Physical characterization of the stratum corneum of an in vitro human skin equivalent produced by tissue engineering and its comparison with normal human skin by ATR-FTIR spectroscopy and thermal analysis (DSC)

Assessing Barrier Function in Psoriasis and Cornification Models of Artificial Skin Using Non-Invasive Impedance Spectroscopy

Reconstructed epidermal equivalents (REEs) consist of two distinct cell layers - the stratum corneum (SC) and the keratinocyte layer (KL). The interplay of these layers is particularly crucial in pruritic inflammatory disorders, like psoriasis, where a defective SC barrier is associated with immune dysregulation. However, independent evaluation of the skin barrier function of the SC and KL in REEs is highly challenging because of the lack of quantitative methodologies that do not disrupt the counter layer. Here, a non-invasive impedance spectroscopy technique is introduced for dissecting the distinct contributions of the SC and KL to overall skin barrier function without disrupting the structure. These findings, inferred from the impedance spectra, highlight the individual barrier resistances and maturation levels of each layer. Using an equivalent circuit model, a correlation between impedance parameters and specific skin layers, offering insights beyond traditional impedance methods that address full-thickness skin only is established. This approach successfully detects subtle changes, such as increased paracellular permeability due to mild irritants and the characterization of an immature SC in psoriatic models. This research has significant implications, paving the way for detailed mechanistic investigations and fostering the development of therapies for skin irritation and inflammatory disorders.

Characterization of skin barrier defects using infrared spectroscopy in patients with atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is characterized by skin barrier defects that are often measured by biophysical tools that observe the functional properties of the stratum corneum (SC).

OBJECTIVES

To employ in vivo infrared spectroscopy alongside biophysical measurements to analyse changes in the chemical composition of the SC in relation to AD severity.

METHODS

We conducted an observational cross-sectional cohort study where attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy measurements were collected on the forearm alongside surface pH, capacitance, erythema and transepidermal water loss (TEWL), combined with tape stripping, in a cohort of 75 participants (55 patients with AD stratified by phenotypic severity and 20 healthy controls). Common FLG variant alleles were genotyped.

RESULTS

Reduced hydration, elevated TEWL and redness were all associated with greater AD severity. Spectral analysis showed a reduction in 1465 cm-1 (full width half maximum) and 1340 cm-1 peak areas, indicative of less orthorhombic lipid ordering and reduced carboxylate functional groups, which correlated with clinical severity (lipid structure r = -0.59, carboxylate peak area r = -0.50).

CONCLUSIONS

ATR-FTIR spectroscopy is a suitable tool for the characterization of structural skin barrier defects in AD and has potential as a clinical tool for directing individual treatment based on chemical structural deficiencies.

Molecular interactions of hesperidin with DMPC/cholesterol bilayers

Since cell membranes are complex systems, the use of model lipid bilayers is quite important for the study of their interactions with bioactive molecules. Mammalian cell membranes require cholesterol (CHOL) for their structure and function. For this reason, the mixtures of phospholipid and cholesterol are necessary to use in model membrane studies to better simulate the real systems. In the present study, we investigated the effect of the incorporation of hesperidin in model membranes consisting of dimyristoylphosphatidylcholine (DMPC) and CHOL by using differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM). ATR-FTIR results demonstrated that hesperidin increases the fluidity of the DMPC/CHOL binary system. DSC findings indicated that the presence of 5 mol% hesperidin induces a broadening of the main phase transition consisting of three overlapping components. AFM experiments showed that hesperidin increases the thickness of DMPC/CHOL lipid bilayer model membranes. In addition to experimental results, molecular docking studies were conducted with hesperidin and human lanosterol synthase (LS), which is an enzyme found in the final step of cholesterol synthesis, to characterize hesperidin's interactions with its surrounding via its hydroxyl and oxygen groups. Then, hesperidin's ADME/Tox (absorption, distribution, metabolism, excretion and toxicity) profile was computed to see the potential impact on living system. In conclusion, considering the data obtained from experimental studies, this work ensures molecular insights in the interaction between a flavonoid, as an antioxidant drug model, and lipids mimicking those found in mammalian membranes. Moreover, computational studies demonstrated that hesperidin may be a great potential for use as a therapeutic agent for hypercholesterolemia due to its antioxidant property.

α-Linolenic acid and linoleic acid modulate the lipidome and the skin barrier of a tissue-engineered skin model

Polyunsaturated fatty acids (PUFAs) play an important role in the establishment and the maintenance of the skin barrier function. However, the impact of their derived lipid mediators remains unclear. Skin substitutes were engineered according to the self-assembly method with a culture medium supplemented with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA). The supplementation with ALA and LA decreased testosterone absorption through a tissue-engineered reconstructed skin model, thus indicating an improved skin barrier function following supplementation. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes. Indeed, the dual supplementation increased the levels of eicosapentaenoic acid (EPA) (15-fold), docosapentaenoic acid (DPA) (3-fold), and LA (1.5-fold) in the epidermal phospholipids while it increased the levels of ALA (>20-fold), DPA (3-fold) and LA (1.5-fold) in the epidermal triglycerides. The bioactive lipid mediator profile of the skin substitutes, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols, was next analyzed using liquid chromatography-tandem mass spectrometry. The lipid supplementation further modulated bioactive lipid mediator levels of the reconstructed skin substitutes, leading to a lipid mediator profile more representative of the one found in normal human skin. These findings show that an optimized supply of PUFAs via culture media is essential for the establishment of improved barrier function in vitro. STATEMENT OF SIGNIFICANCE: Supplementation of the culture medium with 10 μM of both α-linolenic acid (ALA) and linoleic acid (LA) improved the skin barrier function of a tissue-engineered skin model. The exogenously provided fatty acids were incorporated into the phospholipid and triglyceride fractions of the skin substitutes and further modulated bioactive lipid mediator levels, including prostaglandins, hydroxy-fatty acids, N-acylethanolamines and monoacylglycerols. These findings highlight the important role of ALA and LA in skin homeostasis and show that an optimized supply of polyunsaturated fatty acids via culture media is essential for the establishment of improved barrier function in vitro.

Thermal analysis of mammalian stratum corneum using differential scanning calorimetry for advancing skin research and drug delivery

For effective topical and transdermal drug delivery, it is necessary for most actives to penetrate and permeate through the stratum corneum (SC). Extensive investigation of the thermal behaviour of mammalian SC has been performed to understand the barrier function of the skin. However, little attention has been paid to the related experimental variables in thermal analysis of the SC using differential scanning calorimetry that may influence the results obtained from such studies. In this review, we provide a comprehensive overview of the thermal transitions of the SC of both porcine and human skin. More importantly, the selection and impact of the experimental and instrumental parameters used in thermal analysis of the SC are critically evaluated. New opportunities for the use of thermal analysis of mammalian SC in advancing skin research, particularly for elucidation of the actions of excipients employed in topical and transdermal formulations on the skin are also highlighted.

Effective Transcutaneous Delivery of Hyaluronic Acid Using an Easy-to-Prepare Reverse Micelle Formulation

The skin loses its moisture with advancing age, causing cosmetic issues such as wrinkles. In addition, the loss of moisture leads to hypersensitivity to external stimuli such as UV light. Transcutaneous supplementation with hyaluronic acid (HA) is an effective and safe method of recovering the moisturizing function and elasticity of the skin. However, the transcutaneous delivery of HA remains challenging owing to the barrier function of the stratum corneum (SC) layer. To penetrate the SC barrier, we used a reverse micelle formulation that does not require high energy consumption processes for preparation. We aimed to enhance the skin permeability of HA by incorporating glyceryl monooleate—a skin permeation enhancer—into the formulation. A fluorescently-labeled HA-loaded reverse micelle formulation showed significantly enhanced permeation across Yucatan micro pig skin. Fourier transform infra-red spectroscopy of the surface of the skin treated with the reverse micelle formulation showed blue shifts of the CH2 symmetric/asymmetric stretching peaks, indicating a reduction in the barrier function of the SC. Further study revealed that HA was released from the reverse micelles at the hydrophobic/hydrophilic interface between the SC and the living epidermis. The results demonstrated that our reverse micellar system is an easy-to-prepare formulation for the effective transcutaneous delivery of HA.

Transbuccal delivery of benznidazole associated with monoterpenes: permeation studies and mechanistic insights

Benznidazole (BZN) represents the only drug currently available for the treatment of Chagas disease in most endemic countries. When administered orally, high doses are required due to its extensive hepatic metabolism and its toxicity represents the main reason for treatment withdrawals. Because of these complications, transbuccal administration of BZN was investigated. This route avoids the first-pass hepatic metabolism and presents high permeability, with direct access to the systemic circulation. BZN was applied on porcine buccal mucosa after pretreatment with pure eugenol, carvacrol or limonene. Thermal (DSC) and spectroscopic (FT-IR) analyzes were performed to investigate the mechanisms of drug absorption enhancement. The permeability coefficient values of BZN increased 2.6, 2.9 and 4.9-fold after pretreatment with eugenol, carvacrol and limonene, respectively. The lag time, in turn, was shortened in the pretreated samples. The DSC and FT-IR analyzes suggested that transport of BZN through the buccal mucosa is associated with log P and size of monoterpenes. Limonene, the most effective absorption enhancer, contributed to greater interaction with non-polar domains of the buccal epithelium. Overall, BZN showed to be efficiently transported through the buccal route, but in vivo pharmacokinetic studies should be performed to confirm these findings.

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.

Qualifying X-ray and Stimulated Raman Spectromicroscopy for Mapping Cutaneous Drug Penetration

Research on topical drug delivery relies on reconstructed human skin (RHS) in addition to ex vivo human and animal skin, each with specific physiological features. Here, we compared the penetration of dexamethasone from an ethanolic hydroxyethyl cellulose gel into ex vivo human skin, murine skin, and RHS. For comprehensive insights into skin morphology and penetration enhancing mechanisms, scanning transmission X-ray microscopy (STXM), liquid chromatography tandem-mass spectrometry (LC-MS/MS), and stimulated Raman spectromicroscopy (SRS) were combined. STXM offers high spatial resolution with label-free drug detection and is therefore sensitive to tissue damage. Despite differences in sample preparation and data analysis, the amounts of dexamethasone in RHS, detected and quantified by STXM and LC-MS/MS, were very similar and increased during the first 100 min of exposure. SRS revealed interactions between the gel and the stratum corneum or, more specifically, its protein and lipid structures. Similar to both types of ex vivo skin, higher protein-to-lipid ratios within the stratum corneum of RHS indicated reduced lipid amounts after 30 min of ethanol exposure. Extended ethanol exposure led to a continued reduction of lipids in the ex vivo matrixes, while protein integrity appeared to be compromised in RHS, which led to declining protein signals. In conclusion, LC-MS/MS proved the predictive capability of STXM for label-free drug detection. Combining STXM with SRS precisely dissected the penetration enhancing effects of ethanol. Further studies on topical drug delivery should consider the potential of these complementary techniques.

ATR-IR study of skin components: Lipids, proteins and water. Part I: Temperature effect

In this work we report the studies of the effect of temperature on skin components, such as lipids, proteins and water. Modifications of lipids structure induced by increasing temperature (from 20 to 90°C) have been studied using ATR-IR (Attenuated Total Reflectance Infrared) spectroscopy, which is a powerful tool for characterization of the molecular structure and properties of tissues, such as skin. Due to the small depth of penetration (0.6-5.6μm), ATR-IR spectroscopy probes only the outermost layer of the skin, i.e. the stratum corneum (SC). The assignment of main spectral features of skin components allows for the determination of phase transitions from the temperature dependencies of band intensities [e.g. ν_as(CH₂) and ν_s(CH₂)]. The phase transitions were determined by using two methods: the first one was based on the first derivative of the Boltzmann function and the second one employed tangent lines of sigmoidal, aforementioned dependencies. The phase transitions in lipids were correlated with modifications of the structure of water and proteins.

Development, characterization, and in vitro biological performance of fluconazole-loaded microemulsions for the topical treatment of cutaneous leishmaniasis

Cutaneous leishmaniasis (CL) is a resistant form of leishmaniasis that is caused by a parasite belonging to the genus Leishmania. FLU-loaded microemulsions (MEs) were developed by phase diagram for topical administration of fluconazole (FLU) as prominent alternative to combat CL. Three MEs called F1, F2, and F3 (F1—60% 50 M phosphate buffer at pH 7.4 (PB) as aqueous phase, 10% cholesterol (CHO) as oil phase, and 30% soy phosphatidylcholine/oil polyoxyl-60 hydrogenated castor oil/sodium oleate (3/8/6) (S) as surfactant; F2—50% PB, 10% CHO, and 40% S; F3—40% PB, 10% CHO, and 50 % S) were characterized by droplet size analysis, zeta potential analysis, X-ray diffraction, continuous flow, texture profile analysis, and in vitro bioadhesion. MEs presented pseudoplastic flow and thixotropy was dependent on surfactant concentration. Droplet size was not affected by FLU. FLU-loaded MEs improved the FLU safety profile that was evaluated using red cell haemolysis and in vitro cytotoxicity assays with J-774 mouse macrophages. FLU-unloaded MEs did not exhibit leishmanicidal activity that was performed using MTT colourimetric assays; however, FLU-loaded MEs exhibited activity. Therefore, these MEs have potential to modulate FLU action, being a promising platform for drug delivery systems to treat CL.

Inhibitors of thermally induced burn incidents – characterization by microbiological procedure, electrophoresis, SEM, DSC and IR spectroscopy

(DSC) and (TGA) investigations, acetate electrophoresis (CAE), infrared spectrometry (FTIR), scanning electron microscopy (SEM) and microbiological procedures were all carried out after heating the samples to a temperature sufficient for simulating a burn incident.

Molecular organization of the lipid matrix in intact Stratum corneum using ATR-FTIR spectroscopy

ATR-FTIR spectroscopy is useful in investigating the lateral organization of Stratum corneum (SC) lipids in full-thickness skin. Based on studies of the thermotropic phase transitions in n-tricosane and in excised human skin, the temperature dependence of the CH2 scissoring bandwidth emerged as a measure of the extent of orthorhombic and hexagonal phases. This dependence provides a simpler measure of the lateral order in lipid assemblies than the common spectroscopic approaches based on difference spectra, curve fitting of the CH2 scissoring region, and the position of the CH2 stretching vibrations. It has the advantages of ease of determination, relatively low variability, and high discriminative power for the type of lateral intermolecular chain packing. A comparison of the lateral organization of the lipids at the SC surface of mammalian skin using the scissoring bandwidth revealed considerable differences between human abdominal skin (containing mostly orthorhombic phases), porcine ear skin (containing mostly hexagonal phases), and reconstructed human epidermis (containing mostly disordered phases). This parameter also correctly described the different effects of propylene glycol (minimally disturbing) and oleic acid (formation of a highly disordered phase) on the SC lipids in excised human skin. The procedure described here is applicable to in vivo studies in the areas of dermatology, transdermal drug delivery, and skin biophysics.

Physical characterization of the stratum corneum of an in vitro psoriatic skin model by ATR-FTIR and Raman spectroscopies

The stratum corneum is an important permeability barrier for the skin. The disorganization of the skin protective barrier characterizes some skin diseases such as psoriasis. Indeed, psoriatic skin is known to be more permeable than normal human skin. An in vitro human skin substitute may be obtained by the auto-assembly method. This method was adapted to produce psoriatic substitutes. FTIR spectroscopy is a well-established method to evaluate the order of hydrocarbon chains in terms of population of trans and gauche conformers. Using ATR-FTIR, we have compared the physicochemical properties of the stratum corneum in skin models derived from uninvolved and involved psoriatic cells with those derived from normal cells. Our results suggest that the stratum corneum of involved psoriatic skin substitutes is less organized than that of normal skin substitutes. Also, it seems that the properties of uninvolved psoriatic skin may vary with seriousness of the disease. The development of a new psoriatic skin model would be helpful in the design of new treatments and to increase the understanding of the mechanisms of this pathology.

Thermal Behaviour of Human Stratum Corneum

PURPOSE

To use high-speed differential scanning calorimetry (DSC) in the identification of transitions in human stratum corneum (SC). Several scanning rates (100 degrees C/min to 400 degrees C/min) are used.

RESULTS

Eight transitions from 0 to 120 degrees C are detected in a significant number of samples. Most of these transitions have already been identified in previous studies, but have been labeled considering essentially that only four are present. Results also indicate some degree of reversibility for transitions occurring at temperatures above 90 degrees C. Dehydrated SC samples displayed slightly more defined transition peaks and a less frequent presence of the transitions below 50 degrees C. In turn, the delipidised SC matrix showed two major endothermic signals, centered around 55 and 100 degrees C, in conjunction with other much less marked features.

CONCLUSIONS

The interpretation of DSC traces in terms of four main transition temperatures must be complemented having in mind the occurrence of other transitions, some of them at physiological temperatures. This work further suggests that transitions at temperatures above 90 degrees C may to a large degree be associated to lipids, while transition at approx. 55 degrees C is probably related to lipids covalently linked to proteins, as previously suggested.

Effect of the composition of lecithin/n-propanol/isopropyl myristate/water microemulsions on barrier properties of mice skin for transdermal permeation of tetracaine hydrochloride: In vitro

Effect of composition of lecithin water-in-oil and oil-in-water microemulsion on in vitro transdermal permeation of tetracaine hydrochloride was studied on mice model. The results were compared with an aqueous solution of tetracaine hydrochloride (2.7 mg/ml). In vitro skin flux and permeability coefficients were obtained using the Franz diffusion cell. Differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) were used to study the mechanism of action of the microemulsion. Micrographs of TEM and CLSM studies were analyzed by using Image Pro Plus image software. Skin flux of tetracaine hydrochloride was found to be dependent on the composition of lecithin/n-propanol/isopropyl myristate/water microemulsions. At lower Km ratio (i.e. 0.5:1 and 0.8:1) of microemulsion, the rate of permeation of tetracaine hydrochloride was higher when compared to the microemulsion of higher Km ratio (1:1 and 1.5:1). Image analysis of TEM micrograph, 6h after application of lecithin microemulsion, showed 3.5+/-0.75-fold (p<0.001) increase in the intercellular space in the epidermis and 3.8+/-0.4-fold (p<0.001) enhancement in upper dermis. CLMS results show that sweat gland and hair follicles also provided path for permeation of the drug through the skin.

Study of human stratum corneum and extracted lipids by thermomicroscopy and DSC

A study on the thermal behavior of human stratum corneum and lipids is described. The use of high scanning rate DSC for both SC and extracted lipids allows the consistent determination of transition temperatures, including those of lower energy. Changes are found both at physiological and higher temperatures. There is a clear correspondence between the thermotropic behavior of these two systems. However, one of the transitions found in human SC (approximately 55 degrees C) is absent in extracted lipids and may be ascribed to those covalently-linked to corneocytes. Lipidic thermotropic behavior is clearly found above 100 degrees C, in which proteins do not play an exclusive role. Changes related to most transitions are observed directly by polarized light thermal microscopy in extracted lipids. This technique also allowed for the observation of large segregated domains in the extracted lipids. A drastic change is observed at approximately 60 degrees C, corresponding to the disruption of the lamellar structure.

New advances on the functions of epidermal growth factor receptor and ceramides in skin cell differentiation, disorders and cancers

Recent advances in understanding of the biological functions of the epidermal growth factor and epidermal growth factor receptor (EGF-EGFR) system and ceramide production for the maintenance of skin integrity and barrier function are reported. In particular, the opposite roles of EGFR and ceramide cascades in epithelial keratinocyte proliferation, migration and terminal differentiation are described. Moreover, the functions of ceramides in the epidermal permeability barrier are reviewed. The alterations in EGFR signaling and ceramide metabolism, which might be involved in the etiopathogenesis of diverse skin disorders and cancers, are described. New progress in understanding of skin organization, which might provide the basis for the design of new transcutaneous drug delivery techniques as well as for the development of new therapies of skin disorders and cancers, are reported.

Epidermal cell culture model derived from rat keratinocytes with permeability characteristics comparable to human cadaver skin

The permeability characteristics of an organotypic epidermal culture model derived from rat epidermal keratinocytes, ROC, and isolated human cadaver epidermis, HEM, were compared. Rat epidermal keratinocyte (REK) cell line was grown for 3 weeks on collagen gel in the absence of feeder cells in culture inserts at an air-liquid interface. Transdermal permeabilities of 18 compounds ranging from 92 to 504 in molecular weight and from -4.3 to 3.9 in log of octanol-water partition coefficient, charged or uncharged, were measured in the culture model and isolated human epidermis. The REK organotypic culture model (ROC) provided a close estimate of human epidermal permeabilities over the whole range of the solutes used with on the average of 2-fold higher permeability coefficients (range 0.3-5.2) than those obtained from isolated human cadaver epidermis. The easily maintained and reproducible ROC model may be useful in screening transepidermal drug permeabilities together with possessing potential for research on dermal formulations, irritation, toxicity and gene therapy.

Xenobiotic bioconversion in human epidermis models

There is a great need for alternative experimental methods for measuring percutaneous xenobiotic biotransformation. Animal testing and excised human skin studies have been the historical standards for confirmation of therapeutic and toxic effects that occur in the skin as a result of drug and other chemical metabolism. Human skin epidermal bioequivalents have become progressively more used for these types of pharmacological/toxicological studies in recent years. These epidermal models have been used in the form of cell culture, tissue sheets, and highly differentiated epidermal and epidermal/dermal systems. This review highlights the existing published data on the utility of these skin bioequivalent models for various types of metabolism and toxicology studies that should be of interest to the dermatopharmaceutical scientist.

Optical non-invasive approaches to diagnosis of skin diseases

A number of noninvasive approaches have been developed over the years to provide objective evaluation of the skin both in health and in disease. The advent of computers, as well as of lasers and photonics, has made it possible to develop additional techniques that were impossible a few years ago. These approaches provide the dermatologist with sensitive tools to measure the skin's condition in terms of physiologic parameters (e.g., color, erythema and pigmentation, induration, sebaceous and stratum corneum lipids, barrier function, etc.). Yet, a typical dermatologic diagnosis relies primarily on the trained eyes of the physician and to a lesser extent on information from other senses, such as touch and smell. The trained senses of the dermatologist backed by his/her brain form a powerful set of tools for evaluating the skin. The golden rule in diagnosis remains the histologic examination of a skin biopsy, a rather invasive method. These tools have served the profession well. The advent of ever faster and cheaper computers and of sensitive, inexpensive optical instrumentation of minimal dimensions provides the professional with the possibility of making objective measures of a number of skin parameters.

Influence of the lipid composition on the organization of skin lipid model mixtures: an infrared spectroscopy investigation

The polymorphism of the lipids of the stratum corneum (SC), the top layer of the epidermis, has a fundamental impact on the permeability properties of the skin barrier. In this work, we have examined by infrared spectroscopy the thermal behavior of model mixtures involving ceramide, palmitic acid and cholesterol, the three main components of the SC lipids, to gain a refined description of the participation of the various lipid species in the different phases observed as a function of temperature. The results show that below 40 degrees C ceramide, cholesterol and palmitic acid exist mainly in crystalline domains and the lipidic species show very limited miscibility. Between 40 and 50 degrees C, a transition from the crystalline to a liquid ordered (lo) phase occurs and it involves ceramides, cholesterol and palmitic acid. When the mixture has a high cholesterol content, this lo phase is stable up to 75 degrees C. For low cholesterol content, the mixtures undergo a second transition toward a more disordered phase which is likely not lamellar. The formation of these phases is critically dependent on the lipid composition and, therefore, it is likely that composition changes of SC lipids affect the phase behavior and, consequently, the skin barrier properties.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.