Electroperturbation of human stratum corneum fine structure by high voltage pulses: a freeze-fracture electron microscopy and differential thermal analysis study

Application of high voltage pulses (HVP) to the skin has been shown to promote the transdermal drug delivery by a mechanism involving skin electroporation. The aim of this study was to detect potential changes in lipid phase and ultrastructure induced in human stratum corneum by various HVP protocols, using differential thermal analysis and freeze-fracture electron microscopy. Due to the time involved between the moment the electric field is switched off and the analysis, only "secondary" phenomena rather than primary events could be observed. A decrease in enthalpies for the phase transitions observed at 70 degrees C and 85 degrees C was detected by differential thermal analysis after HVP treatment. No changes in transition temperature could be seen. The freeze-fracture electron microscopy study revealed a dramatic perturbation of the lamellar ordering of the intercellular lipid after application of HVP. Most of the planes displayed rough surfaces. The lipid lamellae exhibited rounded off steps or a vanished stepwise order. There was no evidence for perturbation of the corneocytes content. In conclusion, the freeze-fracture electron microscopy and differential thermal analysis studies suggest that HVP application induces a general perturbation of the stratum corneum lipid ultrastructure.

Stratum corneum lipid composition and structure in cultured skin substitutes is restored to normal after grafting onto athymic mice

Restoration of an epidermal barrier is a definitive requirement for wound closure. Cultured skin substitutes grafted onto athymic nude mice were used as a model for a long-term study of stratum corneum barrier lipid metabolism and organization. Samples of stratum corneum collected after 12 and 21 d in vitro and 6, 11, and 24 mo postgrafting were examined for their lipid and fatty acid composition, and their lipid organization and structure using electron microscopy and small angle X-ray diffraction, respectively. All of these methods confirm the impaired barrier function of cultured skin substitutes in vitro, as judged from the deviations in lipid composition and from poor organization of the stratum corneum lipids that show no lamellar structure. At 6 mo postgrafting, the total stratum corneum lipid profiles of the epidermal grafts is close to that of the human stratum corneum with the exception of the presence of mouse specific lipids. The increase of ceramides 4-7 in cultured skin substitutes after grafting indicates restored activity of processes involved in the hydroxylation of fatty acids and sphingoid bases. Conversely, the ceramide profile still reveals some abnormalities (elevated content of ceramide 2 and slightly lower content of ceramide 3) and the content of long-chain fatty acids remains below its physiologic level at 6 mo postgrafting, but normalizes by 2 y postgrafting. The ultramicroscopic observations revealed the formation of lamellar extracellular lipid domains by 4 mo postgrafting. Despite these findings, the X-ray diffraction showed differences in the diffraction pattern at 2 y after grafting, suggesting that the organization of stratum corneum lipids in all epidermal grafts differs from that of the native skin.

Study on the lipid organization of stratum corneum lipid models by (cryo-) electron diffraction

The barrier function of the skin resides in the stratum corneum (SC). This outermost layer consists of protein-rich corneocytes and lipid-rich intercellular domains. These domains form the rate-limiting step for transepidermal water loss and the penetration of substances from the environment. To study the nature of the barrier function, stratum corneum lipid models have been examined with wide-angle X-ray diffraction. A disadvantage of this technique is that it requires bulk quantities of lipid and thus information on variations in the lateral packing cannot be obtained in the microm-range. To the best of our knowledge, this is the first study in which electron diffraction is applied on SC lipid model systems. Using this technique, local structural information was obtained about mixtures prepared from isolated pig ceramides, cholesterol, and long-chain free fatty acids. It appeared that addition of free fatty acids caused a transition from a hexagonal to an orthorhombic packing and that electron diffraction can be applied to distinguish between these two lattices. The results are in good agreement with wide-angle X-ray diffraction data and suggest that application of electron diffraction in skin studies can provide new information on the lipid organization in well-defined areas of the stratum corneum.

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

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

Interactions between liposomes and human skin in vitro, a confocal laser scanning microscopy study

One major problem in (trans)dermal drug delivery is the low penetration rate of drugs through the barrier of the skin. Encapsulation of a drug in lipid vesicles is one strategy to increase the penetration rate of a drug across the skin. In this study, the interactions between fluorescent-labelled liposomes and skin are visualized by confocal laser scanning microscopy (CLSM). Bilayer labelled gel-state and liquid-state liposomes (conventional or with flexible bilayers) were non-occlusively applied on human skin in vitro. The penetration pathway and penetration depth of the lipophilic fluorescent label into the skin were visualized. From the CLSM images, it was clear that the label applied in micelles and gel-state liposomes did not penetrate as deep into the skin as the label applied in liquid-state vesicles. Among the liquid-state vesicles, the suspensions with the flexible bilayers showed the highest fluorescence intensity in the dermis. Thus, the thermodynamic state of the bilayer and, to a smaller extent, the flexibility of the bilayer influence, strongly the penetration depth of the label into the skin. The label applied non-occlusively in flexible liposomes penetrated deeper into the skin than after occlusive application.

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

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

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.

Enhanced permeability of freeze-dried liposomal bilayers upon rehydration

Until now, studies on the protection of liposomes against freeze-drying damage have mainly focused on the bilayer integrity during the freezing or drying step of this process. Here, we investigated the bilayer permeability of freeze-dried, lyoprotected liposomes to a nonbilayer interacting compound after rehydration, by monitoring the leak-in kinetics of externally added carboxyfluorescein (CF). The results showed that freeze-drying and rehydration of DPPC:DPPG 10:1 liposomes with sucrose in- and outside the vesicles caused a temporary increase in the bilayer permeability for CF, which leveled off after approximately 20 h. The amount of CF/mol phospholipid which leaked into the vesicles increased with vesicle size (range 0.1-1 micro m) / lamellarity. Reduction of the number of bilayers in 1-1 micro m) vesicles enhanced the permeability to CF after freeze-drying and rehydration. The presence of CHOL decreased CF leak-in rates into 1 micro m MLVs consisting of DPPC:DPPG 10:1, but not into 0.1-micro m unilamellar vesicles. In the absence of sucrose similar leak-in profiles as a function of time were found after rehydration, suggesting that repacking processes of the bilayer were responsible for the enhanced permeability after freeze-drying and dehydration both with and without sucrose. The effect of size and lamellarity on the CF leak-in correlated with the retention of encapsulated CF after freeze-drying and rehydration, but no correlation was found with the effect of lipid composition. Both small (0.1 micro m) lyoprotected liposomes made of DPPC:DPPG 10:1 and DPPC:DPPG:CHOL 10:1:4 were highly permeable during the rehydration step itself. The results indicate that, despite the presence of the lyoprotectant, "repacking" of the bilayer components takes place both during and after rehydration. This eventually leads to regaining of its barrier function.

Development of an optimal protocol for the ultrastructural examination of skin by transmission electron microscopy

The intercellular lipid bilayers of the stratum corneum provide the permeability barrier of the skin. To perform an electron microscopical examination of the ultrastructure of these bilayers, ruthenium tetroxide fixation is required. In this study an optimal fixation protocol was developed and selected upon comparing seven different fixation procedures, using glutaraldehyde (GA) and the postfixatives ruthenium red, osmium tetroxide (OsO4) and ruthenium tetroxide (RuO4) in combination with potassium ferrocyanide (K4Fe(CN)6) and potassium ferricyanide (K3Fe(CN)6). Instead of fixing skin with either OsO4 or RuO4, these two fixatives were combined in one protocol. In addition, the use of RuRed was introduced and the influence of both K4Fe(CN)6 and K3Fe(CN)6 in combination with RuO4 were examined. Furthermore, we compared two dehydration solvents, methanol and acetone. The most satisfying results were obtained when the skin was prefixed in GA and postfixed in OsO4 and RuO4 with K3Fe(CN)6, i.e. with Fe in its highest oxidation state (Fe3+). No differences were observed between dehydration in methanol and acetone.

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

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

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

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

In vitro human skin barrier modulation by fatty acids: skin permeation and thermal analysis studies

PURPOSE

This study aims to elucidate the skin permeation enhancement and the skin perturbation effects of a number of fatty acids, i.e. straight-chain saturated (SFA), monounsaturated (MUFA) and polyunsaturated acids (PUFA).

METHODS

The skin permeation enhancement effects were studied using human stratum comeum (SC) and p-aminobenzoic acid (PABA) as a model permeant. The fatty acids in propylene glycol (FA/PG) were applied according to a pre-treatment/co-treatment protocol. The perturbation effects were studied using differential thermal analysis (DTA) on SC after pretreatment with FA/PG.

RESULTS

SFA with 6 to 12 carbons exhibit a parabolic correlation between enhancement effect and chain-length, with a maximum at nonanoic-decanoic acids (with 9 and 10 carbons). Nonanoic and decanoic acids exert barely noticeable effects on the thermal behaviour of SC, suggesting that they easily mix with the skin lipids. All cis-6-, 9-, 11- or 13-octadecenoic acids (MUFA) enhance the permeation of PABA to the same extent. DTA revealed that the cis-9- and 13-isomers form a separate domain containing mostly the pure fatty acids within the SC lipids and suppress the lipid transitions at 70 degrees/80 degrees C. PUFA--linoleic (LA), alpha-linolenic (ALA) and arachidonic acids--enhance PABA permeation stronger than MUFA but additional double bonds do not further increase the degree of enhancement. LA and ALA form separate domains but do not completely suppress the SC lipid transitions at 70 degrees/80 degrees C. Increase in the enthalpy changes of 70 degrees/80 degrees transitions linearly correlates to the decrease in the permeability coefficients, suggesting that an increased perturbation of the skin lipids not necessarily has to yield an increased PABA permeation.

CONCLUSIONS

The enhancement effects of fatty acids on the PABA penetration through SC are structure-dependent, associated with the existence of a balance between the permeability of pure fatty acids across SC and the interaction of the acids to skin lipids.

The use of vibratome sections for the ruthenium tetroxide protocol: a key for optimal visualization of epidermal lipid bilayers of the entire human stratum corneum in transmission electron microscopy

In this paper we describe a preparative procedure which allows maximal visualization of all lipid bilayers in the human stratum corneum (SC). We used 50-microns-thick vibratome sections of paraformaldehyde/glutaraldehyde-fixed human skin. The sections were post-fixed in 1% osmium tetroxide and 0.5% ruthenium tetroxide. The vibratome sections were dehydrated only in 70% ethanol in order to prevent dissolution of the lipids. Lipid bilayers, including the alternating electron-dense and electron-lucent lamellae, were visible between all cell layers of the SC. In addition, this preparative procedure also appeared to be excellent for the ultrastructural preservation of lamellar bodies.

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.

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

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

Phase behavior of isolated skin lipids

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

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).

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.

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.

Lipid organization in pig stratum corneum

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

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

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

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.

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.

Estradiol permeation from nonionic surfactant vesicles through human stratum corneum in vitro

The permeation of estradiol from vesicular formulations through human stratum corneum was studied in vitro. The vesicles were composed of nonionic n-alkyl polyoxyethylene ether surfactants (CnEOm). The thermodynamic activity of estradiol present in each formulation was kept constant by saturating all formulations with estradiol. The effects of both the particle size and the composition of the formulation on estradiol permeation across excised human stratum corneum were investigated. Stratum corneum that was pretreated with empty surfactant carriers allowed for significantly higher estradiol fluxes compared with untreated stratum corneum. However, estradiol fluxes obtained in these pretreatment experiments appeared to be significantly lower than those obtained by the direct application of the estradiol-saturated carrier formulation on top of the stratum corneum. Furthermore, in the case of pretreatment of the stratum corneum, an increase in carrier size resulted in a decrease in estradiol flux. For direct application the opposite was found. Two mechanisms are proposed to play an important role in vesicle-skin interactions, i.e., the penetration enhancing effect of surfactant molecules and the effect of the vesicular structures that are most likely caused by adsorption of the vesicles at the stratum corneum-suspension interface.

Effect of divalent cations on lipid organization of cardiolipin isolated from Escherichia coli strain AH930

Escherichia coli strain AH930 is a lipid biosynthetic mutant, which is unable to synthesize phosphatidylethanolamine. Instead it produces large amounts of phosphatidylglycerol and cardiolipin and has an absolute requirement for certain divalent cations. Cardiolipin was isolated from this mutant strain and its interaction with divalent cations was studied by various biophysical techniques. Monolayer measurements showed that the cations decrease the molecular surface area of cardiolipin in the order Ca2+ approximately Mg2+ > Sr2+ > Ba2+. 31P-NMR and X-ray diffraction measurements demonstrated a comparable sequence for the ability of the cations to promote HII phase formation in dispersions of the E. coli cardiolipin: Ca2+ and Mg2+ induced HII phase formation at 50 degrees C, Sr2+ at 75 degrees C, while Ba2+ was found to be unable to promote HII phase formation in the temperature range measured. Furthermore, all divalent cations were found to increase the temperature at which the transition to the liquid-crystalline phase takes place, which was below 5 degrees C for the lipid in the absence of divalent cations. In the presence of Sr2+, Mg2+ and Ba2+ and at 25 degrees C two lamellar phases were observed, one corresponding to a liquid-crystalline phase, the other to either a gel or a crystalline phase. In the presence of Ca2+ at 25 degrees C and even at 45 degrees C no evidence for a liquid-crystalline phase was obtained and only a crystalline phase could be observed. The ability of the different cations to promote HII phase formation in the isolated E. coli cardiolipin was found to correlate with their ability to support growth of the mutant strain (De Chavigny, A., Heacock, P.N., Dowhan, W. (1991) J. Biol. Chem. 266, 5323-5332), suggesting that cardiolipin with divalent cations can replace the role of phosphatidylethanolamine in the mutant strain, and that this role involves the preference of these lipids for organization in non-bilayer lipid structures.

Water mobility and structure in poly[2-hydroxyethylmethacrylate] hydrogels by means of the pulsed field gradient NMR technique

The translational mobility of water in poly[2-hydroxyethylmethacrylate] (pHEMA) hydrogels, cross-linked with ethyleneglycoldimethacrylate, was studied by means of the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique, which offers the opportunity to study the molecular displacements directly under well-defined equilibrium conditions, resulting in a determination of the self-diffusion coefficient. It is possible to check whether coexisting water phases with different mobilities (on a timescale of ca. 10 ms) are present. The dependence of the diffusion coefficient of water on the degree of hydration and the cross-linker concentration was measured. Magnetic interaction is found to cause cross-relaxation between the protons of water and those of the polymer matrix. This affects the data, rendering the evaluation by the standard equation invalid. An equation taking cross-relaxation into account has been derived. Amplitude measurements have shown that all the water in the gels contributes to the NMR signal. The PFG measurements have shown that the total water phase in a gel diffuses as one homogeneous phase, which can be characterized by a diffusion coefficient. The self-diffusion coefficient is strongly dependent on the degree of hydration of the gel; the cross-linker concentration has no measurable effect. The strong dependence of the diffusion of water on its concentration in the gel has consequences for the modelling of the swelling and drug-release dynamics of pHEMA and necessitates a revision of the present models describing these processes.

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

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

Bioadhesion by means of specific binding of tomato lectin

The possibility of developing bioadhesive drug delivery systems on the basis of molecules which selectively bind to the small intestinal epithelium by specific, receptor-mediated mechanisms was investigated using a lectin isolated from tomato fruits (Lycopersicum esculentum). The tomato lectin (TL) was found to bind specifically onto both isolated, fixed pig enterocytes and monolayers of human Caco-2 cell cultures with a similar affinity. TL-coated polystyrene microspheres (0.98 micron) also showed specific binding to enterocytes in vitro. Lectin binding was found to be favored at neutral pH and to be reduced in an acidic environment. Crude pig gastric mucin, however showed a marked cross-reactivity in vitro, indicating that lectin binding to the cell surface in vivo might be inhibited by mucus.

A surface energy analysis of mucoadhesion: contact angle measurements on polycarbophil and pig intestinal mucosa in physiologically relevant fluids

The possible role of surface energy thermodynamics in mucoadhesion was investigated with Polycarbophil and pig intestinal mucosa. In separate experiments, the surface energy parameters of the substrate (mucosa) and the adhesive (polymer film) were determined by contact angle measurements on captive air/octane bubbles in three physiologically relevant test fluids (isotonic saline, artificial gastric fluid, and artificial intestinal fluid). Whereas the swollen Polycarbophil films were relatively hydrophilic as indicated by small water contact angles (22, 23, and 16 degrees), the water contact angles measured on mucosal tissue were significantly larger (61, 48, and 57 degrees). Hence, mucus was found to possess an appreciable hydrophobicity. The measured adhesive performance (force of detachment) between Polycarbophil and pig small intestinal mucosa was highest in nonbuffered saline medium, intermediate in gastric fluid, and minimal in intestinal fluid. In agreement with this trend, the mismatch in surface polarities between substrate and adhesive, calculated from the contact angle data, increased in the same order.

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

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

Transport of peptide and protein drugs across biological membranes

The transport characteristics of peptide and proteins drugs across various epithelial membrane barriers are outlines. These include transport through the intestinal, buccal, nasal and pulmonary absorptive mucosae, as well as transdermal penetration. Because peptides and proteins are hydrophilic and high molecular weight compounds, they commonly show minor permeability across the mentioned biological membranes. In order to improve their transport properties and thereby their systemic bioavailability, several strategies can be undertaken, such as the synthesis of stabilized and lipophilic analogues, the application of absorption enhancers and protease inhibitors, and the design of suitable dosage forms (e.g., liposomes, biodegradable nanocapsules, bioadhesive microspheres).

Thermal analysis of water in p(HEMA) hydrogels

Hydrogels composed of poly(hydroxyethyl methacrylate) (pHEMA) and water were investigated using differential thermal analysis (DTA) and adiabatic calorimetry (AC). The results show that the crystallization of water in the gels is a very gradual process, leading to the development of a metastable, non-equilibrium state. They are not in agreement with models that assume the actual presence of thermodynamically different classes of water in the gels, based on the abnormal melting behaviour of this water. The results indicate that the internal structure of these gels can be described as that of an elastic solution, in which the water molecules are distributed continuously over all possible orientations to and interactions with the polymer.

Thermal behavior of poly hydroxy ethyl methacrylate (pHEMA) hydrogels

The freezing and melting behavior of water in poly hydroxy ethyl methacrylate (pHEMA) hydrogels of different cross-linker and water contents was investigated in relation to the glass transition temperature (Tg) of the gels. After prolonged cooling at -15 degrees C a constant amount of 1.7 mol water per monomeric unit did not freeze, regardless of both the cross-linker and the water content of the gels. At this water content and temperature, pHEMA gels were below their Tg, and the water molecules were prevented from diffusing to the ice crystals formed in the gel. Therefore, the inability of part of the water in pHEMA gels to freeze is not a thermodynamic phenomenon but is caused by kinetic factors.

Synthetic hydrogels as drug delivery systems

Hydrogels are widely studied materials for the preparation of sustained release drug dosage forms. Their soft, tissue-like consistency and their high biocompatibility in a number of applications make them promising candidates for this purpose. The water and the polymer in the gel form intricate structures and much research has been devoted to the elucidation of these structures, and of the interactions involved in their formation. Simple, drug-loaded hydrogels normally give a matrix-type delivery profile, in which the release rate is proportional to the square root of time; a number of approaches has been used to change this profile to other types of delivery, for instance to zero-order release. A number of in vivo tests using hydrogel delivery systems has given favourable results.