Characterization of reconstructed skin models

3D human foreskin model for testing topical formulations of sildenafil citrate

Sildenafil citrate is an approved drug used for the treatment of erectile dysfunction and premature ejaculation. Despite a widespread application, sildenafil citrate shows numerous adverse cardiovascular effects in high-risk patients. Local transdermal drug delivery of this drug is therefore being explored as an interesting and noninvasive alternative administration method that avoids adverse effects arised from peak plasma drug concentrations. Although human and animal skin represents the most reliable models to perform penetration studies, they involve a series of ethical issues and restrictions. For these reasons new in vitro approaches based on artificially reconstructed human skin or "human skin equivalents" are being developed as possible alternatives for transdermal testing. There is little information, however, on the efficiency of such new in vitro methods on cutaneous penetration of active ingredients. The objective of the current study was to investigate the sildenafil citrate loaded in three commercial transdermal vehicles using 3D full-thickness skin equivalent and compare the results with the permeability experiments using porcine skin. Our results demonstrated that, while the formulation plays an imperative role in an appropriate dermal uptake of sildenafil citrate, the D coefficient results obtained by using the 3D skin equivalent are comparable to those obtained by using the porcine skin when a simple drug suspension is applied (1.17 × 10-10 ± 0.92 × 10-10 cm2/s vs 3.5 × 102 ± 3.3 × 102 cm2/s), suggesting that in such case, this 3D skin model can be a valid alternative for ex-vivo skin absorption experiments.

Applications of Engineered Skin Tissue for Cosmetic Component and Toxicology Detection

The scale of the cosmetic market is increasing every day. There are many safety risks to cosmetics, but they benefit people at the same time. The skin can become red, swollen, itchy, chronically toxic, and senescent due to the misuse of cosmetics, triggering skin injuries, with contact dermatitis being the most common. Therefore, there is an urgent need for a system that can scientifically and rationally detect the composition and perform a toxicological assessment of cosmetic products. Traditional detection methods rely on instrumentation and method selection, which are less sensitive and more complex to perform. Engineered skin tissue has emerged with the advent of tissue engineering technology as an emerging bioengineering technology. The ideal engineered skin tissue is the basis for building good in vitro structures and physiological functions in this field. This review introduces the existing cosmetic testing and toxicological evaluation methods, the current development status, and the types and characteristics of engineered skin tissue. The application of engineered skin tissue in the field of cosmetic composition detection and toxicological evaluation, as well as the different types of tissue engineering scaffold materials and three-dimensional (3D) organoid preparation approaches, is highlighted in this review to provide methods and ideas for constructing the next engineered skin tissue for cosmetic raw material component analysis and toxicological evaluation.

Fundamentals and Applications of Raman-Based Techniques for the Design and Development of Active Biomedical Materials

Raman spectroscopy is an analytical method based on light-matter interactions that can interrogate the vibrational modes of matter and provide representative molecular fingerprints. Mediated by its label-free, non-invasive nature, and high molecular specificity, Raman-based techniques have become ubiquitous tools for in situ characterization of materials. This review comprehensively describes the theoretical and practical background of Raman spectroscopy and its advanced variants. The numerous facets of material characterization that Raman scattering can reveal, including biomolecular identification, solid-to-solid phase transitions, and spatial mapping of biomolecular species in bioactive materials, are highlighted. The review illustrates the potential of these techniques in the context of active biomedical material design and development by highlighting representative studies from the literature. These studies cover the use of Raman spectroscopy for the characterization of both natural and synthetic biomaterials, including engineered tissue constructs, biopolymer systems, ceramics, and nanoparticle formulations, among others. To increase the accessibility and adoption of these techniques, the present review also provides the reader with practical recommendations on the integration of Raman techniques into the experimental laboratory toolbox. Finally, perspectives on how recent developments in plasmon- and coherently-enhanced Raman spectroscopy can propel Raman from underutilized to critical for biomaterial development are provided.

A Comparative Analysis of Biological and Synthetic Skin Models for Drug Transport Studies

Ethical restrictions as well as practical or economic issues related to use of animal and human skin has been the main reason the growth in the number of investigations with alternative models. Reconstructed skin models, for example, have been useful to evaluate the in vitro toxicity of compounds; however, these models usually overestimate the amount of drug permeated due to impaired barrier properties. In this review, the performance of synthetic and biological skin models in transport studies was compared by considering two compounds with different physicochemical properties. The advantages and limitations of each skin model are discussed in detail. Although synthetic and reconstructed skin models have shown to be useful in the formulation optimization step, they present many limitations: (1) impaired barrier properties; (2) lack of follicular transport; (3) no metabolism in synthetic membranes; (4) differences in terms of lipid organization; (5) more affected by formulation constituents. Therefore, animal and human tissues should still be prioritized in drug transport studies until new advances in alternative models are achieved. Investigations of the impact of cell-culture conditions on skin formation, in turn, bring perspectives related to the development of unhealthy/injured skin models (an aspect that still deserves attention).

Improved Tool for Predicting Skin Irritation on Reconstructed Human Epidermis Models Based on Electrochemical Impedance Spectroscopy

The rabbit skin irritation test has been the standard for evaluating the irritation potential of chemicals; however, alternative methods that do not use animal testing are actively encouraged. Reconstructed human epidermis (RhE) models mimic the biochemical and physiological properties of the human epidermis and can be used as an alternative method. On RhE methods, the metabolic activity of RhE models is used to predict skin irritation, with a reduction in metabolic activity indicating a reduced number of viable cells and linking cell death to skin irritation processes. However, new challenges have emerged as the use of RhE models increases, including the need for non-invasive and marker-free methodologies to assess cellular states. Electrochemical impedance spectroscopy (EIS) is one such methodology that can meet these requirements. In this study, our results showed that EIS can differentiate between irritant and non-irritant chemicals, with a significant increase in the capacitance values observed in the irritant samples. A ROC curve analysis showed that the prediction method based on EIS met OECD TG 439 requirements at all time points and had 95% within-laboratory reproducibility. Comparison with the MTT viability assay showed that prediction using EIS achieved higher sensitivity, specificity, and accuracy. These results suggest that EIS could potentially replace animal testing in the evaluation of irritation potential and could be a valuable addition to in vitro testing strategies.

Oral Absorption across Organotypic Culture Models of the Human Buccal Epithelium after E-cigarette Aerosol Exposure

Inhaled aerosols are absorbed across the oral cavity, respiratory tract, and gastrointestinal tract. The absorption across the oral cavity, which is one of the exposure routes, plays an important role in understanding pharmacokinetics and physiological effects. After aerosol exposure from e-cigarettes, tissue viability studies, morphological observation, and chemical analyses at the inner and outer buccal tissues were performed using organotypic 3D in vitro culture models of the buccal epithelium to better understand the deposition and absorption on the inner and outer buccal tissues. The aerosol exposures did not affect the tissue viability and had no change to the tissue morphology and structure. The deposition ratio at the buccal tissue surface is relatively low. This shows that majority of aerosol transfers to the airway tissues. The distribution from the inner tissue to the outer tissue has selectivity among various compounds, depending on the affinity with the liquid crystal structure of phospholipids and glucosylceramide. Although nicotine absorption in the aqueous solution was well known to increase as the unprotonated state of nicotine increased, the nicotine absorption after the aerosol exposure is irrelevant to the protonated-unprotonated state. Furthermore, the results showed that half of nicotine that adhered to the oral cavity transferred to the inner tissue via the oral epithelium and the other half transferred to the gastrointestinal tract accompanying multiple executions of swallowing, while majority of the water-soluble compounds with the hydroxyl group such as propylene glycol and benzoic acid that adhered to the oral cavity were eluted with the saliva and transferred to the gastrointestinal tract by swallowing.

The proliferative and multipotent epidermal progenitor cells for human skin reconstruction in vitro and in vivo

OBJECTIVES

The skin exhibits tremendous regenerative potential, as different types of progenitor and stem cells regulate skin homeostasis and damage. However, in vitro primary keratinocytes present with several drawbacks, such as high donor variability, short lifespan, and limited donor tissue availability. Therefore, more stable primary keratinocytes are needed to generate multiple uniform in vitro and in vivo skin models.

RESULTS

We identified epidermal progenitor cells from primary keratinocytes using Integrin beta 1 (ITGB1) an epidermal stem cell marker markedly decreased after senescence in vitro. Epidermal progenitor cells exhibited unlimited proliferation and the potential for multipotent differentiation capacity. Moreover, they could completely differentiate to form an organotypic skin model including conversed mesenchymal cells in the dermis and could mimic the morphologic and biochemical processes of human epidermis. We also discovered that proliferation and the multipotent differentiation capacity of these cells relied on ITGB1 expression. Eventually, we examined the in vitro and in vivo wound healing capacity of these epidermal progenitor cells.

CONCLUSIONS

Overall, the findings suggest that these stable and reproducible cells can differentiate into multiple lineages, including human skin models. They are a potentially powerful tool for studying skin regeneration, skin diseases, and are an alternative for in vivo experiments.

Wavelength-dependent DNA Photodamage in a 3-D Human Skin Model over the far-UVC and Germicidal-UVC Wavelength Ranges from 215 to 255 nm

The effectiveness of UVC to reduce airborne‐mediated disease transmission is well established. However, conventional germicidal UVC (~254 nm) cannot be used directly in occupied spaces because of the potential for damage to the skin and eye. A recently studied alternative with the potential to be used directly in occupied spaces is far UVC (200–235 nm, typically 222 nm), as it cannot penetrate to the key living cells in the epidermis. Optimal far‐UVC use is hampered by limited knowledge of the precise wavelength dependence of UVC‐induced DNA damage, and thus we have used a monochromatic UVC exposure system to assess wavelength‐dependent DNA damage in a realistic 3‐D human skin model. We exposed a 3‐D human skin model to mono‐wavelength UVC exposures of 100 mJ/cm², at UVC wavelengths from 215 to 255 nm (5 nm steps). At each wavelength, we measured yields of DNA‐damaged keratinocytes, and their distribution within the layers of the epidermis. No increase in DNA damage was observed in the epidermis at wavelengths from 215 to 235 nm, but at higher wavelengths (240–255 nm) significant levels of DNA damage was observed. These results support use of far‐UVC radiation to safely reduce the risk of airborne disease transmission in occupied locations.

A review of alternative methods to the use of animals in safety evaluation of cosmetics

Alternative methods to the use of animals in research have been a global trend, mainly after the publication of the 3R’s principle (Replacement, Reduction, and Refinement), proposed by Russel and Burch. In the cosmetic sector, safety and efficacy assessments using animals have generated controversial debates. For this reason, in vitro research techniques are widely used to assess acute toxicity; corrosivity and irritation; skin sensitization; dermal and percutaneous absorption; repeated dose toxicity; reproductive toxicity; mutagenicity and genotoxicity; carcinogenicity; toxicokinetic studies; photo-induced toxicity; and human data. Although there are many methodologies described, validated, and widely used in the cosmetic area, the evaluation of the safety of cosmetic ingredients and products is still an expanding field. It needs global collaboration among regulatory agencies, universities, and industry, to meet several unmet needs in the fields of sensitization, carcinogenicity, systemic action, among other issues involving safety of users of cosmetic products. This review article will cover the currently most relevant in vitro models regarding irritation, corrosion, sensitization, mutagenicity, genotoxicity, and phototoxicity, to help to choose the most appropriate test for evaluating the safety and toxicity of cosmetic ingredients and products.

Hybrid skin chips for toxicological evaluation of chemical drugs and cosmetic compounds

Development of drugs and cosmetics for topical application require safety tests in skin models. However, current skin models, such as skin cell sheets and artificial tissue-engineered skin, do not allow sophisticated toxicological evaluations (e.g., sensory irritation, hepatotoxicity). Animal models are prohibited worldwide for testing cosmetics. Therefore, reliable human skin models that recapitulate physiological events in skin tissue need to be established under in vitro settings. In this study, hybrid human skin models that enable delicate toxicological evaluations of drugs and cosmetic compounds are demonstrated. To recapitulate skin cornification, keratinocytes in the top layer of a vertical microfluidic chip were cultured at the air-liquid interface. For the skin-nerve hybrid model, differentiated neural stem cells in 3D collagen were positioned adjacent to and right below the skin layer. This model enables real-time quantitative skin sensitization analysis following chemical treatments by detecting alterations in neuronal activity in combination with a calcium imaging technique. For the skin-liver model, hepatic cells derived from pluripotent stem cells were cultured in 3D collagen distant from the skin layer. Potential hepatotoxicity of cutaneously applied chemicals in this model can be evaluated by quantification of glutathione and reactive oxygen species. Our study suggests that 3D hybrid skin chips would provide useful human skin models in pharmaceutical and cosmetic industries.

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.

Assessment of the skin barrier function in the reconstructed human epidermis using a multimodal approach at molecular, tissue and functional levels

Reconstructed human epidermis models are used as epidermis alternatives in skin research studies. It is necessary to provide molecular and functional characterization in order to assess these models. Our aim is to establish a link between the barrier function and the structure and composition of the stratum corneum using several complementary techniques. The following three studies were performed on reconstructed human epidermis during the keratinocyte differentiation process: (i) caffeine percutaneous penetration kinetics, (ii) epidermis thickness measurement, stratum corneum formation and lipid organization by Raman microspectroscopy and (iii) lipid composition evolution by liquid chromatography coupled to high-resolution mass spectrometry. The results demonstrated that the caffeine penetration decreased along the differentiation process. Raman in-depth images demonstrated an increase in stratum corneum and RHE thickness accompanied by the evolution of lipid organization. Lipid analysis showed an increase of the ceramide amount and an inverse relationship between ceramide and its precursor levels during the differentiation process. Different behaviors between several ceramide subclasses are highlighted and they relied on the corresponding differentiation stages. The generation of the most important ceramides for the barrier function is closely followed. A period shift between lipid generation and their organization was found. Our analytical data allowed identifying the following 3 groups of maturation days: before day 15, between days 15 and 19, and after day 19. The chemical and physiological states of the barrier function for each group are described thanks to a multimodal approach.

Progress on Reconstructed Human Skin Models for Allergy Research and Identifying Contact Sensitizers

Contact with the skin is inevitable or desirable for daily life products such as cosmetics, hair dyes, perfumes, drugs, household products, and industrial and agricultural products. Whereas the majority of these products are harmless, a number can become metabolized and/or activate the immunological defense via innate and adaptive mechanisms resulting in sensitization and allergic contact dermatitis upon following exposures to the same substance. Therefore, strict safety (hazard) assessment of actives and ingredients in products and drugs applied to the skin is essential to determine I) whether the chemical is a potential sensitizer and if so II) what is the safe concentration for human exposure to prevent sensitization from occurring. Ex vivo skin is a valuable model for skin penetration studies but due to logistical and viability limitations the development of in vitro alternatives is required. The aim of this review is to give a clear overview of the organotypic in vitro skin models (reconstructed human epidermis, reconstructed human skin, immune competent skin models incorporating Langerhans Cells and T-cells, skin-on-chip) that are currently commercially available or which are being used in a laboratory research setting for hazard assessment of potential sensitizers and for investigating the mechanisms (sensitization key events 1-4) related to allergic contact dermatitis. The limitations of the models, their current applications, and their future potential in replacing animals in allergy-related science are discussed.

Influence of silymarin components on keratinocytes and 3D reconstructed epidermis

Silymarin is a flavonoid complex isolated from the plant Silybum marianum which is well known for its antioxidant, hepatoprotective and immunomodulatory effects. Since little is known about its anti-inflammatory properties and healing effects, our study focused on whether or not silymarin components reduce inflammation and support epidermis regeneration. Lipopolysaccharides (LPS) and sodium dodecyl sulfate (SDS) were used to induce inflammation in normal human epidermal keratinocytes (NHEKs) and reconstructed epidermis (RHE), respectively. The expression of pro-inflammatory cytokines (IL-1, IL-6 and IL-8) in NHEKs and RHE was measured by enzyme - linked immunosorbent assay (ELISA). The expression of cytokeratin 14 and loricrin in RHE was detected by immunofluorescent analysis. Hematoxylin and eosin staining was used for the morphological evaluation of RHE. It was determined that 2, 3 - dehydrosilybin (DHSB) downregulated the production of selected pro-inflammatory cytokines produced by NHEKs. Although all layers of RHE displayed full thickness, when SDS was applied, cell detachment was seen in the stratum corneum and loricrin expression was diminished.

Freezing Weakens the Barrier Function of Reconstructed Human Epidermis as Evidenced by Raman Spectroscopy and Percutaneous Permeation

The development and characterization of reconstructed human epidermis (RHE) is an active area of R&D. RHE can replace animal tissues in pharmaceutical, toxicological and cosmetic sciences, yielding scientific and ethical advantages. RHEs remain costly, however, due to consumables and time required for their culture and a short shelf-life. Storing, i.e., freezing RHE could help reduce costs but to date, little is known on the effects of freezing on the barrier function of RHE. We studied such effects using commercial EpiSkin™ RHE stored at -20, -80 and -150 °C for 1 and 10 weeks. We acquired intrinsic Raman spectra in the stratum corneum (SC) of the RHEs as well as spectra obtained following topical application of resorcinol in an aqueous solution. In parallel, we quantified the effects of freezing on the permeation kinetics of resorcinol from time-dependent permeation experiments. Principal component analyses discriminated the intrinsic SC spectra and the spectra of resorcinol-containing RHEs, in each case on the basis of the freezing conditions. Permeation of resorcinol through the frozen RHE increased 3- to 6-fold compared to fresh RHE, with the strongest effect obtained from freezing at -20 °C for 10 weeks. Due to the extensive optimization and standardization of EpiSkin™ RHE, the effects observed in our work may be expected to be more pronounced with other RHEs.

Monitoring calcium-induced epidermal differentiation in vitro using multiphoton microscopy

SIGNIFICANCE

Research in tissue engineering and in vitro organ formation has recently intensified. To assess tissue morphology, the method of choice today is restricted primarily to histology. Thus novel tools are required to enable noninvasive, and preferably label-free, three-dimensional imaging that is more compatible with futuristic organ-on-a-chip models.

AIM

We investigate the potential for using multiphoton microscopy (MPM) as a label-free in vitro approach to monitor calcium-induced epidermal differentiation.

APPROACH

In vitro epidermis was cultured at the air-liquid interface in varying calcium concentrations. Morphology and tissue architecture were investigated using MPM based on visualizing cellular autofluorescence.

RESULTS

Distinct morphologies corresponding to epidermal differentiation were observed. In addition, Ca2  +  -induced effects could be distinguished based on the architectural differences in stratification in the tissue cultures.

CONCLUSIONS

Our study shows that MPM based on cellular autofluorescence enables visualization of Ca2  +  -induced differentiation in epidermal skin models in vitro. The technique has potential to be further adapted as a noninvasive, label-free, and real-time tool to monitor tissue regeneration and organ formation in vitro.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

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.

Engineered Skin Tissue Equivalents for Product Evaluation and Therapeutic Applications

The current status of skin tissue equivalents that have emerged as relevant tools in commercial and therapeutic product development applications is reviewed. Due to the rise of animal welfare concerns, numerous companies have designed skin model alternatives to assess the efficacy of pharmaceutical, skincare, and cosmetic products in an in vitro setting, decreasing the dependency on such methods. Skin models have also made an impact in determining the root causes of skin diseases. When designing a skin model, there are various chemical and physical considerations that need to be considered to produce a biomimetic design. This includes designing a structure that mimics the structural characteristics and mechanical strength needed for tribological property measurement and toxicological testing. Recently, various commercial products have made significant progress towards achieving a native skin alternative. Further research involve the development of a functional bilayered model that mimics the constituent properties of the native epidermis and dermis. In this article, the skin models are divided into three categories: in vitro epidermal skin equivalents, in vitro full-thickness skin equivalents, and clinical skin equivalents. A description of skin model characteristics, testing methods, applications, and potential improvements is presented.

Identification of lipids of the stratum corneum by high performance thin layer chromatography and mass spectrometry

The stratum corneum, the outermost layer of the epidermis, is the most important skin barrier against exogenous physical and chemical effects, in addition to protecting against dehydration. Ceramides are integral parts of the intercellular lipid lamellae of the stratum corneum and play an important role in the barrier function of mammalian skin. Ceramides are sphingolipids consisting of sphingoid bases linked to fatty acids by an amide bond. Typical sphingoid bases in the skin are composed of dihydrosphingosine, sphingosine, phytosphingosine, and 6-hydroxysphingosine, and the fatty acid acyl chains are composed of non-hydroxy fatty acid, α-hydroxy fatty acid, ω-hydroxy fatty acid, and esterified ω-hydroxy fatty acid. Analytical methods, such as gas chromatography/mass spectrometry, high performance thin layer chromatography with UV detection, and liquid chromatography/mass spectrometry, have been developed for the identification and quantification of ceramides in the stratum corneum. However, only a few publications relate to the mass fragmentation patterns specific to ceramide types to determine the structure of skin ceramides. Moreover, these studies provide very limited structural information and only for some ceramides. Therefore, the aim of our study was to develop a quick and easy method of quantification of ceramides, cholesterol, and free fatty acids by high performance thin layer chromatography with ultraviolet detection. High performance thin layer chromatography with ultraviolet detection was also coupled with mass spectrometry using negative ionization by electrospray and tandem mass spectrometry (MS/MS) for identification of ceramides' structure.

Bioengineering the microanatomy of human skin

Recreating the structure of human tissues in the laboratory is valuable for fundamental research, testing interventions, and reducing the use of animals. Critical to the use of such technology is the ability to produce tissue models that accurately reproduce the microanatomy of the native tissue. Current artificial cell-based skin systems lack thorough characterisation, are not representative of human skin, and can show variation. In this study, we have developed a novel full thickness model of human skin comprised of epidermal and dermal compartments. Using an inert porous scaffold, we created a dermal construct using human fibroblasts that secrete their own extracellular matrix proteins, which avoids the use of animal-derived materials. The dermal construct acts as a foundation upon which epidermal keratinocytes were seeded and differentiated into a stratified keratinised epithelium. In-depth morphological analyses of the model demonstrated very close similarities with native human skin. Extensive immunostaining and electron microscopy analysis revealed ultrastructural details such as keratohyalin granules and lamellar bodies within the stratum granulosum, specialised junctional complexes, and the presence of a basal lamina. These features reflect the functional characteristics and barrier properties of the skin equivalent. Robustness and reproducibility of in vitro models are important attributes in experimental practice, and we demonstrate the consistency of the skin construct between different users. In summary, a new model of full thickness human skin has been developed that possesses microanatomical features reminiscent of native tissue. This skin model platform will be of significant interest to scientists researching the structure and function of human skin.

Quantification and characterization of radical production in human, animal and 3D skin models during sun irradiation measured by EPR spectroscopy

Sun radiation is indispensable to our health, however, a long term and high exposure could lead to erythema, premature skin aging and promotion of skin tumors. An underlying pathomechanism is the formation of free radicals. First, reactive oxygen species (*OH, *O₂^-) and then, secondary lipid oxygen species (C centered radicals, CCR) are formed. A high amount of free radicals results in oxidative stress with subsequent cell damage. In dermatological research different skin models are used, however, comparative data about the cutaneous radical formation are missing. In this study, the radical formation in porcine-, (SKH-1) murine-, human- ex vivo skin and reconstructed human skin (RHS) were investigated during simulated sun irradiation (305-2200 nm), with X-band EPR spectroscopy. The amount of radical formation was investigated with the spin probe PCA exposed to a moderate sun dose below one minimal erythema dose (MED, ~25 mJ/cm² UVB) in all skin models. Furthermore, the *OH and *CCR radical concentrations were measured with the spin trap DMPO within 0-4 MED (porcine-, human skin and RHS). The highest amount of radicals was found in RHS followed by murine and porcine, and the lowest amount in human ex vivo skin. In all skin models, more *OH than CCR radicals were found at 0-4 MED. Additionally, this work addresses the limitations in the characterization with the spin trap DMPO. The measurements have shown that the most comparable skin model to in vivo human skin could differ depending on the focus of the investigation. If the amount of radial production is regarded, RHS seems to be in a similar range like in vivo human skin. If the investigation is focused on the radical type, porcine skin is most comparable to ex vivo human skin, at an irradiation dose not exceeding 1 MED. Here, no comparison to in vivo human skin is possible.

Establishment of keratinocyte cell lines from human hair follicles

The advent of organotypic skin models advanced the understanding of complex mechanisms of keratinocyte differentiation. However, these models are limited by both availability of primary keratinocytes and donor variability. Keratinocytes derived from cultured hair follicles and interfollicular epidermis were immortalized by ectopic expression of SV40 and hTERT. The generated keratinocyte cell lines differentiated into stratified epidermis with well-defined stratum granulosum and stratum corneum in organotypic human skin models. They behaved comparable to primary keratinocytes regarding the expression of differentiation-associated proteins, cell junction components and proteins associated with cornification and formed a barrier against biotin diffusion. Mechanistically, we found that SV40 large T-antigen expression, accompanied by a strong p53 accumulation, was only detectable in the basal layer of the in vitro reconstructed epidermis. Inhibition of DNA-methylation resulted in expression of SV40 large T-antigen also in the suprabasal epidermal layers and led to incomplete differentiation of keratinocyte cell lines. Our study demonstrates the generation of keratinocyte cell lines which are able to fully differentiate in an organotypic skin model. Since hair follicles, as source for keratinocytes, can be obtained by minimally invasive procedures, our approach enables the generation of cell lines also from individuals not available for skin biopsies.

Confocal Raman spectroscopic imaging for in vitro monitoring of active ingredient penetration and distribution in reconstructed human epidermis model

Topically applied active cosmetic ingredients (ACI) or active pharmaceutical ingredients (API) efficacy is directly related to their efficiency of penetration in the skin. In vitro reconstructed human epidermis surrogate models offer in vivo like skin samples for transdermal studies. Using Delipidol®, an ACI currently used in the cosmetics industry, the capabilities to deliver accurate distribution maps and penetration profiles of this molecule by means of confocal Raman spectroscopic imaging have been demonstrated. Using a non-negative constrained least squares (NCLS) approach, contribution of specific molecules can be estimated at each point of spectral maps in order to deliver semi-quantitative heat maps representing the ACI levels in the different skin layers. The concentration profiles obtained are approximately single exponential for all 3 time points evaluated, with a consistent decay constant, which is independent of the sublayer structure. Notably, however, there is no significant penetration into the lower basal layers until a critical concentration is built up, after 3 hours. Combination of Raman confocal imaging with spectral unmixing methods such as NCLS is demonstrated to be a relevant approach for in vitro biological evaluation of cosmetic and pharmaceutical active ingredients and could easily be implemented as a screening tool for industrial use.

Improved epidermal barrier formation in human skin models by chitosan modulated dermal matrices

Full thickness human skin models (FTMs) contain an epidermal and a dermal equivalent. The latter is composed of a collagen dermal matrix which harbours fibroblasts. Current epidermal barrier properties of FTMs do not fully resemble that of native human skin (NHS), which makes these human skin models less suitable for barrier related studies. To further enhance the resemblance of NHS for epidermal morphogenesis and barrier formation, we modulated the collagen dermal matrix with the biocompatible polymer chitosan. Herein, we report that these collagen-chitosan FTMs (CC-FTMs) possess a well-organized epidermis and maintain both the early and late differentiation programs as in FTMs. Distinctively, the epidermal cell activation is reduced in CC-FTMs to levels observed in NHS. Dermal-epidermal interactions are functional in both FTM types, based on the formation of the basement membrane. Evaluation of the barrier structure by the organization of the extracellular lipid matrix of the stratum corneum revealed an elongated repeat distance of the long periodicity phase. The ceramide composition exhibited a higher resemblance of the NHS, based on the carbon chain-length distribution and subclass profile. The inside-out barrier functionality indicated by the transepidermal water loss is significantly improved in the CC-FTMs. The expression of epidermal barrier lipid processing enzymes is marginally affected, although more restricted to a single granular layer. The novel CC-FTM resembles the NHS more closely, which makes them a promising tool for epidermal barrier related studies.

A Trichophyton Rubrum Infection Model Based on the Reconstructed Human Epidermis - Episkin®

BACKGROUND

Trichophyton rubrum represents the most common infectious fungus responsible for dermatophytosis in human, but the mechanism involved is still not completely understood. An appropriate model constructed to simulate host infection is the prerequisite to study the pathogenesis of dermatophytosis caused by T. rubrum. In this study, we intended to develop a new T. rubrum infection model in vitro, using the three-dimensional reconstructed epidermis - EpiSkin ®, and to pave the way for further investigation of the mechanisms involved in T. rubrum infection.

METHODS

The reconstructed human epidermis (RHE) was infected by inoculating low-dose (400 conidia) and high-dose (4000 conidia) T. rubrum conidia to optimize the infection dose. During the various periods after infection, the samples were processed for pathological examination and scanning electron microscopy (SEM) observation.

RESULTS

The histological analysis of RHE revealed a fully differentiated epidermis with a functional stratum corneum, which was analogous to the normal human epidermis. The results of hematoxylin and eosin staining and the periodic acid-Schiff staining showed that the infection dose of 400 conidia was in accord with the pathological characteristics of host dermatophytosis caused by T. rubrum. SEM observations further exhibited the process of T. rubrum infection in an intuitionistic way.

CONCLUSIONS

We established the T. rubrum infection model on RHE in vitro successfully. It is a promising model for further investigation of the mechanisms involved in T. rubrum infection.

Quantitative analysis of ceramides using a novel lipidomics approach with three dimensional response modelling

In the outermost layer of the skin, the stratum corneum (SC), ceramides form a diverse and essential pool of lipids. Due to their diversity and the limited availability of synthetic standards it is challenging to quantitatively analyse all SC ceramides independently. We aim to perform a detailed analysis of ceramides on SC harvested from in vivo and ex vivo skin, therefore, a LC/MS method was developed in which all steps from sample acquisition until data analysis were examined and optimized. Improving extraction efficiency of ceramides resulted in an increase in efficiency from 71.5% to 99.3%. It was shown that sample harvesting by tape-stripping in vivo was accurate and precise. A full scan MS method was developed, compatible with all sample types, enabling simultaneously qualitative and quantitative data analysis. A novel three dimensional response model was constructed to quantify all detected ceramides from full scan data using a limited amount of synthetic ceramides. The application is demonstrated on various SC sample types. When ex vivo SC was regenerated during human skin culture, increases are observed in the amount of the ceramide sphingosine subclasses, in mono unsaturated ceramides (which have an cis-double bond in the acyl chain), and ceramides with a short C34 carbon chain (ceramides with a total carbon chain of 34 carbon atoms), compared with native human skin. These changes in ceramide levels are also often encountered in diseased skin.

Glutathione metabolism in the HaCaT cell line as a model for the detoxification of the model sensitisers 2,4-dinitrohalobenzenes in human skin

Glutathione (GSH) is the most prominent antioxidant in cells and the co-factor of an important set of enzymes involved in the skin metabolic clearance system, glutathione S-transferases (GST). Here, we describe an LC-MS (liquid chromatography-mass spectroscopy) method to measure GSH and its disulfide form (GSSG) in HaCaT cells and a 3D Reconstructed Human Epidermis (RHE) model. In our assay, the basal level of GSH in both systems was in the low nmol/mg soluble protein range, while the level of GSSG was systematically below our limit of quantification (0.1 μM). We found that 2,4-dinitrohalobenzenes deplete the GSH present in HaCaT cells within the first hour of exposure, in a dose dependent manner. The level of GSH in HaCaT cells treated with a single non-toxic dose of 10 μM of dinitrohalobenzene was also shown to increase after two hours. While cells treated with 1-chloro-2,4-dinitrobenzene (DNCB) and 1-fluoro-2,4-dinitrobenzene (DNFB) repleted GSH to levels similar to untreated control cells within 24h, 1-bromo-2,4-dinitrobenzene (DNBB) seemed to prevent such a repletion and appeared to be the most toxic compound in all assays. A mathematical modelling of experimental results was performed to further rationalise the differences observed between test chemicals. For this purpose the biological phenomena observed were simplified into two sequential events: the initial depletion of the GSH stock after chemical treatment followed by the repletion of the GSH once the chemical was cleared. Activation of the nuclear factor E2-related factor 2 (Nrf2) pathway was observed with all compounds within two hours, and at concentrations less than 10 μM. These data show that GSH depletion and repletion occur rapidly in skin cells and emphasize the importance of conducting kinetic studies when performing in vitro experiments exploring skin sensitization.

Performance of the N/TERT epidermal model for skin sensitizer identification via Nrf2-Keap1-ARE pathway activation

Animal testing of chemical ingredients for cosmetic purposes is prohibited. Therefore there is an urgent need for in vitro models to identify chemical allergens. In human skin, keratinocytes (KCs) are abundantly present and are key players in initiation of allergic contact dermatitis. One of the pathways that has been shown to be induced by sensitizers is the Keap1-Nrf2-ARE pathway. In this study we compared the response of four keratinocyte-based models including (a) primary human KCs, (b) N/TERT monolayer cultures, (c) the Leiden Epidermal models (LEMs) and (d) the N/TERT epidermal models (NEMs). All keratinocyte-based models were subjected to chemical exposure of the sensitizer 2,4-dinitrochlorobenzene (DNCB) and irritant Sodium dodecyl sulfate (SDS) at nontoxic concentrations. Activation of the Keap1-Nrf2-ARE pathway was evaluated by measuring Nrf2 protein levels as well as nuclear translocation and activation of transcriptional targets of Nrf2. Results show that the Keap1-Nrf2-ARE pathway is activated by the sensitizer DNCB in monolayer keratinocytes and as well as the LEMs and NEMs and not by the irritant SDS. Collectively our data demonstrate that the N/TERT models respond similarly as primary KCs and could therefore serve as an alternative model for skin sensitizer identification, thereby overcoming the need for primary skin tissue.

Critical analysis of 3-D organoid in vitro cell culture models for high-throughput drug candidate toxicity assessments

Drug failure due to toxicity indicators remains among the primary reasons for staggering drug attrition rates during clinical studies and post-marketing surveillance. Broader validation and use of next-generation 3-D improved cell culture models are expected to improve predictive power and effectiveness of drug toxicological predictions. However, after decades of promising research significant gaps remain in our collective ability to extract quality human toxicity information from in vitro data using 3-D cell and tissue models. Issues, challenges and future directions for the field to improve drug assay predictive power and reliability of 3-D models are reviewed.

The use of skin models in drug development

Three dimensional (3D) tissue models of the human skin are probably the most developed and understood in vitro engineered constructs. The motivation to accomplish organotypic structures was driven by the clinics to enable transplantation of in vitro grown tissue substitutes and by the cosmetics industry as alternative test substrates in order to replace animal models. Today a huge variety of 3D human skin models exist, covering a multitude of scientific and/or technical demands. This review summarizes and discusses different approaches of skin model development and sets them into the context of drug development. Although human skin models have become indispensable for the cosmetics industry, they have not yet started their triumphal procession in pharmaceutical research and development. For drug development these tissue models may be of particular interest for a) systemically acting drugs applied on the skin, and b) drugs acting at the site of application in the case of skin diseases or disorders. Although quite a broad spectrum of models covering different aspects of the skin as a biologically acting surface exists, these are most often single stand-alone approaches. In order to enable the comprehensive application into drug development processes, the approaches have to be synchronized to allow a cross-over comparison. Besides the development of biological relevant models, other issues are not less important in the context of drug development: standardized production procedures, process automation, establishment of significant analytical methods, and data correlation. For the successful routine use of engineered human skin models in drug development, major requirements were defined. If these requirements can be accomplished in the next few years, human organotypic skin models will become indispensable for drug development, too.

Transcriptional changes in organoculture of full-thickness human skin following topical application of all-trans retinoic acid

Objective

Retinoids are used as therapeutic agents for numerous skin diseases, for example, psoriasis, acne and keratinization disorders. The same substances have also been recognized in the treatment for hyperpigmentation disorders such as melasma.

Other studies on photo-damaged skin have shown that retinoids reduce wrinkles, surface roughness, mottled pigmentation, and visual skin appearance as a whole. We tested the hypothesis that an organoculture of full-thickness human skin could be used as a preclinical model to investigate the retinoid transcriptional profile in human skin in vitro.

Methods

Full-thickness skin explants were exposed to topically applied all-trans retinoic acid (RA) for 24 h. The gene expression profile was analysed using oligonucleotide microarrays, and data were validated with real-time (RT) PCR.

Results

We showed that the expression of 93 genes was significantly altered more than twofold. Several of the altered genes, for example, KRT4, CYP26 and LCN2, have previously been shown to be affected by RA in keratinocyte monocultures, reconstructed epidermis and skin biopsies from patients treated topically or orally with RA. In addition, genes, such as SCEL, NRIP1, DGAT2, RDH12 EfnB2, MAPK14, SAMD9 and CEACAM6 not previously reported to be affected by RA in human skin, were identified for the first time in this study.

Conclusion

The results in the present study show that full-thickness human explants represent a valuable pre-clinical model for studying the effects of retinoids in skin.

Résumé

Human skin organ culture for assessment of chemically induced skin damage

The move away from animal models for skin safety testing is inevitable. It is a question of when, not if. As skin safety studies move away from traditional animal-based approaches, a number of replacement technologies are becoming available. Human skin in organ culture is one such technology. Organ-cultured skin has several features that distinguish it from other technologies. First and foremost, organ-cultured skin is real skin. Almost by definition, therefore, it approximates the intact skin better than other alternative models. Organ culture is an easy-to-use and relatively inexpensive approach to preclinical safety assessment. Although organ culture is not likely to replace high-throughput enzyme assays or monolayer culture/skin equivalent cultures for initial compound assessment, organ culture should find use when the list of compounds to be evaluated is small and when simpler models have narrowed the dose range. Organ-cultured skin also provides a platform for mechanistic studies.

Raman spectroscopy: in vivo quick response code of skin physiological status

Dermatologists need to combine different clinically relevant characteristics for a better understanding of skin health. These characteristics are usually measured by different techniques, and some of them are highly time consuming. Therefore, a predicting model based on Raman spectroscopy and partial least square (PLS) regression was developed as a rapid multiparametric method. The Raman spectra collected from the five uppermost micrometers of 11 healthy volunteers were fitted to different skin characteristics measured by independent appropriate methods (transepidermal water loss, hydration, pH, relative amount of ceramides, fatty acids, and cholesterol). For each parameter, the obtained PLS model presented correlation coefficients higher than R2=0.9. This model enables us to obtain all the aforementioned parameters directly from the unique Raman signature. In addition to that, in-depth Raman analyses down to 20 μm showed different balances between partially bound water and unbound water with depth. In parallel, the increase of depth was followed by an unfolding process of the proteins. The combinations of all these information led to a multiparametric investigation, which better characterizes the skin status. Raman signal can thus be used as a quick response code (QR code). This could help dermatologic diagnosis of physiological variations and presents a possible extension to pathological characterization.

Practical problems encountered during the cultivation of an open-source reconstructed human epidermis model on a polycarbonate membrane and protein quantification

During recent years, the importance of in vitro technology in skin research has increased significantly. A variety of skin culture models have been developed and commercialized. In this respect, the availability of reconstructed human epidermis (RHE) equivalents represents a significant improvement compared to the use of monolayer cultures. However, when an in-house RHE model is being developed, researchers might encounter some difficulties during cultivation. The scope of this paper is to report our experiences and practical problems with the development of a three-dimensional RHE model cultured on a polycarbonate membrane. Some important issues including cell density, the use of lysing enzymes, culture media, cell storage and viability, cell confluency and protein extraction are reported and optional solutions are given.

Reconstructed skin models as emerging tools for drug absorption studies

INTRODUCTION

As humans can come into contact with xenobiotics intentionally or accidentally, knowledge about the skin absorption of these substances is crucial and requires reliable models and test procedures. Animal experiments should be avoided whenever possible, instead of making the use of in vitro systems. Furthermore, due to limited availability of normal and especially diseased human skin, alternative test systems such as reconstructed skin models are urgently required.

AREAS COVERED

This article discusses the advantages and limitations of excised human skin, animal skin and reconstructed skin models for absorption testing in vitro. Furthermore, the authors also describe the standard procedure for skin absorption testing and give an excursion to the applicability of artificial membranes. Finally, the article highlights the progress in the development of reconstructed disease models and provides an extensive overview about past and ongoing research in this field.

EXPERT OPINION

The development and validation of in vitro systems for skin absorption testing is inevitable. More research efforts are required for the development of reconstructed disease models. Reconstructed skin models need to be improved, especially in terms of complexity to mimic the in vivo situation better. It should not, however, be the main goal to imitate the in vivo situation exactly, but to establish reliable systems that ensure predictive and reliable data.

Histamine suppresses epidermal keratinocyte differentiation and impairs skin barrier function in a human skin model

Background

Defects in keratinocyte differentiation and skin barrier are important features of inflammatory skin diseases like atopic dermatitis. Mast cells and their main mediator histamine are abundant in inflamed skin and thus may contribute to disease pathogenesis.

Methods

Human primary keratinocytes were cultured under differentiation-promoting conditions in the presence and absence of histamine, histamine receptor agonists and antagonists. The expression of differentiation-associated genes and epidermal junction proteins was quantified by real-time PCR, Western blot, and immunofluorescence labeling. The barrier function of human skin models was tested by the application of biotin as tracer molecule.

Results

The addition of histamine to human keratinocyte cultures and organotypic skin models reduced the expression of the differentiation-associated proteins keratin 1/10, filaggrin, and loricrin by 80–95%. Moreover, the addition of histamine to skin models resulted in the loss of the granular layer and thinning of the epidermis and stratum corneum by 50%. The histamine receptor H1R agonist, 2-pyridylethylamine, suppressed keratinocyte differentiation to the same extent as did histamine. Correspondingly, cetirizine, an antagonist of H1R, virtually abrogated the effect of histamine. The expression of tight junction proteins zona occludens-1, occludin, claudin-1, and claudin-4, as well as that of desmosomal junction proteins corneodesmosin and desmoglein-1, was down-regulated by histamine. The tracer molecule biotin readily penetrated the tight junction barrier of skin cultures grown in the presence of histamine, while their diffusion was completely blocked in nontreated controls.

Conclusions

Our findings suggest a new mechanism by which mast cell activation and histamine release contribute to skin barrier defects in inflammatory skin diseases.