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

Chemical enhancement of percutaneous absorption in relation to stratum corneum structural alterations

The outermost layer of the skin, stratum corneum (SC), provides an outstanding barrier against the external environment and is also responsible for skin impermeability toward most solutes. The barrier function is related to the unique composition of the SC lipids and their complex structural arrangement. The lipoidal matrix of the SC, therefore, is a target of penetration enhancer action. The literature on the skin barrier structure and function and on the mechanisms of action of some well established permeation promoters, with a focus on their impact on SC structural alterations, is reviewed. Data obtained from infrared, thermal, and fluorescence spectroscopic examinations of the SC and its components imply enhancer improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and X-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers.

Structural parameters involved in the permeation of propranolol HCl by iontophoresis and enhancers

The purpose of this work was to study the effect of iontophoretic transport of Propranolol hydrochloride on the lipidic organization of the stratum corneum pretreated under passive or iontophoretic conditions by two model penetration enhancers, sodium lauryl sulfate and hexadecyl trimethylammonium bromide. Characterization of human stratum corneum was performed by X-ray diffraction, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and differential scanning calorimetry (DSC). The structural properties were compared with the iontophoretic permeability of propranolol hydrochloride on human stratum corneum. The iontophoretic fluxes of propranolol hydrochloride were effectively increased by two-fold in stratum corneum pretreated with sodium lauryl sulfate. In contrast, they were halved in stratum corneum pretreated with hexadecyl trimethylammonium bromide. These results could be related to changes in the electrical and structural properties of the stratum corneum after incorporation of these enhancers.

Effects of iontophoresis and electroporation on the stratum corneum. Review of the biophysical studies

This review focuses on the effects induced by iontophoresis and electroporation on the stratum corneum of the skin. Hence, the aims were: (1) to contribute to the understanding of the mechanisms of drug transport by these methods; (2) to evaluate the safety issues associated with current application. Complementary biophysical methods were used to provide a complete picture of the stratum corneum. Even though the mechanism of drug transport is believed to be different, i.e., electrophoresis for iontophoresis and creation of new aqueous pathways for electroporation, the effects on the stratum corneum detected minutes after current application are very similar. For both methods, the major findings were: (1) a disorganisation of the lipid bilayers of the stratum corneum; (2) an increase in skin hydration; (3) a larger decrease in skin resistance induced by electroporation as compared to iontophoresis. These changes were partly reversible and depended on the amount of electrical charges transferred. The mechanisms of stratum corneum perturbations are discussed. These perturbations could explain partly the increase in drug transport. If iontophoresis is considered as a safe method of drug delivery, the data augurs for the safety of electroporation.

Non-uniform cellular packing of the stratum corneum and permeability barrier function of intact skin: a high-resolution confocal laser scanning microscopy study using highly deformable vesicles (Transfersomes)

Novel, functional skin staining with fluorescent, ultradeformable lipid vesicles (Transfersomes, IDEA, Munich, Germany) was developed and combined with confocal laser scanning microscopy. This revealed the structural and barrier characteristics of intact skin to a resolution of > or = 0.2 micron, that is, to the limit of light microscopy. Different routes of penetration into the stratum corneum were visualized and new details in the skin anatomy and barrier were unveiled. Most prominent was the lateral inhomogeneity of the stratum corneum, where three to 10 neighbouring corneocyte 'columns' were found to form a cluster. Corneocyte edges inside each cluster intercalated extensively, but adjacent clusters were separated by 'gorges' a few micrometers deep; lipid packing was also less regular and tight in the intercluster region. Two quantitatively different hydrophilic pathways were found in the horny layer: an intercluster route with low penetration resistance comprising < or = 1% of the total or < or = 20% of the pathway area in the skin, and an intercorneocyte pathway that resists penetration better and is more abundant (> or = 3% of the skin or > or = 80% of the pathway area). This latter route is strongly tortuous, as it goes between all the corneocytes in a cluster. It traces the irregularities between the intercellular lipid lamellae and/or the adjacent corneocyte envelopes which may act as virtual channels in the skin. It was inferred that such channels coincide with the route of water evaporation through the skin and exhibit the permeability barrier maximum in the stratum corneum conjunctum.

Transdermal drug delivery. Clinical aspects

The delivery of drugs into and through the skin is a recognized and effective means of therapy for dermatologic, regional, and systemic disease. The selection of drug candidates and the rational design of suitable formulations depends upon the biological make-up of the skin's barrier, and the physiochemical interactions between the membrane, the delivery system technology, and the active agent. This article summarizes the state of the art and examines more recent developments that are the subject of considerable research at this time. In addition, the potential to use the skin as a portal for noninvasive clinical chemistry (e.g., for glucose monitoring in diabetics) is discussed.

Morphological observations on liposomes bearing covalently bound protein: studies with freeze-fracture and cryo electron microscopy and small angle X-ray scattering techniques

The appearance of protein bound to the surface of intact and microfluidized liposomes and its possible influence on their morphology was examined by freeze-fracture electron microscopy, cryo electron microscopy and small angle X-ray scattering (SAXS) techniques. Results obtained by the two microscopy techniques were in agreement with one another in terms of vesicle size and localization of protein (tetanus toxoid or immunoglobulin G) on the surface of vesicles. Surface-bound protein was observed as particles (10-12 nm diameter) by freeze-fracture electron microscopy and was confirmed by immunogold cryo microscopy. SAXS was shown to be a suitable means to further characterize liposomes with, or without bound protein.

Cryo-electron diffraction as a tool to study local variations in the lipid organization of human stratum corneum

The human skin provides the body with a barrier against transepidermal water loss and the penetration of harmful agents (e.g. microbes) from outside. This barrier function is produced mainly by the outermost, nonviable layer of the epidermis, the stratum corneum (s.c.). The s.c. consists of terminally differentiated corneocytes surrounded by a continuous intercellular lipid domain, which contains mostly ceramides, cholesterol and free fatty acids. Small- and wide-angle X-ray diffraction studies have elucidated the lamellar and lateral lipid organizations in these domains. However, these techniques require bulk quantities of SC, as a result of which local structure information on the lipids cannot be obtained. Insights to these local lipid arrangements are important when new transdermal drug delivery systems have to be developed. Therefore, the technique of electron diffraction arose as a tool to study the lateral packing of the lipids in the intercellular domains of SC, locally. In a previous study, the suitability of electron diffraction was demonstrated using a lipid model system that resembled the lipid composition of the SC. The spacings calculated from the electron diffraction patterns were in good agreement with the spacings revealed by wide-angle X-ray diffraction. The results presented here succeed this previous study. We improved the microscope settings and developed a new preparation method to study ex vivo human s.c. by cryo-electron diffraction. The method is based on the conventional tape-stripping method and offers the possibility to study depth-related changes in the lipid organization of human SC. Diffraction patterns of both hexagonal and orthorhombic lipid lattices have been recorded with spacings that resembled those found in human s.c. by wide-angle X-ray diffraction. After lipid extraction, such diffraction patterns could no longer be detected in the samples.

Validation of freeze-drying to visualize percutaneous 3H-estradiol transport: the influence of skin hydration on the efficacy of the method

A study was made of the validity of freeze-drying to visualize the distribution of 3H-estradiol in human stratum corneum after topical application of a dry dose, a patch or a buffer solution. Each of these donor formulations was applied to human dermatomed skin for 24 h using Franz permeation cells. Subsequently, small pieces of skin were subjected to cryofixation, freeze-drying, osmium tetroxide vapor fixation, Spurr resin embedding and electron microscopic autoradiography. Stratum corneum from dry dose and patch application experiments was well preserved by freeze-drying, allowing an accurate localization of 3H-estradiol. In contrast, stratum corneum from buffer solution experiments suffered from cryofixation artifacts due to excessive hydration of the skin. The corresponding autoradiographs showed strong redistribution of 3H-estradiol. Thus, the visualization method under investigation has its limitations regarding the hydration level of the skin.

Evaluation of solute permeation through the stratum corneum: lateral bilayer diffusion as the primary transport mechanism

Solute permeation across human stratum corneum (SC) was examined in terms of the fundamental bilayer transport properties. A mathematical model was developed to describe the macroscopic SC permeation via the interkeratinocyte lipid domain in terms of (i) the structure and dimensions of the SC, and (ii) the microscale lipid bilayer transport properties, which include the bilayer/water partition coefficient, the lateral diffusion coefficient, the interfacial transbilayer mass transfer coefficient, and the intramembrane transbilayer mass transfer coefficient. The relative importance of the diffusive resistances associated with the bilayer transport properties was evaluated with the model and experimental data. Lateral diffusion coefficients in SC lipid bilayers were calculated from 120 human skin permeability measurements, and compared with previously reported measurements made in SC-extracted lipids. Good qualitative and quantitative agreement was observed, indicating that, in the context of the model, the diffusive resistance associated with lateral diffusion is sufficient to explain the overall resistance of solute permeation through the SC. A similar analysis shows that the diffusive resistance associated with interfacial transbilayer transport is not capable of explaining the experimental permeation values, thus supporting this finding. The lateral diffusion analysis also revealed a bifunctional size dependence of transport within the SC, with a strong size dependence for small solutes (<300 Da) and a weak size dependence for larger solutes.

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

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

Lipid microdomains in dimyristoylphosphatidylcholine-ceramide liposomes

Binary membranes composed of dimyristoylphosphatidylcholine (DMPC) and natural ceramide (up to a mole fraction Xcer = 0.25) were investigated by measuring the excimer:monomer fluorescence emission intensity ratio IE:IM for the pyrene labeled phospholipid probe 1-palmitoyl-2[(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC), by monitoring fluorescence polarization of diphenylhexatriene (DPH), as well as using differential scanning calorimetry (DSC). Increasing Xcer > 0.10 both below and above the main transition temperature Tm increased IE:IM for PPDPC maximally approximately 1.6-fold at Xcer = 0.25. Above Tm, and when Xcer approaches 0.10, fluorescence polarization P for DPH increases steeply, reflecting an overall decrease in acyl chain motions. At Xcer = 0.10 there is a discontinuity in P and upon further increase in the content of ceramide a smaller, yet significant increase in P is evident. DSC revealed ceramide to increase in the pretransition temperature until at Xcer exceeding 0.07 this transition was no longer evident. Simultaneously, increasing Xcer up to 0.05 increased Tm from 23.9 to 24.6 degrees C. Total enthalpy delta Hm of the main transition diminished progressively upon increase in Xcer up to approximately 0.10. Above this concentration of ceramide a new endotherm became evident at 22.5 degrees C, and exceeding Xcer = 0.14 this endotherm became dominant. Our results indicate an enrichment of the pyrene labeled phospholipid analog into microdomains concomitant with the formation of a distinct ceramide-enriched phase at Xcer > 0.10.

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

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

Thermotropic phase behavior of in vivo extracted human stratum corneum lipids

The thermotropic phase behavior of lipids extracted either in vivo from inner forearm (SCLE) or plantar callus (PC) was investigated by differential scanning calorimetry and small angle X-ray diffraction. PC composition was chromatographically modified (MPC) by eliminating the more polar lipids in order to evaluate their role. Analysis of composition confirms the potential use of PC as a source of stratum corneum lipids. MPC and SCLE exhibit similar differential scanning calorimetry (DSC) profiles with a main transition around 50 degrees C attributed to the solid-to-liquid phase transition of the ceramides. The absence of a transition around 50 degrees C for PC suggests the possible perturbation of ceramide packing by the significantly high proportion of phospholipids. X-ray data suggest a high miscibility of sebum components in stratum corneum lipids with possible modification of chain packing. The MPC patterns show a lipid phase separation which underscores the role of polar lipids in cholesterol/free fatty acids/sterol esters/ceramides structural cohesion.

Homogeneous transport in a heterogeneous membrane: water diffusion across human stratum corneum in vivo

The objective of this study was to determine whether a structurally heterogeneous biomembrane, human stratum corneum (SC), behaved as a homogeneous barrier to water transport. The question is relevant because the principal function of the SC in vivo is to provide a barrier to the insensible loss of tissue water across the skin. Impedance spectra (IS) of the skin and measurements of the rate of transepidermal water loss (TEWL) were recorded sequentially in vivo in human subjects as layers of the SC were progressively removed by the serial application of adhesive tape strips. The low-frequency (< or = 100 rad s-1) impedance of skin was much more significantly affected by tape stripping than the higher frequency values; removal of the outermost SC layer had the largest effect. In contrast, TEWL changed little as the outer SC layers were stripped off, but increased dramatically when 6-8 microns of the tissue had been removed. It follows that the two noninvasive techniques probe SC barrier integrity in somewhat different ways. After SC removal, recovery of barrier function, as assessed by increasing values of the low-frequency impedance, apparently proceeded faster than TEWL decreased to the prestripping control. The variation of TEWL as a function of SC removal behaved in a manner entirely consistent with a homogeneous barrier, thereby permitting the apparent SC diffusivity of water to be found. Skin impedance (low frequency) was correlated with the relative concentration of water within the SC, thus providing an in vivo probe for skin hydration. Finally, the SC permeability coefficient to water, as a function of SC thickness, was calculated and correlated with the corresponding values of skin admittance derived from IS.

Lateral diffusion of small compounds in human stratum corneum and model lipid bilayer systems

An image-based technique of fluorescence recovery after photobleaching (video-FRAP) was used to measure the lateral diffusion coefficients of a series of nine fluorescent probes in two model lipid bilayer systems, dimyristoylphosphatidylcholine (DMPC) and DMPC/cholesterol (40 mol%), as well as in human stratum corneum-extracted lipids. The probes were all lipophilic, varied in molecular weight from 223 to 854 Da, and were chosen to characterize the lateral diffusion of small compounds in these bilayer systems. A clear molecular weight dependence of the lateral diffusion coefficients in DMPC bilayers was observed. Values ranged from 6.72 x 10(-8) to 16.2 x 10(-8) cm2/s, with the smaller probes diffusing faster than the larger ones. Measurements in DMPC/cholesterol bilayers, which represent the most thorough characterization of small-solute diffusion in this system, exhibited a similar molecular weight dependence, although the diffusion coefficients were lower, ranging from 1.62 x 10(-8) to 5.60 x 10(-8) cm2/s. Lateral diffusion measurements in stratum corneum-extracted lipids, which represent a novel examination of diffusion in this unique lipid system, also exhibited a molecular weight dependence, with values ranging from 0.306 x 10(-8) to 2.34 x 10(-8) cm2/s. Literature data showed that these strong molecular weight dependencies extend to even smaller compounds than those examined in this study. A two-parameter empirical expression is presented that describes the lateral diffusion coefficient in terms of the solute's molecular weight and captures the size dependence over the range examined. This study illustrates the degree to which small-molecule lateral diffusion in stratum corneum-extracted lipids can be represented by diffusion in DMPC and DMPC/cholesterol bilayer systems, and may lead to a better understanding of small-solute transport across human stratum corneum.

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.

Stratum corneum lipids of human epidermal keratinocyte air-liquid cultures: implications for barrier function

PURPOSE

The purpose of this study is to investigate the permeability barrier, i.e., the stratum corneum (SC) lipids, of human epidermal keratinocyte air-liquid cultures and compare them with those of human SC.

METHOD

The SC lipids composition was analyzed by TLC technique, the organization by electron microscopic procedure, and the phase transition temperature by infrared spectroscopic method.

RESULTS

Electron microscopy demonstrated that The SC lipids of cultures were largely retained inside the comeocytes, and that the intercellular lipids lack both the basic unit repetition (i.e., broad: narrow: broad: broad: narrow: broad of electron lucent bands) and the covalently-bound lipid envelope normally found in human SC. These characteristics are similar to those found in SC from patients with atopic dermatitis or psoriasis, or from animals with essential fatty acid deficiency, suggesting that the cultures may be hyperproliferative. In addition, the high free sterol content and the altered fatty acid/ceramide composition of these cultures argue that the compromised barrier function is linked to hyperproliferation and lipid synthesis, or vice versa. Infrared spectroscopic analyses confirm that there are major conformational differences between the lipids of human and cultured SC.

CONCLUSIONS

The profound differences in SC lipid composition, organization and conformational properties attest that permeability alone is not a sufficiently sensitive marker to define barrier equivalence between cultures and human skin.

X-ray diffraction analysis of isolated skin lipids: reconstitution of intercellular lipid domains

Low- and wide-angle X-ray diffraction were used to determine the structural organization of lipids isolated from the stratum corneum extracellular matrix that forms the major water permeability barrier in mammalian epidermis. Hydrated pig skin ceramides gave a single low-angle reflection of about 62 angstroms and a wide-angle-reflection at 4.15 angstroms. The addition of either cholesterol or fatty acid, the other major lipid components of the skin stratum corneum extracellular matrix, modified this diffraction pattern, depending on the lipid mole ratios. In the absence of water, lipid mixtures exhibited lipid phase separation, as shown by low- and wide-angle reflections typical of a separate cholesterol phase. However, a hydrated 2:1:1 mole ratio of ceramide:cholesterol:palmitic acid (similar to that found in stratum corneum) produced a diffraction pattern with a single sharp wide-angle reflection at 4.10 angstroms and low-angle reflections which indexed as the first eight orders of a single repeat period of 130 angstroms. The repeat period and intensity distribution of the low-angle data were similar to those found in intact stratum corneum [White et al. (1988) Biochemistry 27, 3725-3732; Bouwstra et al. (1994) Biochim. Biophys. Acta 1212, 183-192]. Higher concentrations of cholesterol or palmitic acid resulted in lipid phase separations. The 130 angstrom repeat period decreased only about 3 angstroms as water was removed by incubation in low-relative humidity atmospheres. The 130 angstrom repeat period depended on the presence of a particular ceramide, N-(omega-acyloxy)-acylsphingosine, which is found only in the epidermis. In contrast, 2:1:1 mixtures of brain ceramide:cholesterol:palmitic acid gave reflections of 56 and 34 angstroms. These results indicate that a structure with dimensions similar to those of the lamellar repeating unit found in skin stratum corneum does not depend on the presence of protein but does depend on the presence of specific skin ceramides and appropriate concentrations of cholesterol and fatty acid.

Structure of fully hydrated human stratum corneum: a freeze-fracture electron microscopy study

The structure of fully hydrated human stratum corneum was investigated by means of freeze-fracture electron microscopy. Mammary and abdominal stratum corneum were incubated for 48 h with phosphate-buffered saline, pH 7.4, occlusively or phosphate buffer, pH 7.4, occlusively and non-occlusively. The micrographs showed the corneocytes aligned parallel to the surface of the stratum corneum embedded in intercellular lipids. The corneocytes were swollen by the uptake of water. New features located in the intercellular lamellar regions were rough structures, water pools, and occasionally vesicle-like structures. The nature of the vesicle-like structures was not completely clear. The presence of water pools, mostly in close contact with the rough structures, suggests that a lipid-water phase separation occurred. The localization of water in the intercellular region and the corneocytes offers new insights into the penetration enhancement property of water (and into the pathways of drug penetration).

Regional variation in content, composition and organization of porcine epithelial barrier lipids revealed by thin-layer chromatography and transmission electron microscopy

Epidermis and oral epithelia provide permeability barriers that limit penetration of potentially harmful agents. Barrier function is determined by lipids in the superficial epithelial layers and varies regionally by more than 10-fold. The purpose of this study was to determine whether differences in lipid content, composition or organization could account for this variation in barrier function. Stratum corneum from skin, gingiva and palate and superficial layers from buccal regions and the floor of the mouth were isolated, and lipids were extracted and analysed by thin-layer chromatography. Tissue from each region was examined by electron microscopy. There was an inverse correlation between permeability and ceramide content and a direct correlation with triglyceride content. Electron microscopy revealed that the intercellular space in epidermal stratum corneum contained multiple lipid lamellae displaying an alternating broad-narrow-broad spacing. In palatal and gingival stratum corneum, uniformly spaced lamellae were present at the periphery of dilations of the intercellular space, but the interiors of the dilations contained disorganized lamellae and electron-dense material. In the non-keratinized barriers, there was a single, broad lamella at the cell periphery and occasional short stacks of lamellae traversing the intercellular space. These intercellular lamellae may be derived from a population of membrane-coating granules that contain internal lamellae. The results suggest that ceramides may be important barrier components, even in non-keratinizing epithelia where they are very minor components. Regional differences in the physical organization of barrier lipids may also contribute to differences in barrier function.

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

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

Interaction of phosphatidylcholine liposomes with the human stratum corneum

The interaction of dimyristoylphosphatidylcholine liposomes with the human stratum corneum was investigated by confocal laser scanning microscopy and differential scanning calorimetry. Human skin is characterized by a high autofluorescence. By introducing appropriate optical filters the autofluorescence of the skin was depressed and the penetration profile of fluorescence labelled vesicles was investigated. From optical sectioning it was obvious that neither the vesicles nor the fluorophore N-(lissamine rhodamine B sulfonyl)diacylphophatidylethanolamine (Rho-PE) penetrates in detectable amounts into the human skin. Differential scanning calorimetry of human stratum corneum revealed, that the peak positions of the human stratum corneum specific endothermic transitions at 10 degrees C, 35 degrees C, 50 degrees C, 62 degrees C, 73 degrees C and 81 degrees C did not change significantly after 18 h of non-occlusive vesicle application. However, the enthalpy of the transitions at 35 degrees C, 50 degrees C, 62 degrees C and 73 degrees C, estimated through peak heights increased, relative to the protein related peak at 81 degrees C. A novel transition at 10 degrees C was observed. From these data we conclude that DMPC liposomes do not penetrate intact into the human skin. We deduce, however, that the vesicles disintegrate at the surface of stratum corneum after non-occlusive application. The individual lipid molecules then interact with the lipid barrier of the stratum corneum and penetrate into the latter, which results in an increase of the enthalpy, related to the lipid components of the SC.

Effect of hydration upon the fluidity of intercellular membranes of stratum corneum: an EPR study

The principal mechanisms controlling the molecular permeability through the skin are associated to the intercellular membranes of stratum corneum (SC), the outermost layer of mammalian skin. It is generally accepted that an increase in fluidity of these membranes leads to a reduction of the physical barrier exerted by SC with a consequent enhancement in permeation of different compounds. It is known that water diffusion in SC increases with the increase in the water content in SC. Using the spin labeling method we evaluate the effect of hydration on the fluidity of intercellular membranes at three depths of the alkyl chain. Increase in the water content in SC leads to a drastic increase in membrane fluidity especially in the region near the membrane/water interface; the effect decreases on going deeper inside the hydrophobic core. Analysis of electron paramagnetic resonance (EPR) parameters as a function of temperature showed that the rotational motion at depth of the 16th carbon atom of the chain experienced a phase transition at 45 and 60 degrees C. These phase transition temperatures were not altered by changes in the water content of SC. A phase transition between 28 and 48 degrees C was observed from the segmental motion in the region near the polar headgroup (up to 12th carbon in the chain) and was strongly dependent upon the hydration of SC. Our results give a better characterization of the fluidity of SC, the main parameter involved in the mechanisms that control the permeability of different compounds through skin.

Interactions between liposomes and human stratum corneum in vitro: freeze fracture electron microscopical visualization and small angle X-ray scattering studies

The interactions between three liposomal formulations and human stratum corneum were visualized using freeze fracture electron microscopy. A new replica cleaning method was introduced. Human stratum corneum was submerged for 48 h in liposome suspensions prepared from commercially available phospholipid mixtures. The size, lamellarity and lipid moieties of the liposomes were similar. The main difference between the three phospholipid formulations was the hydrophilicity of the headgroups. The composition dependence of the interactions between these vesicles and human stratum corneum was investigated. In essence, two types of interaction were observed: adsorption of the liposomes on to the outer surface of the stratum corneum, and ultrastructural changes in deeper layers of the stratum corneum caused by mixing of the liposomal constituents and the stratum corneum lipids. The electron microscopic observations were verified with small-angle X-ray scattering. It was found that liposomes composed of phospholipids containing relatively small hydrophilic headgroups showed a marked interaction with the skin lipids of human stratum corneum in vitro. The complexity of the phospholipid mixtures, however, made it very difficult to determine the exact effect each of these headgroups has on the interactions between these vesicles and human stratum corneum.

Wide-angle X-ray diffraction of human stratum corneum: effects of hydration and terpene enhancer treatment

Wide-angle X-ray-diffraction experiments were used to investigate the molecular organization of barrier components of human stratum corneum. Diffraction lines related to the side-by-side lipid packing arrangements in the intercellular bilayers were identified as were patterns arising from secondary protein structures in intracellular keratin. Reflections were also identified which may be produced by proteins in the corneocyte envelopes. The effects of hydration on stratum corneum structure were monitored using 0, 20-40, 40-60, 60-80 and approximately 300% hydrated samples. The packing arrangements in the intercellular lipid bilayers remained the same over the entire hydration range, as did keratin structures. A new diffraction ring, attributable to liquid water, was produced by 300% hydrated samples with a repeat spacing of 0.35 to 0.30-0.29 nm. The effects of three terpene enhancers, (+)-limonene, nerolidol and 1,8-cineole, on stratum corneum structure were monitored. Treatment with each of the terpenes produced additional reflections which were attributed to the presence of the respective liquid enhancers within the stratum corneum. (+)-Limonene produced an additional reflection at 0.503-0.489 nm, nerolidol, an additional reflection at 0.486-0.471 nm and 1,8-cineole, an intense reflection at 0.583-0.578 nm. Reflections characteristic of gel-phase lipids and crystalline lipids also remained after all terpene treatments. These results provide no clear evidence of lipid bilayer disruption by the terpenes and suggest that areas of liquid terpene exist within the stratum corneum. The mechanisms underlying propylene glycol synergy with terpene enhancers were investigated. Treatment of stratum corneum with each terpene mixed with propylene glycol gave rise to two additional reflections. One reflection, always positioned at 0.452-0.448 nm, had been observed in control studies following propylene glycol treatment and may have been associated with bilayer structures disrupted by propylene glycol or altered keratin structures. The second reflection was developed by the respective terpene enhancer. For example, treatment with a 1,8-cineole/propylene glycol mixture produced reflections at 0.457-0.451 nm (propylene glycol-disrupted lipids or altered keratin) and 0.591-0.578 nm (liquid 1,8-cineole). Since the reflection at 0.452-0.448 nm was unaffected by co-application of propylene glycol with terpene enhancers, this study offers no evidence to support the theory that propylene glycol synergy with the terpenes occurs through enhanced lipid disruption.

Subzero thermal analysis of human stratum corneum

The thermal behaviour of human stratum corneum was studied using differential thermal analysis within the temperature range of -130 degrees C to 120 degrees C. Aside from thermal transitions at around 40 degrees C, 70 degrees C, 85 degrees C and 100 degrees C, which have been reported before, a particular transition below 0 degree C (subzero), at approx. -9 degrees C (264 K), was noticed. This transition was present in the analysis curves of dehydrated as well as hydrated stratum corneum sheets and could be distinguished from the water peak found only in hydrated stratum corneum samples. To further characterize this transition, thermal analysis was performed on stratum corneum sheets: (i) after lipid extraction, (ii) after pretreatment of propylene glycol and (iii) after pretreatment of oleic acid/propylene glycol solution. From the results, it was concluded that the subzero transition (-9 degrees C) belongs to low melting lipid components of stratum corneum.

Organization of stratum corneum lipids in relation to permeability: influence of sodium lauryl sulfate and preheating

The role of the structural organization of intercorneocyte lipids in the barrier function of human stratum corneum was evaluated by treatment with heat and sodium lauryl sulfate. Measurement of transepidermal water loss in treated samples was used to quantify variations in stratum corneum permeability. Thermodynamic transition of lamellar lipids and their degree of organization were evaluated by differential scanning calorimetry and small-angle X-ray diffraction, respectively. Progressively preheating stratum corneum samples from 75 degrees C to 90 degrees C increased stratum corneum permeability to water vapor, while the fusion temperature of lamellar lipids and the intensity of the X-ray diffraction peaks of the polar lipids decreased. Sodium lauryl sulfate induced similar variations of these three parameters. These results support the hypothesis that, in addition to the chemical nature of intercorneocyte lipids, their structural arrangement and thermodynamic properties play an important role in the barrier function of the stratum corneum to water vapor.

Models of stratum corneum intercellular membranes: 2H NMR of macroscopically oriented multilayers

Deuterium NMR was used to characterize model membrane systems approximating the composition of the intercellular lipid lamellae of mammalian stratum corneum (SC). The SC models, equimolar mixtures of ceramide:cholesterol:palmitic acid (CER:CHOL:PA) at pH 5.2, were contrasted with the sphingomyelin:CHOL:PA (SPM:CHOL:PA) system, where the SPM differs from the CER only in the presence of a phosphocholine headgroup. The lipids were prepared both as oriented samples and as multilamellar dispersions, and contained either perdeuterated palmitic acid (PA-d31) or [2,2,3,4,6-2H5]CHOL (CHOL-d5). SPM:CHOL:PA-d31 formed liquid-ordered membranes over a wide range of temperatures, with a maximum order parameter of approximately 0.4 at 50 degrees C for positions C3-C10 (the plateau region). The quadrupolar splitting at C2 was significantly smaller, suggesting an orientational change at this position, possibly because of hydrogen bonding with water and/or other surface components. A comparison of the longitudinal relaxation times obtained at theta = 0 degrees and 90 degrees (where theta is the angle between the normal to the glass plates and the magnetic field) revealed a significant T1Z anisotropy for all positions. In contrast to the behavior observed with the SPM system, lipid mixtures containing CER exhibited a complex polymorphism. Between 20 and 50 degrees C, a significant portion of the entire membrane (as monitored by both PA-d31 and CHOL-d5) was found to exist as a solid phase, with the remainder either a gel or liquid-ordered phase. The proportion of solid decreased as the temperature was increased and disappeared entirely above 50 degrees C. Between 50 and 70 degrees C, the membrane underwent a liquid-ordered to isotropic phase transition. These transitions were reversible but displayed considerable hysteresis, especially the conversion from a fluid phase to solid. The order profiles, relaxation behavior, and angular dependence of these parameters suggest strongly that both the liquid-ordered CER- and SPM-membranes are bilayers. The unusual phase behavior observed for the CER-system, particularly the observation of solid-phase lipid at physiological temperatures, may provide insight into the functioning of the permeability barrier of stratum corneum.

Liposomes as carriers for topical and transdermal delivery

The delivery of active agents to the skin by liposome carriers is an interdisciplinary topic of great interest today. Data accumulated over the last decade strongly point to important advantages of these drug delivery systems. A symposium devoted to classic and new approaches in the use of liposomal systems was organized and chaired by M. Mezei and E. Touitou as a part of the Jerusalem Conference on Pharmaceutical Sciences and Clinical Pharmacology, held on May 24-30, 1992, in Jerusalem, Israel. The presentations focused on liposomes as tools in the mechanistic study of absorption promoters (T. Nagai), drug liposomal delivery in the skin strata and structures (N. Weiner), interaction of liposomes and niosomes with the human skin (H.E. Junginger), and design and characterization of caffeine liposomal systems for use in hyperproliferative diseases (E. Touitou). Mezei reviewed biodisposition and clinical studies on liposomal dosage forms containing various drugs.

Characterization of low-temperature (i.e., < 65 degrees C) lipid transitions in human stratum corneum

This study aims to characterize human stratum corneum (SC), focusing on those lipid transitions that occur at or below physiologically relevant temperatures. In the past, a lipid transition near 35 degrees C had been thought to be variable and a consequence of superficial sebaceous lipid contamination. However, analysis here indicates that it is widely present, and cannot be attributed to sebum production. We demonstrate that this transition represents a solid-to-fluid phase change for a discrete subset of SC lipids. The reversibility of this transition upon reheating, and its absence in extracted lipid samples imply that these lipids are not uniformly present throughout the SC, but would appear to be differentially distributed in response to terminal differentiation. Further, such an arrangement could involve a close association with other nonlipid (e.g., protein) components. Evidence for a new transition at approximately 55 degrees C is presented that suggests the loss of crystalline orthorhombic lattice structure. The existence of orthorhombic structure at physiologic temperature is reasoned to involve ceramides and/or free fatty acids. Localization of these lipids at the level of the corneocyte envelope supports a comprehensive picture of water transport across the SC, whereby diffusion occurs primarily via the intercellular lipids. This view, coupled with the hydration-induced changes in lipid disorder observed here provides additional insight into the mechanism by which skin occlusion increases permeability. Summarily, these results i) emphasize the inherent danger of over-interpreting experiments with isolated SC lipids, ii) emphasize the potential advantage(s) of employing several biophysical techniques to study SC structure, and iii) indicate that a full characterization of lipid phase behavior is requisite to our eventual understanding of SC structure and permeability function, particularly those phase transitions that occur near or at normal skin temperature.

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

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

Polymorphism in stratum corneum lipids

Fourier transform infrared spectroscopy (FTIR) was employed to investigate the thermotropic phase behavior of stratum corneum lipid multilamellae. Stratum corneum (SC), the uppermost layer of mammalian skin, is unusual in many respects. It has been demonstrated that the lipids of the stratum corneum provide the primary electrical and transport resistance in the skin. These lipids are unusual in their composition, structure and localization; they contain only cholesterol, fatty acids and ceramides and they form broad, multi-lamellar sheets which are located extracellularly. The FTIR results from both the symmetric CH2 stretching and the CH2 scissoring vibrations suggest that the SC lipids exhibit polymorphic phase behavior below the main phase transition temperature. The multiple phases are most likely crystalline mixtures of different alkyl chain packings, along with solid-liquid phases. Similarities between the FTIR results reported here for SC lipids and those obtained for cholesterol-containing gel phase phospholipids suggest that the non-uniform distribution of cholesterol occurs in each system.

Improvement of epidermal differentiation and barrier function in reconstructed human skin after grafting onto athymic nude mice

To determine whether epidermis reconstructed in vitro at the air-liquid interface on de-epidermized dermis has the capacity to normalize the expression of differentiation-specific markers, its lipid composition and stratum corneum barrier properties, human skin equivalents were transplanted onto athymic nude mice and investigated at different stages ranging from 1 to 4 months after grafting. Indirect immunofluorescence with species- or non-species-specific antibodies revealed that as early as 1 month after transplantation keratinization, and involucrin, loricrin and transglutaminase patterns were normalized. Human melanocytes were observed in the basal layer of the pigmented graft. As revealed by high-performance thin-layer chromatography and transmission electron microscopy after ruthenium tetroxide fixation, the lipid profile and the intracellular lamellar organization were similar to those found in natural epidermis. Transepidermal water loss measurements and penetration studies showed that the barrier properties of the reconstructed epidermis after transplantation were comparable to those of normal human skin.

A study of the percutaneous absorption-enhancing effects of cyclodextrin derivatives in rats

2-Hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) and 2,6-dimethyl-beta-cyclodextrin (D-beta-CyD) were studied for transdermal penetration enhancement of the cytochrome P450 inhibitor liarozole by an in vivo transdermal absorption rat model. The mode of action of penetration enhancement was investigated by differential scanning calorimetry (DSC). In-vivo, HP-beta-CyD, as a 20% aqueous solution, increased the absorption of liarozole approximately threefold and a 20% aqueous solution of D-beta-CyD decreased the percutaneous absorption of liarozole in blood by a factor of 0.6. However, pretreatment with D-beta-CyD (20%, 4 h) enhanced the transdermal absorption 9.4-fold. In the DSC experiments the thermal profile of human stratum corneum was practically unchanged after treatment with HP-beta-CyD, but treatment with D-beta-CyD revealed an interaction of D-beta-CyD with the protein and lipid fraction. Thus the results from DSC and those from the permeability experiments revealed that D-beta-CyD acts as a transdermal absorption enhancer by changing the stratum corneum barrier whereas HP-beta-CyD influences the partitioning behaviour of the drug in the skin.

A critical comparison of some Raman spectroscopic techniques for studies of human stratum corneum

This study evaluates a variety of techniques and sampling conditions for Raman spectroscopic investigations of human stratum corneum. Using a Fourier-transform Raman system and samples of stratum corneum in vitro, we demonstrated minimal inter- and intracadaver variations in molecular vibrations. We have also shown Raman spectroscopy to be relatively insensitive to the hydration state of human stratum corneum, indicating that the technique should be valuable for monitoring the transdermal delivery of drugs from aqueous solutions. The stability of human stratum corneum to near-infrared laser excitation was verified by spectral collection for approximately 1 hr. We have also compared FT-Raman spectra from human stratum corneum in vitro and in vivo. Of the different types of Raman instruments used in this study (visible-light excitation microprobe, visible-light excitation macroscopic sampling, and Fourier-transform Raman), the FT-Raman system provided good-quality spectra with high sample throughput, but systems using visible-light excitation should provide unique information for use in specialist applications.