Stratum corneum ceramides

FIGURE 3.1 Representative structures of human stratum corneum ceramides. 

Melnik, B.C., Hollmann, J., and Plewig, G., Decreased stratum corneum ceramides in atopic individuals-pathobiochemical factor in xerosis, Br. J. Dermatol., 119, 547, 1988. 

Farwanah H, Nuhn P, Neubert R, Raith K. Normal phase LC separation of stratum corneum ceramides with detection by evaporative light scattering and APCl massspectrometry. Anal. Chim. Acta 2003 492 233-239. 

Conti, A. Rogers, J. Verdejo, P. Harding, C.R. Rawlings, A.V. Seasonal influences on stratum corneum ceramide 1 fatty acids and the influence of topical essential fatty acids. Int J. Cosmet. Sci. 1996, 18, 1-12. 

The stratum corneum consists of separated, nonviable, cornified, almost nonpermeable corneocytes embedded into a continuous lipid bilayer made of various classes of lipids, for example, ceramides, cholesterol, cholesterol esters, free fatty acids, and triglycerides [6], Structurally, this epidermis layer is best described by the so-called brick-and-mortar model [7], The stratum corneum is crucial for the barrier function of the skin, controlling percutaneous absorption of dermally applied substances and regulating fluid homeostasis. The thickness of the stratum corneum is usually 10-25 /an, with exceptions at the soles of the feet and the palms, and swells several-fold when hydrated. All components of the stratum corneum originate from the basal layer of the epidermis, the stratum germinativum. 

M. W. de Jager, G. S. Gooris, I. P. Dolbnya, W. Bras, M. Ponec, and J. A. Bouwstra. Novel lipid mixtures based on synthetic ceramides reproduce the unique stratum corneum lipid organization. J. Lipid Res. 45 923-932 (2004). 

The success of the Potts-Guy equation led many authors to advocate a single mechanism as the rate determining step for permeation through the skin barrier for all or at least a wide range of solutes diffusion was assumed to occur primarily via the interkeratinocyte lipids of the stratum corneum, a mixture of ceramides, fatty acids, and sterols. While from a macroscopic point of view these lipids may be modeled as a bulk solvent, on a microscopic scale they... 

Using a similar approach, Notman et al. [81], determined the free energy for pore formation in bilayers composed of ceramide, as a model for the stratum corneum of the skin, both in the presence and in the absence of DMSO. Without DMSO, the bilayer was in the gel phase, and interestingly, a hydrophobic pore was observed with a high free-energy barrier ( 60 kj/mol). In the presence of DMSO, the bilayer was more fluid, and the more typical hydrophilic pore was observed, with a much smaller activation energy of 20kJ/mol. This work provided a thermodynamic and structural explanation for the enhanced permeability of skin by DMSO. 

As an emollient, squalene is expected to increase skin hydration due to skin surface occlusions. In addition, squalene is a substance believed to maintain moisture in the stratum corneum. Novel substitutes were researched for vernix caseosa which is a reported highly efficient barrier cream for facilitating stratum corneum hydration for barrier-deficient skins. For this purpose, various lipid fractions were mixed with squalene, triglycerides, cholesterol, ceramides, and fatty acids to produce a mixture that can generate similar compositions of vernix caseosa (Rissmann et ah,... 

The Lipid Organization in Stratum Corneum and Model Systems Based on Ceramides... 

The lipid composition changes dramatically during terminal differentiation. After extrusion from the lamellar bodies, the polar lipid precursors are enzymatically converted into more hydrophobic lipids. As a result, phospholipids are almost absent in the stratum corneum. The lipid lamellae surrounding the corneocytes are predominantly composed of ceramides, cholesterol, and free fatty acids. It is generally assumed that these lipids are present in nearly equimolar ratios. However, inspection of literature data shows that there is a high interindividual variability in the lipid composition [37],... 

FIGURE 11.2 Molecular structures of the ceramides (CER) present in human stratum corneum (a) and pig stratum corneum (b). 

There are several genetic skin diseases with known defects in the lipid metabolism. Atopic dermatitis, lamellar ichthyosis, and psoriasis have been the most widely studied with respect to epidermal barrier function and alterations in the lipid profile. Deviations in the lipid profile have been linked with an impaired stratum corneum barrier function. Atopic dermatitis is characterized by inflammatory, dry and easily irritable skin, and overall reduced ceramide levels in the stratum corneum [58-60]. In particular a significant decrease in the ceramide 1 level is observed, whereas the levels of oleate that is esterified to ceramide 1 are elevated [59]. Both aberrations may be responsible for the reduced order of the lamellar phases as observed with freeze fracture electron microscopy [61]. It has further been established that, in comparison to healthy stratum corneum, the fraction of lipids forming a hexagonal packing is increased [61]. A recent study reveals that the level of free fatty acids... 

Psoriasis is a chronic skin disorder, in which an abnormally fast transition of basal cells into corneocytes results in a thickening of the stratum corneum. Transmission electron microscopy studies show an aberrant stratum corneum lipid ultrastructure in psoriatic skin [66], which is expected to be related to abnormalities in the lipid profile. Particularly, a significant reduction in ceramide 1 and a predominance of sphingosine ceramides at the expense of phytosphingosine ceramides are reported in psoriatic stratum corneum [67,68],... 

In recessive X-linked ichthyosis, the amount of cholesterol sulfate in the stratum corneum is increased due to a deficiency in cholesterol sulfatase deficiency [69,70], Lipid analysis of scales reveals a nearly 10-fold increase in the cholesterol sulfate to free cholesterol ratio as compared to healthy stratum corneum [71]. Previous x-ray diffraction studies on isolated ceramide mixtures revealed that increased cholesterol sulfate levels induce the formation of a fluid phase, which is likely to reduce the skin barrier function [72]. 

The above results demonstrate that the unique stratum corneum lipid organization can be reproduced in vitro with mixtures based on cholesterol, free fatty acids, and a limited number of synthetic ceramides. The results further reveal that the formation of the LPP is rather insensitive toward changes in the total composition of cholesterol, CER, and free fatty acids over a wide range of molar ratios. This is in excellent agreement with the in vivo situation, in which a high interindividual variability in stratum corneum lipid composition usually does not lead to substantial changes in the lipid organization. 

The lipid organization in stratum corneum is very unusual. Two lamellar phases are present with periodicities of approximately 6 and 13 nm. This lipid organization can be reproduced in vitro with mixtures prepared from either isolated ceramides (from pig and human stratum corneum) or synthetic ceramides. However, for the formation of the long periodicity phase the presence of acyl ceramides in the lipid mixtures is very crucial. The phase behavior of the lipid mixtures provides important information to explain the deviation in phase behavior in diseased skin. 

Wertz, P.W., et al. 1985. The composition of the ceramides from human stratum corneum and from comedones. J Invest Dermatol 84 410. 

Imokawa, G., et al. 1991. Decreased level of ceramides in stratum corneum of atopic dermatitis An etiologic factor in atopic dry skin J Invest Dermatol 96 523. 

Bouwstra, J.A., et al. 2002. Phase behavior of stratum corneum lipid mixtures based on human ceramides The role of natural and synthetic ceramide 1. J Invest Dermatol 118 606. 

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