In vitro activation of the neuro-transduction mechanism in sensitive organotypic human skin model

Fabrication of 3D engineered intestinal tissue producing abundant mucus by air-liquid interface culture using paper-based dual-layer scaffold

Fabrication of engineered intestinal tissues with the structures and functions as humans is crucial and promising as the tools for developing drugs and functional foods. The aim of this study is to fabricate an engineered intestinal tissue from Caco-2 cells by air-liquid interface culture using a paper-based dual-layer scaffold and analyze its structure and functions. Just by simply placing on a folded paper soaked in the medium, the electrospun gelatin microfiber mesh as the upper cell adhesion layer of the dual-layer scaffold was exposed to the air, while the lower paper layer worked to preserve and supply the cell culture medium to achieve stable culture over several weeks. Unlike the flat tissue produced using the conventional commercial cultureware, Transwell, the engineered intestinal tissue fabricated in this study formed three-dimensional villous architectures. Microvilli and tight junction structures characteristic of epithelial tissue were also formed at the apical side. Furthermore, compared to the tissue prepared by Transwell, mucus production was significantly larger, and the enzymatic activities of drug metabolism and digestion were almost equivalent. In conclusion, the air-liquid interface culture using the paper-based dual-layer scaffold developed in this study was simple but effective in fabricating the engineered intestinal tissue with superior structures and functions.

Biomechanical properties of articular cartilage in different regions and sites of the knee joint: acquisition of osteochondral allografts

Osteochondral allograft (OCA) transplantation involves grafting of natural hyaline cartilage and supporting subchondral bone into the cartilage defect area to restore its biomechanical and tissue structure. However, differences in biomechanical properties and donor-host matching may impair the integration of articular cartilage (AC). This study analyzed the biomechanical properties of the AC in different regions of different sites of the knee joint and provided a novel approach to OCA transplantation. Intact stifle joints from skeletally mature pigs were collected from a local abattoir less than 8 h after slaughter. OCAs were collected from different regions of the joints. The patella and the tibial plateau were divided into medial and lateral regions, while the trochlea and femoral condyle were divided into six regions. The OCAs were analyzed and compared for Young's modulus, the compressive modulus, and cartilage thickness. Young's modulus, cartilage thickness, and compressive modulus of OCA were significantly different in different regions of the joints. A negative correlation was observed between Young's modulus and the proportion of the subchondral bone (r =  - 0.4241, P < 0.0001). Cartilage thickness was positively correlated with Young's modulus (r = 0.4473, P < 0.0001) and the compressive modulus (r = 0.3678, P < 0.0001). During OCA transplantation, OCAs should be transplanted in the same regions, or at the closest possible regions to maintain consistency of the biomechanical properties and cartilage thickness of the donor and recipient, to ensure smooth integration with the surrounding tissue. A 7 mm depth achieved a higher Young's modulus, and may represent the ideal length.

Rapid innervation and physiological epidermal regeneration by bioengineered dermis implanted in mouse

Tissue-engineered skin substitutes are promising tools to cover large and deep skin defects. However, the lack of a synergic and fast regeneration of the vascular network, nerves, and skin appendages limits complete skin healing and impairs functional recovery. It has been highlighted that an ideal skin substitute should mimic the structure of the native tissue to enhance clinical effectiveness. Here, we produced a pre-vascularized dermis (PVD) comprised of fibroblasts embedded in their own extracellular matrix (ECM) and a capillary-like network. Upon implantation in a mouse full-thickness skin defect model, we observed a very early innervation of the graft in 2 weeks. In addition, mouse capillaries and complete epithelialization were detectable as early as 1 week after implantation and, skin appendages developed in 2 weeks. These anatomical features underlie the interaction with the skin nerves, thus providing a further cue for reinnervation guidance. Further, the graft displays mechanical properties, collagen density, and assembly features very similar to the host tissue. Taken together our data show that the pre-existing ECM components of the PVD, physiologically organized and assembled similarly to the native tissue, support a rapid regeneration of dermal tissue. Therefore, our results suggest a promising potential for PVD in skin regeneration.

The frontline of alternatives to animal testing: novel in vitro skin model application in drug development and evaluation

The FDA Modernization Act 2.0 has brought nonclinical drug evaluation into a new era. In vitro models are widely used and play an important role in modern drug development and evaluation, including early candidate drug screening and preclinical drug efficacy and toxicity assessment. Driven by regulatory steering and facilitated by well-defined physiology, novel in vitro skin models are emerging rapidly, becoming the most advanced area in alternative testing research. The revolutionary technologies bring us many in vitro skin models, either laboratory-developed or commercially available, which were all built to emulate the structure of the natural skin to recapitulate the skin's physiological function and particular skin pathology. During the model development, how to achieve balance among complexity, accessibility, capability, and cost-effectiveness remains the core challenge for researchers. This review attempts to introduce the existing in vitro skin models, align them on different dimensions, such as structural complexity, functional maturity, and screening throughput, and provide an update on their current application in various scenarios within the scope of chemical testing and drug development, including testing in genotoxicity, phototoxicity, skin sensitization, corrosion/irritation. Overall, the review will summarize a general strategy for in vitro skin model to enhance future model invention, application, and translation in drug development and evaluation.

Layer-by-Layer Analysis of In Vitro Skin Models

In vitro human skin models are evolving into versatile platforms for the study of skin biology and disorders. These models have many potential applications in the fields of drug testing and safety assessment, as well as cosmetic and new treatment development. The development of in vitro skin models that accurately mimic native human skin can reduce reliance on animal models and also allow for more precise, clinically relevant testing. Recent advances in biofabrication techniques and biomaterials have led to the creation of increasingly complex, multilayered skin models that incorporate important functional components of skin, such as the skin barrier, mechanical properties, pigmentation, vasculature, hair follicles, glands, and subcutaneous layer. This improved ability to recapitulate the functional aspects of native skin enhances the ability to model the behavior and response of native human skin, as the complex interplay of cell-to-cell and cell-to-material interactions are incorporated. In this review, we summarize the recent developments in in vitro skin models, with a focus on their applications, limitations, and future directions.

Co-culture system of human skin equivalents with mouse neural spheroids

This study describes a co-culture system of human skin equivalents (HSEs) and dorsal root ganglion (DRG) neurons. We prepared spheroids of mouse DRG neurons with or without Schwann cells (SCs). Spheroids comprising DRG neurons and SCs showed longer neurite extensions than those comprising DRG neurons alone. Neurite extension of more than 1 mm was observed from spheroids cultured inside HSEs, whereas neurite extension was primarily observed on the surface of HSEs from spheroids cultured on HSEs. We propose that our model may be a useful tool for studying neurite extension in the human skin.

Modeling of three-dimensional innervated epidermal like-layer in a microfluidic chip-based coculture system

Reconstruction of skin equivalents with physiologically relevant cellular and matrix architecture is indispensable for basic research and industrial applications. As skin-nerve crosstalk is increasingly recognized as a major element of skin physiological pathology, the development of reliable in vitro models to evaluate the selective communication between epidermal keratinocytes and sensory neurons is being demanded. In this study, we present a three-dimensional innervated epidermal keratinocyte layer as a sensory neuron-epidermal keratinocyte co-culture model on a microfluidic chip using the slope-based air-liquid interfacing culture and spatial compartmentalization. Our co-culture model recapitulates a more organized basal-suprabasal stratification, enhanced barrier function, and physiologically relevant anatomical innervation and demonstrated the feasibility of in situ imaging and functional analysis in a cell-type-specific manner, thereby improving the structural and functional limitations of previous coculture models. This system has the potential as an improved surrogate model and platform for biomedical and pharmaceutical research.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Organotypic Human Skin Cultures Incorporating Primary Melanocytes

Three-dimensional (3D) human organotypic skin cultures provide a physiologically relevant model that recapitulates in vivo skin features. Most commonly, organotypic skin cultures are created by seeding isolated epidermal keratinocytes onto a collagen/fibroblast plug and lifting to an air liquid interface. These conditions are sufficient to drive stratification and differentiation of the keratinocytes to form an epidermal-like sheet with remarkable similarities to human epidermis. Coupled with genetic or pharmacological treatments, these cultures provide a powerful tool for elucidating keratinocyte biology. Recent focus has been placed on increasing the utility of organotypic skin cultures by incorporating other cell types that are present in the skin, such as melanocytes, immune cells, and other cells. Here we describe a step-by-step protocol for the isolation of neonatal human epidermal keratinocytes and melanocytes from foreskins, and the creation of organotypic skin cultures that include both cell types. We also describe methods that can be used to assess melanocyte behavior in these organotypic cultures, including methods for whole mount staining, measurement of melanocyte dendricity, staining for pigment, and 5-bromo-2'-deoxyuridine (BrdU) labeling for identification of proliferating cells. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Isolation of primary cells Alternate Protocol: Isolation of primary cells using differential trypsinization Basic Protocol 2: Organotypic culture protocol Support Protocol 1: Culture and maintenance of NHEKs and melanocytes Support Protocol 2: Lentiviral transduction of melanocytes Support Protocol 3: Retroviral transduction of NHEKs Support Protocol 4: Whole mount immunostaining protocol Support Protocol 5: Measuring melanocyte dendricity Support Protocol 6: Fontana-Masson staining protocol Support Protocol 7: BrdU labeling and staining.

Cellular Pathogenesis of Chemotherapy-Induced Peripheral Neuropathy: Insights From Drosophila and Human-Engineered Skin Models

Chemotherapy-induced peripheral neuropathy (CIPN) is a highly prevalent and complex condition arising from chemotherapy cancer treatments. Currently, there are no treatment or prevention options in the clinic. CIPN accompanies pain-related sensory functions starting from the hands and feet. Studies focusing on neurons in vitro and in vivo models significantly advanced our understanding of CIPN pathological mechanisms. However, given the direct toxicity shown in both neurons and non-neuronal cells, effective in vivo or in vitro models that allow the investigation of neurons in their local environment are required. No single model can provide a complete solution for the required investigation, therefore, utilizing a multi-model approach would allow complementary advantages of different models and robustly validate findings before further translation. This review aims first to summarize approaches and insights from CIPN in vivo models utilizing small model organisms. We will focus on Drosophila melanogaster CIPN models that are genetically amenable and accessible to study neuronal interactions with the local environment in vivo. Second, we will discuss how these findings could be tested in physiologically relevant vertebrate models. We will focus on in vitro approaches using human cells and summarize the current understanding of engineering approaches that may allow the investigation of pathological changes in neurons and the skin environment.

Modeling an Optimal 3D Skin-on-Chip within Microfluidic Devices for Pharmacological Studies

Preclinical research remains hampered by an inadequate representation of human tissue environments which results in inaccurate predictions of a drug candidate’s effects and target’s suitability. While human 2D and 3D cell cultures and organoids have been extensively improved to mimic the precise structure and function of human tissues, major challenges persist since only few of these models adequately represent the complexity of human tissues. The development of skin-on-chip technology has allowed the transition from static 3D cultures to dynamic 3D cultures resembling human physiology. The integration of vasculature, immune system, or the resident microbiome in the next generation of SoC, with continuous detection of changes in metabolism, would potentially overcome the current limitations, providing reliable and robust results and mimicking the complex human skin. This review aims to provide an overview of the biological skin constituents and mechanical requirements that should be incorporated in a human skin-on-chip, permitting pharmacological, toxicological, and cosmetic tests closer to reality.

In Vitro Sensitive Skin Models: Review of the Standard Methods and Introduction to a New Disruptive Technology

The skin is a protective organ, able to decode a wide range of tactile, thermal, or noxious stimuli. Some of the sensors belonging to the transient receptor potential (TRP) family, for example, TRPV1, can elicit capsaicin-induced heat pain or histamine-induced itching sensations. The sensory nerve fibers, whose soma is located in the trigeminal or the dorsal root ganglia, are able to carry signals from the skin’s sensory receptors toward the brain via the spinal cord. In some cases, in response to environmental factors, nerve endings might be hyper activated, leading to a sensitive skin syndrome (SSS). SSS affects about 50% of the population and is correlated with small-fiber neuropathies resulting in neuropathic pain. Thus, for cosmetical and pharmaceutical industries developing SSS treatments, the selection of relevant and predictive in vitro models is essential. In this article, we reviewed the different in vitro models developed for the assessment of skin and neuron interactions. In a second part, we presented the advantages of microfluidic devices and organ-on-chip models, with a focus on the first model we developed in this context.

Sensitive Skins May Be Neuropathic Disorders: Lessons from Studies on Skin and Other Organs

Sensitive skin can be considered a neuropathic disorder. Sensory disorders and the decrease in intra-epidermal nerve ending density are strong arguments for small-fiber neuropathies. Sensitive skin is frequently associated with irritable bowel syndrome or sensitive eyes, which are also considered neuropathic disorders. Consequently, in vitro co-cultures of skin and neurons are adequate models for sensitive skin.

Skin-Nerve Co-Culture Systems for Disease Modeling and Drug Discovery

Prominent clinical problems related to the skin-nerve interface include barrier dysfunction and erythema, but it is the symptoms of pain and itch that most often lead patients to seek medical treatment. Tissue-engineered innervated skin models provide an excellent solution for studying the mechanisms underlying neurocutaneous disorders for drug screening, and cutaneous device development. Innervated skin substitutes provide solutions beyond traditional monolayer cultures and have advantages that make them preferable to in vivo animal studies for certain applications, such as measuring somatosensory transduction. The tissue-engineered innervated skin models replicate the complex stratified epidermis that provides barrier function in native skin, a feature that is lacking in monolayer co-cultures, while allowing for a level of detail in measurement of nerve morphology and function that cannot be achieved in animal models. In this review, the advantages and disadvantages of different cell sources and scaffold materials will be discussed and a presentation of the current state of the field is reviewed. Impact statement A review of the current state of innervated skin substitutes and the considerations that need to be addressed when developing these models. Tissue-engineered skin substitutes are customizable and provide barrier function allowing for screening of topical drugs and for studying nerve function.

Indentation probe with optical fibre array-based optical coherence tomography for material deformation

We present a new optomechanical probe for mechanical testing of soft matter. The probe consists of a micromachined cantilever equipped with an indenting sphere, and an array of 16 single‐mode optical fibres, which are connected to an optical coherence tomography (OCT) system that allows subsurface analysis of the sample during the indentation stroke. To test our device and its capability, we performed indentation on a PDMS‐based phantom. Our findings demonstrate that Common Path (CP)‐OCT via lensed optical fibres can be successfully combined with a microindentation sensor to visualise the phantom's deformation profile at different indentation depths and locations in real time.

Lay Description

This work presents a new approach to simultaneously perform micro‐indentation experiments and OCT imaging. An optical fiber array‐based sensor is used to develop a new hybrid tool where micro‐indentation is combined with optical coherence tomography. The sensor is therefore capable of compressing a sample with a small force and simultaneously collecting OCT depth profiles underneath and around the indentation point. This method offers the opportunity to characterize the mechanical properties of soft materials and simultaneously visualize their deformation profile. The ability to integrate OCT imaging with indentation technology is promising for the non‐invasive and precise characterization of different soft materials.

Recent innovations in artificial skin

The skin is a "smart", multifunctional organ that is protective, self-healing and capable of sensing and many forms of artificial skins have been developed with properties and functionalities approximating those of natural skin. Starting from specific commercial products for the treatment of burns, progress in two fields of research has since allowed these remarkable materials to be viable skin replacements for a wide range of dermatological conditions. This review maps out the development of bioengineered skin replacements and synthetic skin substitutes, including electronic skins. The specific behaviors of these skins are highlighted, and the performances of both types of artificial skins are evaluated against this. Moving beyond mere replication, highly advanced artificial skin materials are also identified as potential augmented skins that can be used as flexible electronics for health-care monitoring and other applications.

Lab-On-Fiber Technology: A Roadmap toward Multifunctional Plug and Play Platforms

This review presents an overview of the “lab-on-fiber technology” vision and the main milestones set in the technological roadmap to achieve the ultimate objective of developing flexible, multifunctional plug and play fiber-optic platforms designed for specific applications. The main achievements, obtained with nanofabrication strategies for unconventional substrates, such as optical fibers, are discussed here. The perspectives and challenges that lie ahead are highlighted with a special focus on full spatial control at the nanoscale and high-throughput production scenarios. The rapid progress in the fabrication stage has opened new avenues toward the development of multifunctional plug and play platforms, discussed here with particular emphasis on new functionalities and unparalleled figures of merit, to demonstrate the potential of this powerful technology in many strategic application scenarios. The paper also analyses the benefits obtained from merging lab-on-fiber (LOF) technology objectives with the emerging field of optomechanics, especially at the microscale and the nanoscale. We illustrate the main advances at the fabrication level, describe the main achievements in terms of functionalities and performance, and highlight future directions and related milestones. All achievements reviewed and discussed clearly suggest that LOF technology is much more than a simple vision and could play a central role not only in scenarios related to diagnostics and monitoring but also in the Information and Communication Technology (ICT) field, where optical fibers have already yielded remarkable results.

[Use of 2D and 3D cell cultures in dermatology]

The skin is a complex organ that performs a number of vital functions, including forming a physical barrier that protects our body from the penetration of pathogens and irritants and from excessive transepidermal water loss. In addition to its passive properties, the skin is also actively involved in the immune process. A complex structure of different cell types and structures allows the skin to fulfil these functions. In vitro research often faces the problem that simple 2D cell cultures are not able to adequately map these functions. Here 3D skin models offer a possible solution. In recent years, there has been significant development in this field; the reproducibility of the method as well as the physiological structure and tissue architecture of the 3D skin models have been improved. Depending on the research question, protocols for 3D skin models have been published, ranging from simple multilayer epidermis models to highly complex vascularized 3D full skin models.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

Intestine-on-chip device increases ECM remodeling inducing faster epithelial cell differentiation

An intestine-on-chip has been developed to study intestinal physiology and pathophysiology as well as intestinal transport absorption and toxicity studies in a controlled and human similar environment. Here, we report that dynamic culture of an intestine-on-chip enhances extracellular matrix (ECM) remodeling of the stroma, basement membrane production and speeds up epithelial differentiation. We developed a three-dimensional human intestinal stromal equivalent composed of human intestinal subepithelial myofibroblasts embedded in their own ECM. Then, we cultured human colon carcinoma-derived cells in both static and dynamic conditions in the opportunely designed microfluidic system until the formation of a well-oriented epithelium. This low cost and handy microfluidic device allows to qualitatively and quantitatively detect epithelial polarization and mucus production as well as monitor barrier function and ECM remodeling after nutraceutical treatment.

Patient-derived in vitro skin models for investigation of small fiber pathology

Objective

To establish individually expandable primary fibroblast and keratinocyte cultures from 3‐mm skin punch biopsies for patient‐derived in vitro skin models to investigate of small fiber pathology.

Methods

We obtained 6‐mm skin punch biopsies from the calf of two patients with small fiber neuropathy (SFN) and two healthy controls. One half (3 mm) was used for diagnostic intraepidermal nerve fiber density (IENFD). From the second half, we isolated and cultured fibroblasts and keratinocytes. Cells were used to generate patient‐derived full‐thickness three‐dimensional (3D) skin models containing a dermal and epidermal component. Cells and skin models were characterized morphologically, immunocyto‐ and ‐histochemically (vimentin, cytokeratin (CK)‐10, CK 14, ki67, collagen1, and procollagen), and by electrical impedance.

Results

Distal IENFD was reduced in the SFN patients (2 fibers/mm each), while IENFD was normal in the controls (8 fibers/mm, 7 fibers/mm). Two‐dimensional (2D) cultured skin cells showed normal morphology, adequate viability, and proliferation, and expressed cell‐specific markers without relevant difference between SFN patient and healthy control. Using 2D cultured fibroblasts and keratinocytes, we obtained subject‐derived 3D skin models. Morphology of the 3D model was analogous to the respective skin biopsy specimens. Both, the dermal and the epidermal layer carried cell‐specific markers and showed a homogenous expression of extracellular matrix proteins.

Interpretation

Our protocol allows the generation of disease‐specific 2D and 3D skin models, which can be used to investigate the cross‐talk between skin cells and sensory neurons in small fiber pathology.

Neural microphysiological systems for in vitro modeling of peripheral nervous system disorders

PNS disease pathology is diverse and underappreciated. Peripheral neuropathy may result in sensory, motor or autonomic nerve dysfunction and can be induced by metabolic dysfunction, inflammatory dysfunction, cytotoxic pharmaceuticals, rare hereditary disorders or may be idiopathic. Current preclinical PNS disease research relies heavily on the use of rodent models. In vivo methods are effective but too time-consuming and expensive for high-throughput experimentation. Conventional in vitro methods can be performed with high throughput but lack the biological complexity necessary to directly model in vivo nerve structure and function. In this review, we survey in vitro PNS model systems and propose that 3D-bioengineered microphysiological nerve tissue can improve in vitro–in vivo extrapolation and expand the capabilities of in vitro PNS disease modeling.

Using Artificial Skin Devices as Skin Replacements: Insights into Superficial Treatment

Artificial skin devices are able to mimic the flexibility and sensory perception abilities of the skin. They have thus garnered attention in the biomedical field as potential skin replacements. This Review delves into issues pertaining to these skin-deep devices. It first elaborates on the roles that these devices have to fulfill as skin replacements, and identify strategies that are used to achieve such functionality. Following which, a comparison is done between the current state of these skin-deep devices and that of natural skin. Finally, an outlook on artificial skin devices is presented, which discusses how complementary technologies can create skin enhancements, and what challenges face such devices.

Instructive microenvironments in skin wound healing: Biomaterials as signal releasing platforms

Skin wound healing aims to repair and restore tissue through a multistage process that involves different cells and signalling molecules that regulate the cellular response and the dynamic remodelling of the extracellular matrix. Nowadays, several therapies that combine biomolecule signals (growth factors and cytokines) and cells are being proposed. However, a lack of reliable evidence of their efficacy, together with associated issues such as high costs, a lack of standardization, no scalable processes, and storage and regulatory issues, are hampering their application. In situ tissue regeneration appears to be a feasible strategy that uses the body's own capacity for regeneration by mobilizing host endogenous stem cells or tissue-specific progenitor cells to the wound site to promote repair and regeneration. The aim is to engineer instructive systems to regulate the spatio-temporal delivery of proper signalling based on the biological mechanisms of the different events that occur in the host microenvironment. This review describes the current state of the different signal cues used in wound healing and skin regeneration, and their combination with biomaterial supports to create instructive microenvironments for wound healing.

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.

Dual-Component Gelatinous Peptide/Reactive Oligomer Formulations as Conduit Material and Luminal Filler for Peripheral Nerve Regeneration

Toward the next generation of nerve guidance conduits (NGCs), novel biomaterials and functionalization concepts are required to address clinical demands in peripheral nerve regeneration (PNR). As a biological polymer with bioactive motifs, gelatinous peptides are promising building blocks. In combination with an anhydride-containing oligomer, a dual-component hydrogel system (cGEL) was established. First, hollow cGEL tubes were fabricated by a continuous dosing and templating process. Conduits were characterized concerning their mechanical strength, in vitro and in vivo degradation and biocompatibility. Second, cGEL was reformulated as injectable shear thinning filler for established NGCs, here tyrosine-derived polycarbonate-based braided conduits. Thereby, the formulation contained the small molecule LM11A-31. The biofunctionalized cGEL filler was assessed regarding building block integration, mechanical properties, in vitro cytotoxicity, and growth permissive effects on human adipose tissue-derived stem cells. A positive in vitro evaluation motivated further application of the filler material in a sciatic nerve defect. Compared to the empty conduit and pristine cGEL, the functionalization performed superior, though the autologous nerve graft remains the gold standard. In conclusion, LM11A-31 functionalized cGEL filler with extracellular matrix (ECM)-like characteristics and specific biochemical cues holds great potential to support PNR.

In vitro skin models to study epithelial regeneration from the hair follicle

The development of dermal equivalents (DEs) for the treatment of burns has contributed toward efficient wound closure. A collagen-glycosaminoglycan DE (C-GAG) grafted with hair follicles converted a full-thickness wound to partial-thickness resulting in complete wound closure and restored hair. In this study we compared the ability of both intact pilosebaceous units (PSU) or truncated hair follicles (THF) to regenerate a multilayered epidermis in vitro when implanted into de-epidermalized dermis (DED) or C-GAG with the epidermis generated in vivo using C-CAG. Keratinocytes explanted from the outer root sheath of PSU and THF in both DED and C-GAG but only formed a multilayered epidermis with PSU in DED. PSU were more effective at forming multilayered epidermis in DED than THF. Both DED and C-GAG skin expressed proliferation (PCNA), differentiation (K1, K10), hyperproliferation (K6, K16), basal (K14), putative stem cell (p63), extracellular matrix protein (Collagen IV), mesenchymal (vimentin) and adherens junction (β-catenin) markers. These data suggest DEs supported initial maintenance of the implanted hair follicles, in particular PSU, and provide an excellent model with which to investigate the regulation of hair follicle progenitor epithelial cells during epidermal regeneration. Although neither PSU nor THF formed multilayered epidermis in C-CAG in vitro, hair follicles implanted into engrafted C-GAG on a burns patient resulted in epithelial regeneration and expression of proliferation and differentiation markers in a similar manner to that seen in vitro. However, the failure of C-GAG to support epidermal regeneration in vitro suggests in vivo factors are essential for full epidermal regeneration using C-GAG.