Characterization of the living skin equivalent as a model of cutaneous re-epithelialization

Keratinocyte Migration in a Three-Dimensional In Vitro Wound Healing Model Co-Cultured with Fibroblasts

Background

Because three-dimensional (3D) models more closely mimic native tissues, one of the goals of 3D in vitro tissue models is to aid in the development and toxicity screening of new drug therapies. In this study, a 3D skin wound healing model comprising of a collagen type I construct with fibrin-filled defects was developed.

Methods

Optical imaging was used to measure keratinocyte migration in the presence of fibroblasts over 7 days onto the fibrin-filled defects. Additionally, cell viability and growth of fibroblasts and keratinocytes was measured using the alamarBlue^® assay and changes in the mechanical stiffness of the 3D construct was monitored using compressive indentation testing.

Results

Keratinocyte migration rate was significantly increased in the presence of fibroblasts with the cells reaching the center of the defect as early as day 3 in the co-culture constructs compared to day 7 for the control keratinocyte monoculture constructs. Additionally, constructs with the greatest rate of keratinocyte migration had reduced cell growth. When fibroblasts were cultured alone in the wound healing construct, there was a 1.3 to 3.4-fold increase in cell growth and a 1.2 to 1.4-fold increase in cell growth for keratinocyte monocultures. However, co-culture constructs exhibited no significant growth over 7 days. Finally, mechanical testing showed that fibroblasts and keratinocytes had varying effects on matrix stiffness with fibroblasts degrading the constructs while keratinocytes increased the construct's stiffness.

Conclusion

This 3D in vitro wound healing model is a step towards developing a mimetic construct that recapitulates the complex microenvironment of healing wounds and could aid in the early studies of novel therapeutics that promote migration and proliferation of epithelial cells.

Multimodal label-free imaging of living dermal equivalents including dermal papilla cells

BACKGROUND

Despite the significant progress in the development of skin equivalents (SEs), the problem of noninvasively assessing the quality of the cell components and the collagen structure of living SEs both before and after transplantation remains. Undoubted preference is given to in vivo methods of noninvasive, label-free monitoring of the state of the SEs. Optical bioimaging methods, such as cross-polarization optical coherence tomography (CP OCT), multiphoton tomography (MPT), and fluorescence lifetime imaging microscopy (FLIM), present particular advantages for the visualization of such SEs.

METHODS

In this study, we simultaneously applied several visualization techniques for skin model examination. We investigated the structure and quality of dermal equivalents containing dermal papilla (DP) cells and dermal fibroblasts (FBs) using CP OCT, MPT, and FLIM. Both the energy metabolism of the cell components and the structuring of the collagen fibrils were addressed.

RESULTS

Based on the data from the fluorescence lifetimes and the contributions of protein-bound NAD(P)H, a bias toward oxidative metabolism was indicated, for the first time, in both the DP cells and FBs on day 14 of SE cultivation. The CP OCT and MPT data also indicated that both DP cells and FBs structured the collagen gel in a similar manner.

CONCLUSION

In this study, multimodal label-free imaging of the structure and quality of living dermal equivalents was implemented for the first time with the use CP OCT, MPT, and FLIM of NAD(P)H. Our data suggest that the combination of different imaging techniques provides an integrated approach to data acquisition regarding the structure and quality of dermal equivalents, minimizes the potential disadvantages of using a single method, and provides an ideal information profile for clinical and research applications.

Synthetic polymeric biomaterials for wound healing: a review

Wounds are of a variety of types and each category has its own distinctive healing requirements. This realization has spurred the development of a myriad of wound dressings, each with specific characteristics. It is unrealistic to expect a singular dressing to embrace all characteristics that would fulfill generic needs for wound healing. However, each dressing may approach the ideal requirements by deviating from the 'one size fits all approach', if it conforms strictly to the specifications of the wound and the patient. Indeed, a functional wound dressing should achieve healing of the wound with minimal time and cost expenditures. This article offers an insight into several different types of polymeric materials clinically used in wound dressings and the events taking place at cellular level, which aid the process of healing, while the biomaterial dressing interacts with the body tissue. Hence, the significance of using synthetic polymer films, foam dressings, hydrocolloids, alginate dressings, and hydrogels has been reviewed, and the properties of these materials that conform to wound-healing requirements have been explored. A special section on bioactive dressings and bioengineered skin substitutes that play an active part in healing process has been re-examined in this work.

Spatiotemporal Evolution of the Wound Repairing Process in a 3D Human Dermis Equivalent

Several skin equivalent models have been developed to investigate in vitro the re-epithelialization process occurring during wound healing. Although these models recapitulate closure dynamics of epithelial cells, they fail to capture how a wounded connective tissue rebuilds its 3D architecture until the evolution in a scar. Here, the in vitro tissue repair dynamics of a connective tissue is replicated by using a 3D human dermis equivalent (3D-HDE) model composed of fibroblasts embedded in their own extracellular matrix (ECM). After inducing a physical damage, 3D-HDE undergoes a series of cellular and extracellular events quite similar to those occurring in the native dermis. In particular, fibroblasts differentiation toward myofibroblasts phenotype and neosynthesis of hyaluronic acid, fibronectin, and collagen during the repair process are assessed. Moreover, tissue reorganization after physical damage is investigated by measuring the diameter of bundles and the orientation of fibers of the newly formed ECM network. Finally, the ultimate formation of a scar-like tissue as physiological consequence of the repair and closure process is demonstrated. Taking together, the results highlight that the presence of cell-assembled and responsive stromal components enables quantitative and qualitative in vitro evaluation of the processes involved in scarring during wound healing.

Regeneration and repair of human digits and limbs: fact and fiction

A variety of digit and limb repair and reconstruction methods have been used in different clinical settings, but regeneration remains an item on every plastic surgeon's "wish list." Although surgical salvage techniques are continually being improved, unreplantable digits and limbs are still abundant. We comprehensively review the structural and functional salvage methods in clinical practice, from the peeling injuries of small distal fingertips to multisegmented amputated limbs, and the developmental and tissue engineering approaches for regenerating human digits and limbs in the laboratory. Although surgical techniques have forged ahead, there are still situations in which digits and limbs are unreplantable. Advances in the field are delineated, and the regeneration processes of salamander limbs, lizard tails, and mouse digits and each component of tissue engineering approaches for digit- and limb-building are discussed. Although the current technology is promising, there are many challenges in human digit and limb regeneration. We hope this review inspires research on the critical gap between clinical and basic science, and leads to more sophisticated digit and limb loss rescue and regeneration innovations.

Re-epithelialization: advancing epithelium frontier during wound healing

The first function of the skin is to serve as a protective barrier against the environment. Its loss of integrity as a result of injury or illness may lead to a major disability and the first goal of healing is wound closure involving many biological processes for repair and tissue regeneration. In vivo wound healing has four phases, one of them being the migration of the healthy epithelium surrounding the wound in the direction of the injury in order to cover it. Here, we present a theoretical model of the re-epithelialization phase driven by chemotaxis for a circular wound. This model takes into account the diffusion of chemoattractants both in the wound and the neighbouring tissue, the uptake of these molecules by the surface receptors of epithelial cells, the migration of the neighbour epithelium, the tension and proliferation at the wound border. Using a simple Darcy's law for cell migration transforms our biological model into a free-boundary problem, which is analysed in the simplified circular geometry leading to explicit solutions for the closure and making stability analysis possible. It turns out that for realistic wound sizes of the order of centimetres and from experimental data, the re-epithelialization is always an unstable process and the perfect circle cannot be observed, a result confirmed by fully nonlinear simulations and in agreement with experimental observations.

Alternatives for animal wound model systems

In this chapter a review of animal model systems already being utilized to study normal and pathologic wound healing is provided. We also go into details on alternatives for animal wound model systems. The case is made for limitations in the various approaches. We also discuss the benefits/limitations of in vitro/ex vivo systems bringing everything up to date with our current work on developing a cell-based reporter system for diabetic wound healing.

Skin substitutes: a brief review of types and clinical applications

Replacing skin defects has witnessed several developments over the centuries. It started with the introduction of skin grafting by Reverdin in 1871. Since then, varieties of skin grafting techniques have been used successfully. Despite being clinically useful, skin grafts have many limitations including the availability of the donor site especially in circumstances of extensive skin loss, immune rejection in allogenic skin grafts, pain, scarring, slow healing and infection.(1,2) For these reasons, scientist have worked hard to find skin substitutes to replace skin defects without the need for a "natural" skin graft. These materials which are used to cover skin defects are called "Skin substitutes". This article briefly discusses the common types of skin substitutes and their clinical uses.

Effects of hydrogen peroxide in a keratinocyte-fibroblast co-culture model of wound healing

Recently, there has been renewed interest in the role of reactive oxygen species (ROS), especially H(2)O(2), in wound healing. We previously showed that H(2)O(2) stimulates healing in a keratinocyte scratch wound model. In this paper, we used a more complex and physiologically relevant model that involves co-culturing primary keratinocytes and fibroblasts. We found that the two main cell types within the skin have different sensitivities to H(2)O(2) and to the widely used "antioxidant"N-acetyl-l-cysteine (NAC). Keratinocytes were very resistant to the toxicity of H(2)O(2) (250 and 500 μM) or NAC (5 mM). However, the viability of fibroblasts was decreased by both compounds. Using the co-culture model, we also found that H(2)O(2) increases re-epithelialization while NAC retards it. Our data further illustrate the possible role of ROS in wound healing and the co-culture model should be useful for screening agents that may influence the wound healing process.

In vitro study of the effects of plaunotol on oral cell proliferation and wound healing

Plaunotol is an acyclic diterpene alcohol extracted from a medicinal plant called plau-noi, Croton stellatopilosus Ohba, and has been widely used for the treatment of gastric ulcers in Japan. The aim of this study was to examine the effects of plaunotol on human gingival fibroblasts (HGFs) and human oral keratinocytes (HOKs). To assess the cytotoxic effect, HGFs and HOKs were treated with plaunotol. Subsequently, the morphology of cells was recorded and cells were subjected to MTT assay. To investigate cell proliferation effect, cells were treated with plaunotol and counted with a haemocytometer. To determine wound healing effect, the number of cells repopulated into the wounded areas in monolayer culture and in fibroblast-populated collagen lattice (FPCL) was measured. The results showed that 10 and 1 μg/ml (33 and 3.3 μmol/l) plaunotol induced toxicity in HGFs and HOKs, respectively. However, 0.1 μg/ml (0.33 μmol/l) plaunotol promoted HGF proliferation and wound healing in monolayer and FPCL models. In contrast, 0.1 μg/ml plaunotol could not induce HOK proliferation nor in vitro wound healing using monolayer culture, but it induced wound healing in a modified FPCL model. Our data suggested that plaunotol could promote oral cell proliferation and wound healing in vitro and may have an implication on oral wound healing.

Engineering skin to study human disease--tissue models for cancer biology and wound repair

Recent advances in the engineering of three-dimensional tissues known as skin equivalents, that have morphologic and phenotypic properties of human skin, have provided new ways to study human disease processes. This chapter will supply an overview of two such applications--investigations of the incipient development of squamous cell cancer, and studies that have characterized the response of human epithelium during wound repair. Using these novel tools to study cancer biology, it has been shown that cell-cell interactions inherent in three-dimensional tissue architecture can suppress early cancer progression by inducing a state of intraepithelial dormancy. This dormant state can be overcome and cancer progression enabled by altering tissue organization in response to tumor promoters or UV irradiation or by modifying the interaction of tumor cells with extracellular matrix proteins or their adjacent epithelia. By adapting skin equivalent models of human skin to study wound reepithelialization, it has been shown that several key responses, including cell proliferation, migration, differentiation, growth-factor responsiveness and protease expression, will mimic the response seen in human skin. In this light, these engineered models of human skin provide powerful new tools for studying disease processes in these tissues as they occur in humans.

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.

Epithelial regeneration from bioengineered skin explants in culture

BACKGROUND

Artificial skin substitutes are beneficial in the treatment of chronic wounds although their performance relative to authentic human skin is unclear.

OBJECTIVES

We compared the rate of outgrowth and morphology of neoepidermis from a bioengineered skin construct (Apligraf) with normal adult human skin explants on de-epidermized human dermal growth substrate with or without intact epidermal basement membrane zone.

METHODS

Epithelial outgrowth of air-exposed cultures in serum-supplemented keratinocyte medium was quantified by fluorescence imaging, morphology by light microscopy, biomarkers of keratinocyte activation, proliferation and migration by immunohistochemical analysis, and gelatinases by zymography.

RESULTS

Resurfacing from bioengineereed skin explants started earlier than from normal skin but subsequently, from day 3 to day 9, the rate of epidermalization from bioengineered skin was only 40% (206 +/- 23 microm day(-1), mean +/- SEM) of that of authentic skin (521 +/- 17 microm day(-1), P < 0.001). At culture termination at day 11, normal human skin had formed a multilayered and well-structured neoepidermis covering 41.0 +/- 1.2 mm2 of the dermal substrate while bioengineered skin produced a thinner, less organized epithelium covering 20.4 +/- 3.0 mm2. At this later stage, a higher expression of beta-defensin-2, keratin 16, Ki67 and matrix metalloproteinase (MMP)-9 was found in neoepidermis formed from authentic skin than from bioengineered skin. Activated MMP-2 was elevated in bioengineered skin-derived neoepidermis. Minor epithelial outgrowth was noted with either skin type on the dermal substrate devoid of basement membrane zone.

CONCLUSIONS

Cultured normal skin explants produced a more uniform and expansive in vivo-like neoepidermis than bioengineered skin explants.

Upside-down transfer of porcine keratinocytes from a porous, synthetic dressing to experimental full-thickness wounds

Currently, the use of cultured epithelial autografts as an alternative to split-thickness skin autografts for coverage of full-thickness wounds is limited due to fragility of the sheet and variability in the outcome of healing. This could be circumvented by the transfer of proliferating keratinocytes, instead of differentiated sheets, to the wound bed and the "in vivo" regeneration of epidermis. The aim of this study was to achieve re-epithelialization on experimental full-thickness wounds in the pig using a porous, synthetic carrier seeded with proliferating keratinocytes. Porcine keratinocytes were isolated by enzymatic digestion and cultured in Optimem basal medium with mitogens. In a full-thickness wound model, carriers with different seeding densities were transplanted upside down onto the wound bed. Keratinocytes were labeled using a fluorescent red membrane marker, PKH-26 GL. Transfer of keratinocytes and re-epithelialization were recorded macroscopically and histologically. On day 4 after transplantation, transfer of fluorescently labeled keratinocytes was shown by their presence in the granulation tissue. An immature epidermis, as well as epithelial cords and islands, formed as early as day 8. At day 12 a stratified epidermis and wound closure were established and epithelial cysts were formed by differentiation of epithelial islands. Wounds treated with seeding densities as low as 50,000 cells/cm(2) showed wound closure within 12 days, whereas wounds treated with 10,000 cells/cm(2) or the nonseeded (acellular) carriers did not show complete re-epithelialization before day 17 after treatment. This study showed that porcine keratinocytes, transplanted "upside down" in experimental full-thickness wounds using a synthetic carrier, continued to proliferate and started to differentiate, enabling the formation of a new epidermis in a time frame of 12 days.

Stage-dependent modulation of limb regeneration by caffeic acid phenethyl ester (CAPE)--immunocytochemical evidence of a CAPE-evoked delay in mesenchyme formation and limb regeneration

Caffeic acid phenethyl ester (CAPE), a natural compound of bee propolis, selectively inhibits proliferation of transformed cells in several cancer models in vitro. To examine in vivo CAPE function, we used the newt regeneration blastema as a model system wherein the processes of de-differentiation and subsequent proliferation of undifferentiated cells mimic changes associated with oncogenic transformation and tumorigenesis. We have shown that a single dose of CAPE significantly increased cell proliferation at the stages of blastema growth and re-differentiation. At the de-differentiation stage, CAPE significantly stimulated proliferation of wound epidermis keratinocytes, but decreased proliferation in the blastema mesenchyme. Immunohistochemistry with a mesenchymal cell marker, vimentin, revealed a highly significant reduction of vimentin staining in the mesenchyme of CAPE-treated regenerates (p<0.001). These results, together with morphological observations indicate that, at the de-differentiation stage, CAPE stimulated wound re-epithelization, increased keratinocyte proliferation and increased thickness of the wound epidermis. However, CAPE inhibited mesenchyme formation and proliferation. The functional consequence of the CAPE inhibitory action was a delay in limb regeneration.

Fucoidan Modulates the Effect of Transforming Growth Factor (TGF)-.BETA.1 on Fibroblast Proliferation and Wound Repopulation in in Vitro Models of Dermal Wound Repair

Aberrant wound healing, either causing scarring or chronic wounds, is a significant cause of morbidity. There is therefore, considerable interest in agents which can modulate certain aspects of the wound healing process. Fucoidans, sulphated polyfucose polysaccharides which may be extracted from Fucus spp., have been shown to modulate the effects of a variety of growth factors through mechanisms thought to be similar to the action of heparin. We investigated the interaction between two commercial preparations of fucoidan and transforming growth factor (TGF)-beta(1). These preparations of fucoidan, as well as heparin, inhibited fibroblast proliferation at concentrations from 0.01 to 100 mg/ml. The anti-proliferative effects of 1 ng/ml TGF-beta(1) on dermal fibroblasts were abrogated by fucoidan preparation F7 when used at concentrations over 1 mg/ml. In a three dimensional in vitro model of wound repair, the fibroblast populated collagen lattice or "dermal equivalent", TGF-beta(1) reduced the rate of fibroblast repopulation of a wound defect created by punch biopsy. Addition of fucoidan to the model in the presence of TGF-beta(1) increased the rate of fibroblast repopulation of the wound and at 10 mg/ml of fucoidan the number of cells which had migrated into the wounded defect was similar to that of control cultures. These data suggest that fucoidan has properties which may be beneficial in the treatment of wound healing.