Human stratum corneum lipids have a distorted orthorhombic packing at the surface of cohesive failure

Effect of Surfactant-Keratin Hydrolysate Interactions on the Hydration Properties of a Stratum Corneum Substitute

A novel stratum corneum substitute (SCS) has been developed, and the fundamental mechanism of the dehydration process has been studied using the SCS. After washing with cleansers which contain surfactants, our skin "feels" dehydrated (or hydrated). Although many studies have focused on the effect of surfactants on the regulation of the water loss by the lipid bilayers in the stratum corneum (SC) for a long timescale or at equilibrium, only few studies have focused on the acute effect of the surfactant interaction on dehydration. In addition, the interaction between the surfactant and keratin has been often underappreciated compared to lipid bilayers although keratin is the major nonaqueous component of the SC. Here, we have developed novel SCS models, nonkeratinized (lipid only) and keratinized, to study the effect of keratin hydrolysates on the dehydration rate. We have confirmed that the lipid organizational structure of the SCS was similar to that of the human SC using X-ray scattering. We have revealed that keratin hydrolysates play a significant role in the dehydration rate, accelerating the rate for the short term. We have also demonstrated that the effect of surfactants on dehydration is more pronounced for keratinized samples than that for the nonkeratinized sample. However, the dehydration rate for the nonkeratinized SCS with the surfactant became faster than the that for the keratinized SCS after the 20 min evaporation process, suggesting that the water binding sites of keratin hydrolysates slowed down evaporation, while the surfactant interacting with the lipids accelerated the water loss. Lastly, the study demonstrated that the SCS model can be a great platform to test macroscopic properties and analyze the underlying mechanism at the molecular level for various chemicals.

Studying the penetration of fatty acids into human skin by ex vivo TOF-SIMS imaging

Fatty acids classified as chemical penetration enhancers (CPEs) might cause the fluidization and perturbation of stratum corneum (SC) lipid matrix. The penetration of oleic, linoleic, lauric and capric acids into human skin was studied by time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging and related to fatty acids enhancing effect on lipophilic model drug tolnaftate penetration into human epidermis and dermis ex vivo. Fatty acid enhancing effect on tolnaftate penetration into human skin was evaluated using Bronaugh-type flow-through diffusion cells. After in vitro penetration studies visualization and spatial localization of fatty acid molecules in human skin were performed using TOF-SIMS. Penetration of oleic, linoleic, lauric and capric acids into human skin was compared to the control skin sections by ion images and intensity profiles. Only oleic acid significantly (P<0.05) enhanced tolnaftate penetration into epidermis (enhancing ratio equal to 1.867). CPE might have no effect on model drug penetration enhancement, but might penetrate itself into the skin.

Hydration effects on skin microstructure as probed by high-resolution cryo-scanning electron microscopy and mechanistic implications to enhanced transcutaneous delivery of biomacromolecules

Although hydration is long known to improve the permeability of skin, penetration of macromolecules such as proteins is limited and the understanding of enhanced transport is based on empirical observations. This study uses high-resolution cryo-scanning electron microscopy to visualize microstructural changes in the stratum corneum (SC) and enable a mechanistic interpretation of biomacromolecule penetration through highly hydrated porcine skin. Swollen corneocytes, separation of lipid bilayers in the SC intercellular space to form cisternae, and networks of spherical particulates are observed in porcine skin tissue hydrated for a period of 4-10 h. This is explained through compaction of skin lipids when hydrated, a reversal in the conformational transition from unilamellar liposomes in lamellar granules to lamellae between keratinocytes when the SC skin barrier is initially established. Confocal microscopy studies show distinct enhancement in penetration of fluorescein isothiocyanate-bovine serum albumin (FITC-BSA) through skin hydrated for 4-10 h, and limited penetration of FITC-BSA once skin is restored to its natively hydrated structure when exposed to the environment for 2-3 h. These results demonstrate the effectiveness of a 4-10 h hydration period to enhance transcutaneous penetration of large biomacromolecules without permanently damaging the skin.

Phase behavior of an equimolar mixture of N-palmitoyl-D-erythro-sphingosine, cholesterol, and palmitic acid, a mixture with optimized hydrophobic matching

The phase behavior and lipid mixing properties of an equimolar mixture of nonhydroxylated palmitoyl ceramide (Cer16), palmitic acid (PA), and cholesterol have been investigated using 2H NMR and vibrational spectroscopy. This mixture is formed by the three main classes of lipids found in the stratum corneum (SC), the top layer of the epidermis, and provides an optimized hydrophobic matching. Therefore, its behavior highlights the role played by hydrophobic matching on the phase behavior of SC lipids. We found that, below 45 degrees C, the mixture is essentially formed of coexisting crystalline domains with a small fraction of lipids (less than 20%) that forms a gel or fluid phase, likely ensuring cohesion between the solid domains. Upon heating, there is the formation of a liquid ordered phase mainly composed of PA and cholesterol, including a small fraction of Cer16. This finding is particularly highlighted by correlation vibrational microspectroscopy that indicates that domains enriched in cholesterol and PA include more disordered Cer16 than those found in the Cer16-rich domains. Solubilization of Cer16 in the fluid phase occurs progressively upon further heating, and this leads to the formation of a nonlamellar self-assembly where the motions are isotropic on the NMR time scale. It is found that the miscibility of Cer16 with cholesterol and PA is more limited than the one previously observed for ceramide III extracted from bovine brain, which is heterogeneous in chain composition and includes, in addition to Cer16, analogous ceramide with longer alkyl chains that are not hydrophobically matched with cholesterol and PA. Therefore, it is inferred that, in SC, the chain heterogeneity is a stronger criteria for lipid miscibility than chain hydrophobic matching.

Atomic force microscopy as an innovative tool for nanoanalysis of native stratum corneum

This study demonstrates an innovative application of atomic force microscopy (AFM). The combination of high-resolution AFM technology and tape stripping is presented as a tool for the structure analysis of human stratum corneum (SC) at a nanometer scale. Topographic images with a vertical resolution of about 10 nm of the SC are presented. Topographical and structural differences between aged and young skin can be observed. Aged skin SC is characterized by an increased single-cell surface area, prominent intercellular gaps and enhanced cell surface roughness. The use of AFM in combination with other already established methods, e.g. tape stripping in the field of dermatological research will give new insights to the structure, function and morphodynamics of SC.

Study of human stratum corneum and extracted lipids by thermomicroscopy and DSC

A study on the thermal behavior of human stratum corneum and lipids is described. The use of high scanning rate DSC for both SC and extracted lipids allows the consistent determination of transition temperatures, including those of lower energy. Changes are found both at physiological and higher temperatures. There is a clear correspondence between the thermotropic behavior of these two systems. However, one of the transitions found in human SC (approximately 55 degrees C) is absent in extracted lipids and may be ascribed to those covalently-linked to corneocytes. Lipidic thermotropic behavior is clearly found above 100 degrees C, in which proteins do not play an exclusive role. Changes related to most transitions are observed directly by polarized light thermal microscopy in extracted lipids. This technique also allowed for the observation of large segregated domains in the extracted lipids. A drastic change is observed at approximately 60 degrees C, corresponding to the disruption of the lamellar structure.

Influence of the lipid composition on the organization of skin lipid model mixtures: an infrared spectroscopy investigation

The polymorphism of the lipids of the stratum corneum (SC), the top layer of the epidermis, has a fundamental impact on the permeability properties of the skin barrier. In this work, we have examined by infrared spectroscopy the thermal behavior of model mixtures involving ceramide, palmitic acid and cholesterol, the three main components of the SC lipids, to gain a refined description of the participation of the various lipid species in the different phases observed as a function of temperature. The results show that below 40 degrees C ceramide, cholesterol and palmitic acid exist mainly in crystalline domains and the lipidic species show very limited miscibility. Between 40 and 50 degrees C, a transition from the crystalline to a liquid ordered (lo) phase occurs and it involves ceramides, cholesterol and palmitic acid. When the mixture has a high cholesterol content, this lo phase is stable up to 75 degrees C. For low cholesterol content, the mixtures undergo a second transition toward a more disordered phase which is likely not lamellar. The formation of these phases is critically dependent on the lipid composition and, therefore, it is likely that composition changes of SC lipids affect the phase behavior and, consequently, the skin barrier properties.

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

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

Electron diffraction provides new information on human stratum corneum lipid organization studied in relation to depth and temperature

The outermost layer of mammalian skin, the stratum corneum, provides the body with a barrier against transepidermal water loss and penetration of agents from outside. The lipid-rich extracellular matrix surrounding the corneocytes in the stratum corneum is mainly responsible for this barrier function. In this study (cryo-) electron diffraction was applied to obtain information about the local lateral lipid organization in the extracellular matrix in relation to depth in human stratum corneum. For this purpose, stratum corneum grid-strips were prepared from native skin in vivo and ex vivo. It was found that the lipid packing in samples prepared at room temperature is predominantly orthorhombic. In samples prepared at 32 degrees C the presence of a hexagonal packing is more pronounced in the outer layers of the stratum corneum. Gradually increasing the specimen temperature from 30 to 40 degrees C induced a further transition from an orthorhombic to a hexagonal sublattice. At 90 degrees C all lipids were present in a fluid phase. These results are in good agreement with previously reported wide angle X-ray diffraction and Fourier transformed infrared spectroscopy studies. We conclude that the lipids in human stratum corneum are highly ordered throughout the stratum corneum and that electron diffraction allows monitoring of the local lipid organization, which contributes to the understanding of stratum corneum barrier function.

Quantitative detection of silicone in skin by means of electron spectroscopy for chemical analysis (ESCA)

BACKGROUND

Evaluation of silicone-induced morbidity in skin has been hampered by the difficulty of detecting silicone in tissue because conventional methods are nonquantitative and insensitive.

OBJECTIVE

We attempted to determine whether silicone could be identified and quantitated in skin by means of electron spectroscopy for chemical analysis (ESCA).

METHODS

Skin biopsy specimens were obtained from the nose, chin, malar region, and inner arm of a patient who had received injections of silicone gel in his nose and chin. Frozen sections were dried under vacuum and examined by means of ESCA. Contiguous sections were examined by light microscopy.

RESULTS

The surface concentrations of silicone were as follows: chin, 20.6% +/- 3.6%; nose, 19.0%; malar region, 2.6% +/- 1.6%; inner arm, 0.0% +/- 0.0%. Light microscopy revealed homogeneous "globules" consistent with silicone in the chin and nose sections only; the malar region and inner arm sections showed no evidence of silicone.

CONCLUSION

ESCA can be used to detect silicone in skin in a specific, highly sensitive, and quantitative manner. This is the first report of quantification of silicone in skin by means of ESCA.

Mechanistic studies of molecular transdermal transport due to skin electroporation

The application of electrical high voltage pulses has been shown to greatly enhance the transdermal transport of water-soluble compounds. The resistance of the skins most important barrier, the stratum corneum, drops within less than 1 µs by orders of magnitude. This effect is attributed to electroporation, a nonthermic phenomena known to occur in phospholipid double layers. The striking difference between the stratum corneum lipid layers and the usually investigated phospholipid systems is the phase transition temperature. While lipid layers used for electroporation experiments are in liquid crystal phase above the phase transition temperature, the stratum corneum lipids (phase transition at approximately 70 degrees C) form a rigid quasi-crystalline membrane at room temperature.After the electrical stimulus a recovery of the passive flux was found making high voltage pulsing a suitable tool for controlling transdermal drug delivery. By ordinary light microscopy no dramatic changes in skin structure were found supporting the thesis of electroporation. However the microstructure shows clearly persistent structural changes. Recently the involvement of Joule heating due to the electric stimulus was shown as an important factor for skin permeabilization and molecular transport.

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.

Scanning probe microscopy

During the past year, scanning probe microscopy, especially atomic force microscopy (AFM), has taken root in the biological sciences community, as is evident from the large number of publications and from the variety of specialized journals in which these papers appear. Furthermore, there is a strong indication that the technique is evolving from a qualitative imaging tool to a probe of the critical dimensions and properties of biomolecules and living cells. The next stage of the evolution involves the development of microinstruments for process control and sensing applications. Recent advances have been reported in AFM instrumentation and method. For example, the tapping mode of operation is becoming the method of choice to image biological molecules; work to extend tapping-mode operation in liquids has been reported. Biological molecules can also be imaged at low temperature in a cryo-AFM with improved resolution. The measurement of recognition forces between individual molecules continues to attract much attention and has spawned new concepts for ultra-sensitive biosensors. The AFM is being used increasingly for property measurements such as determining the viscoelastic properties of biological molecules. Finally, structural studies using the AFM abound. Some specific highlights include the mapping of DNA using restriction enzymes, imaging during DNA transcription and determining the mode of drug binding to DNA.

Lipid domains and orthorhombic phases in model stratum corneum: evidence from Fourier transform infrared spectroscopy studies

A three component model for the lipid barrier of the stratum corneum (SC) consisting of ceramide III, cholesterol, and perdeuterated palmitic acid, has been characterized by Fourier transform infrared spectroscopy. At physiological temperature the CD2 scissoring mode of the palmitic acid methylenes, and the CH2 rocking mode of the ceramide methylenes, are each split into two components. This indicates that both components exist in separate, conformationally ordered phases, probably with orthorhombic perpendicular subcells. The magnitude of the splitting indicates that the domains are at least 100 chains in size. The thermotropic behavior of the CD2 stretching vibrations demonstrates that conformational disordering of the palmitic acid commences at 42 degrees C with a transition midpoint of 50 degrees C. The CH2 stretching frequency indicates the ceramide chains remain ordered until 50 degrees C then disorder with a midpoint of 67 degrees C. The results provide a molecular characterization for the complex low temperature (10-40 degrees C) dynamic behavior suggested by recent 2H NMR experiments.