The molecular arrangement of ceramides in the unit cell of the long periodicity phase of stratum corneum models shows a high adaptability to different ceramide head group structures

The stratum corneum (SC) lipid matrix, composed primarily of ceramides (CERs), cholesterol and free fatty acids (FFA), has an important role for the skin barrier function. The presence of the long periodicity phase (LPP), a unique lamellar phase, is characteristic for the SC. Insight into the lipid molecular arrangement within the LPP unit cell is imperative for understanding the relationship between the lipid subclasses and the skin barrier function. In this study, the impact of the CER head group structure on the lipid arrangement and barrier functionality was investigated using lipid models forming the LPP. The results demonstrate that the positions of CER N-(tetracosanoyl)-sphingosine (CER NS) and CER N-(tetracosanoyl)-phytosphingosine (CER NP), two essentials CER subclasses, are not influenced by the addition of another CER subclass (N-(tetracosanoyl)-dihydrosphingosine (CER NdS), N-(2R-hydroxy-tetracosanoyl)-sphingosine (CER AS) or D-(2R-hydroxy-tetracosanoyl)-phytosphingosine (CER AP)). However, differences are observed in the lipid organization and the hydrogen bonding network of the three different models. A similar localization of CER NP and CER NS is also observed in a more complex lipid model, with the CER subclass composition mimicking that of human SC. These studies show the adaptability and insensitivity of the LPP unit cell structure to changes in the lipid head group structures of the CER subclasses.

The skin barrier: An extraordinary interface with an exceptional lipid organization

The barrier function of the skin is primarily located in the stratum corneum (SC), the outermost layer of the skin. The SC is composed of dead cells with highly organized lipid lamellae in the intercellular space. As the lipid matrix forms the only continuous pathway, the lipids play an important role in the permeation of compounds through the SC. The main lipid classes are ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). Analysis of the SC lipid matrix is of crucial importance in understanding the skin barrier function, not only in healthy skin, but also in inflammatory skin diseases with an impaired skin barrier. In this review we provide i) a historical overview of the steps undertaken to obtain information on the lipid composition and organization in SC of healthy skin and inflammatory skin diseases, ii) information on the role CERs, CHOL and FFAs play in the lipid phase behavior of very complex lipid model systems and how this knowledge can be used to understand the deviation in lipid phase behavior in inflammatory skin diseases, iii) knowledge on the role of both, CER subclasses and chain length distribution, on lipid organization and lipid membrane permeability in complex and simple model systems with synthetic CERs, CHOL and FFAs, iv) similarity in lipid phase behavior in SC of different species and complex model systems, and vi) future directions in modulating lipid composition that is expected to improve the skin barrier in inflammatory skin diseases.

Effect of sphingosine and phytosphingosine ceramide ratio on lipid arrangement and barrier function in skin lipid models

The lipids in the uppermost layer of the skin, the stratum corneum (SC), play an important role in the skin barrier function. The three main subclasses in the SC lipid matrix are ceramides (CER), cholesterol, and free fatty acids. In inflammatory skin diseases, such as atopic dermatitis and psoriasis, the SC lipid composition is modulated compared to the composition in healthy SC. One of the main alterations is the molar ratio between the concentration of CER N-(tetracosanoyl)-sphingosine (CER NS) and CER N-(tetracosanoyl)-phytosphingosine (CER NP), which correlated with an impaired skin barrier function. In the present study, we investigated the impact of varying the CER NS:CER NP ratios on the lipid organization, lipid arrangement, and barrier functionality in SC lipid model systems. The results indicate that a higher CER NS:CER NP ratio as observed in diseased skin did not alter the lipid organization or lipid arrangement in the long periodicity phase encountered in SC. The trans-epidermal water loss, an indication of the barrier functionality, was significantly higher for the CER NS:CER NP 2:1 model (mimicking the ratio in inflammatory skin diseases) compared to the CER NS:CER NP 1:2 ratio (in healthy skin). These findings provide a more detailed insight into the lipid organization in both healthy and diseased skin and suggest that in vivo the molar ratio between CER NS:CER NP contributes to barrier impairment as well but might not be the main factor.

The effect of PPAR isoform (de)activation on the lipid composition in full-thickness skin models

Human skin equivalents (HSEs) are 3D-cultured human skin models that mimic many aspects of native human skin (NHS). Although HSEs resemble NHS very closely, the barrier located in the stratum corneum (SC) is impaired. This is caused by an altered lipid composition in the SC of HSEs compared with NHS. One of the most pronounced changes in this lipid composition is a high level of monounsaturation. One key enzyme in this change is stearoyl-CoA desaturase-1 (SCD1), which catalyses the monounsaturation of lipids. In order to normalize the lipid composition, we aimed to target a group of nuclear receptors that are important regulators in the lipid synthesis. This group of receptors are known as the peroxisome proliferating activating receptors (PPARs). By (de)activating each isoform (PPAR-α, PPAR-δ and PPAR-γ), the PPAR isoforms may have normalizing effects on the lipid composition. In addition, another PPAR-α agonist Wy14643 was included as this supplement demonstrated normalizing effects in the lipid composition in a more recent study. After PPAR (ant)agonists supplementation, the mRNA of downstream targets, lipid synthesis genes and lipid composition were investigated. The PPAR downstream targets were activated, indicating that the supplements reached the keratinocytes to trigger their effect. However, minimal impact was observed on the lipid composition after PPAR isoform (de) activation. Only the highest concentration Wy14643 resulted in strong, but negative effects on CER composition. Although the novel tested modifications did not result in an improvement, more insight is gained on the nuclear receptors PPARs and their effects on the lipid barrier in full-thickness skin models.

Phytosphingosine ceramide mainly localizes in the central layer of the unique lamellar phase of skin lipid model systems

Understanding the lipid arrangement within the skin's outermost layer, the stratum corneum (SC), is important for advancing knowledge on the skin barrier function. The SC lipid matrix consists of ceramides (CERs), cholesterol, and free fatty acids, which form unique crystalline lamellar phases, referred to as the long periodicity phase (LPP) and short periodicity phases. As the SC lipid composition is complex, lipid model systems that mimic the properties of native SC are used to study the SC lipid organization and molecular arrangement. In previous studies, such lipid models were used to determine the molecular organization in the trilayer structure of the LPP unit cell. The aim of this study was to examine the location of CER N-(tetracosanoyl)-phytosphingosine (CER NP) in the unit cell of this lamellar phase and compare its position with CER N-(tetracosanoyl)-sphingosine (CER NS). We selected CER NP as it is the most prevalent CER subclass in the human SC, and its location in the LPP is not known. Our neutron diffraction results demonstrate that the acyl chain of CER NP was positioned in the central part of the trilayer structure, with a fraction also present in the outer layers, the same location as determined for the acyl chain of CER NS. In addition, our Fourier transformed infrared spectroscopy results are in agreement with this molecular arrangement, suggesting a linear arrangement for the CER NS and CER NP. These findings provide more detailed insight into the lipid organization in the SC lipid matrix.

Increased Levels of Short-Chain Ceramides Modify the Lipid Organization and Reduce the Lipid Barrier of Skin Model Membranes

The skin barrier function is attributed to the stratum corneum (SC) intercellular lipid matrix, which is composed primarily of ceramides (CERs), free fatty acids, and cholesterol. These lipids are organized in two lamellar phases: the short and long periodicity phases (SPP and LPP), respectively. The LPP is considered important for the skin barrier function. High levels of short-chain CERs are observed in various inflammatory skin diseases and have been correlated with barrier dysfunction. In this research, we investigated how the increase in the fraction of the short-chain CER with a nonhydroxy C16 acyl chain linked to a C18 sphingosine base CER NS(C16) at the expense of the physiological chain length CER NS with a C24 acyl chain (CER NS(C24)) impacts the microstructure and barrier function of a lipid model that mimicked certain characteristics of the SC lipid organization. The permeability and lipid organization of the model membranes were compared with that of a control model without CER NS(C16). The permeability increased significantly when ≥50% of CER NS(C24) was substituted with CER NS(C16). Employing biophysical techniques, we showed that the lipid packing density reduced with an increasing proportion of CER NS(C16). Substitution of 75% of CER NS(C24) by CER NS(C16) resulted in the formation of phase-separated lipid domains and alteration of the LPP structure. Using deuterium-labeled lipids enabled simultaneous characterization of the C24 and C16 acyl chains in the lipid models, providing insight into the mechanisms underlying the reduced skin barrier function in diseased skin.

Improved organotypic skin model with reduced quantity of monounsaturated ceramides by inhibiting stearoyl-CoA desaturase-1

Full thickness models (FTM) are 3D in vitro skin cultures that resemble the native human skin (NHS) to a great extent. However, the barrier function of these skin models is reduced. The skin barrier is located in the stratum corneum (SC) and consists of corneocytes embedded in a lipid matrix. In this matrix, deviations in the composition of the FTMs lipid matrix may contribute to the impaired skin barrier when compared to NHS. One of the most abundant changes in lipid composition is an increase in monounsaturated lipids for which stearoyl-CoA desaturase-1 (SCD-1) is responsible. To improve the SC lipid composition, we reduced SCD-1 activity during the generation of the FTMs. These FTMs were subsequently assessed on all major aspects, including epidermal homeostasis, lipid composition, lipid organization, and barrier functionality. We demonstrate that SCD-1 inhibition was successful and resulted in FTMs that better mimic the lipid composition of FTMs to NHS by a significant reduction in monounsaturated lipids. In conclusion, this study demonstrates an effective approach to normalize SC monounsaturated lipid concentration and may be a valuable tool in further optimizing the FTMs in future studies.

High concentration of the ester-linked ω-hydroxy ceramide increases the permeability in skin lipid model membranes

The ester-linked ω-hydroxy acyl chain linked to a sphingosine base referred to as CER EOS is essential for the skin barrier lipid organization. While the majority of the skin lipids form a dense, crystalline structure, associated with low permeability, the unsaturated moiety of CER EOS, (either the linoleate or the oleate chain) exists in a liquid phase at the skin's physiological temperature. Thus, the relationship between CER EOS and barrier function is not entirely comprehended. We studied the permeability and lipid organization in skin lipid models, gradually increasing in CER EOS concentration, mixed with non-hydroxy sphingosine-based ceramide (CER NS) in an equimolar ratio of CERs, cholesterol, and free fatty acids (FFAs) mimicking the ratio in the native skin. A significant increase in the orthorhombic-hexagonal phase transition temperature was recorded when CER EOS concentration was raised to 70 mol% of the total CER content and higher, rendering a higher fraction of lipids in the orthorhombic phase at the expense of the hexagonal phase at physiological temperature. The model's permeability did not differ when CER EOS concentration ranged between 10 and 30% but increased significantly at 70% and higher. Using CER EOS with a perdeuterated oleate chain, it was shown that the fraction of lipids in a liquid phase increased with CER EOS concentration, while the neighboring CERs and FFAs remained in a crystalline state. The increased fraction of the liquid phase therefore, had a stronger effect on permeability than the increased fraction of lipids forming an orthorhombic phase.

Barrier Capability of Skin Lipid Models: Effect of Ceramides and Free Fatty Acid Composition

The skin is an effective barrier that prevents the influx of harmful substances from the environment and the efflux of body fluid. This barrier function is ascribed to the intercellular lipids present in the outermost layer of the skin referred to as the stratum corneum (SC). These lipids are composed mainly of ceramides (CERs), cholesterol, and free fatty acids (FFAs). Alterations in the SC lipid composition and barrier function impairment occur in several skin diseases including atopic dermatitis (AD). As the etiology of AD is multifactorial, establishing the relationship between the changes in SC lipid composition and barrier function impairment in the patients remains a challenge. Here, we employed model membrane systems to investigate the contribution of various anomalies in the SC CER and FFA composition observed in AD patients' skin to the barrier dysfunction. Using ethyl-p-aminobenzoate permeation and transepidermal water loss values as markers for barrier function, we determined that the alterations in SC lipid composition contribute to the impaired barrier function in AD patients. By the use of biophysical techniques, we established that the largest reduction in barrier capability was observed in the model with an increased fraction of short-chain FFAs, evident by the decrease in chain packing density. Modulations in the CER subclass composition impacted the lamellar organization while having a smaller effect on the barrier function. These findings provide evidence that AD therapies normalizing the FFA composition are at least as important as normalizing CER composition.

The effects of LXR agonist T0901317 and LXR antagonist GSK2033 on morphogenesis and lipid properties in full thickness skin models

Full thickness models (FTMs) are 3D-cultured human skin models that mimic many aspects of native human skin (NHS). However, their stratum corneum (SC) lipid composition differs from NHS causing a reduced skin barrier. The most pronounced differences in lipid composition are a reduction in lipid chain length and increased monounsaturated lipids. The liver-X-receptor (LXR) activates the monounsaturated lipid synthesis via stearoyl-CoA desaturase-1 (SCD-1). Therefore, the aim was to improve the SC lipid synthesis of FTMs by LXR deactivation. This was achieved by supplementing culture medium with LXR antagonist GSK2033. LXR agonist T0901317 was added for comparison. Subsequently, epidermal morphogenesis, lipid composition, lipid organization and the barrier functionality of these FTMs were assessed. We demonstrate that LXR deactivation resulted in a lipid composition with increased overall chain lengths and reduced levels of monounsaturation, whereas LXR activation increased the amount of monounsaturated lipids and led to a reduction in the overall chain length. However, these changes did not affect the barrier functionality. In conclusion, LXR deactivation led to the development of FTMs with improved lipid properties, which mimic the lipid composition of NHS more closely. These novel findings may contribute to design interventions to normalize SC lipid composition of atopic dermatitis patients.

New insight into phase behavior and permeability of skin lipid models based on sphingosine and phytosphingosine ceramides

The intercellular lipid matrix of the stratum corneum (SC), which consist mainly of ceramides (CERs), free fatty acids and cholesterol, is fundamental to the skin barrier function. These lipids assemble into two lamellar phases, known as the long and short periodicity phases (LPP and SPP respectively). The LPP is unique in the SC and is considered important for the skin barrier function. Alterations in CER composition, as well as impaired skin barrier function, are commonly observed in diseased skin, yet the understanding of this relationship remains insufficient. In this study, we have investigated the influence of non-hydroxy and α-hydroxy sphingosine-based CERs and their phytosphingosine counterparts on the permeability and lipid organization of model membranes, which were adjusted in composition to enhance formation of the LPP. The permeability was compared by diffusion studies using ethyl-p-aminobenzoate as a model drug, and the lipid organization was characterized by X-ray diffraction and infrared spectroscopy. Both the sphingosine- and phytosphingosine-based CER models formed the LPP, while the latter exhibited a longer LPP repeat distance. The ethyl-p-aminobenzoate flux across the sphingosine-based CER models was higher when compared to the phytosphingosine counterparts, contrary to the fact that the α-hydroxy phytosphingosine-based CER model had the lowest chain packing density. The unanticipated low permeability of the α-hydroxy phytosphingosine-based model is probably associated with a stronger headgroup hydrogen bonding network. Our findings indicate that the increased level of sphingosine-based CERs at the expense of phytosphingosine-based CERs, as observed in the diseased skin, may contribute to the barrier function impairment.

Interactions of dipalmitoylphosphatidylcholine with ceramide-based mixtures

The outermost layer of the skin, the stratum corneum (SC), acts as the natural physical barrier. The SC consists of corneocytes embedded in a crystalline lipid matrix consisting of ceramides, free fatty acids and cholesterol. Although phospholipids are frequently present in topical formulations, no detailed information is reported on the interactions between phospholipids and SC lipids. The aim of this study was to examine the interactions between a model phospholipid, dipalmitoylphosphatidylcholine (DPPC) and synthetic ceramide-based mixtures (referred to as SC lipids). (Perdeuterated) DPPC was mixed with SC lipids and the lipid organization and mixing properties were examined. The studies revealed that DPPC participates in the same lattice as SC lipids thereby enhancing a hexagonal packing. Even at a high DPPC level, no phase separated pure DPPC was observed. When a DPPC containing formulation is applied to the skin surface it must partition into the SC lipid matrix prior to any mixing with the SC lipids. To mimic this, DPPC was applied on top of a SC lipid membrane. DPPC applied in a liquid crystalline state was able to mix with the SC lipids and participated in the same lattice as the SC lipids. However, when DPPC was applied in a rippled gel-state very limited partitioning of DPPC into the SC lipid matrix occurred. Thus, when applied to the skin, liquid crystalline DPPC will have very different interactions with SC lipids than DPPC in a (rippled-)gel phase.

Phase behavior of skin lipid mixtures: the effect of cholesterol on lipid organization

The lipid matrix in the stratum corneum (SC), the upper layer of the skin, plays a critical role in the skin barrier. The matrix consists of ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). In human SC, these lipids form two coexisting crystalline lamellar phases with periodicities of approximately 6 and 13 nm. In the studies reported here, we investigated the effect of CHOL on lipid organization in each of these lamellar phases separately. For this purpose, we used lipid model mixtures. Our studies revealed that CHOL is imperative for the formation of each of the lamellar phases. At low CHOL levels, the formation of the lamellar phases was dramatically changed: a minimum 0.2 CHOL level in the CER/CHOL/FFA (1 : 0.2 : 1) mixture is required for the formation of each of the lamellar phases. Furthermore, CHOL enhances the formation of the highly dense orthorhombic lateral packing. The gradual increment of CHOL increases the fraction of lipids forming the very dense orthorhombic lateral packing. Therefore, these studies demonstrate that CHOL is an indispensable component of the SC lipid matrix and is of fundamental importance for appropriate dense lipid organization and thus important for the skin barrier function.

The role of ceramide chain length distribution on the barrier properties of the skin lipid membranes

The skin barrier function is provided by the stratum corneum (SC). The lipids in the SC are composed of three lipid classes: ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) which form two crystalline lamellar structures. In the present study, we investigate the effect of CER chain length distribution on the barrier properties of model lipid membranes mimicking the lipid composition and organization of SC. The membranes were prepared with either isolated pig CERs (PCERs) or synthetic CERs. While PCERs have a wide chain length distribution, the synthetic CERs are quite uniform in chain length. The barrier properties were examined by means of permeation studies using hydrocortisone as a model drug. Our studies revealed a reduced barrier in lipid membranes prepared with PCERs compared to synthetic CERs. Additional studies revealed that a wider chain length distribution of PCERs results in an enhanced hexagonal packing and increased conformational disordering of the lipid tails compared to synthetic CERs, while the lamellar phases did not change. This demonstrates that the chain length distribution affects the lipid barrier by reducing the lipid ordering and density within the lipid lamellae. In subsequent studies, the effect of increased levels of FFAs or CERs with a long acyl chain in the PCERs membranes was also studied. These changes in lipid composition enhanced the level of orthorhombic packing, reduced the conformational disordering and increased the barrier of the lipid membranes. In conclusion, the CER chain length distribution is an important key factor for maintaining a proper barrier.

Localization of cholesterol and fatty acid in a model lipid membrane: a neutron diffraction approach

The intercellular lipid matrix of the skin's stratum corneum serves to protect the body against desiccation and simultaneously limits the passage of drugs and other xenobiotics into the body. The matrix is made up of ceramides, free fatty acids, and cholesterol, which are organized as two coexisting crystalline lamellar phases. In studies reported here, we sought to use the technique of neutron diffraction, together with the device of isotopic (H/D) substitution, to determine the molecular architecture of the lamellar phase having a repeat distance of 53.9 ± 0.3 Å. Using hydrogenous samples as well as samples incorporating perdeuterated (C24:0) fatty acids and selectively deuterated cholesterol, the diffraction data obtained were used to construct neutron scattering length density profiles. By this means, the locations within the unit cell were determined for the cholesterol and fatty acids. The cholesterol headgroup was found to lie slightly inward from the unit cell boundary and the tail of the molecule located 6.2 ± 0.2 Å from the unit cell center. The fatty acid headgroups were located at the unit cell boundary with their acyl chains straddling the unit cell center. Based on these results, a molecular model is proposed for the arrangement of the lipids within the unit cell.

The important role of stratum corneum lipids for the cutaneous barrier function

The skin protects the body from unwanted influences from the environment as well as excessive water loss. The barrier function of the skin is located in the stratum corneum (SC). The SC consists of corneocytes embedded in a lipid matrix. This lipid matrix is crucial for the lipid skin barrier function. This paper provides an overview of the reported SC lipid composition and organization mainly focusing on healthy and diseased human skin. In addition, an overview is provided on the data describing the relation between lipid modulations and the impaired skin barrier function. Finally, the use of in vitro lipid models for a better understanding of the relation between the lipid composition, lipid organization and skin lipid barrier is discussed. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

The important role of stratum corneum lipids for the cutaneous barrier function

The skin protects the body from unwanted influences from the environment as well as excessive water loss. The barrier function of the skin is located in the stratum corneum (SC). The SC consists of corneocytes embedded in a lipid matrix. This lipid matrix is crucial for the lipid skin barrier function. This paper provides an overview of the reported SC lipid composition and organization mainly focusing on healthy and diseased human skin. In addition, an overview is provided on the data describing the relation between lipid modulations and the impaired skin barrier function. Finally, the use of in vitro lipid models for a better understanding of the relation between the lipid composition, lipid organization and skin lipid barrier is discussed. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Disposition of ceramide in model lipid membranes determined by neutron diffraction

The lipid matrix present in the uppermost layer of the skin, the stratum corneum, plays a crucial role in the skin barrier function. The lipids are organized into two lamellar phases. To gain more insight into the molecular organization of one of these lamellar phases, we performed neutron diffraction studies. In the diffraction pattern, five diffraction orders were observed attributed to a lamellar phase with a repeat distance of 5.4 nm. Using contrast variation, the scattering length density profile could be calculated showing a typical bilayer arrangement. To obtain information on the arrangement of ceramides in the unit cell, a mixture that included a partly deuterated ceramide was also examined. The scattering length density profile of the 5.4-nm phase containing this deuterated ceramide demonstrated a symmetric arrangement of the ceramides with interdigitating acyl chains in the center of the unit cell.

Infrared spectroscopy studies of mixtures prepared with synthetic ceramides varying in head group architecture: coexistence of liquid and crystalline phases

The barrier function of the skin is provided by the stratum corneum (SC), the outermost layer of the skin.Ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) are present in SC and form highly ordered crystalline lipid lamellae. These lamellae are crucial for a proper skin barrier function. In the present study,Fourier transform infrared spectroscopy was used to examine the lipid organization of mixtures prepared from synthetic CERs with CHOL and FFAs. The conformational ordering and lateral packing of these mixtures showed great similarities to the lipid organization in SC and lipid mixtures prepared with native CERs.Therefore, mixtures with synthetic CERs serve as an excellent tool for studying the effect of molecular architecture of CER subclasses on the lipid phase behavior. In SC the number of OH-groups in the head groups of CER subclasses varies. Furthermore, acylCERs with a linoleic acid chemically bound to a long acyl chain are also identified. The present study revealed that CER head group architecture affects the lateral packing and conformational ordering of the CER:CHOL:FFA mixtures. Furthermore, while the majority of the lipids form a crystalline packing, the linoleate moiety of the acylCERs participates in a "pseudo fluid" phase.

Interaction of Lipophilic Moisturizers on Stratum Corneum Lipid Domains in vitro and in vivo

Dry skin symptoms such as scaling and itching are often treated with lipophilic moisturizers. The aim of this study was to investigate the effect of lipophilic moisturizers on the stratum corneum (SC) ultra-structure and lipid organization. Lipophilic moisturizers were applied on the forearms of 4 healthy volunteers for 3 h. Subsequently, the application sites were tape stripped, and selected tape strips prepared for Freeze Fracture Electron Microscopy (FFEM), a method to visualize the SC intercellular lipid parallel to the skin surface. To investigate the effect of lipid moisturizers on the lipid lamellae, isolated SC was pretreated with the lipophilic moisturizers for 24 h prior to performing small angle X-ray diffraction (SAXD) measurements. Additionally, the lipid organization of mixtures prepared with ceramides, cholesterol, free fatty acids and lipophilic moisturizer in a 2:1:1:1 molar ratio were studied using SAXD. The FFEM data (in vivo) as well as the SAXD data (in vitro) show that the lipophilic moisturizers do not change the lipid lamellar organization in the SC. Addition of 20% m/m lipophilic moisturizer to the ceramide:cholesterol:free fatty acids mixture did not inhibit the formation of the long periodicity phase, the characteristic lamellar phase in the SC, even though there was clear evidence that two of the three moisturizers were at least partially incorporated in the long periodicity phase. Concluding, all findings suggest that the lipophilic moisturizers investigated in this study do not drastically change the lamellar organization of the SC intracellular lipid matrix, but that the moisturizers form separate domains in the SC, as was visualized by FFEM.

Infrared spectroscopic study of stratum corneum model membranes prepared from human ceramides, cholesterol, and fatty acids

The outermost layer of the skin, the stratum corneum, consists of corneocytes surrounded by lipid domains. The main lipid classes in stratum corneum are cholesterol, ceramides (CER), and free fatty acids forming two crystalline lamellar phases. However, only limited information is available on whether the various lipid classes participate in the same crystalline lattices or if separate domains are formed within the lipid lamellae. In this article infrared spectroscopic studies are reported of hydrated mixtures prepared from cholesterol, human CER, and free fatty acids. Evaluation of the methylene stretching vibrations revealed a conformational disordering starting at approximately 60 degrees C for all mixtures. Examination of the rotational ordering (scissoring and rocking vibrations) of mixtures prepared from equimolar cholesterol and CER with a variation in the level of free fatty acids showed that at lower free fatty acid content orthorhombic and hexagonal domains coexist in the lipid lamellae. Increasing the fatty acid level to an equimolar cholesterol/CER/fatty acid mixture reveals the dominant presence of an orthorhombic lattice, confirming x-ray diffraction studies. Replacing the protonated free fatty acid chains by their perdeuterated counterparts demonstrates that free fatty acids and CER participate in the same orthorhombic lattice up to a level of slightly less than 1:1:0.75 cholesterol/CER/free fatty acids molar ratio but that free fatty acids also form separate domains within the lipid lamellae at equimolar ratios at room temperature. However, no evidence for this has been observed at 32 degrees C. Extrapolating these findings to the situation in stratum corneum led us conclude that in stratum corneum, fatty acids and CER participate in the orthorhombic lattice at 32 degrees C, the skin temperature.

The phase behaviour of skin lipid mixtures based on synthetic ceramides

The lipid lamellae present in the outermost layer of the skin, the stratum corneum (SC), form the main barrier for diffusion of molecules across the skin. The main lipid classes in SC are cholesterol (CHOL), free fatty acids (FFA) and at least nine classes of ceramides (CER), referred to as CER1 to CER9. In the present study the phase behaviour of four synthetic CER, either single or mixed with CHOL or CHOL and FFA, has been studied using small and wide angle X-ray diffraction. The lipid mixtures showed complex phase behaviour with coexistence of several phases. The results further revealed that the presence of synthetic CER1 as well as a proper composition of the other CER in the mixture were crucial for the formation of a phase with a long periodicity, characteristic for SC lipid phase behaviour. Only a mixture containing synthetic CER1 and CER3, CHOL and FFA showed similar phase behaviour to that of SC.

Phase behavior of lipid mixtures based on human ceramides: coexistence of crystalline and liquid phases

The lipid regions in the outermost layer of the skin (stratum corneum) form the main barrier for diffusion of substances through the skin. In this layer the main lipid classes are ceramides, cholesterol (CHOL), and FFA. Previous studies revealed a coexistence of two crystalline lamellar phases with periodicities of approximately 13 nm (referred to as long periodicity phase) and 6 nm (short periodicity phase). Additional studies showed that lipid mixtures prepared with isolated pig ceramides (pigCER) mimic lipid phase behavior in stratum corneum closely. Because the molecular structure of pigCER differs in some important aspects from that of human ceramides (HCER), in the present study the phase behavior of mixtures prepared with HCER has been examined. Phase behavior studies of mixtures based on HCER revealed that in CHOL:HCER mixtures the long periodicity phase dominates. In the absence of HCER1 the short periodicity phase is dominant. Addition of FFA promotes the formation of the short periodicity phase and induces a transition from a hexagonal sublattice to an orthorhombic sublattice. Furthermore, the presence of FFA promotes the formation of a liquid phase. Finally, cholesterol sulfate, a minor but important lipid in the stratum corneum, reduces the amount of cholesterol that phase separates in crystalline domains. From these observations it can be concluded that the phase behavior of mixtures prepared from HCER differs in some important aspects from that prepared from pigCER. The most prevalent differences are the following: i) the addition of FFA promotes the formation of the short periodicity phase; and ii) liquid lateral packing is obviously present in CHOL:HCER:FFA mixtures. These changes in phase behavior might be due to a larger amount of linoleic acid moiety in HCER mixtures compared with that in pigCER mixtures.

The influence of two azones and sebaceous lipids on the lateral organization of lipids isolated from human stratum corneum

The main problem with topical application of compounds to administer drugs to and regulate drug levels in a human body, is the barrier formed by the intercellular lipid matrix of the stratum corneum (SC). In a search for possibilities to overcome this barrier function, a good understanding of the organization and phase behavior of these lipids is required. SC lipid model studies especially provide a wealth of information with respect to the lipid organization and the importance of certain subclasses of lipids for the structure. Previously, we have shown that electron diffraction (ED) provides detailed information on the lateral lipid packing in both intact SC (G.S.K. Pilgram et al., J. Invest. Dermatol. 113 (1999) 403) and SC lipid models (G.S.K. Pilgram et al., J. Lipid Res. 39 (1998) 1669). In the present study, we used ED to examine the influence of two azones and sebaceous lipids on the lateral phase behavior of lipids isolated from human SC. We established that human SC lipids are arranged in an orthorhombic packing pattern. Upon mixing with the two enhancers the orthorhombic packing pattern was still observed; however, an additional fluid phase became more apparent. In mixtures with sebaceous lipids, the presence of the hexagonal lattice increased. These findings provide a basis for the mechanism by which these enhancers and sebaceous lipids interact with human SC lipids.

The lipid organisation in the skin barrier

The main function of the skin is to protect the body against exogenous substances. The skin barrier is located in the outermost layer of the skin, the stratum corneum. This layer consists of keratin enriched cells embedded in lipid lamellae. These lamellae form the main barrier for diffusion of substances through the skin. In diseased skin the barrier function is often impaired. For a full understanding of the properties of the human skin barrier, insight in the stratum corneum lipid organisation is of great importance. In this paper a short description of the lipid organisation in normal human stratum corneum will be given, after which the role the main lipid classes play in the stratum corneum lipid organisation will be described. In addition the effect of cholesterol sulfate and calcium on the lipid organisation will be discussed. Finally a new model, the "sandwich model", will be proposed that describe the localisation of the fluid phases in the stratum corneum.

Phase behaviour of skin barrier model membranes at pH 7.4

The main function of the skin is to protect the body against exogenous substances. The skin barrier is located in the outermost layer of the skin, the stratum corneum (SC). This layer consists of keratin enriched cells embedded in lipid lamellae that form the main barrier for diffusion of substances through the skin. The main lipid classes in this barrier are ceramides, cholesterol and free fatty acids. Cholesterol sulfate and calcium are also present in SC. Furthermore it has been suggested that a pH gradient exists. In a previous paper the effect of cholesterol sulfate and calcium on the lipid phase behaviour of mixtures prepared from cholesterol, ceramides and free fatty acids at pH 5 was reported (approximate pH at the skin surface). In the present study the phase behaviour of mixtures prepared from cholesterol, ceramides and free fatty acids prepared at pH 7.4 (the pH of viable cells) has been examined between 25 and 95 degrees C. Our studies reveal that a reversed hexagonal phase has been formed at elevated temperatures. Addition of calcium inhibits the formation of the reversed hexagonal phase, while cholesterol sulfate promotes the presence of the reversed hexagonal phase at increased temperatures. From our results we can conclude that the lipid mixtures prepared at pH 5 resemble more closely the lipid phase behaviour in intact SC than the lipid mixtures prepared at pH 7.4.

Barrier characteristics of different human skin types investigated with X-ray diffraction, lipid analysis, and electron microscopy imaging

The stratum corneum requires ceramides, cholesterol, and fatty acids to provide the cutaneous permeability barrier. The lipids are organized in intercellular membranes exhibiting short- and long-periodicity lamellar phases. In recent years, the phase behavior of barrier lipid mixtures has been studied in vitro. The relationship of human stratum corneum lipid composition to membrane organization in vivo, however, has not been clearly established. Furthermore, the special function of the different ceramide species in the stratum corneum is largely unknown. We examined lipid organization and composition of stratum corneum sheets from different subtypes of healthy human skin (normal, dry, and aged skin). Lipid organization was investigated using X-ray diffraction and demonstrated that the 4.4 nm peak attributed to the long periodicity phase was frequently missing for skin with a low Cer(EOS)/Cer(total) ratio, indicating an important part for Cer(EOS), which contains omega-hydroxy fatty acid (O) ester-linked to linoleic acid (E) and amide-linked to sphingosine (S). A deficiency in the 4. 4 nm peak was predominantly observed in young dry skin. In one case of aged skin, however, and less often in young normal skin this peak was also missing. Furthermore, the ceramide composition of samples without the 4.4 nm peak showed a deficiency of Cer(EOH), which contains 6-hydroxy-4-sphingenine (H), and an increase in Cer(NS) and Cer(AS), which contain nonhydroxy (N) or alpha-hydroxy fatty acids (A). In addition, a 3.4 nm peak attributed to crystalline cholesterol occurred in most cases of aged and dry skin, but was not observed in young normal skin. Our results do not indicate a definite pattern of correlation between lipid organization and types of human skin. They demonstrate, however, that Cer(EOS) and Cer(EOH) are key elements for the molecular organization of the long periodicity lamellar phase in the human stratum corneum.

Cholesterol sulfate and calcium affect stratum corneum lipid organization over a wide temperature range

The main diffusion barrier for drugs penetrating through the skin is located in the intercellular lipid matrix in the upper layer of the skin, the stratum corneum (SC). The main lipid classes in the SC are ceramides (CER), free fatty acids (FFA) and cholesterol (CHOL). The lipids in SC are organized into two lamellar phases with periodicities of approximately 13 and 6 nm, respectively. Similar lipid organization has been found with equimolar CHOL:CER:FFA mixtures in SAXD studies performed at room temperature. However, one may conclude that the phase behavior of the mixtures is similar to that in SC only when the lipid organization of the lipid mixtures resembles that in SC over a wide temperature range. Therefore, in the present study, the organization of the lipid mixtures has been studied in a temperature range between 20 degrees and 95 degrees C. From these experiments it appeared that at elevated temperatures in equimolar CHOL:CER:FFA mixtures a new prominent 4.3 nm phase is formed between 35;-55 degrees C, which is absent or only weakly formed in intact human and pig SC, respectively. As it has been suggested that gradients of pH and cholesterol sulfate exist in the SC and that Ca(2+) is present only in the lowest SC layers, the effect of pH, cholesterol sulfate, and Ca(2+) on the lipid phase behavior has been investigated with lipid mixtures. Both an increase in pH from 5 (pH at the skin surface) to 7.4 (pH at the SC;-stratum granulosum interface) and the presence of cholesterol sulfate promote the formation of the 13 nm lamellar phase. Furthermore, cholesterol sulfate reduces the amount of CHOL that is present in crystalline domains, causes a shift in the formation of the 4.3 nm phase to higher temperatures, and makes this phase less prominent at higher temperatures. The finding that Ca(2+) counteracts the effects of cholesterol sulfate indicates the importance of a proper balance of minor SC components for appropriate SC lipid organization. In addition, when the findings are extrapolated to the in vivo situation, it seems that cholesterol sulfate is required to dissolve cholesterol in the lamellar phases and to stabilize SC lipid organization. Therefore, a drop in cholesterol sulfate content in the superficial layers of the SC is expected to destabilize the lipid lamellar phases, which might facilitate the desquamation process.

The role of ceramide composition in the lipid organisation of the skin barrier

The lipid lamellae in the stratum corneum (SC) play a key role in the barrier function of the skin. The major lipids are ceramides (CER), cholesterol (CHOL) and free fatty acids (FFA). In pig SC at least six subclasses of ceramides (referred to as CER 1, 2-6) are present. Recently it was shown that in mixtures of isolated pig SC ceramides (referred to as CER(1-6)) and CHOL two lamellar phases are formed, which mimic SC lipid organisation very closely [J.A. Bouwstra et al., 1996, J. Lipid Res. 37, 999-1011] [1]. Since the CER composition in SC originating from different sources/donors often varies, information on the effect of variations in CER composition on the SC lipid organisation is important. The results of the present study with mixtures of CHOL including two different CER mixtures that lack CER 6 (CER(1-5) mixtures) revealed that at an equimolar molar ratio their lipid organisation was similar to that of the equimolar CHOL:CER(1-6) and CHOL:CER(1,2) mixtures, described previously. These observations suggest that at an equimolar CHOL:CER ratio the lipid organisation is remarkably insensitive toward a change in the CER composition. Similar observations have been made with equimolar CHOL:CER:FFA mixtures. The situation is different when the CHOL:CER molar ratio varies. While in the CHOL:CER(1-6) mixture the lamellar organisation hardly changed with varying molar ratio from 0.4 to 2, the lamellar organisation in the CHOL:CER(1-5) mixtures appeared to be more sensitive to a change in the relative CHOL content, especially concerning the changes in the periodicities of the lamellar phases. In summary, these findings clearly indicate that at an equimolar CHOL:CER molar ratio the lamellar organisation is least sensitive to a variation in CER composition, while at a reduced CHOL:CER molar ratio the CER composition plays a more prominent role in the lamellar phases. This observation may have an implication for the in vivo situation when both the CER composition and the CHOL:CER molar ratio change simultaneously.

pH, cholesterol sulfate, and fatty acids affect the stratum corneum lipid organization

Lipid mixtures prepared from cholesterol (CHOL), isolated ceramides (CER), and free fatty acids can serve as attractive tools to study the role various stratum corneum (SC) lipids or microenvironmental conditions play in the SC lipid organization, as the phase behavior in these mixtures and in SC are similar: two lamellar phases with periodicities of approximately 6 and 13 nm are present. Because pH and cholesterol sulfate (CSO4) gradients exist in SC and may affect the local SC lipid organization, the effects of pH and CSO4 on lipid phase behavior was examined. X-ray diffraction studies with CHOL:CER mixtures revealed that the lamellar ordering at pH 5 and 7.4 were similar: both the short and the long periodicity phases were present. Upon addition of free fatty acids the phase behavior became pH dependent; the long periodicity phase being more dominant at pH 7.4 than at pH 5. Similar observations have been made upon addition of CSO4. Furthermore, only in the presence of CSO4 did phase-separated CHOL disappear, indicating that CHOL completely dissolves in the lamellar phases. A major phase change from an hexagonal to an orthorhombic lateral packing has been observed in the presence of free fatty acids. Furthermore, in the presence of CSO4 next to orthorhombic also liquid lateral packing could be detected. In contrast to lamellar ordering, changes in pH did not affect the lateral packing in any of the lipid mixtures studied.

Role of ceramide 1 in the molecular organization of the stratum corneum lipids

The main barrier of the skin is formed by the lipids in the apical skin layer, the stratum corneum (SC). In SC mainly ceramides (CER), free fatty acids (FFA) and cholesterol (CHOL) are present. The CER are composed of at least six different fractions. CER 1 has an exceptional molecular structure as it contains a linoleic acid linked to a long-chain omega-hydroxy acid (C > 30). The SC lipids are organized in two lamellar phases with periodicities of approximately 6 and 13 nm, respectively. Recent studies revealed that ceramides isolated from pig SC mixed with cholesterol in confined ratios mimic stratum corneum lipid phase behavior closely (Bouwstra, J.A., et al. 1996. J. Lipid Res. 37: 999-1011). In this paper the role of CER 1 for the SC lipid lamellar organization was studied. For this purpose lipid phase behavior of mixtures of CHOL and total ceramide fraction was compared with that of mixtures of CHOL and a ceramide mixture lacking CER 1. These studies showed that in the absence of CER 1 almost no long periodicity phase was formed over a wide CHOL/CER molar ratio. A model is proposed for the molecular arrangement of the two lamellar phases. This model is based on the dominant role CER 1 plays in the formation of the long periodicity phase, electron density distribution calculations, and observations, such as i) the bimodal distribution of the fatty acid chain lengths of the ceramides, ii) the phase separation between long-chain ceramides and short-chain ceramides in a monolayer approach, and iii) the absence of swelling of the lamellae upon increasing the water content organization in SC. In this molecular model the short periodicity phase is composed of only two high electron density regions indicating the presence of only one bilayer, similar to that often found in phospholipid membranes. The molecular arrangement in the long periodicity phase is very exceptional. This phase most probably consists of two broad and one narrow low electron density regions. The two broad regions are formed by partly interdigitating ceramides with long-chain fatty acids of approximately 24-26 C atoms, while the narrow low-electron density region is formed by fully interdigitating ceramides with a short free fatty acid chain of approximately 16 to 18 C atoms.

The formation of competent barrier lipids in reconstructed human epidermis requires the presence of vitamin C

Our analysis of epidermal lipids revealed that (glucosyl)ceramide profiles in various human skin equivalents are different from those of native tissue. The main difference is the reduced content in skin equivalents of ceramides 4-7 and especially the very low content of the most polar ceramides 6 and 7, which contain hydroxylated sphingoid base and/or fatty acid. To facilitate hydroxylation, the culture medium was supplemented with vitamins C and E. Although in vitamin E-supplemented medium lipogenesis was not affected, in vitamin C-supplemented medium the content of glucosylceramides and of ceramides 6 and 7 was markedly increased, both in the presence and absence of serum and irrespective the substrate used (inert or natural, populated or not with fibroblasts). The improvement of the lipid profile was accompanied by a marked improvement of the barrier formation as judged from extensive production of lamellar bodies, their complete extrusion at the stratum granulosum/stratum corneum interface, and the formation of multiple broad lipid lamellar structures in the intercorneocyte space. The presence of well-ordered lipid lamellar phases was confirmed by small-angle x-ray diffraction. Some differences between native and reconstructed epidermis, however, were noticed. Although the long-range lipid lamellar phase was present in both the native and the reconstructed epidermis, the short lamellar phase was present only in native tissue. It remains to be established whether these differences can be ascribed to small differences in relative amounts of individual ceramides, to differences in fatty acid profiles, or to differences in cholesterol sulfate, pH, or calcium gradients. The results indicate the key role vitamin C plays in the formation of stratum corneum barrier lipids.

A model membrane approach to the epidermal permeability barrier: an X-ray diffraction study

The permeability of mammalian skin is determined in large part by lamellar lipid domains packed between cells of the upper layer of the epidermis, the stratum comeum. Although these lamellae have features in common with typical biological membranes, they differ in having a lipid population composed mainly of ceramides, cholesterol, and free fatty acids. In our initial studies of the relationship between lipid composition and phase behavior in this unusual system, we used deuterium NMR [Kitson et al. (1994) Biochemistry 33, 6707-6715] to examine aqueous dispersions of nonhydroxylated bovine brain ceramide, cholesterol, and perdeuterated palmitic acid, and found complex phase behavior as a function of temperature and pH, whereas analogous dispersions in which sphingomyelin replaced ceramide resulted in spectra consistent with a fluid lamellar phase under the same conditions. To extend these observations, we examined the same dispersions at pH 5.2 by means of X-ray diffraction. The significant findings are as follows: (1) the ceramide dispersions form complex crystalline phases between room temperature and about 40 degrees C; (2) the majority of the crystalline cholesterol is not in a separate phase; and (3) the analogous sphingomyelin dispersions form a fluid lamellar phase under the same conditions. We conclude that ceramides, even in the presence of considerable mole fractions of cholesterol, can form crystalline lamellar structures. We suggest that the existence of such structures in stratum corneum may be important in the function of the epidermal permeability barrier, and that the interaction between ceramide and cholesterol in other biological membranes may result in regions having unique physical properties.

Electroperturbation of the human skin barrier in vitro: II. Effects on stratum corneum lipid ordering and ultrastructure

In transdermal iontophoresis, drugs can be driven across the skin by electrorepulsion, but their transport can also be enhanced by electrical perturbation of the skin barrier. Our objective was to study perturbing effects of electrical current on human stratum corneum lipid fine structure combining techniques including freeze-fracture electron microscopy. Human stratum corneum was subjected to pulsed constant currents, varying from 0.013-13 mA.cm-2. The voltage across the stratum corneum was high-frequency-sampled and s.c. impedence values derived from it. Upon termination of the current, skin samples were rapidly frozen and processed for freeze-fracture electron microscopy or subjected to X-ray diffraction analysis. Initially a rapid decrease of the resistance and, overall, a rapid increase of the capacitances was observed; generally, these effects became more pronounced with increasing current density. Wide- and small-angle X-ray diffractograms of human stratum corneum exposed for 1 h to the highest current indicated a disordering of both the lateral packaging arrangement and long-range lamellar stacking of the intercellular lipids of stratum corneum. Furthermore, an increase in the stratum corneum hydration level as a result of electrical current application was observed. On electron micrographs of freeze-fracture replicas of human stratum corneum, exposed for 1 h to current densities between 0.013 and 13 mA.cm-2, perturbations of the intercellular lipid structure were observed in accordance with the results of X-ray diffraction; these perturbations aggravated with increasing current density. Together, the data suggest that both the lateral and the longitudinal disordering of the intercellular lipids observed with X-ray diffraction may be responsible for the appearance of perturbed structures observed with freeze-fracture electron microscopy. The lipid disordering may be due to polarization of the lipid head groups induced by the electrical field, followed by mutual repulsion.

The role of ceramides 1 and 2 in the stratum corneum lipid organisation

A mixture of ceramide 1 and ceramide 2 (CER(1 + 2)) was isolated from pig stratum corneum and mixed in various molar ratios with cholesterol (CHOL) or with CHOL and palmitic acid (PA). The mixtures were hydrated in a buffer solution of pH 5.0 and their phase behaviour was studied by wide- and small-angle X-ray diffraction. The small-angle diffraction curve of the CHOL/CER(1 + 2) mixture at a molar ratio of 0.4 revealed the presence of only one peak at a spacing of 6.7 nm. Increasing the amount of CHOL to a molar ratio of 0.6 was accompanied by a shift of this peak to a smaller spacing (5.7 nm) and the appearance of two weak peaks at 11.8 and 4.1 nm spacings. Increasing the CHOL content to an equimolar ratio resulted in the appearance of two lamellar phases with periodicities of 5.5 and 12 nm, respectively. In a CHOL/CER(1 + 2) mixture at a molar ratio of 2 the periodicities of the two phases were 5.6 and 12 nm, respectively. From these observations it was concluded that the CHOL/CER(1 + 2) mixtures exerted similar phase behaviour, as reported earlier for intact SC (Bouwstra et al. (1995) J. Lipid Res. 36, 496-504) and for mixtures (Bouwstra et al. (1996) J. Lipid Res., in press) prepared from CHOL and total ceramide fraction (CER) isolated from pig stratum corneum. However, in the CHOL/CER mixtures a lower relative amount of CHOL was required to acquire these lamellar phases, indicating that at low CHOL contents, CER 3, 4, 5 and 6 play a crucial role in the formation of the lamellar phases. Furthermore, the solubility of CHOL in the mixtures increased in the presence of CER 1, suggesting its important role for the barrier function of the skin. When palmitic acid (PA) was included, the phase behaviour of the CHOL/CER(1 + 2)/PA mixture was more complex. Next to two lamellar phases, an additional phase with a spacing of 3.77 nm was observed, never seen in intact stratum corneum. In the absence of CHOL, the wide-angle diffraction pattern of the CER(1 + 2) revealed one sharp reflection at 0.456 nm and two diffuse reflections at 0.430, 0.417 nm and 0.395 nm, indicating the presence of a crystalline sublattice. In an equimolar mixture of CHOL/CER(1 + 2) no sharp 0.456 nm reflection was observed indicating a more disordered packing. Furthermore, phase separation of CHOL occurred, this conclusion is based on the presence of reflections corresponding to polycrystalline cholesterol monohydrate. These findings indicate that the lateral packing of mixtures of CHOL/CER(1 + 2) is more complex than that of the CHOL/CER mixtures that reveals a hexagonal lateral packing.

Phase behavior of isolated skin lipids

Ceramides were isolated from the pig stratum corneum (SC) and mixed in varying molar ratios with either cholesterol or with cholesterol and free fatty acids. The phase behavior of the mixtures was studied by small-(SAXD) and wide-angle (WAXD) X-ray diffraction. Ceramides alone did not exhibit a long range ordering. Upon addition of cholesterol to ceramides, lamellar phases were formed and a hexagonal lateral packing was detected similar to that seen in intact SC. At a cholesterol/ceramide molar ratio of 0.1, only one reflection at 5.9 nm was observed. At a cholesterol/ceramide molar ratio of 0.2, three reflections corresponding to 12.3, 5.56, and 4.26 nm appeared. The reflections were based on two phases. Increasing the cholesterol/ceramide ratio to 0.4, the peak positions were slightly shifted. The diffraction pattern revealed the presence of two lamellar phases with periodicities of 12.2 and 5.2 nm, respectively. The positions of the peaks remained unchanged when the cholesterol/ceramide ratio was increased up to 1.0. At a cholesterol/ceramide molar ratio of 2.0, the intensity of various peaks based on the 12.2 nm phase decreased in intensity. The phase behavior of the cholesterol/ceramide mixtures in a ratio between 0.4 and 1.0 was very similar to that found in intact pig SC in which two lamellar phases with periodicities of 6.0 and 13.2 nm are present. Our data further indicate that the formation of the 5.2 nm lamellar phase requires a higher cholesterol content than the formation of the 12.2 nm lamellar phase. Furthermore, when the relative amount of cholesterol is very high, the 5.2 nm phase is the most pronounced one. Addition of free fatty acids increased the solubility of cholesterol, indicating the role free fatty acids may play for the skin barrier function. The phase behavior of cholesterol/ceramide/fatty acid mixtures was found to be dependent on the chain length of fatty acids used. Namely, addition of short-chain free fatty acids (C14-C18) did not change the periodicity of the 12.2 and 5.2 nm phases, but induced the formation of an additional 4.2 nm phase. In the presence of long-chain free fatty acids (C16-C26), the periodicity of the lamellar phases was slightly increased (to 13.0 and 5.3 nm, respectively) but no additional 4.2 nm phase was formed. These results indicate that the lipid phase behavior of the cholesterol/ceramide/free fatty acid mixtures closely mimics that of the intact stratum corneum only in the presence of long-chain free fatty acids.

Reduced skin barrier function parallels abnormal stratum corneum lipid organization in patients with lamellar ichthyosis

Most patients with autosomal recessive lamellar ichthyosis are known to have markedly impaired skin barrier function. We hypothesize that this may be due to imperfections in the composition and fine structure of the intercellular stratum corneum lipids. The aim of the present study was to test this hypothesis. To characterize the barrier properties in three female patients with lamellar ichthyosis, the following parameters were used and compared with those of healthy volunteers: transepidermal water loss, stratum corneum lipid profiles after topical acetone/ether extraction on the flexure side of the forearm, and small-angle x-ray diffraction. The extracted lipids were separated using high performance thin-layer chromatography and quantified, and the ceramide profile was determined. Small-angle x-ray diffraction was used to obtain information on the molecular structure and organization of the intercellular lipid domains of stratum corneum using stratum corneum scales collected by scraping. Transepidermal water loss was significantly increased in all three patients. Lipid analysis showed significant differences in the relative amounts of ceramide fractions 2-3a-3b-4-5, free fatty acid-ceramide ratio, and free fatty acid-cholesterol ratio. Small-angle x-ray diffraction showed smaller repeated distances of lipid bilayers in stratum corneum samples of the patients compared with the healthy volunteers. An additional diffraction peak was found in the patients compared with the healthy volunteers, which can be ascribed to crystalline cholesterol. These data suggest that there might be a relation between the impaired barrier function and stratum corneum lipid structural and composition changes.

Lipid organization in pig stratum corneum

The lipid and keratin structure of pig stratum corneum has been elucidated by small- and wide-angle X-ray diffraction. The measurements were carried out as a function of hydration and temperature. In addition, the stratum corneum was measured after recrystallization of the lipids at various temperatures. The results led us to conclude that the intercellular lipids in the stratum corneum are organized in at least two different lamellar structures with repeat distances of 6 and 13.2 nm. There is an indication for the presence of a third phase with a periodicity of 9 nm. The wide-angle pattern revealed a hexagonal (0.414 nm spacing) and liquid lateral packing (approximately 0.46 nm spacing). The 0.414 nm reflection started to decrease in intensity between 60 and 66 degrees C and disappeared between 72 and 95 degrees C. Furthermore, crystalline cholesterol has been indicated by both, wide- and small-angle X-ray diffraction, while the reflections of alpha-keratin were observed in the wide-angle X-ray diffraction pattern.

Characterization of stratum corneum structure in reconstructed epidermis by X-ray diffraction

The intercellular lipid regions in the stratum corneum (SC), the outermost layer of the skin, form the major barrier for diffusion of substances through the skin. The barrier function of in vitro reconstructed epidermis is still impaired. With respect to further optimization of the model, it is necessary to characterize its stratum corneum lipid structure. In this study, small and wide angle X-ray diffraction were used to characterize the lipid organization in stratum corneum isolated from 14-day-old reconstructed epidermis. The measurements were carried out at room temperature, and subsequently as a function of temperature between 25 degrees C and 109 degrees C, followed by measurements after cooling to room temperature. The results of the X-ray diffraction measurements revealed the following in reconstructed epidermis. 1) The lamellar ordering of stratum corneum lipids was much lower than that observed in native stratum corneum. 2) Crystalline anhydrous cholesterol was present. 3) Orthorhombic packing was present, but the corresponding reflections were very weak. The orthorhombic packing disappeared between 30 degrees C and 45 degrees C. 4) A hexagonal packing was present and disappeared between 60 degrees C and 75 degrees C. 5) Soft keratin is present. 6) A higher extent of lamellar ordering could be achieved by heating to 109 degrees C and cooling down to room temperature. Analysis of SC lipids revealed the presence of high amounts of triglycerides, the level of which could be decreased by lowering the glucose content. However, modulation of culture medium composition did not significantly affect lipid lamellae structures or hydrocarbon chain packing.

The lipid and protein structure of mouse stratum corneum: a wide and small angle diffraction study

The structure of mouse stratum corneum was investigated using small and wide angle X-ray scattering. Diffraction patterns were collected as a function of temperature and hydration. The lipid lamellar structure is characterized by a repeat distance of 13.4 nm. Occasionally a second lipid lamellar phase has been found with a repeat distance of 6.1 nm. Upon hydration neither swelling of the lamellae nor lateral swelling of the lipids was found. On the basis of these facts it was concluded that the size of the crystallographic unit cell of the lipid structure is insensitive to the water content. The 13.4 nm lamellar phase disappeared upon heating to 55 degrees C. At 45 degrees C the orthorhombic lateral packing disappeared. At this temperature only an hexagonal and liquid lateral packing of the lipids was observed. The hexagonal lateral packing transformed to a liquid one between 45 degrees C and 80 degrees C. Model calculations were carried out to obtain the electron density profile of the lamellar structure. In all models three electron lucent regions were fitted between which electron dense regions are located indicating that the 13.4 nm lamellar structure consist of three bilayers.

Small angle X-ray scattering: possibilities and limitations in characterization of vesicles

The use of small angle X-ray scattering (SAXS) for characterization of lipid vesicle dispersions is described. The effect of curvature of the membrane, the presence of proteins in the core and on the surface of the membrane, variations in membrane thickness and distribution in the number of bilayers of the vesicles in the dispersion on the scattering curve is discussed. Concerning unilamellar vesicles, either the membrane curvature of vesicles smaller than 50 nm or variations in membrane thickness result in a disappearance of the first node in the scattering curve, even if the bilayer is symmetric with respect to the electron density distribution. In the case of dispersion in which unilamellar as well as multilamellar vesicles are present it is shown that a small fraction of multilamellar liposomes changes the scattering curve dramatically. Liposomes were prepared from various compositions of dipalmitoylphosphatidylcholine (DPPC) and cholesterol hemisuccinate (CHEMS) by the film method. The electron density profile of the bilayers and distribution in the number of bilayers of the liposome dispersions were determined. The average number of bilayers increased as a function of the decrease in CHEMS content. Liposomes with higher CHEMS content than 10 mol% were unilamellar. It seems that increase in charge intercalated in the bilayers resulted in unilamellar vesicles.

Structural investigations of human stratum corneum by small-angle X-ray scattering

The structure of human stratum corneum was investigated with small-angle X-ray scattering (SAXS). At room temperature the scattering curve was characterized by a strong intensity at low scattering vector (Q less than 0.8 nm-1) and two complicated diffraction peaks originating from a lamellar structure of the lipids. The lamellar lipid structure in the stratum corneum transformed to a disordered structure between 65 degrees C and 75 degrees C, the same temperature region at which a thermal lipid transition occurred. After cooling down to room temperature a recrystallization of at least a part of the lipids took place, after which only one unit cell with a repeat distance of 13.4 nm could be detected. Comparison of the scattering curve of the stratum corneum after crystallization with the scattering curve of the stratum corneum before recrystallization leads to the conclusion that in the original curve the lipids are arranged in two unit cells with repeat distances of 6.4 nm and 13.4 nm. From model calculations it appears that the latter unit cell consists of more than one bilayer. The scattering curves of stratum corneum hydrated to various levels were measured. A change in the water content of stratum corneum between 6% w/w and 60% w/w (fully hydrated) did not result in swelling of the bilayers, but the scattering curve obtained with stratum corneum hydrated to 60% w/w differed from those at lower hydration levels: the scattering curve at 60% w/w showed only the diffraction peaks corresponding to a unit cell with a repeat distance of 6.4 nm. This observation implies that the ordering of a part of the lipids is reduced at very high water contents, which may explain the strong penetration-enhancing effects of water in the stratum corneum.