Stratum corneum barrier function

To further increase our insight into the stratum corneum barrier function, stratum corneum lipid models have been developed. The main advantage of these lipid models is that the... 

Barrier function, stratum corneum general discussion, 241-242 ways to decrease for enhanced drug delivery, 283... 

The human skin model assay involves measuring the effects of corrosives on viable cells in a reconstituted human skin equivalent. To be accepted as a valid human skin model, several criteria must be met. The artificial skin must comprise a functional stratum corneum with an underlying layer of viable cells. Furthermore, the barrier function of the stratum corneum, as well as the viability of the epidermis, must be verified with appropriate experimental setups. The chemicals to be tested are applied up to 4 h as a liquid or a wet powder onto the skin model. Afterwards, careful washing has to be performed, followed by investigation of the cell viability [e.g., with a (MTT)] reduction assay). 

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. 

Because of the possible effects of active and carrier-mediated processes and metabolic biotransformation, the issue of tissue viability is important for in vitro buccal mucosal experiments. The barrier nature of the buccal mucosa resides in the upper layers of the epithelium, where unlike in the stratum corneum, the cells contain a variety of functional organelles [119, 122, 125, 150], and so tissue viability may be an important component of the barrier function of the tissue. Various methods have been employed to assess the viability of excised buccal mucosa, including measurement of biochemical markers, microscopic methods, and linearity of transport data [42], While biochemical methods, including measurement of adenosine 5 -triphosphate (ATP) levels and utilization of glucose, provide information on the metabolic activity of the tissue, this does not necessarily relate to the barrier function of the tissue. In excised rabbit buccal mucosa, levels of ATP were measured and found to decline by 40% in 6 h, and this correlated well with transmission electron microscopic evaluation of the tissue (intact superficial cells) [32], In addition, the permeability of a model peptide was unaltered up to 6 h postmortem, but at 8 h, a significant change in permeability was observed [32], These investigators therefore claimed that excised rabbit buccal mucosa could be used for diffusion studies for 6 h. 

Compounds that penetrate the stratum corneum via the transepidermal route may follow a transcellular (or intracellular) or intercellular pathway (see Figure 11.1). Because of the highly impermeable character of the cornified envelope (see previous section), the tortuous intercellular pathway has been suggested to be the route of preference for most drug molecules [32], This is confirmed by several microscopic transport studies, in which compounds have been visualized in the intercellular space of the stratum corneum [33-35]. Moreover, it has been demonstrated that drug permeation across stratum corneum increases many folds after lipid extraction [36], Hence, knowledge of the structure and physical properties of the intercellular lipids is crucial to broaden our insight into the skin barrier function. 

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... 

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]. 

All the above-mentioned changes in lipid composition and organization in diseased and dry skin likely contribute to an impaired stratum corneum barrier function and increased susceptibility to dry skin. However, as previously indicated, abnormalities in the process of envelope formation may also strongly influence the stratum corneum barrier integrity. Therefore, more information is required to elucidate the precise mechanisms by which stratum corneum structure and function are altered. 

Bommannan, D., R.O. Potts, and R.H. Guy. 1990. Examination of stratum corneum barrier function in vivo by infrared spectroscopy. J Invest Dermatol 95 403. 

Lavrijsen, A.P.M., et al. 1995. Reduced skin barrier function parallels abnormal stratum corneum lipid organization in patients with lamellar ichthyosis. J Invest Dermatol 105 619. 

Urea is a hydrating agent (a hydrotrope) used to treat scaling conditions such as psoriasis, ichthyosis, and other hyperkeratotic skin conditions. Applied in a water-in-oil vehicle, urea alone or in combination with ammonium lactate hydrated stratum corneum and improved barrier function when compared to the vehicle alone in human volunteers in vivo [45], Urea also has keratolytic properties, usually when combined with salicylic acid for keratolysis. The somewhat modest penetration-enhancing activity of urea probably arises from a combination of increasing stratum corneum water content (water is a valuable penetration enhancer) and through the keratolytic activity. 

A permeation enhancer can be defined as a compound that alters the skin barrier function so that a desired drug can permeate at a faster rate. Dozens of enhancers are patented each year, and several books have been written summarizing the work and proposing mechanisms of enhancement.70-72 The permeation enhancers may be classified simply as polar and nonpolar ones. They can be used individually or in combination, such as binary mixtures. For several drugs, the flux across skin was observed to be linear with that of the most widely used enhancer, ethanol.73-75 Another polar enhancer, isopropanol, facilitated ion association of charged molecules and enhanced the transport of both neutral and ionic species across the stratum corneum.76 77 While polar enhancers traverse the skin, nonpolar enhancers are largely retained in the stratum corneum both aspects make the combination a superior enhancer to the individual enhancers.78... 

A. Schatzlein and G. Cevc. 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). Br. J. Dermatol. 138 583-592 (1998). 

This chapter will deal with the stratum corneum barrier with a special focus on structure-function relationships. For this reason our approach has been to describe some details of the epidermal physiology that have a bearing on upholding the barrier function. We see it as important that skin barrier function is regarded as part of the dynamic processes of cellular transformation during the differentiation of epidermal keratinocytes, hence dependent on the status of the skin. 

This is in sharp contrast to the conditions in stratum corneum where the lipid membranes are almost impermeable to water. As a consequence of these facts, we expect the bulk of lipids that form the skin barrier to be in a crystalline (gel) state, that is, to have long carbon chains (C > 20 0) to comply with the physical requirement that the transition temperature should be higher than normal skin temperature (>35°C). A physiological mixture of ceramides, free fatty acids (FFA), and cholesterol is indeed needed for a normal barrier function. 

The third class of lipids found in stratum corneum extracts is represented by cholesterol and cholesteryl esters. The actual role of cholesterol remains enigmatic, and no clear reason for its role in the barrier function has been proposed so far. However, it is possible that contrary to what is the role in cell membranes where cholesterol increases close packing of phospholipids, it can act as kind of a detergent in lipid bilayers of long-chain, saturated lipids.30,31 This would allow some fraction of the barrier to be in a liquid crystalline state, hence water permeable in spite of the fact that not only ceramides, but also fatty acids found in the barrier are saturated, long-chain species.28,32... 

In 1975, Michaels et al.33 presented a conceptual model of the arrangement of corneocytes and lipids in stratum corneum. They envisaged stratum corneum as a brick and mortar structure with the keratin filled corneocytes as bricks and the intercellular lipids as mortar. This model was further explored by Elias and co-worker.34-37 This model does not per se include a structure-function perspective on the barrier but has had a tremendous impact on the research on stratum corneum and its composition, function, and the regulation of homeostasis. 

In 1994 Forslind presented a more structure-function orientated model, the domain mosaic model.38 With the background given previously, the requirements on the stratum corneum barrier can be summarized as follows the barrier should be watertight but still allow a small, controlled amount of water to leak from the system in order to keep the corneocyte keratin hydrated. 

The lipid bilayers of the stratum corneum not only constitute a barrier, but may also function as a pool from which substances can slowly penetrate into the system on a downhill gradient. The actual effect of solvents and detergents on barrier lipid structure is not known in any satisfactory detail. Likewise, we are only starting to understand how different moisturizers might influence the structure and function of the barrier. We still lack an understanding of how the composition of the ceramide, FFA, and cholesterol influences the defect barrier in some pathological disorders, for example, dry atopic skin. 

Norlen, L. et al., Differences in human stratum corneum lipid content related to physical parameters of skin barrier function in vivo, J. Invest. Dermatol., 112, 72, 1999. 

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