The importance of ceramide headgroup for lipid localisation in skin lipid models

The stratum corneum's lipid matrix is a critical for the skin's barrier function and is primarily composed of ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). The lipids form a long periodicity phase (LPP), a unique trilayer unit cell structure. An enzyme driven pathway is implemented to synthesize these key lipids. If these enzymes are down- or upregulated as in inflammatory diseases, the final lipid composition is affected often altering the barrier function. In this study, we mimicked down regulation of enzymes involved in the synthesis of the sphingosine and CER amide bond. In a LPP lipid model, we substituted CER N-(tetracosanoyl)-sphingosine (CER NS) with either i) FFA C24 and free sphingosine, to simulate the loss of the CER amide bond, or ii) with FFA C24 and C18 to simulate the loss of the sphingosine headgroup. Our study shows the lipids in the LPP would not phase separate until at least 25% of the CER NS is substituted keeping the lateral packing and conformational ordering unaltered. Neutron diffraction studies showed that free sphingosine chains localized at the outer layers of the unit cell, while the remaining CER NS head group was concentrated in the inner headgroup layers. However, when FFA C18 was inserted, CER NS was dispersed throughout the LPP, resulting in an even distribution between the inner and outer water layers. The presented results highlight the importance of the CER NS headgroup structure and its interaction in combination with the carbon chain invariability for optimal lipid arrangement.

Effects of ozone on stratum corneum lipid integrity and assembly

The stratum corneum (SC) acts as the main barrier of the skin against exogenous substances (e.g. air pollutants) and against the loss of endogenous substances such as water. The SC consists of keratin-rich dead cells surrounded by crystalline lamellar lipid regions. The main lipid classes are ceramides (CERs), free fatty acids (FFAs), and cholesterol (CHOL). Tropospheric ozone (O₃) is a potent oxidant compound that reacts instantly with biological molecules such as lipids and proteins. Although it has been reported that O₃ induces biological responses at the cellular level, to the best of our knowledge, there is no information related to the damages O₃ can cause at the level of the SC extracellular lipid matrix. The aim of our work was to investigate which SC lipid subclasses are prone to oxidation when exposed to O₃ and how the changes in chemical structures affect the lipid organization in a stratum corneum substitute (SCS) membrane. Ultimately the barrier properties of the SCS were examined. Our studies reveal that O₃ induces chemical modifications of the unsaturated bonds in CERs and CHOL. The appearance of carbonyl groups at the headgroup level and the removal of the linoleate moiety of omega acylceramides (CER EOS) impact the lamellar organization of the lipid assembly and to a lesser extent the lateral packing of the lipids. Unexpectedly, the modifications improved the barrier function of the SCS.

Multitargeted Approach for the Optimization of Morphogenesis and Barrier Formation in Human Skin Equivalents

In vitro skin tissue engineering is challenging due to the manifold differences between the in vivo and in vitro conditions. Yet, three-dimensional (3D) human skin equivalents (HSEs) are able to mimic native human skin in many fundamental aspects. However, the epidermal lipid barrier formation, which is essential for the functionality of the skin barrier, remains compromised. Recently, HSEs with an improved lipid barrier formation were generated by (i) incorporating chitosan in the dermal collagen matrix, (ii) reducing the external oxygen level to 3%, and (iii) inhibiting the liver X receptor (LXR). In this study, we aimed to determine the synergic effects in full-thickness models (FTMs) with combinations of these factors as single-, double-, and triple-targeted optimization approaches. The collagen–chitosan FTM supplemented with the LXR inhibitor showed improved epidermal morphogenesis, an enhanced lipid composition, and a better lipid organization. Importantly, barrier functionality was improved in the corresponding approach. In conclusion, our leading optimization approach substantially improved the epidermal morphogenesis, barrier formation, and functionality in the FTM, which therefore better resembled native human skin.

The Importance of Free Fatty Chain Length on the Lipid Organization in the Long Periodicity Phase

The skin's barrier ability is an essential function for terrestrial survival, which is controlled by intercellular lipids within the stratum corneum (SC) layer. In this barrier, free fatty acids (FFAs) are an important lipid class. As seen in inflammatory skin diseases, when the lipid chain length is reduced, a reduction in the barrier's performance is observed. In this study, we have investigated the contributing effects of various FFA chain lengths on the lamellar phase, lateral packing. The repeat distance of the lamellar phase increased with FFA chain length (C20-C28), while shorter FFAs (C16 to C18) had the opposite behaviour. While the lateral packing was affected, the orthorhombic to hexagonal to fluid phase transitions were not affected by the FFA chain length. Porcine SC lipid composition mimicking model was then used to investigate the proportional effect of shorter FFA C16, up to 50% content of the total FFA mixture. At this level, no difference in the overall lamellar phases and lateral packing was observed, while a significant increase in the water permeability was detected. Our results demonstrate a FFA C16 threshold that must be exceeded before the structure and barrier function of the long periodicity phase (LPP) is affected. These results are important to understand the lipid behaviour in this unique LPP structure as well as for the understanding, treatment, and development of inflammatory skin conditions.

Arrangement of Ceramides in the Skin: Sphingosine Chains Localize at a Single Position in Stratum Corneum Lipid Matrix Models

Understanding the structure of the stratum corneum (SC) is essential to understand the skin barrier process. The long periodicity phase (LPP) is a unique trilayer lamellar structure located in the SC. Adjustments in the composition of the lipid matrix, as in many skin abnormalities, can have severe effects on the lipid organization and barrier function. Although the location of individual lipid subclasses has been identified, the lipid conformation at these locations remains uncertain. Contrast variation experiments via small-angle neutron diffraction were used to investigate the conformation of ceramide (CER) N-(tetracosanoyl)-sphingosine (NS) within both simplistic and porcine mimicking LPP models. To identify the lipid conformation of the twin chain CER NS, the chains were individually deuterated, and their scattering length profiles were calculated to identify their locations in the LPP unit cell. In the repeating trilayer unit of the LPP, the acyl chain of CER NS was located in the central and outer layers, while the sphingosine chain was located exclusively in the middle of the outer layers. Thus, for the CER NS with the acyl chain in the central layer, this demonstrates an extended conformation. Electron density distribution profiles identified that the lipid structure remains consistent regardless of the lipid's lateral packing phase, this may be partially due to the anchoring of the extended CER NS. The presented results provide a more detailed insight on the internal arrangement of the LPP lipids and how they are expected to be arranged in healthy skin.

One Peptide for Them All: Gold Nanoparticles of Different Sizes Are Stabilized by a Common Peptide Amphiphile

The functionalization of gold nanoparticles (GNPs) with peptidic moieties can prevent their aggregation and facilitate their use for applications both in vitro and in vivo. To date, no peptide-based coating has been shown to stabilize GNPs larger than 30 nm in diameter; such particles are of interest for applications including vaccine development, drug delivery, and sensing. Here, GNPs with diameters of 20, 40, and 100 nm are functionalized with peptide amphiphiles. Using a combination of transmission electron microscopy, UV-vis spectroscopy, and dynamic light scattering, we show that GNPs up to 100 nm in size can be stabilized by these molecules. Moreover, we demonstrate that these peptide amphiphiles form curvature-dependent, ordered structures on the surface of the GNPs and that the GNPs remain disperse at high-salt concentrations and in the presence of competing thiol-containing molecules. These results represent the development of a peptide amphiphile-based coating system for GNPs which has the potential to be beneficial for a wide range of biological applications, in addition to image enhancement and catalysis.

Hyperalphalipoproteinemic scavenger receptor BI knockout mice exhibit a disrupted epidermal lipid barrier

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of cholesteryl esters (CE) from high-density lipoproteins (HDL). An impaired SR-BI function leads to hyperalphalipoproteinemia with elevated levels of cholesterol transported in the HDL fraction. Accumulation of cholesterol in apolipoprotein B (apoB)-containing lipoproteins has been shown to alter skin lipid composition and barrier function in mice. To investigate whether these hypercholesterolemic effects on the skin also occur in hyperalphalipoproteinemia, we compared skins of wild-type and SR-BI knockout (SR-BI^-/-) mice. SR-BI deficiency did not affect the epidermal cholesterol content and induced only minor changes in the ceramide subclasses. The epidermal free fatty acid (FFA) pool was, however, enriched in short and unsaturated chains. Plasma CE levels strongly correlated with epidermal FFA C18:1 content. The increase in epidermal FFA coincided with downregulation of cholesterol and FFA synthesis genes, suggesting a compensatory response to increased flux of plasma cholesterol and FFAs into the skin. Importantly, the SR-BI^-/- epidermal lipid barrier showed increased permeability to ethyl-paraminobenzoic acid, indicating an impairment of the barrier function. In conclusion, increased HDL-cholesterol levels in SR-BI^-/- mice can alter the epidermal lipid composition and lipid barrier function similarly as observed in hypercholesterolemia due to elevated levels of apoB-containing lipoproteins.

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.

Preferential arrangement of lipids in the long-periodicity phase of a stratum corneum matrix model

The lipid matrix of the stratum corneum, the outermost skin layer, consists primarily of ceramides, cholesterol, and FFAs. These lipids form a trilayer long-periodicity phase (LPP) that is unique to this barrier. Knowledge about the LPP is essential in understanding the barrier function. Previous studies of LPP lipid models have identified the position of the major lipid classes and suggested that a large fraction of FFAs and the ceramide acyl chain are present in the central region. However, the precise arrangement, such as lipid subclass mixing (isolated or mixed) and ceramide conformation (extended or hairpin), remains unknown. Here, we deuterated FFAs and the ceramide acyl chain to study CD₂ and CH₂ interactions with Fourier-transform infrared spectroscopy. The ceramide and FFAs of various chain lengths were not in separate domains but had mixed together. The larger number of CD₂-CD₂ lipid chain interactions in the LPP than in a symmetrical bilayer structure implied that the ceramide had primarily adopted an extended conformation. Shorter FFAs were present in the central region of the LPP. This model explores the biophysical properties of the stratum corneum's LPP to improve the understanding of the barrier function of this layer.

Degree of Skin Barrier Disruption Affects Lipid Organization in Regenerated Stratum Corneum

Previously, a skin barrier repair model was developed to examine the effect of formulations on the lipid properties of compromised skin. In this model, the lipid organization mimics that of several skin diseases with impaired skin barrier and less dense lateral lipid organization. In addition, parakeratosis was occasionally observed. The present study investigated whether the extent of initial barrier disruption affects lipid organization and parakeratosis in regenerated stratum corneum. After barrier disruption and stratum corneum regeneration the fraction of lipids adopting a less dense lateral organization gradually increased with increasing degree of barrier disruption. Only when 75% of the stratum corneum was removed, were parakeratosis and a change in lamellar organization observed. This demonstrates the possibility of using the skin barrier repair model to study the effects of formulations on compromised skin in which the presence of parakeratosis and lipid organization can be modified by the extent of barrier disruption.

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.

Topically Applied Ceramides Interact with the Stratum Corneum Lipid Matrix in Compromised Ex Vivo Skin

PURPOSE

To determine whether formulations containing ceramides (including a ceramide with a long hydroxyl acyl chain linked to a linoleate, CER EOS) and fatty acids are able to repair the skin barrier by normalizing the lipid organization in stratum corneum (SC).

METHODS

The formulations were applied on a skin barrier repair model consisting of ex vivo human skin from which SC was removed by stripping. The effect of formulations on the lipid organization and conformational ordering in the regenerated SC were analyzed using Fourier transform infrared spectroscopy and small angle X-ray diffraction.

RESULTS

Application of the formulation containing only one ceramide on regenerating SC resulted in a higher fraction of lipids adopting an orthorhombic organization. A similar fraction of lipids forming an orthorhombic organization was observed after application of a formulation containing two ceramides and a fatty acid on regenerating SC. No effects on the lamellar lipid organization were observed.

CONCLUSIONS

Application of a formulation containing either a single ceramide or two ceramides and a fatty acid on regenerating SC, resulted in a denser lateral lipid packing of the SC lipids in compromised skin. The strongest effect was observed after application of a formulation containing a single ceramide.

Free fatty acids chain length distribution affects the permeability of skin lipid model membranes

The lipid matrix in the stratum corneum (SC) plays an important role in the barrier function of the skin. The main lipid classes in this lipid matrix are ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs). The aim of this study was to determine whether a variation in CER subclass composition and chain length distribution of FFAs affect the permeability of this matrix. To examine this, we make use of lipid model membranes, referred to as stratum corneum substitute (SCS). We prepared SCS containing i) single CER subclass with either a single FFA or a mixture of FFAs and CHOL, or ii) a mixture of various CER subclasses with either a single FFA or a mixture of FFAs and CHOL. In vitro permeation studies were performed using ethyl-p-aminobenzoic acid (E-PABA) as a model drug. The flux of E-PABA across the SCS containing the mixture of FFAs was higher than that across the SCS containing a single FA with a chain length of 24 C atoms (FA C24), while the E-PABA flux was not effected by the CER composition. To select the underlying factors for the changes in permeability, the SCSs were examined by Fourier transform infrared spectroscopy (FTIR) and Small angle X-ray scattering (SAXS). All lipid models demonstrated a similar phase behavior. However, when focusing on the conformational ordering of the individual FFA chains, the shorter chain FFA (with a chain length of 16, 18 or 20 C atoms forming only 11m/m% of the total FFA level) had a higher conformational disordering, while the conformational ordering of the chains of the CER and FA C24 and FA C22 hardly did not change irrespective of the composition of the SCS. In conclusion, the conformational mobility of the short chain FFAs present only at low levels in the model SC lipid membranes has a great impact on the permeability of E-PABA.

Skin lipids: localization of ceramide and fatty acid in the unit cell of the long periodicity phase

The lipid matrix of the skin's stratum corneum plays a key role in the barrier function, which protects the body from desiccation. The lipids that make up this matrix consist of ceramides, cholesterol, and free fatty acids, and can form two coexisting crystalline lamellar phases: the long periodicity phase (LPP) and the short periodicity phase (SPP). To fully understand the skin barrier function, information on the molecular arrangement of the lipids in the unit cell of these lamellar phases is very desirable. To determine this arrangement in previous studies, we examined the molecular arrangement of the SPP. In this study, neutron diffraction studies were performed to obtain information on the molecular arrangement of the LPP. The diffraction pattern reveals nine diffraction orders attributed to the LPP with a repeating unit of 129.4 ± 0.5 Å. Using D2O/H2O contrast variation, the scattering length density profiles were calculated for protiated samples and samples that included either the perdeuterated acyl chain of the most abundant ceramide or the most abundant perdeuterated fatty acid. Both perdeuterated chains are predominantly located in the central part of the unit cell with substantial interdigitation of the acyl chains in the unit cell center. However, a fraction of the perdeuterated chains is also located near the border of the unit cell with their acyl chains directing toward the center. This arrangement of lipids in the LPP unit cell corresponds with the location of their lipid headgroups at the border and also inside of the unit cell at a well-defined position (±21 Å from the unit cell center), indicative of a three-layer lipid arrangement within the 129.4 ± 0.5 Å repeating unit.

Modulation of stratum corneum lipid composition and organization of human skin equivalents by specific medium supplements

Our in-house human skin equivalents contain all stratum corneum (SC) barrier lipid classes, but have a reduced level of free fatty acids (FAs), of which a part is mono-unsaturated. These differences lead to an altered SC lipid organization and thereby a reduced barrier function compared to human skin. In this study, we aimed to improve the SC FA composition and, consequently, the SC lipid organization of the Leiden epidermal model (LEM) by specific medium supplements. The standard FA mixture (consisting of palmitic, linoleic and arachidonic acids) supplemented to the medium was modified, by replacing protonated palmitic acid with deuterated palmitic acid or by the addition of deuterated arachidic acid to the mixture, to determine whether FAs are taken up from the medium and are incorporated into SC of LEM. Furthermore, supplementation of the total FA mixture or that of palmitic acid alone was increased four times to examine whether this improves the SC FA composition and lipid organization of LEM. The results demonstrate that the deuterated FAs are taken up into LEMs and are subsequently elongated and incorporated in their SC. However, a fourfold increase in palmitic acid supplementation does not change the SC FA composition or lipid organization of LEM. Increasing the concentration of the total FA mixture in the medium resulted in a decreased level of very long chain FAs and an increased level of mono-unsaturated FAs, which lead to deteriorated SC lipid properties. These results indicate that SC lipid properties can be modulated by specific medium supplements.

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.

Monounsaturated fatty acids reduce the barrier of stratum corneum lipid membranes by enhancing the formation of a hexagonal lateral packing

The effectiveness of the skin barrier underlies the outer layer of the skin: the stratum corneum (SC). However, in several skin diseases this barrier is impaired. In two inflammatory skin diseases, atopic eczema and Netherton syndrome, an increased level of monounsaturated fatty acids (MUFAs) has been observed as opposed to healthy skin. In the present study, we aimed to investigate the effect of MUFAs on the lipid organization and skin lipid barrier using an in vitro model membrane system, the stratum corneum substitute (SCS), mimicking the SC lipid composition and organization. To achieve our goal, the SCS has been prepared with increasing levels of MUFAs using various chain length. Permeation studies and trans-epidermal water loss measurements show that an increment of MUFAs reduces the lipid barrier in the SCS. The increased level of unsaturation exerts its effect by reducing the packing density in the lipid organization, while the lamellar phases are not affected. Our findings indicate that increased levels of MUFAs may contribute to the impaired skin barrier in diseased skin.

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.

Increase in short-chain ceramides correlates with an altered lipid organization and decreased barrier function in atopic eczema patients

A hallmark of atopic eczema (AE) is skin barrier dysfunction. Lipids in the stratum corneum (SC), primarily ceramides, fatty acids, and cholesterol, are crucial for the barrier function, but their role in relation to AE is indistinct. Filaggrin is an epithelial barrier protein with a central role in the pathogenesis of AE. Nevertheless, the precise causes of AE-associated barrier dysfunction are largely unknown. In this study, a comprehensive analysis of ceramide composition and lipid organization in nonlesional SC of AE patients and control subjects was performed by means of mass spectrometry, infrared spectroscopy, and X-ray diffraction. In addition, the skin barrier and clinical state of the disease were examined. The level of ceramides with an extreme short chain length is drastically increased in SC of AE patients, which leads to an aberrant lipid organization and a decreased skin barrier function. Changes in SC lipid properties correlate with disease severity but are independent of filaggrin mutations. We demonstrate for the first time that changes in ceramide chain length and lipid organization are directly correlated with the skin barrier defects in nonlesional skin of AE patients. We envisage that these insights will provide a new therapeutic entry in therapy and prevention of AE.

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.

Investigating the barrier function of skin lipid models with varying compositions

The lipids in the uppermost layer of the skin, the stratum corneum (SC), play an important role in the barrier function. The main lipid classes in stratum corneum are ceramides, cholesterol, and free fatty acids. In previous publications, a lipid model was presented, referred to as the stratum corneum substitute (SCS), that closely mimics the SC lipid organization and SC barrier function. In the present study, we use the SCS to study the effect of changes in lipid organization on the lipid barrier function using benzoic acid as permeation compound. First, in the SCS, we increased the level of one of the three major lipid classes keeping the ratio between the other lipid classes constant. An increased cholesterol level resulted in an increase in phase-separated cholesterol and a reduction in the permeability. An increase in ceramide or free fatty acid level resulted in the formation of additional phases, but had no significant influence on the permeability. We also examined models that mimic selected changes in lipid composition reported for dry or diseased skin. The SCS that mimics the composition in recessive X-linked ichthyosis skin displayed a twofold increase in permeability. This increase is possibly related to the formation of an additional, less ordered phase in this model.

Is an orthorhombic lateral packing and a proper lamellar organization important for the skin barrier function?

The lipid organization in the stratum corneum (SC), plays an important role in the barrier function of the skin. SC lipids form two lamellar phases with a predominantly orthorhombic packing. In previous publications a lipid model was presented, referred to as the stratum corneum substitute (SCS), that closely mimics the SC lipid organization and barrier function. Therefore, the SCS serves as a unique tool to relate lipid organization with barrier function. In the present study we examined the effect of the orthorhombic to hexagonal phase transition on the barrier function of human SC and SCS. In addition, the SCS was modified by changing the free fatty acid composition, resulting in a hexagonal packing and perturbed lamellar organization. By measuring the permeability to benzoic acid as function of temperature, Arrhenius plots were constructed from which activation energies were calculated. The results suggest that the change from orthorhombic to hexagonal packing in human SC and SCS, does not have an effect on the permeability. However, the modified SCS revealed an increased permeability to benzoic acid, which we related to its perturbed lamellar organization. Thus, a proper lamellar organization is more crucial for a competent barrier function than the presence of an orthorhombic lateral packing.

Model membranes prepared with ceramide EOS, cholesterol and free fatty acids form a unique lamellar phase

The lipid matrix present in the human stratum corneum (the thin, uppermost layer of the skin) is considered to play a crucial role in the skin barrier function. The lipid matrix consists of ceramides, cholesterol, and free fatty acids. The 13 nm lamellar phase present in the lipid matrix of the stratum corneum is very characteristic and plays an important role in the skin barrier function. One subclass of ceramides with a linoleic acid linked to a very long acyl (referred to as EOS) plays a crucial role in the formation of the 13 nm lamellar phase. In this article, we focus on the lipid phase behavior of EOS mixed with cholesterol or with cholesterol and free fatty acids. Our studies reveal that an equimolar ratio of EOS, cholesterol, and free fatty acids forms a lamellar phase with a very long repeat distance of approximately 14.7 nm. This phase exhibits exceptional behavior in that in the thermotropic response the fatty acid chains and the ceramide chains undergo an order-disorder transition in different temperature ranges while part of the hydrocarbon chains of ceramides and fatty acids are mixing in the orthorhombic lattice. On the basis of these observations, a molecular model for the 14.7 nm phase has been proposed in which the lipids are organized in a lamellar phase with three different lipid layers in a symmetric unit cell.

Hydrophilic and lipophilic moisturizers have similar penetration profiles but different effects on SC water distribution in vivo

Dry skin is often treated with hydrophilic and/or lipophilic moisturizers. Hydrophilic moisturizers must penetrate the stratum corneum (SC) deeply to function properly, whereas lipophilic moisturizers should remain in the upper SC layers. In this study, both types of moisturizers were applied on volunteers for 3 h, after which the relative amount of moisturizer and the water distribution in the SC were determined using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy in combination with tape-stripping. The results show that while hydrophilic moisturizers penetrate much more readily than lipophilic moisturizers, the latter are abundantly present in the upper regions of the SC. It was also observed that a 3-h treatment with lipophilic moisturizer did not result in increased water levels in the SC, whereas hydrophilic moisturizers retained water where they are located. The results suggest that upon prolonged application, adequate amounts of moisturizer can be obtained in those regions where they may cause moisturization in the central part of the SC. However, a single application of 3 h is probably too short to exert increased hydration as measured with ATR-FTIR.

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.

Two new methods for preparing a unique stratum corneum substitute

Stratum corneum lipids play an important role in the barrier function of the skin. An in vitro permeation model consisting of synthetic lipids has previously been developed to replace human stratum corneum (SC) in permeation studies. This model is referred to as the stratum corneum substitute (SCS). In order to improve its reproducibility and to increase the efficiency in preparing the SCS, two new preparation methods are developed. Subsequently the properties of the SCS prepared by the various methods, i.e. the manual airbrush method, the rotor airbrush method and the linomat method, are investigated. The results show that the SCS prepared with the various methods share the properties of a uniform lipid composition and lipid distribution. Furthermore, irrespective of the preparation method, the lipids form crystalline lamellar phases, mimicking the lipid organization and orientation in human SC. As a result, permeation profiles of benzoic acid through SCS are very similar to human SC. The rotor method increases the efficiency and reproducibility of the manual airbrush method, while the linomat method reduces the lipid loss during preparation and results in SCS with a more uniform membrane thickness. In conclusion, the linomat method was chosen as the preferred method for preparing the substitute.

FTIR studies show lipophilic moisturizers to interact with stratum corneum lipids, rendering the more densely packed

Lipophilic moisturizers are widely used to treat dry skin. However, their interaction with the lipids in the upper layer of the skin, the stratum corneum (SC), is largely unknown. In the present study this interaction of three moisturizers, isostearyl isostearate (ISIS), isopropyl isostearate (IPIS) and glycerol monoisostearate (GMIS), has been elucidated using lipid mixtures containing isolated ceramides (CER), cholesterol (CHOL) and free fatty acids (FFA), mimicking the lipid composition and organization in SC. The conformational ordering and the lateral packing of the lipid mixtures were examined by Fourier transformed infrared spectroscopy. Equimolar CER:CHOL:FFA mixtures show an orthorhombic to hexagonal phase transition between 22 and 30 degrees C and an ordered-disordered phase transition between 46 and 64 degrees C. Addition of 20% m/m ISIS or IPIS increased the thermotropic stability of the orthorhombic lateral packing, while GMIS had no influence. Furthermore, small amounts of all three moisturizers are incorporated into the CER:CHOL:FFA lattice, while the majority of the moisturizer exists in separate domains. Especially the thermotropic stabilization of the orthorhombic lateral packing, which might reduce water loss from the skin, is considered to contribute to the moisturizing effect of IPIS and ISIS in stratum corneum.

Lipid organization in human and porcine stratum corneum differs widely, while lipid mixtures with porcine ceramides model human stratum corneum lipid organization very closely

The conformational disordering and lateral packing of lipids in porcine and human isolated stratum corneum (SC) was compared using Fourier transform infrared spectroscopy (FTIR). It was shown that SC of both species differ markedly, porcine SC lipids being arranged predominantly in a hexagonal lattice while lipids in human SC are predominantly packed in the denser orthorhombic lattice. However, the lipid organization of equimolar ceramide:cholesterol:free fatty acid (CER:CHOL:FFA) mixtures prepared with isolated porcine CER or human CER is very similar, only the transition temperatures differed being slightly lower in mixtures with porcine CER. Therefore, the difference in lateral packing between human and porcine stratum corneum is not due to the difference in CER composition. Furthermore, it is possible to use more readily available porcine CER in model lipid mixtures to mimic lipid organization in human SC. As the equimolar porcine CER:CHOL:FFA mixtures closely mimic the lipid organization in human SC, both human SC and this mixture were selected to examine the effect of glycerol on the lipid phase behaviour. It was found that high concentrations of glycerol change the lamellar organization slightly, while domains with an orthorhombic lateral packing are still observed.

Temperature-Induced Changes in Structural and Physicochemical Properties of Vernix Caseosa

The skin of the third trimester fetus and early newborn exhibits a complex, multifunctional, highly hydrated but viscous skin-surface biofilm called vernix caseosa (VC). During birth, VC undergoes a substantial change from an aqueous and warm surrounding into a gaseous and colder environment postnatally. The aim of this study was to investigate the structural and physicochemical changes in VC, which accompany physiologically relevant variations in environment parameters, such as temperature and humidity. A remarkable difference was observed in water release and uptake properties: dehydration and rehydration processes take place two to four times faster at 37 degrees C than at room temperature (RT). The dehydration was irreversible; rehydration was only possible to a final weight of 55% (37 degrees C) and 46% (RT) of the pre-desiccation weight. Differential scanning calorimetry showed two different overlapping phase transitions within physiological temperature range. Investigation of the lipid organization by Fourier transform infrared spectroscopy and small-angle X-ray diffraction revealed a more disordered state of lipids at 37 degrees C than at RT, which might explain the faster dehydration and rehydration process at 37 degrees C as well as the changes in thermotropic rheological behavior. In conclusion, we demonstrated that VC properties adjust to the fundamental change from the intrauterine to the post-natal environment.

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.

Lipid mixtures prepared with well-defined synthetic ceramides closely mimic the unique stratum corneum lipid phase behavior

Lipid lamellae present in the outermost layer of the skin, the stratum corneum, form the main barrier for the diffusion of molecules through the skin. The presence of a unique 13 nm lamellar phase and its high crystallinity are characteristic for the stratum corneum lipid phase behavior. In the present study, small-angle and wide-angle X-ray diffraction were used to examine the organization in lipid mixtures prepared with a unique set of well-defined synthetic ceramides, varying from each other in head group architecture and acyl chain length. The results show that equimolar mixtures of cholesterol, free fatty acids, and synthetic ceramides (resembling the composition of pig ceramides) closely resemble the lamellar and lateral stratum corneum lipid organization, both at room and higher temperatures. Exclusion of several ceramide classes from the mixture does not affect the lipid organization. However, complete substitution of ceramide 1 (acylceramide with a sphingosine base) with ceramide 9 (acylceramide with a phytosphingosine base) reduces the formation of the long periodicity lamellar phase. This indicates that the head group architecture of acylceramides affects the lipid organization. In conclusion, lipid mixtures prepared with well-defined synthetic ceramides offer an attractive tool with which to unravel the importance of the molecular structure of individual ceramides for proper lipid organization.

Quantitative Assessment of the Transport of Elastic and Rigid Vesicle Components and a Model Drug from these Vesicle Formulations into Human Skin In Vivo

The aim of this study was to quantitatively assess the distribution profiles of elastic and rigid vesicle material in human skin in vivo. Furthermore, the distribution profiles of the model drug ketorolac applied in these vesicle formulations was investigated. A deuterium-labelled phospholipid was incorporated into these vesicles to serve as a marker for the vesicle material. The vesicles were loaded with ketorolac at saturated concentrations. Vesicle solutions were applied non-occlusively onto the skin and the treated site was sequentially tape-stripped. Tape-strips were analyzed for vesicle material using attenuated total reflectance-Fourier transform infrared spectroscopy and for ketorolac by extraction of the tape-strips followed by high pressure liquid chromatography. Distribution profiles in the stratum corneum (SC) were obtained for the elastic and rigid vesicle material and for the ketorolac. These profiles have suggested that elastic vesicle material can rapidly enter the deeper layers of the SC and can reach almost the SC-viable epidermal junction. Rigid vesicle material, however, did not penetrate deep into the SC. Furthermore, the elastic vesicles were better than the rigid vesicles in the enhancement of ketorolac transport into human SC. The distribution profile of ketorolac in the deeper SC layers was, however, different from that of the vesicle material. This suggests that once the elastic vesicles partition into the SC, the ketorolac is released from the vesicles. The elastic vesicles are superior to the rigid vesicles both in terms of vesicular transport into the SC and in terms of therapeutic potential as a skin delivery vehicle.

Modelling the stratum corneum lipid organisation with synthetic lipid mixtures: the importance of synthetic ceramide composition

Cholesterol (CHOL), free fatty acids (FFA) and nine classes of ceramides (CER1-CER9) form the main constituents of the intercellular lipid lamellae in stratum corneum (SC), which regulate the skin barrier function. Both the presence of a unique 13-nm lamellar phase, of which the formation depends on the presence of CER1, and its dense lateral packing are characteristic for the SC lipid organisation. The present study focuses on the lipid organisation in mixtures prepared with CHOL, FFA and a limited number of synthetic CER, namely CER1, CER3 and bovine brain CER type IV (SigmaCERIV). The main objective is to determine the optimal molar ratio of CER3 to SigmaCERIV for the formation of the 13-nm lamellar phase. CER3 contains a uniform acyl chain length, whereas SigmaCERIV contains fatty acids with varying chain lengths. Using small angle X-ray diffraction (SAXD), it is demonstrated that the CER3 to SigmaCERIV ratio affects the formation of the 13-nm lamellar phase and that the optimal ratio depends on the presence of FFA. Furthermore, the formation of the 13-nm lamellar phase is not very sensitive to variations in the total CER level, which is similar to the in vivo situation.

Novel lipid mixtures based on synthetic ceramides reproduce the unique stratum corneum lipid organization

Lipid lamellae present in the outermost layer of the skin protect the body from uncontrolled water loss. In human stratum corneum (SC), two crystalline lamellar phases are present, which contain mostly cholesterol, free fatty acids, and nine types of free ceramides. Previous studies have demonstrated that the SC lipid organization can be mimicked with model mixtures based on isolated SC lipids. However, those studies are hampered by low availability and high interindividual variability of the native tissue. To elucidate the role of each lipid class in the formation of a competent skin barrier, the use of synthetic lipids would offer an alternative. The small- and wide-angle X-ray diffraction results of the present study show for the first time that synthetic lipid mixtures, containing only three synthetic ceramides, reflect to a high extent the SC lipid organization. Both an appropriately chosen preparation method and lipid composition promote the formation of two characteristic lamellar phases with repeat distances similar to those found in native SC. From all synthetic lipid mixtures examined, equimolar mixtures of cholesterol, ceramides, and free fatty acids equilibrated at 80 degrees C resemble to the highest extent the lamellar and lateral SC lipid organization, both at room and increased temperatures.

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.

Water distribution and related morphology in human stratum corneum at different hydration levels

This study focused on the water distribution in human stratum corneum and on the swelling of the corneocytes. For this purpose stratum corneum was hydrated to various levels and used either for Fourier transform infrared spectroscopy or for cryo-scanning electron microscopy. The images were analyzed with respect to water localization and cell shape. The Fourier transform infrared spectra were measured to study the water-lipid interactions. The results show that water only slightly changes the lipid transitions in the stratum corneum even at a hydration level of 300% wt/wt compared to stratum corneum and that water is inhomogeneously distributed in the stratum corneum. No gradual increase in water level was observed in depth. At 57%-87% wt/wt water content the hydration level in the central part of stratum corneum is higher than in the superficial and deeper cell layers. Water domains are mainly present within the corneocytes and not in the intercellular regions. At a very high hydration level (300% wt/wt), the corneocytes are strongly swollen except for the deepest cell layers adjacent to the viable epidermis. The corneocytes in these layers are not swollen. At 300% wt/wt hydration level water domains are also present in intercellular regions. Between 17% wt/wt and 300% wt/wt the cell thickness increases linearly with the hydration level suggesting that swelling of cells mainly occurs in the direction perpendicular to the skin surface. At an increased hydration level, the corneocyte envelope more efficiently surrounds the cell content compensating for the increased cell volume. The changes in stratum corneum morphology with increasing water level have also been observed in dermatomed skin.

Structure of the skin barrier and its modulation by vesicular formulations

The natural function of the skin is to protect the body from unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. For this reason the organization in the lipid domains is considered to be very important for the skin barrier function. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid phase behavior is different from that of other biological membranes. In stratum corneum crystalline phases are predominantly present, but most probably a subpopulation of lipids forms a liquid phase. Both the crystalline nature and the presence of a 13 nm lamellar phase are considered to be crucial for the skin barrier function. Since it is impossible to selectively extract individual lipid classes from the stratum corneum, the lipid organization has been studied in vitro using isolated lipid mixtures. These studies revealed that mixtures prepared with isolated stratum corneum lipids mimic to a high extent stratum corneum lipid phase behavior. This indicates that proteins do not play an important role in the stratum corneum lipid phase behavior. Furthermore, it was noticed that mixtures prepared only with ceramides and cholesterol already form the 13 nm lamellar phase. In the presence of free fatty acids the lattice density of the structure increases. In stratum corneum the ceramide fraction consists of various ceramide subclasses and the formation of the 13 nm lamellar phase is also affected by the ceramide composition. Particularly the presence of ceramide 1 is crucial. Based on these findings a molecular model has recently been proposed for the organization of the 13 nm lamellar phase, referred to as "the sandwich model", in which crystalline and liquid domains coexist. The major problem for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Therefore, several methods have been assessed to increase the permeation rate of drugs temporarily and locally. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state vesicles, liquid-state vesicles and elastic vesicles.

Phase behavior of stratum corneum lipid mixtures based on human ceramides: the role of natural and synthetic ceramide 1

In a recent study the lipid phase behavior of mixtures of human ceramides, cholesterol, and free fatty acids has been examined. We observed in cholesterol: human ceramide mixtures a prominent formation of the 12.8 nm lamellar phase (referred to as the long periodicity phase). Addition of free fatty acids promoted the formation of a 5.6 nm lamellar phase (referred to as the short periodicity phase) and increased the subpopulation of lipids forming a fluid phase. In this study we focused on the role of human ceramide 1, as the presence of this ceramide appeared to be crucial for proper lipid phase behavior in mixtures prepared with ceramide isolated from pig stratum corneum. In order to do this, mixtures of cholesterol and free fatty acids were prepared with human ceramides, in which natural human ceramide 1 was replaced by either synthetic CER1-linoleate (CER1-lin), or CER1-oleate (CER1-ol), or CER1-stearate (CER1-ste). After substitution of natural human ceramide 1 by synthetic ceramide 1 the following observations were made. (i) In the presence of synthetic CER1-ste no long periodicity phase and no liquid phase could be detected. (ii) In the presence of HCER1-ol a liquid phase was more prominently formed than in the presence of HCER1-lin. (iii) In cholesterol:human ceramide mixtures in the presence of CER1-lin the long periodicity phase was more prominently present than in the presence of CER1-ol. (iv) In the presence of CER1-ste neither a long periodicity phase nor a liquid lateral packing could be detected. The results of these studies further indicate that for the formation of the long periodicity phase a certain (optimal) fraction of lipids has to form a liquid phase. When the fraction forming this liquid phase is either too low or too high, the formation of the short periodicity phase is increased at the expense of the formation of the long periodicity phase. Based on the results of this and previous studies we offer an explanation for the deviation in lipid organization in diseased and in dry skin compared to normal skin.

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.