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.     

The Lipid Organisation of the Skin Barrier: Liquid and Crystalline Domains Coexist in Lamellar Phases

The superficial layer of the skin, the stratum corneum, is the main barrier for diffusion of substances across the skin. The stratum corneum is composed of corneocytes embedded in lipid lamellae. In previous studies two lamellar phases have been identified with periodicities of 6.4 and 13.4 nm of which the 13.4 nm phase (long periodicity phase = LPP) is considered to be very important for the skin banier function. The main lipid classes in stratumcorneum are ceramides, free fatty acids and cholesterol. Until now 8 subclassesof ceramides are identified in human stratum corneum referred to as ceramide 1 to 8. Studies with mixtures prepared with isolated human ceramides revealed that cholesterol and ceramides are very important for the formation of the lamellar phases. After addition of free fatty acids the lipids are organised in an orthorhombic packing with a small proportion of lipids in a liquid phase. Our most recent results show that the presence of ceramide 1 and the formation of a liquid phase are crucial elements for the formation of the LPP. These observations and the broad-narrowbroad sequence of lipid layers in the LPP led us to propose a molecular model for this phase. This consists of one narrow central lipid layer with fluid domains with on both sides a broad layer with a crystalline structure. This model is referred to as `the sandwich model'.     

Role of ceramide structure and its microenvironment on the conformational order of model stratum corneum lipids mixtures: an approach by FTIR spectroscopy

The aim of this study is to investigate the influence of ceramide head group architecture and free fatty acid (another main class of stratum corneum lipids) or protein (keratin), on the lamellar organization of the ceramide auto-associated in model films mimicking lipid organization within the stratum corneum. FTIR spectroscopy is a powerful technique for investigating the structure of such systems. This technique has already been used to characterize phase transitions of the SC and of related model systems. As temperature is known to modify the conformational order of lipids, we used it as a variable parameter to monitor the differences in the conformational stability of ceramides. Our study included four ceramides: ceramide 2, 3, 5 and 6 which differ by their head group architecture. Two kinds of lipid-lipid interactions were studied: non-polar and polar. We noted some structural factors which participated to the organizational behavior: insaturation of alkyl chain, alpha-hydroxyl on fatty acid moiety and sphingosine or phytosphingosine head group. There is a direct interaction of palmitic acid on alkyl chains organization and a weak interaction with polar head group in presence of keratin, both provoking a destabilization of the ceramidic orthorhombic organization.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

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.     

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.     

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.     

Comparison of rat epidermal keratinocyte organotypic culture (ROC) with intact human skin: lipid composition and thermal phase behavior of the stratum corneum.

The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while alpha-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified omega-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T2) at a slightly lower temperature (68 degrees C) than human skin (74 degrees C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.     

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.     

Structural Transitions in Ceramide Cubic Phases during Formation of the Human Skin Barrier

The stratum corneum is the outermost layer of human skin and the primary barrier toward the environment. The barrier function is maintained by stacked layers of saturated long-chain ceramides, free fatty acids, and cholesterol. This structure is formed through a reorganization of glycosylceramide-based bilayers with cubic-like symmetry into ceramide-based bilayers with stacked lamellar symmetry. The process is accompanied by deglycosylation of glycosylceramides and dehydration of the skin barrier lipid structure. Using coarse-grained molecular dynamics simulation, we show the effects of deglycosylation and dehydration on bilayers of human skin glycosylceramides and ceramides, folded in three dimensions with cubic (gyroid) symmetry. Deglycosylation of glycosylceramides destabilizes the cubic lipid bilayer phase and triggers a cubic-to-lamellar phase transition. Furthermore, subsequent dehydration of the deglycosylated lamellar ceramide system closes the remaining pores between adjacent lipid layers and locally induces a ceramide chain transformation from a hairpin-like to a splayed conformation.     

Deuterium NMR investigation of polymorphism in stratum corneum lipids

The intercellular lipid lamellae of stratum corneum constitute the major barrier to percutaneous penetration. Deuterium magnetic resonance and freeze-fracture electron microscopic investigation of hydrated lipid mixtures consisting of ceramides, cholesterol, palmitic acid and cholesteryl sulfate and approximating the stratum corneum intercellular lipid composition, revealed thermally induced polymorphism. The transition temperature of bilayer to hexagonal transition decreased as the ratio of cholesterol to ceramides in these mixtures was lowered. Lipid mixtures in which the stratum corneum ceramides were replaced by synthetic dipalmitoylphosphatidylcholine did not show any polymorphism throughout the temperature range used in the present study. The ability of the ceramide-containing samples to form hexagonal structures establishes a plausible mechanism for the assembly of the stratum corneum intercellular lamellae during the final stages of epidermal differentiation. Also, the bilayer to hexagonal phase transition of these nonpolar lipid mixtures could be used to enhance the penetration of drugs through skin.     

Comparison of rat epidermal keratinocyte organotypic culture (ROC) with intact human skin: Lipid composition and thermal phase behavior of the stratum corneum

The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while α-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified ω-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T₂) at a slightly lower temperature (68 °C) than human skin (74 °C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.     

Influence of different ceramides on the structure of in vitro model lipid systems of the stratum corneum lipid matrix

Human stratum corneum (SC) consists of several layers of keratinized corneocytes embedded in a lipid matrix of ordered lamellar structure which is considered to constitute the major barrier to percutaneous penetration. Artificial mixtures of SC lipids are often used as model systems to mimic the skin barrier or to investigate the effects of substances on the phase behaviour of the models. In the present study a SC lipid model composed of cholesterol, fatty acids and ceramides was used to investigate the effect of three different commercially available ceramide types on the microstructure and the physicochemical behaviour of the lipids. Polarized light microscopy, transmission electron microscopy, small-angle X-ray diffraction, wide-angle X-ray diffraction and differential scanning calorimetry (DSC) were used for physicochemical characterization. The results revealed a lamellar structure for all models but showed differences with regard to the thermal and optical behaviour depending obviously on the composition of the ceramide mixtures. A model containing a mixture of Cer[AS] was comparable to human SC lipids.     

Effect of ceramide acyl chain length on skin permeability and thermotropic phase behavior of model stratum corneum lipid membranes

Stratum corneum ceramides play an essential role in the barrier properties of skin. However, their structure-activity relationships are poorly understood. We investigated the effects of acyl chain length in the non-hydroxy acyl sphingosine type (NS) ceramides on the skin permeability and their thermotropic phase behavior. Neither the long- to medium-chain ceramides (8-24 C) nor free sphingosine produced any changes of the skin barrier function. In contrast, the short-chain ceramides decreased skin electrical impedance and increased skin permeability for two marker drugs, theophylline and indomethacin, with maxima in the 4-6C acyl ceramides. The thermotropic phase behavior of pure ceramides and model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesterol sulfate was studied by differential scanning calorimetry and infrared spectroscopy. Differences in thermotropic phase behavior of these lipids were found: those ceramides that had the greatest impact on the skin barrier properties displayed the lowest phase transitions and formed the least dense model stratum corneum lipid membranes at 32°C. In conclusion, the long hydrophobic chains in the NS-type ceramides are essential for maintaining the skin barrier function. However, this ability is not shared by their short-chain counterparts despite their having the same polar head structure and hydrogen bonding ability.     

Phase behavior of stratum corneum lipids in mixed Langmuir-Blodgett monolayers.

The lipids found in the bilayers of the stratum corneum fulfill the vital barrier role of mammalian bodies. The main classes of lipids found in stratum corneum are ceramides, cholesterol, and free fatty acids. For an investigation of their phase behavior, mixed Langmuir-Blodgett monolayers of these lipids were prepared. Atomic force microscopy was used to investigate the structure of the monolayers as a function of the monolayer composition. Three different types of ceramide were used: ceramide extracted from pigskin, a commercially available ceramide with several fatty acid chain lengths, and two synthetic ceramides that have only one fatty acid chain length. In pigskin ceramide-cholesterol mixed monolayers phase separation was observed. This phase separation was also found for the commercially available type III Sigma ceramide-cholesterol mixed monolayers with molar ratios ranging from 1:0.1 to 1:1. These monolayers separated into two phases, one composed of the long fatty acid chain fraction of Sigma ceramide III and the other of the short fatty acid chain fraction of Sigma ceramide III mixed with cholesterol. Mixtures with a higher cholesterol content consisted of only one phase. These observations were confirmed by the results obtained with synthetic ceramides, which have only one fatty acid chain length. The synthetic ceramide with a palmitic acid (16:0) chain mixed with cholesterol, and the synthetic ceramide with a lignoceric acid (24:0) chain did not. Free fatty acids showed a preference to mix with one of these phases, depending on their fatty acid chain lengths. The results of this investigation suggest that the model system used in this study is in good agreement with those of other studies concerning the phase behavior of the stratum corneum lipids. By varying the composition of the monolayers one can study the role of each lipid class in detail.     

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.     

Human stratum corneum lipid organization as observed by atomic force microscopy on Langmuir-Blodgett films

The barrier function of skin ultimately depends on the physical state and structural organisation of the stratum corneum extracellular lipid matrix. Ceramides, cholesterol and a broad distribution of saturated long-chain free fatty acids dominate the stratum corneum lipid composition. Additionally, smaller amounts of cholesterol sulfate and cholesteryl oleate may be present. A key feature determining skin barrier capacity is thought to be whether or not different lipid domains coexist laterally in the stratum corneum extracellular lipid matrix. In this study, the overall tendency for lipid domain formation in different mixtures of extracted human stratum corneum ceramides, cholesterol, free fatty acids, cholesterol sulfate and cholesteryl oleate were studied using atomic force microscopy (AFM) on Langmuir-Blodgett (LB) films on mica. It is shown that the saturated long-chain free fatty acid distribution of human stratum corneum prevents hydrocarbon chain segregation. Further, LB-films of human stratum corneum ceramides express a pattern of connected elongated domains with a granular domain interface. The dominating effect of both cholesterol and cholesterol sulfate is that of increased ceramide domain dispersion. This effect is counteracted by the presence of free fatty acids, which preferentially mix with ceramides and not with cholesterol. Cholesteryl oleate does not mix with other skin lipid components, supporting the hypothesis of an extra-endogenous origin. In the system composed of endogenous human ceramides and cholesterol plus 15 wt% stratum corneum distributed free fatty acids, i.e., the system mimicking most closely the lipid composition of the stratum corneum extracellular space, LB-films on mica express lateral domain formation.     

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.     

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.