vivo. Therefore, use of Kerator for large scale production of ESS can lead to increased availability at reduced cost while maintaining ESS quality for grafting.

Novel insights into wound healing sequence of events

Effective wound healing leads to restoration of tissue integrity and occurs through a highly organized multistage process involving various cell types. Currently, methods for wound healing assessment lack a structured system for analysis of quantitative parameters. We have established a unique quantitative assessment strategy of wound healing stages based on histological criteria. Distinctive immunohistochemical parameters including re-epithelialization, epidermal differentiation, cell migration, proliferation, inflammatory response as well as dermal closure, matrix distribution, and skin remodelling were identified and followed during the timeline of wound healing progression. Assessment was based on various defined characteristics and each stage-specific parameter was independently quantified for complete wound closure. This analysis allowed a follow-up of wound healing dynamics and identified the contribution of critical and specific parameters to wound healing physiology and pathology. In this review we demonstrate our assessment strategy of crucial wound healing events and introduce a unique quantification system for each of the processes involved in wound repair. We believe that our unique method can be utilized as a diagnostic platform for standardizing assessment of wound healing progression as well as a screening tool for potential therapies.

Composite fibrin scaffolds increase mechanical strength and preserve contractility of tissue engineered blood vessels

OBJECTIVES

We recently demonstrated that fibrin-based tissue engineered blood vessels (TEV) exhibited vascular reactivity, matrix remodeling and sufficient strength for implantation into the veins of an ovine animal model, where they remained patent for 15 weeks. Here we present an approach to improve the mechanical properties of fibrin-based TEV and examine the relationship between mechanical strength and smooth muscle cell (SMC) function.

MATERIALS AND METHODS

To this end, we prepared TEV that were composed of two layers: a cellular layer containing SMC embedded in fibrin hydrogel to provide contractility and matrix remodeling; and a second cell-free fibrin layer composed of high concentration fibrinogen to provide mechanical strength.

RESULTS

The ultimate tensile force of double-layered TEV increased with FBG concentration in the cell-free layer in a dose-dependent manner. Double-layered TEV exhibited burst pressure that was ten-fold higher than single-layered tissues but vascular reactivity remained high even though the cells were constricting an additional tissue layer.

CONCLUSION

These results showed that mechanical strength results largely from the biomaterial but contractility requires active cellular machinery. Consequently, they may suggest novel approaches for engineering biomaterials that satisfy the requirement for high mechanical strength while preserving SMC function.

Intravital insights in skin wound healing using the mouse dorsal skin fold chamber

The skin fold chamber is one of the most accepted animal models for studying the microcirculation both in health and disease. Here we describe for the first time the alternative use of the skin fold chamber in mice for intravital microscopic investigation of skin regeneration after creating a full dermal thickness wound. The dorsal skin fold chamber was implanted in hairless SKH1-hr mice and a full dermal thickness wound (area approximately 4 mm2) was created. By means of intravital fluorescence microscopy, the kinetics of wound healing were analyzed for 12 days post wounding with assessment of epithelialization and nutritive perfusion. The morphology of the regenerating skin was characterized by hematoxylin-eosin histology and immunohistochemistry for proliferation and microvessel density. The model allows the continuous visualization of wound closure with complete epithelialization at day 12. Furthermore, a sola cutis se reficientis could be described by an inner circular ring of vessels at the wound margin surrounded by outer radial passing vessels. Inner circular vessels presented initially with large diameters and matured towards diameters of less than 15 microm for conversion into radial spreading outer vessels. Furthermore, wound healing showed all diverse core issues of skin repair. In summary, we were able to establish a model for the analysis of microcirculation in the healing skin of the mouse. This versatile model allows distinct analysis of new vessel formation and maturation in regenerating skin as well as evaluation of skin healing under different pathologic conditions.

Production of a novel fibroblast-populated platelet matrix cocultured with keratinocytes

We have developed a new method for the production of a dermal matrix equivalent. Human platelets were used to dilute human fibroblasts. The platelet mix was placed in a cell culture well. Addition of 200 microL of a thrombin solution caused gel formation. Gels were overlaid with standard Iscove's growth medium supplemented with 10% fetal bovine serum, insulin, and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer. Medium was exchanged regularly. Keratinocytes were plated on top of selected gels and elevated to the air-liquid interface. The gels were harvested weekly, fixed, cut, and stained with hematoxylin and eosin stains and immunostains for collagens I, III, and IV and cytokeratins. Digital image analysis was used to quantitate collagen production. Growth factors, including transforming growth factor-beta (TGF-beta), platelet-derived growth factor, and vitamin C were added. Staining identified fibroblasts within the gels with a surrounding fibrous matrix. Immunostaining for cytokeratin identified keratinocytes on the gel surface. Immunostaining revealed the fibrous matrix to be composed of collagen I and III and some collagen IV. Digital image analysis demonstrated that greater TGF-beta concentration resulted in greater collagen production. These differences were statistically significant. With development of this construct, a viable dermal/epidermal replacement may be possible. TGF-beta enhances collagen production by fibroblasts in this matrix.

Potential of Heterotopic Fibroblasts as Autologous Transplanted Cells for Tracheal Epithelial Regeneration

The tracheal epithelium maintains the health of the respiratory tract through mucociliary clearance and regulation of ion and water balance. When the trachea is surgically removed, artificial grafts have been clinically used by our group to regenerate the trachea. In such cases, the tracheal epithelium needs 2 months for functional regeneration. Previous study has shown that fibroblasts facilitate tracheal epithelial regeneration. In this study, heterotopic fibroblasts originating from the dermis, nasal, and gingival mucosa were cocultured with tracheal epithelial cells to evaluate their potential as autologous transplanted cells for tracheal epithelial regeneration. The epithelia induced by the heterotopic fibroblasts showed differences in structure, cilia development, mucin secretion, and expression of ion and water channels. These results indicated that nasal fibroblasts could not induce mature tracheal epithelium and that dermal fibroblasts induced epidermis-like epithelium. Only the gingival fibroblasts (GFBs) could induce morphologically and functionally normalized tracheal epithelium comparable to the epithelium induced by tracheal fibroblasts. Epithelial cell proliferation and migration were also upregulated by GFBs. These results indicate that GFBs are useful as autologous transplant cells for tracheal epithelial regeneration.

Efficient Production of Bioactive Insulin from Human Epidermal Keratinocytes and Tissue-Engineered Skin Substitutes: Implications for Treatment of Diabetes

Despite many years of research, daily insulin injections remain the gold standard for diabetes treatment. Gene therapy may provide an alternative strategy by imparting the ability to secrete insulin from an ectopic site. The epidermis is a self-renewing tissue that is easily accessible and can provide large numbers of autologous cells to generate insulin-secreting skin substitutes. Here we used a recombinant retrovirus to modify human epidermal keratinocytes with a gene encoding for human proinsulin containing the furin recognition sequences at the A-C and B-C junctions. Keratinocytes were able to process proinsulin and secrete active insulin that promoted glucose uptake. Primary epidermal cells produced higher amounts of insulin than cell lines, suggesting that insulin secretion may depend on the physiological state of the producer cells. Modified cells maintained the ability to stratify into 3-dimensional skin equivalents that expressed insulin at the basal and suprabasal layers. Modifications at the furin recognition sites did not improve proinsulin processing, but a single amino acid substitution in the proinsulin B chain enhanced C-peptide secretion from cultured cells and bioengineered skin substitutes 10- and 28-fold, respectively. These results suggest that gene-modified bioengineered skin may provide an alternative means of insulin delivery for treatment of diabetes.

Migration of the epidermal over the dermal component (epiboly) in a bilayered bioengineered skin construct

A bilayered bioengineered living skin construct (LSC) consisting of viable human neonatal keratinocytes over a collagenous dermis seeded with dermal fibroblasts has been used extensively in difficult-to heal human wounds. Its biological properties include production of several mediators, cytokines, and growth factors and the ability to heal itself upon injury. In this study, we investigated the process of keratinocyte migration in LSC. At baseline, 6-mm punch biopsies of the construct were placed in serum-free medium (AIM-V) or Dulbecco's modified Eagle medium. At varying time points, the LSC samples were processed and analyzed using histology and immunohistochemistry. By 72 h, in a time-dependent manner, the overlying epidermis had migrated over and enveloped the entire underlying dermis, a process known as epiboly. Increasing concentrations of neutralizing antibodies to epidermal growth factor or interleukin-1 alpha down-regulated the extent of epiboly, as measured using computerized planimetry, but antibodies to transforming growth factor-beta 1 did not affect it. The consistent expression of laminin V, alpha3beta1 integrin, and vitronectin (epibolin) and its integrin receptor (alphavbeta5) characterized the tongue of migrating epidermis. Increasing concentrations of antibodies to vitronectin blocked the process of epiboly, as did antibodies to the alphavbeta5 integrin receptor, which mediates vitronectin-driven keratinocyte locomotion. This process of epiboly provides novel mechanisms of action for bioengineered skin constructs.

Experimental models and high-throughput diagnostics for tissue regeneration

During wound healing, cells recreate functional structures to regenerate the injured tissue. Understanding the healing process is essential for the development of new concepts and the design of novel biomimetic approaches for delivery of cells, genes and growth factors to accelerate tissue regeneration. To this end, realistic experimental models and high-throughput diagnostics are necessary to understand the molecular mechanisms of healing and reveal the genetic networks that determine tissue repair versus regeneration. Following a brief overview of the biology of wound healing, this review covers the in vitro and in vivo models that are employed at present to study the healing process. Discussion then covers the application of high-throughput genomic and proteomic technologies in epithelial development, living skin substitutes and wound healing. Finally, this review provides a perspective on novel technologies that should be developed to facilitate the understanding of wound healing complications and the design of therapeutics that target the underlying deficiencies.