Development of microfabricated dermal epidermal regenerative matrices to evaluate the role of cellular microenvironments on epidermal morphogenesis

Sensory neurons increase keratinocyte proliferation through CGRP release in a tissue engineered in vitro model of innervation in psoriasis

Skin denervation has been shown to cause remission of psoriatic lesions in patients, which can reappear if reinnervation occurs. This effect can be induced by the activation of dendritic cells through sensory innervation. However, a direct effect of nerves on the proliferation of keratinocytes involved in the formation of psoriatic plaques has not been investigated. We developed, by tissue engineering, a model of psoriatic skin made of patient skin cells that showed increased keratinocyte proliferation and epidermal thickness compared to healthy controls. When this model was treated with CGRP, a neuropeptide released by sensory neurons, an increased keratinocyte proliferation was observed in the psoriatic skin model, but not in the control. When a sensory nerve network was incorporated in the psoriatic model and treated with capsaicin to induce neuropeptide release, an increase of keratinocyte proliferation was confirmed, which was blocked by a CGRP antagonist while no difference was noticed in the innervated healthy control. We showed that sensory neurons can participate directly to keratinocyte hyperproliferation in the formation of psoriatic lesions through the release of CGRP, independently of the immune system. Our unique tissue-engineered innervated psoriatic skin model could be a valuable tool to better understand the mechanism by which nerves may modulate psoriatic lesion formation in humans. STATEMENT OF SIGNIFICANCE: This study shows that keratinocytes extracted from patients' psoriatic skin retain, at least in part, the disease phenotype. Indeed, when combined in a 3D model of tissue-engineered psoriatic skin, keratinocytes exhibited a higher proliferation rate, and produced a thicker epidermis than a healthy skin control. In addition, their hyperproliferation was aggravated by a treatment with CGRP, a neuropeptide released by sensory nerves. In a innervated model of tissue-engineered psoriatic skin, an increase in keratinocyte hyperproliferation was also observed after inducing neurons to release neuropeptides. This effect was prevented by concomitant treatment with an antagonist to CGRP. Thus, this study shows that sensory nerves can directly participate to affect keratinocyte hyperproliferation in psoriasis through CGRP release.

Dermal stiffness governs the topography of the epidermis and the underlying basement membrane in young and old human skin

The epidermis is a stratified epithelium that forms the outer layer of the skin. It is composed primarily of keratinocytes and is constantly renewed by the proliferation of stem cells and their progeny that undergo terminal differentiation as they leave the basal layer and migrate to the skin surface. Basal keratinocytes rest on a basement membrane composed of an extracellular matrix that controls their fate via integrin-mediated focal adhesions and hemidesmosomes which are critical elements of the epidermal barrier and promote its regenerative capabilities. The distribution of basal cells with optimal activity provides the basement membrane with its characteristic undulating shape; this configuration disappears with age, leading to epidermal weakness. In this study, we present an in-depth imaging analysis of basal keratinocyte anchorage in samples of human skin from participants across the age spectrum. Our findings reveal that skin aging is associated with the depletion of hemidesmosomes that provide crucial support for stem cell maintenance; their depletion correlates with the loss of the characteristic basement membrane structure. Atomic force microscopy studies of skin and in vitro experiments revealed that the increase in tissue stiffness observed with aging triggers mechanical signals that alter the basement membrane structure and reduce the extent of basal keratinocyte anchorage, forcing them to differentiate. Genomic analysis revealed that epidermal aging was associated with mechanical induction of the transcription factor Krüppel-like factor 4. The altered mechanical properties of tissue being a new hallmark of aging, our work opens new avenues for the development of skin rejuvenation strategies.

Biofabrication of biomimetic undulating microtopography at the dermal-epidermal junction and its effects on the growth and differentiation of epidermal cells

The undulating microtopography located at the junction of the dermis and epidermis of the native skin is called rete ridges (RRs), which plays an important role in enhancing keratinocyte function, improving skin structure and stability, and providing three-dimensional (3D) microenvironment for skin cells. Despite some progress in recent years, most currently designed and manufactured tissue-engineered skin models still cannot replicate the RRs, resulting in a lack of biological signals in the manufactured skin models. In this study, a composite manufacturing method including electrospinning, 3D printing, and functional coating was developed to produce the epidermal models with RRs. Polycaprolactone (PCL) nanofibers were firstly electrospun to mimic the extracellular matrix environment and be responsible for cell attachment. PCL microfibers were then printed onto top of the PCL nanofibers layer by 3D printing to quickly prepare undulating microtopography and finally the entire structures were dip-coated with gelatin hydrogel to form a functional coating layer. The morphology, chemical composition, and structural properties of the fabricated models were studied. The results proved that the multi-process composite fabricated models were suitable for skin tissue engineering. Live and dead staining, cell counting kit-8 (CCK-8) as well as histology (haematoxylin and eosin (HE) methodology) and immunofluorescence (primary and secondary antibodies combination assay) were used to investigate the viability, metabolic activity, and differentiation of skin cells forin vitroculturing.In vitroresults showed that each model had high cell viability, good proliferation, and the expression of differentiation marker. It was worth noting that the sizes of the RRs affected the cell growth status of the epidermal models. In addition, the unique undulation characteristics of the epidermal-dermal junction can be reproduced in the developed epidermal models. Overall, thesein vitrohuman epidermal models can provide valuable reference for skin transplantation, screening and safety evaluation of drugs and cosmetics.

Biomimetic human skin model patterned with rete ridges

Rete ridges consist of undulations between the epidermis and dermis that enhance the mechanical properties and biological function of human skin. However, most human skin models are fabricated with a flat interface between the epidermal and dermal layers. Here, we report a micro-stamping method for producing human skin models patterned with rete ridges of controlled geometry. To mitigate keratinocyte-induced matrix degradation, telocollagen-fibrin matrices with and without crosslinks enable these micropatterned features to persist during longitudinal culture. Our human skin model exhibits an epidermis that includes the following markers: cytokeratin 14, p63, and Ki67 in the basal layer, cytokeratin 10 in the suprabasal layer, and laminin and collagen IV in the basement membrane. We demonstrated that two keratinocyte cell lines, one from a neonatal donor and another from an adult diabetic donor, are compatible with this model. We tested this model using an irritation test and showed that the epidermis prevents rapid penetration of sodium dodecyl sulfate. Gene expression analysis revealed differences in keratinocytes obtained from the two donors as well as between 2D (control) and 3D culture conditions. Our human skin model may find potential application for drug and cosmetic testing, disease and wound healing modeling, and aging studies.

Constructing epidermal rete ridges using a composite hydrogel to enhance multiple signaling pathways for the maintenance of epidermal stem cell niche

The undulating microstructure rete ridge (RR) located at the junction between the dermis and epidermis plays a crucial role in improving skin mechanical properties and maintaining skin homeostasis. However, the investigation of RR microstructures is usually neglected in current tissue engineering for skin regeneration. Here, to create an epidermal model with RR microstructures, keratinocytes were cultured on a patterned GelMA-PEGDA hydrogel constructed using molding technology. Furthermore, grafting acryloylated Arg-Gly-Asp (RGD) peptides on the hydrogel surface significantly improved cell adhesion, fusion, and development. RT-PCR, Western blot, and immunofluorescence staining confirmed that cells on RR microstructures exhibited higher gene and protein expression associated with epidermal stem cells. RNA sequencing analysis of cells on RR microstructure showed higher gene expression profiles related to stem cell maintenance, basement membrane formation, and epidermal development. Furthermore, RT-PCR analysis of epidermal models of various dimensions demonstrated that smaller microstructures were more conducive to epidermal stem cell marker gene expression, which is analogous to human skin. Overall, we have successfully developed a method for integrating RR microstructures into an epidermal model that mimics natural skin to maintain epidermal stem cell niche, providing a valuable reference for researching skin regeneration within the fields of tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This study presents a method for precisely fabricating microstructures of skin rete ridges using composite hydrogels, thereby creating a skin model that mimics natural human skin. The findings reveal that this microstructure provides a stem cell niche that regulates the pathways and promotes the expression of proteins related to epidermal stem cells. This work advances the functional properties of tissue engineered skin and holds promise for improving the therapeutic efficacy of artificial skin grafts for the skin wounds.

Micropatterned fibrin scaffolds increase cardiomyocyte alignment and contractility for the fabrication of engineered myocardial tissue

Cardiovascular disease is the leading cause of death in the United States, which can result in blockage of a coronary artery, triggering a myocardial infarction (MI), scar tissue formation in the myocardium, and ultimately heart failure. Currently, the gold-standard solution for total heart failure is a heart transplantation. An alternative to total-organ transplantation is surgically remodeling the ventricle with the implantation of a cardiac patch. Acellular cardiac patches have previously been investigated using synthetic or decellularized native materials to improve cardiac function. However, a limitation of this strategy is that acellular cardiac patches only reshape the ventricle and do not increase cardiac contractile function. Toward the development of a cardiac patch, our laboratory previously developed a cell-populated composite fibrin scaffold and aligned microthreads to recapitulate the mechanical properties of native myocardium. In this study, we explore micropatterning the surfaces of fibrin gels to mimic anisotropic native tissue architecture and promote cellular alignment of human induced pluripotent stem cell cardiomyocytes (hiPS-CM), which is crucial for increasing scaffold contractile properties. hiPS-CMs seeded on micropatterned surfaces exhibit cellular elongation, distinct sarcomere alignment, and circumferential connexin-43 staining at 14 days of culture, which are necessary for mature contractile properties. Constructs were also subject to electrical stimulation during culture to promote increased contractile properties. After 7 days of stimulation, contractile strains of micropatterned constructs were significantly higher than unpatterned controls. These results suggest that the use of micropatterned topographic cues on fibrin scaffolds may be a promising strategy for creating engineered cardiac tissue.

Laser Micropatterning Promotes Rete Ridge Formation and Enhanced Engineered Skin Strength without Increased Inflammation

Rete ridges play multiple important roles in native skin tissue function, including enhancing skin strength, but they are largely absent from engineered tissue models and skin substitutes. Laser micropatterning of fibroblast-containing dermal templates prior to seeding of keratinocytes was shown to facilitate rete ridge development in engineered skin (ES) both in vitro and in vivo. However, it is unknown whether rete ridge development results exclusively from the microarchitectural features formed by ablative processing or whether laser treatment causes an inflammatory response that contributes to rete ridge formation. In this study, laser-micropatterned and non-laser- treated ES grafts were developed and assessed during culture and for four weeks post grafting onto full-thickness wounds in immunodeficient mice. Decreases in inflammatory cytokine secretion were initially observed in vitro in laser-treated grafts compared to non-treated controls, although cytokine levels were similar in both groups five days after laser treatment. Post grafting, rete ridge-containing ES showed a significant increase in vascularization at week 2, and in collagen deposition and biomechanics at weeks 2 and 4, compared with controls. No differences in inflammatory cytokine expression after grafting were observed between groups. The results suggest that laser micropatterning of ES to create rete ridges improves the mechanical properties of healed skin grafts without increasing inflammation.

Rete ridges: Morphogenesis, function, regulation, and reconstruction

Rete ridges (RRs) are distinct undulating microstructures at the junction of the dermis and epidermis in the skin of humans and certain animals. This structure is essential for enhancing the mechanical characteristics of skin and preserving homeostasis. With the development of tissue engineering and regenerative medicine, artificial skin grafts have made great progress in the field of skin healing. However, the restoration of RRs has been often disregarded or absent in artificial skin grafts, which potentially compromise the efficacy of tissue repair and regeneration. Therefore, this review collates recent research advances in understanding the structural features, function, morphogenesis, influencing factors, and reconstruction strategies pertaining to RRs. In addition, the preparation methods and limitations of tissue-engineered skin with RRs are discussed. STATEMENT OF SIGNIFICANCE: The technology for the development of tissue-engineered skin (TES) is widely studied and reported; however, the preparation of TES containing rete ridges (RRs) is often ignored, with no literature reviews on the structural reconstruction of RRs. This review focuses on the progress pertaining to RRs and focuses on the reconstruction methods for RRs. In addition, it discusses the limitations of existing reconstruction methods. Therefore, this review could be a valuable reference for transferring TES with RR structure from the laboratory to clinical applications in skin repair.

Bioactive Materials Promote Wound Healing through Modulation of Cell Behaviors

Skin wound repair is a multistage process involving multiple cellular and molecular interactions, which modulate the cell behaviors and dynamic remodeling of extracellular matrices to maximize regeneration and repair. Consequently, abnormalities in cell functions or pathways inevitably give rise to side effects, such as dysregulated inflammation, hyperplasia of nonmigratory epithelial cells, and lack of response to growth factors, which impedes angiogenesis and fibrosis. These issues may cause delayed wound healing or even non-healing states. Current clinical therapeutic approaches are predominantly dedicated to preventing infections and alleviating topical symptoms rather than addressing the modulation of wound microenvironments to achieve targeted outcomes. Bioactive materials, relying on their chemical, physical, and biological properties or as carriers of bioactive substances, can affect wound microenvironments and promote wound healing at the molecular level. By addressing the mechanisms of wound healing from the perspective of cell behaviors, this review discusses how bioactive materials modulate the microenvironments and cell behaviors within the wounds during the stages of hemostasis, anti-inflammation, tissue regeneration and deposition, and matrix remodeling. A deeper understanding of cell behaviors during wound healing is bound to promote the development of more targeted and efficient bioactive materials for clinical applications.

The Importance of Mimicking Dermal-Epidermal Junction for Skin Tissue Engineering: A Review

There is a distinct boundary between the dermis and epidermis in the human skin called the basement membrane, a dense collagen network that creates undulations of the dermal-epidermal junction (DEJ). The DEJ plays multiple roles in skin homeostasis and function, namely, enhancing the adhesion and physical interlock of the layers, creating niches for epidermal stem cells, regulating the cellular microenvironment, and providing a physical boundary layer between fibroblasts and keratinocytes. However, the primary role of the DEJ has been determined as skin integrity; there are still aspects of it that are poorly investigated. Tissue engineering (TE) has evolved promising skin regeneration strategies and already developed TE scaffolds for clinical use. However, the currently available skin TE equivalents neglect to replicate the DEJ anatomical structures. The emergent ability to produce increasingly complex scaffolds for skin TE will enable the development of closer physical and physiological mimics to natural skin; it also allows researchers to study the DEJ effect on cell function. Few studies have created patterned substrates that could mimic the human DEJ to explore their significance. Here, we first review the DEJ roles and then critically discuss the TE strategies to create the DEJ undulating structure and their effects. New approaches in this field could be instrumental for improving bioengineered skin substitutes, creating 3D engineered skin, identifying pathological mechanisms, and producing and screening drugs.

Fabrication of Topographically Controlled Electrospun Scaffolds to Mimic the Stem Cell Microenvironment in the Dermal-Epidermal Junction

The use of microfabrication techniques for the development of innovative constructs for tissue regeneration is a growing area of research. This area comprises both manufacturing and biological approaches for the development of smart materials aiming to control and direct cell behavior to enhance tissue healing. Many groups have focused their efforts on introducing complexity within these innovative constructs via the inclusion of nano- and microtopographical cues mimicking physical and biological aspects of the native stem cell niche. Specifically, in the area of skin tissue engineering, seminal work has reported replicating the microenvironments located in the dermal-epithelial junction, which are known as rete ridges. The rete ridges are key for both stem cell control and the physiological performance of the skin. In this work, we have introduced complexity within electrospun membranes to mimic the morphology of the rete ridges in the skin. We designed and tested three different patterns, characterized them, and explored their performance in vitro, using 3D skin models. One of the studied patterns (pattern B) was shown to aid in the development of an in vitro rite-ridgelike skin model that resulted in the expression of relevant epithelial markers such as collagen IV and integrin β1. In summary, we have developed a new skin model including synthetic rete-ridgelike structures that replicate both morphology and function of the native dermal-epidermal junction and that offer new insights for the development of smart skin tissue engineering constructs.

The Use of Microfabrication Techniques for the Design and Manufacture of Artificial Stem Cell Microenvironments for Tissue Regeneration

The recapitulation of the stem cell microenvironment is an emerging area of research that has grown significantly in the last 10 to 15 years. Being able to understand the underlying mechanisms that relate stem cell behavior to the physical environment in which stem cells reside is currently a challenge that many groups are trying to unravel. Several approaches have attempted to mimic the biological components that constitute the native stem cell niche, however, this is a very intricate environment and, although promising advances have been made recently, it becomes clear that new strategies need to be explored to ensure a better understanding of the stem cell niche behavior. The second strand in stem cell niche research focuses on the use of manufacturing techniques to build simple but functional models; these models aim to mimic the physical features of the niche environment which have also been demonstrated to play a big role in directing cell responses. This second strand has involved a more engineering approach in which a wide set of microfabrication techniques have been explored in detail. This review aims to summarize the use of these microfabrication techniques and how they have approached the challenge of mimicking the native stem cell niche.

The Human Epidermal Basement Membrane: A Shaped and Cell Instructive Platform That Aging Slowly Alters

One of the most important functions of skin is to act as a protective barrier. To fulfill this role, the structural integrity of the skin depends on the dermal-epidermal junction—a complex network of extracellular matrix macromolecules that connect the outer epidermal layer to the underlying dermis. This junction provides both a structural support to keratinocytes and a specific niche that mediates signals influencing their behavior. It displays a distinctive microarchitecture characterized by an undulating pattern, strengthening dermal-epidermal connectivity and crosstalk. The optimal stiffness arising from the overall molecular organization, together with characteristic anchoring complexes, keeps the dermis and epidermis layers extremely well connected and capable of proper epidermal renewal and regeneration. Due to intrinsic and extrinsic factors, a large number of structural and biological changes accompany skin aging. These changes progressively weaken the dermal–epidermal junction substructure and affect its functions, contributing to the gradual decline in overall skin physiology. Most changes involve reduced turnover or altered enzymatic or non-enzymatic post-translational modifications, compromising the mechanical properties of matrix components and cells. This review combines recent and older data on organization of the dermal-epidermal junction, its mechanical properties and role in mechanotransduction, its involvement in regeneration, and its fate during the aging process.

Fractional CO2 laser micropatterning of cell-seeded electrospun collagen scaffolds enables rete ridge formation in 3D engineered skin

Rete ridges are interdigitations of the epidermis and dermis of the skin that play multiple roles in homeostasis, including enhancing adhesion via increased contact area and acting as niches for epidermal stem cells. These structures, however, are generally absent from engineered skin (ES). To develop ES with rete ridges, human fibroblast-seeded dermal templates were treated with a fractional CO₂ laser, creating consistently spaced wells at the surface. Constructs with and without laser treatment were seeded with keratinocytes, cultured for 10 days, and grafted onto athymic mice for four weeks. Rete-ridge like structures were observed in the laser-patterned (ridged) samples at the time of grafting and were maintained in vivo. Ridged grafts displayed improved barrier function over non-lasered (flat) grafts at the time of grafting and 4 weeks post-grafting. Presence of ridges in vivo corresponded with increased keratinocyte proliferation, epidermal area, and basement membrane length. These results suggest that this method can be utilized to develop engineered skin grafts with rete ridges, that the ridge pattern is stable for at least 4 weeks post-grafting, and that the presence of these ridges enhances epidermal proliferation and establishment of barrier function. STATEMENT OF SIGNIFICANCE: Rete ridges play a role in epidermal homeostasis, enhance epidermal-dermal adhesion and act as niches for epidermal stem cells. Despite their role in skin function, these structures are not directly engineered into synthetic skin. A new method to rapidly and reproducibly generate rete ridges in engineered skin was developed using fractional CO₂ laser ablation. The resulting engineered rete ridges aided in the establishment of epidermal barrier function, basement membrane protein deposition and epidermal regeneration. This new model of engineered skin with rete ridges could be utilized as an in vitro system to study epidermal stem cells, a testbed for pharmaceutical evaluation or translated for clinical use in full-thickness wound repair.

Micropattern-mediated apical guidance accelerates epithelial cell migration to improve healing around percutaneous gastrostomy tubes

Hypergranulation, bacterial infection, and device dislodgment are common complications associated with percutaneous gastronomy (PG) tube placement for enteral feeding largely attributable to delayed stoma tract maturation around the device. Stoma tract maturation is a wound-healing process that requires collective and complete migration of an advancing epithelial layer. While it is widely accepted that micropatterned surfaces enhance cell migration when cells are cultured directly on the substrate, few studies have investigated the influence of apical contact guidance from micropatterned surfaces on cell migration, as occurs during stoma tract formation. Here, we developed 2D and 3D in vitro epithelial cell migration assays to test the effect of various Sharklet micropatterns on apically-guided cell migration. The 2D modified scratch wound assay identified a Sharklet micropattern (+10SK50×50) that enhanced apical cell migration by 4-fold (p = 0.0105) compared to smooth controls over the course of seven days. The best-performing micropattern was then applied to cylindrical prototypes with the same outer diameter as a pediatric PG tube. These prototypes were evaluated in the novel 3D migration assay where magnetic levitation aggregated cells around prototypes to create an artificial stoma. Results indicated a 50% increase (p < 0.0001) in cell migration after seven days along Sharklet-micropatterned prototypes compared to smooth controls. The Sharklet micropattern enhanced apically-guided epithelial cell migration in both 2D and 3D in vitro assays. These data suggest that the incorporation of a Sharklet micropattern onto the surface of a PG tube may accelerate cell migration via apical contact, improve stoma tract maturation, and reduce skin-associated complications.

Re-epithelialization of adult skin wounds: Cellular mechanisms and therapeutic strategies

Cutaneous wound healing in adult mammals is a complex multi-step process involving overlapping stages of blood clot formation, inflammation, re-epithelialization, granulation tissue formation, neovascularization, and remodelling. Re-epithelialization describes the resurfacing of a wound with new epithelium. The cellular and molecular processes involved in the initiation, maintenance, and completion of epithelialization are essential for successful wound closure. A variety of modulators are involved, including growth factors, cytokines, matrix metalloproteinases, cellular receptors, and extracellular matrix components. Here, we focus on cellular mechanisms underlying keratinocyte migration and proliferation during epidermal closure. Inability to re-epithelialize is a clear indicator of chronic non-healing wounds, which fail to proceed through the normal phases of wound healing in an orderly and timely manner. This review summarizes the current knowledge regarding the management and treatment of acute and chronic wounds, with a focus on re-epithelialization, offering some insights into novel future therapies.

Dynamic Culture Substrates That Mimic the Topography of the Epidermal-Dermal Junction

IMPACT STATEMENT

In human skin the junction between the epidermis and dermis undulates. Epidermal stem cells pattern according to their position relative to those undulations. Here we describe a rig in which epidermal cells are cultured on a collagen-coated poly(d,l-lactide-co-glycolide) (PLGA) membrane. When a vacuum is applied the membrane is induced to undulate. Stem cells cluster in response to the vacuum, whereas differentiating cells do not. Rho kinase inhibition results in loss of clustering, suggesting a role for Rho family members in the process. This dynamic platform is a new tool for investigating changes in the skin with age and disease.

A 3D in vitro model of the dermoepidermal junction amenable to mechanical testing

Recessive dystrophic Epidermolysis Bullosa (RDEB) is caused by mutations in collagen‐type VII gene critical for the dermoepidermal junction (DEJ) formation. Neither tissues of animal models nor currently available in vitro models are amenable to the quantitative assessment of mechanical adhesion between dermal and epidermal layers. Here, we created a 3D in vitro DEJ model using extracellular matrix (ECM) proteins of the DEJ anchored to a poly(ethylene glycol)‐based slab (termed ECM composites) and seeded with human keratinocytes and dermal fibroblasts. Keratinocytes and fibroblasts of healthy individuals were well maintained in the ECM composite and showed the expression of collagen type VII over a 2‐week period. The ECM composites with healthy keratinocytes and fibroblasts exhibited yield stress associated with the separation of the model DEJ at 0.268 ± 0.057 kPa. When we benchmarked this measure of adhesive strength with that of the model DEJ fabricated with cells of individuals with RDEB, the yield stress was significantly lower (0.153 ± 0.064 kPa) consistent with our current mechanistic understanding of RDEB. In summary, a 3D in vitro model DEJ was developed for quantification of mechanical adhesion between epidermal‐ and dermal‐mimicking layers, which can be utilized for assessment of mechanical adhesion of the model DEJ applicable for Epidermolysis Bullosa‐associated therapeutics. © 2018 The Authors. Journal Of Biomedical Materials Research Part A Published By Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3231–3238, 2018.

Engineered Microenvironments to Direct Epidermal Stem Cell Behavior at Single-Cell Resolution

Mammalian epidermis is maintained through proliferation of stem cells and differentiation of their progeny. The balance between self-renewal and differentiation is controlled by a variety of interacting intrinsic and extrinsic factors. Although the nature of these interactions is complex, they can be modeled in a reductionist fashion by capturing single epidermal stem cells on micropatterned substrates and exposing them to individual stimuli, alone or in combination, over defined time points. These studies have shown that different extrinsic stimuli trigger a common outcome-initiation of terminal differentiation-by activating different signaling pathways and eliciting different transcriptional responses.

Evaluation of a bilayered, micropatterned hydrogel dressing for full-thickness wound healing

Nearly 12 million wounds are treated in emergency departments throughout the United States every year. The limitations of current treatments for complex, full-thickness wounds are the driving force for the development of new wound treatment devices that result in faster healing of both dermal and epidermal tissue. Here, a bilayered, biodegradable hydrogel dressing that uses microarchitecture to guide two key steps in the proliferative phase of wound healing, re-epithelialization, and revascularization, was evaluated in vitro in a cell migration assay and in vivo in a bipedicle ischemic rat wound model. Results indicate that the Sharklet™-micropatterned apical layer of the dressing increased artificial wound coverage by up to 64%, P = 0.024 in vitro. In vivo evaluation demonstrated that the bilayered dressing construction enhanced overall healing outcomes significantly compared to untreated wounds and that these outcomes were not significantly different from a leading clinically available wound dressing. Collectively, these results demonstrate high potential for this new dressing to effectively accelerate wound healing.

Mimicking the topography of the epidermal-dermal interface with elastomer substrates

In human skin the interface between the epidermis and dermis is not flat, but undulates. The dimensions of the undulations change as a function of age and disease. Epidermal stem cell clusters lie in specific locations relative to the undulations; however, whether their location affects their properties is unknown. To explore this, we developed a two-step protocol to create patterned substrates that mimic the topographical features of the human epidermal-dermal interface. Substrates with negative patterns were first fabricated by exposing a photocurable formulation to light, controlling the topographical features (such as diameter, height and center-to-center distance) by the photomask pattern dimensions and UV crosslinking time. The negative pattern was then translated to PDMS elastomer to fabricate substrates with 8 unique surface topographies on which primary human keratinocytes were cultured. We found that cells were patterned according to topography, and that separate cues determined the locations of stem cells, differentiated cells and proliferating cells. The biomimetic platform we have developed will be useful for probing the effect of topography on stem cell behaviour.

Mechanical cues in orofacial tissue engineering and regenerative medicine

Cleft lip and palate patients suffer from functional, aesthetical, and psychosocial problems due to suboptimal regeneration of skin, mucosa, and skeletal muscle after restorative cleft surgery. The field of tissue engineering and regenerative medicine (TE/RM) aims to restore the normal physiology of tissues and organs in conditions such as birth defects or after injury. A crucial factor in cell differentiation, tissue formation, and tissue function is mechanical strain. Regardless of this, mechanical cues are not yet widely used in TE/RM. The effects of mechanical stimulation on cells are not straight-forward in vitro as cellular responses may differ with cell type and loading regime, complicating the translation to a therapeutic protocol. We here give an overview of the different types of mechanical strain that act on cells and tissues and discuss the effects on muscle, and skin and mucosa. We conclude that presently, sufficient knowledge is lacking to reproducibly implement external mechanical loading in TE/RM approaches. Mechanical cues can be applied in TE/RM by fine-tuning the stiffness and architecture of the constructs to guide the differentiation of the seeded cells or the invading surrounding cells. This may already improve the treatment of orofacial clefts and other disorders affecting soft tissues.

Tissue Engineering the Cornea: The Evolution of RAFT

Corneal blindness affects over 10 million people worldwide and current treatment strategies often involve replacement of the defective layer with healthy tissue. Due to a worldwide donor cornea shortage and the absence of suitable biological scaffolds, recent research has focused on the development of tissue engineering techniques to create alternative therapies. This review will detail how we have refined the simple engineering technique of plastic compression of collagen to a process we now call Real Architecture for 3D Tissues (RAFT). The RAFT production process has been standardised, and steps have been taken to consider Good Manufacturing Practice compliance. The evolution of this process has allowed us to create biomimetic epithelial and endothelial tissue equivalents suitable for transplantation and ideal for studying cell-cell interactions in vitro.

Challenges and promises in modeling dermatologic disorders with bioengineered skin

The tremendous cost of drug development is often attributed to the long time interval between identifying lead compounds in preclinical studies to assessing clinical efficacy in randomized clinical trials. Many candidate molecules show promise in cell culture or animal models, only to fail in late stage in human investigations. There is a need for novel technologies that allow investigators to quickly and reliably predict drug safety and efficacy. The advent of microtechnology has made it possible to integrate multiple microphysiologic organ systems into a single microfabricated chip. This review focuses on three-dimensional engineered skin, which has enjoyed a long history of uses both in clinical treatments of refractory ulcers and as a laboratory model. We discuss current biological and engineering challenges in construction of a robust bioengineered skin and provide a blueprint for its potential utility to model dermatologic disorders such as psoriasis or cutaneous drug reactions.

Micropatterned dermal-epidermal regeneration matrices create functional niches that enhance epidermal morphogenesis

Although tissue engineered skin substitutes have demonstrated some clinical success for the treatment of chronic wounds such as diabetic and venous ulcers, persistent graft take and stability remain concerns. Current bilayered skin substitutes lack the characteristic microtopography of the dermal-epidermal junction that gives skin enhanced mechanical stability and creates cellular microniches that differentially promote keratinocyte function to form skin appendages and enhance wound healing. We developed a novel micropatterned dermal-epidermal regeneration matrix (μDERM) which incorporates this complex topography and substantially enhances epidermal morphology. Here, we describe the use of this three-dimensional (3-D) in vitro culture model to systematically evaluate different topographical geometries and to determine their relationship to keratinocyte function. We identified three distinct keratinocyte functional niches: the proliferative niche (narrow geometries), the basement membrane protein synthesis niche (wide geometries) and the putative keratinocyte stem cell niche (narrow geometries and corners). Specifically, epidermal thickness and keratinocyte proliferation is significantly (p<0.05) increased in 50 and 100 μm channels while laminin-332 deposition is significantly (p<0.05) increased in 400 μm channels compared to flat controls. Additionally, β1(bri)p63(+) keratinocytes, putative keratinocyte stem cells, preferentially cluster in channel geometries (similar to clustering observed in native skin) compared to a random distribution on flats. This study identifies specific target geometries to enhance skin regeneration and graft performance. Furthermore, these results suggest the importance of μDERM microtopography in designing the next generation of skin substitutes. Finally, we anticipate that 3-D organotypic cultures on μDERMS will provide a novel tissue engineered skin substitute for in vitro investigations of skin morphogenesis, wound healing and pathology.

Rapid tissue engineering of biomimetic human corneal limbal crypts with 3D niche architecture

Limbal epithelial stem cells are responsible for the maintenance of the human corneal epithelium and these cells reside in a specialised stem cell niche. They are located at the base of limbal crypts, in a physically protected microenvironment in close proximity to a variety of neighbouring niche cells. Design and recreation of elements of various stem cell niches have allowed researchers to simplify aspects of these complex microenvironments for further study in vitro. We have developed a method to rapidly and reproducibly create bioengineered limbal crypts (BLCs) in a collagen construct using a simple one-step method. Liquid is removed from collagen hydrogels using hydrophilic porous absorbers (HPAs) that have custom moulded micro-ridges on the base. The resulting topography on the surface of the thin collagen constructs resembles the dimensions of the stromal crypts of the human limbus. Human limbal epithelial cells seeded onto the surface of the constructs populate these BLCs and form numerous layers with a high proportion of the cells lining the crypts expressing putative stem cell marker, p63α. The HPAs are produced using a moulding process that is flexible and can be adapted depending on the requirements of the end user. Creation of defined topographical features using this process could be applicable to numerous tissue-engineering applications where varied 3-dimensional niche architectures are required.