Characterization of human skin equivalents developed at body's core and surface temperatures

Recreating Human Skin In Vitro: Should the Microbiota Be Taken into Account?

Skin plays crucial roles in the human body: besides protecting the organism from external threats, it acts as a thermal regulator, is responsible for the sense of touch, hosts microbial communities (the skin microbiota) involved in preventing the invasion of foreign pathogens, contains immunocompetent cells that maintain a healthy immunogenic/tolerogenic balance, and is a suitable route for drug administration. In the skin, four defense levels can be identified: besides the physical, chemical, and immune barriers that are inherent to the tissue, the skin microbiota (i.e., the numerous microorganisms living on the skin surface) provides an additional barrier. Studying the skin barrier function or the effects of drugs or cosmetic agents on human skin is a difficult task since snapshot evidence can only be obtained using bioptic samples where dynamic processes cannot properly be followed. To overcome these limitations, many different in vitro models of human skin have been developed that are characterized by diverse levels of complexity in terms of chemical, structural, and cellular composition. The aim of this review is to summarize and discuss the advantages and disadvantages of the different human skin models so far available and to underline how the insertion of a proper microbiota would positively impact an in vitro human skin model in an attempt to better mimic conditions in vivo.

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.

Single-cell transcriptomics of human-skin-equivalent organoids

Several methods for generating human-skin-equivalent (HSE) organoid cultures are in use to study skin biology; however, few studies thoroughly characterize these systems. To fill this gap, we use single-cell transcriptomics to compare in vitro HSEs, xenograft HSEs, and in vivo epidermis. By combining differential gene expression, pseudotime analyses, and spatial localization, we reconstruct HSE keratinocyte differentiation trajectories that recapitulate known in vivo epidermal differentiation pathways and show that HSEs contain major in vivo cellular states. However, HSEs also develop unique keratinocyte states, an expanded basal stem cell program, and disrupted terminal differentiation. Cell-cell communication modeling shows aberrant epithelial-to-mesenchymal transition (EMT)-associated signaling pathways that alter upon epidermal growth factor (EGF) supplementation. Last, xenograft HSEs at early time points post transplantation significantly rescue many in vitro deficits while undergoing a hypoxic response that drives an alternative differentiation lineage. This study highlights the strengths and limitations of organoid cultures and identifies areas for potential innovation.

The Impact of Diclofenac Gel on Ion Transport in the Rabbit (Oryctolagus cuniculus) Skin: An In Vitro Study

Diclofenac belongs to the non-steroidal anti-inflammatory drugs with analgesic, antipyretic and anti-inflammatory effects. Diclofenac administration on the skin may be associated with the appearance of side effects. The study aimed to evaluate the impact of diclofenac gel on transepithelial electrophysiological parameters of the 55 rabbit abdomen skin specimens. The electric parameters were analyzed in a modified Ussing chamber. The resistance (R) of the skin specimens treated with diclofenac gel significantly increased, which could be related to the reduction in the water content in intercellular spaces and, consequently, tighter adhesion of the cells. Increased electric potential (PD) was also observed in the skin specimens treated with diclofenac gel. The increase in both R and PD measured under stationary conditions was most likely caused by a transient and reversible increase in sodium ion transport, as the R and PD values decreased after the diclofenac gel was washed away. However, diclofenac gel did not affect the maximum and minimum PDs measured during stimulations. Therefore, it seems that diclofenac gel does not affect the perception of stimuli in the model system used.

Could "Islets of Sparing" Be a Clue for Neutral Lipid Storage Disease with Ichthyosis in Patients with Congenital Ichthyosiform Erythroderma?

The prevalent form of ichthyosis in neutral lipid storage disease (NLSDI) is nonbullous congenital ichthyosiform erythroderma (CIE) characterized by fine, whitish scales on erythematous skin over the whole body. Here, we report a late-diagnosed, 25-year-old woman with NLSDI presenting with diffuse erythema and fine whitish scales throughout the body with patches of apparently normal skin, "islets of sparing" on her lower extremities. We observed that the size of the normal skin islets changed with time, and even the entire lower extremity was covered with erythema and desquamation like the rest of the body. Frozen section histopathological examinations were made from lesional skin and normal-looking skin; no difference was observed in terms of lipid accumulation. The only noticeable difference was the thickness of the keratin layer. In CIE patients, observation of patches of apparently normal skin or "islets of sparing" might be a clue for NLSDI to be distinguished from other CIE conditions.

Design considerations of benchtop fluid flow bioreactors for bio-engineered tissue equivalents in vitro

One of the major aims of bio-engineering tissue equivalents in vitro is to create physiologically relevant culture conditions to accurately recreate the cellular microenvironment. This often includes incorporation of factors such as the extracellular matrix, co-culture of multiple cell types and three-dimensional culture techniques. These advanced techniques can recapitulate some of the properties of tissue in vivo, however fluid flow is a key aspect that is often absent. Fluid flow can be introduced into cell and tissue culture using bioreactors, which are becoming increasingly common as we seek to produce increasingly accurate tissue models. Bespoke technology is continuously being developed to tailor systems for specific applications and to allow compatibility with a range of culture techniques. For effective perfusion of a tissue culture many parameters can be controlled, ranging from impacts of the fluid flow such as increased shear stress and mass transport, to potentially unwanted side effects such as temperature fluctuations. A thorough understanding of these properties and their implications on the culture model can aid with a more accurate interpretation of results. Improved and more complete characterisation of bioreactor properties will also lead to greater accuracy when reporting culture conditions in protocols, aiding experimental reproducibility, and allowing more precise comparison of results between different systems. In this review we provide an analysis of the different factors involved in the development of benchtop flow bioreactors and their potential biological impacts across a range of applications.

Advancements in Extracellular Matrix-Based Biomaterials and Biofabrication of 3D Organotypic Skin Models

Over the last decades, three-dimensional (3D) organotypic skin models have received enormous attention as alternative models to in vivo animal models and in vitro two-dimensional assays. To date, most organotypic skin models have an epidermal layer of keratinocytes and a dermal layer of fibroblasts embedded in an extracellular matrix (ECM)-based biomaterial. The ECM provides mechanical support and biochemical signals to the cells. Without advancements in ECM-based biomaterials and biofabrication technologies, it would have been impossible to create organotypic skin models that mimic native human skin. In this review, the use of ECM-based biomaterials in the reconstruction of skin models, as well as the study of complete ECM-based biomaterials, such as fibroblasts-derived ECM and decellularized ECM as a better biomaterial, will be highlighted. We also discuss the benefits and drawbacks of several biofabrication processes used in the fabrication of ECM-based biomaterials, such as conventional static culture, electrospinning, 3D bioprinting, and skin-on-a-chip. Advancements and future possibilities in modifying ECM-based biomaterials to recreate disease-like skin models will also be highlighted, given the importance of organotypic skin models in disease modeling. Overall, this review provides an overview of the present variety of ECM-based biomaterials and biofabrication technologies available. An enhanced organotypic skin model is expected to be produced in the near future by combining knowledge from previous experiences and current research.

The influence of hydration status on ion transport in the rabbit (Oryctolagus cuniculus) skin-An in vitro study

The preservation of physiological transport of ions and water content is particularly important for maintaining the skin barrier, touch and pain stimuli, as well as the initiation of skin regeneration processes, especially after treatments associated with breaking skin continuity and wound healing difficulties. The aim of the study was to assess changes in ion transport, measured as values of transepithelial electric resistance and potential difference in stationary conditions and during mechanical-chemical stimulations, depending on the hydration status of isolated rabbit skin specimens. The specimens were divided into five groups: control (n = 22), dehydrated in 10% NaCl (n = 30), rehydrated after dehydration (n = 26), dried at 37°C (n = 26), and rehydrated after drying (n = 25). Dehydrated tissue samples showed altered resistance compared to the control; this change was maintained regardless of rehydration. In the dehydrated samples, changes in the measured electric potential were also noted, which returned to values comparable with the control after rehydration. Dehydrated skin, regardless of the cause of dehydration, responds with changes in the transport of sodium and chloride ions and the altered cellular microenvironment. It could influence the perception of stimuli, particularly pain, and slow down the regeneration processes.

Human skin equivalents: Impaired barrier function in relation to the lipid and protein properties of the stratum corneum

To advance drug development representative reliable skin models are indispensable. Animal skin as test model for human skin delivery is restricted as their properties greatly differ from human skin. In vitro 3D-human skin equivalents (HSEs) are valuable tools as they recapitulate important aspects of the human skin. However, HSEs still lack the full barrier functionality as observed in native human skin, resulting in suboptimal screening outcome. In this review we provide an overview of established in-house and commercially available HSEs and discuss in more detail to what extent their skin barrier biology is mimicked in vitro focusing on the lipid properties and cornified envelope. Further, we will illustrate how underlying factors, such as culture medium improvements and environmental factors affect the barrier lipids. Lastly, potential improvements in skin barrier function will be proposed aiming at a new generation of HSEs that may replace animal skin delivery studies fully.

TatS: a novel in vitro tattooed human skin model for improved pigment toxicology research

Reports of tattoo-associated risks boosted the interest in tattoo pigment toxicity over the last decades. Nonetheless, the influence of tattoo pigments on skin homeostasis remains largely unknown. In vitro systems are not available to investigate the interactions between pigments and skin. Here, we established TatS, a reconstructed human full-thickness skin model with tattoo pigments incorporated into the dermis. We mixed the most frequently used tattoo pigments carbon black (0.02 mg/ml) and titanium dioxide (TiO2, 0.4 mg/ml) as well as the organic diazo compound Pigment Orange 13 (0.2 mg/ml) into the dermis. Tissue viability, morphology as well as cytokine release were used to characterize TatS. Effects of tattoo pigments were compared to monolayer cultures of human fibroblasts. The tissue architecture of TatS was comparable to native human skin. The epidermal layer was fully differentiated and the keratinocytes expressed occludin, filaggrin and e-cadherin. Staining of collagen IV confirmed the formation of the basement membrane. Tenascin C was expressed in the dermal layer of fibroblasts. Although transmission electron microscopy revealed the uptake of the tattoo pigments into fibroblasts, neither viability nor cytokine secretion was altered in TatS. In contrast, TiO2 significantly decreased cell viability and increased interleukin-8 release in fibroblast monolayers. In conclusion, TatS emulates healed tattooed human skin and underlines the advantages of 3D systems over traditional 2D cell culture in tattoo pigment research. TatS is the first skin model that enables to test the effects of pigments in the dermis upon tattooing.

Hyaluronic acid biomaterial for human tissue-engineered skin substitutes: Preclinical comparative in vivo study of wound healing

BACKGROUND

There is not an ideal biomaterial for tissue-engineered skin substitutes (TESSs), and most of the studies or existing therapies use xenogeneic origin natural biomaterials or biosynthetic scaffolds.

OBJECTIVE

To analyse clinical, histological integration and homeostasis parameters of a human TESS manufactured with fibrin-hyaluronic acid biomaterial (HA-Skin), grafted in immunodeficient mice for 8 weeks, and compared with the gold standard treatment (Autograft), a human TESS manufactured with fibrin-agarose biomaterial (AG-Skin) and secondary wound healing dressings.

METHODS

Human TESSs and autografts were implanted into BALB/c mice after surgical excision. Secondary wound healing approach was achieved with biosynthetic collagen wound dressing (Biobrane^® ) and fibrin-hyaluronic acid or fibrin-agarose biomaterial without cells (Total N = 44). Clinical integration and homeostasis parameters were evaluated every two weeks for two months. Histological and immunohistochemical analyses were performed four and eight weeks after grafting.

RESULTS

HA-Skin, AG-Skin and Autograft groups showed a proper clinical integration and epithelization eight weeks later. Scar evaluation revealed better results for Autograft and HA-Skin. Homeostasis analysis indicated similar values of transepidermal water loss and elasticity between HA-Skin (6.42 ± 0.75 g/h/m² , 0.42 ± 0.08 AU), Autograft (6.91 ± 1.28 g/h/m² , 0.40 ± 0.08 AU) and healthy mouse skin (6.40 ± 0.43 g/h/m² , 0.35 ± 0.03 AU). Histological results showed that human TESSs and autografts presented better skin structuration and higher expression of cytokeratins.

CONCLUSIONS

This study suggests that human TESS based on fibrin-hyaluronic acid biomaterial could be suitable for clinical application in the treatment of several dermatological pathologies (wound healing).

Contribution of Palmitic Acid to Epidermal Morphogenesis and Lipid Barrier Formation in Human Skin Equivalents

The outermost barrier layer of the skin is the stratum corneum (SC), which consists of corneocytes embedded in a lipid matrix. Biosynthesis of barrier lipids occurs de novo in the epidermis or is performed with externally derived lipids. Hence, in vitro developed human skin equivalents (HSEs) are developed with culture medium that is supplemented with free fatty acids (FFAs). Nevertheless, the lipid barrier formation in HSEs remains altered compared to native human skin (NHS). The aim of this study is to decipher the role of medium supplemented saturated FFA palmitic acid (PA) on morphogenesis and lipid barrier formation in HSEs. Therefore, HSEs were developed with 100% (25 μM), 10%, or 1% PA. In HSEs supplemented with reduced PA level, the early differentiation was delayed and epidermal activation was increased. Nevertheless, a similar SC lipid composition in all HSEs was detected. Additionally, the lipid organization was comparable for lamellar and lateral organization, irrespective of PA concentration. As compared to NHS, the level of monounsaturated lipids was increased and the FFA to ceramide ratio was drastically reduced in HSEs. This study describes the crucial role of PA in epidermal morphogenesis and elucidates the role of PA in lipid barrier formation of HSEs.

Human skin equivalents cultured under hypoxia display enhanced epidermal morphogenesis and lipid barrier formation

Human skin equivalents (HSEs) are three-dimensional cell models mimicking characteristics of native human skin (NHS) in many aspects. However, a limitation of HSEs is the altered in vitro morphogenesis and barrier formation. Differences between in vitro and in vivo skin could have been induced by suboptimal cell culture conditions, of which the level of oxygen in vitro (20%) is much higher than in vivo (0.5-8%). Our aim is to study how external oxygen levels affect epidermal morphogenesis and barrier formation in HSEs. In the present study, fibroblast and keratinocyte monocultures, and HSEs were generated under 20% (normoxia) and 3% (hypoxia) oxygen level. In all cultures under hypoxia, expression of hypoxia-inducible factor target genes was increased. Characterization of HSEs generated under hypoxia using immunohistochemical analyses of morphogenesis biomarkers revealed a reduction in epidermal thickness, reduced proliferation, similar early differentiation, and an attenuated terminal differentiation program compared to normoxia, better mimicking NHS. The stratum corneum ceramide composition was studied with liquid chromatography coupled to mass spectrometry. Under hypoxia, HSEs exhibited a ceramide composition that more closely resembles that of NHS. Consequently, the lipid organization was improved. In conclusion, epidermal morphogenesis and barrier formation in HSEs reconstructed under hypoxia better mimics that of NHS.

3D-Organotypic Cultures to Unravel Molecular and Cellular Abnormalities in Atopic Dermatitis and Ichthyosis Vulgaris

Atopic dermatitis (AD) is characterized by dry and itchy skin evolving into disseminated skin lesions. AD is believed to result from a primary acquired or a genetically-induced epidermal barrier defect leading to immune hyper-responsiveness. Filaggrin (FLG) is a protein found in the cornified envelope of fully differentiated keratinocytes, referred to as corneocytes. Although FLG null mutations are strongly associated with AD, they are not sufficient to induce the disease. Moreover, most patients with ichthyosis vulgaris (IV), a monogenetic skin disease characterized by FLG homozygous, heterozygous, or compound heterozygous null mutations, display non-inflamed dry and scaly skin. Thus, all causes of epidermal barrier impairment in AD have not yet been identified, including those leading to the Th2-predominant inflammation observed in AD. Three dimensional organotypic cultures have emerged as valuable tools in skin research, replacing animal experimentation in many cases and precluding the need for repeated patient biopsies. Here, we review the results on IV and AD obtained with epidermal or skin equivalents and consider these findings in the context of human in vivo data. Further research utilizing complex models including immune cells and cutaneous innervation will enable finer dissection of the pathogenesis of AD and deepen our knowledge of epidermal biology.

Characterization of human skin equivalents developed at body's core and surface temperatures

Human skin equivalents (HSEs) are in vitro developed three‐dimensional models resembling native human skin (NHS) to a high extent. However, the epidermal lipid biosynthesis, barrier lipid composition, and organization are altered, leading to an elevated diffusion rate of therapeutic molecules. The altered lipid barrier formation in HSEs may be induced by standardized culture conditions, including a culture temperature of 37°C, which is dissimilar to skin surface temperature. Therefore, we aim to determine the influence of culture temperature during the generation of full thickness models (FTMs) on epidermal morphogenesis and lipid barrier formation. For this purpose, FTMs were developed at conventional core temperature (37°C) or lower temperatures (35°C and 33°C) and evaluated over a time period of 4 weeks. The stratum corneum (SC) lipid composition was analysed using advanced liquid chromatography coupled to mass spectrometry analysis. Our results show that SC layers accumulated at a similar rate irrespective of culture temperature. At reduced culture temperature, an increased epidermal thickness, a disorganization of the lower epidermal cell layers, a delayed early differentiation, and an enlargement of granular cells were detected. Interestingly, melanogenesis was reduced at lower temperature. The ceramide subclass profile, chain length distribution, and level of unsaturated ceramides were similar in FTMs generated at 37°C and 35°C but changed when generated at 33°C, reducing the resemblance to NHS. Herein, we report that culture temperature affects epidermal morphogenesis substantially and to a lesser extent the lipid barrier formation, highlighting the importance of optimized external parameters during reconstruction of skin.