Intercellular lipid layers

As the final outer stratum comeum is formed the phospholipid bilayer deteriorates and intercellular lipid layers are formed.k l These contain principally ceramides, cholesterol, and free fatty acids. Some sphingolipids are covalently attached to proteins.3... 

Fartasch investigated the ultrastructural changes of epidermal lipids resulting from the topical application of sodium lauryl sulfate (SLS) and absolute acetone (Fartasch 1997). SLS caused cell damage to the nucleated cells of the epidermis, with disturbance of lamellar body lipid extrusion and transfer into the lipid bilayers. However, the upper portions of the SC displayed intact intercellular lipid layers. With acetone treatment, the lamellae showed disruption and loss of... 

FIG. 5 Disordering of intercellular lipid layers in stratum comeum Ordered structures of intercellular lipid bilayers in the stratum corneum are measured in terms of the order parameter (S) by the in vitro ESR spin-probe method. Intact stratum comeum showed a higher S value. With SLS treatment, structural deformation is evidenced by a smaller S value. (From Ref. 17.)... 

In addition to the intercellular lipids of the buccal mucosa, there appear to be other barriers which may reduce the ability of an exogenous compound to permeate the buccal mucosa. These include the salivary film and mucus layer, the basement membrane, and a metabolic barrier. 

Madison, K.C., et al. Presence of intact intercellular lipid lamellae in the upper layers of the stratum corneum. J Invest Dermatol 88 714. 

FIGURE 12.1 Penetration enhancer activity, (a) Action at intercellular lipids. Some of the ways by which penetration enhancers attack and modify the well-organized intercellular lipid domain of the stratum comeum. (b) Action at desmosomes and protein structures. Such dramatic disruption by enhancers (particularly potent solvents) as they split the stratum corneum into additional squames and individual cells would be clinically unacceptable, (c) Action within comeocytes. Swelling, further keratin denaturation and vacuolation within individual horny layer cells would not be so drastic but would usually be cosmetically challenging (see Menon and Lee [69] for further details). (Reproduced from Barry, B.W., Nat. Biotechnol. 22, 165, 2004. With permission.)... 

Water homeostasis is a strict requirement for normal physiological function. The most important task of the human skin is thus to create a watertight enclosure of the body to prevent water loss. It is the intercellular lipid matrix of the outermost keratinized horny layer of the skin (possibly together with recently reported claudin-based tight-junctions Furuse et al., 2002) that represents the skin barrier proper as once this lipid matrix (composed foremostly of saturated long chain ceramides ( 50% wt/wt) and cholesterol (—30% wt/wt) (Wertz and Norlen, 2002)) has been removed, substances diffuse freely into or out of the body system (Blank, 1952 Breathnach et al., 1973 Elias and Friend, 1975). At the same time the intercellular lipid matrix ensures that the stratum corneum remains hydrated and thus the skin surface appears healthy and smooth. 

Permeation studies using a number of tracers, including horseradish peroxidase and lanthanum nitrate, have confirmed that the outer third of the epithelium is the rate-limiting barrier for mucosal penetration. When applied to the outer surface of the epithelium, these tracers are seen to penetrate only through the outermost layers of cells. Thus the compacted, flattened cells of the lower superficial layer and intermediate layer present a major physical barrier to transport. The intercellular lipids also play an important role, since extraction of these lipids results in more permeable tissue. Generally, keratinized epithelium appears to be more impermeable than non-keratinized epithelium. 

The stratum comeum is usefully thought of as a brick wall , with the fully differentiated comeocytes comprising the bricks , embedded in the mortar created by the intercellular lipids. A layer of lipid covalently bound to the comified envelope of the comeocyte contributes to this exquisite organization. The intercellular lipids of the stratum comeum include no phosphohpids, comprising an approximately equimolar mixture of ceramides, cholesterol and free fatty acids. These non-polar and somewhat rigid components of the stratum comeum s cement play a critical role in barrier function. On average, there are about 20 cell layers in the stratum comeum, each of which is about 0.5 fim in thickness. Yet, the architecture of the membrane is such that this very thin structure limits, under normal conditions, the passive loss of water across the entire skin surface to only about 250 mL per day, a volume easily replaced in order to maintain homeostasis. 

More recently, Bommannan et al. [61] used ATR-IR to examine the SC as a function of serial tape stripping. The nonuniform and inhomogeneous nature of the membrane has inevitably led to questions concerning the exact location of the barrier within the SC. With ATR-IR in conjunction with tape stripping, it has been possible to probe, at discrete intervals across the forearm SC in human subjects, spectral features associated with both the intercellular lipids and the hydration level of the membrane, key determinants of skin barrier function. On average, each tape-stripping procedure removed one SC layer per strip and enabled the progressive examination of 50-75% of the SC tissue. 

The inclusion of ethanol in commercially available transdermal systems has naturally provoked curiosity concerning its role as an enhancer in human skin, particularly in vivo. The mechanism by which ethanol compromises the human stratum comeum in vivo was investigated by Bommannan et al. [125] using ATR-IR in studies analogous to those described previously in this chapter [61 ]. Those in vivo studies on the untreated ventral forearm of healthy adults had revealed a depth-dependent ordering and reduction of the intercellular lipids relative to the superficial layers. Consequently, in the ethanol experiments, the measurement site (about 20 cm ) was tape stripped four times prior to ethanol treatment in order to isolate the effect of ethanol from the inherent lipid changes in untreated SC. The examination site was treated for 30 minutes with absolute ethanol (10 ml) and then spectrally examined periodically over the... 

The cornified cell envelope is the outermost layer of a corneocyte, and mainly consists of tightly bundled keratin filaments aligned parallel to the main face of the corneocyte. The envelope consists of both protein and lipid components in that the lipid is attached covalently to the protein envelope. The envelope lies adjacent to the interior surface of the plasma membrane. " The corneocyte protein envelope appears to play an important role in the structural assembly of the intercellular lipid lamellae of the stratum corneum. The corneocyte possesses a chemically bound lipid envelope comprised of A-co-hydroxyceramides, which are ester linked to the numerous glutamate side chains provided possibly by both the ot-helical conformation and p-sheet conformation of involucrin in the envelope protein matrix. In the absence of A-oo-hydro-xyceramides, the stratum corneum intercellular lipid lamellae were abnormal and permeability barrier function was disrupted. 

The stratum corneum intercellular lipids exist as a continuous lipid phase occupying about 20% of the stratum corneum volume and arranged in multiple lamellar structures. They are composed of cholesterol (27 /o) and ceramides (41 /o), together with free fatty acids (9 /o), cholesteryl esters (10 /o) and cholesteryl sulfate (2 /o) (Table 1). Phospholipids, which dominate in the basal layer, are converted to glucosylceramides and subsequently to ceramides and free fatty acids, and are virtually absent in the outer layers of the stratum corneum. Eight classes of ceramides have been isolated and identified in human stratum corneum but the functions of the individual ceramide types are not fully understood. Similarly, the exact function of cholesterol esters within the stratum corneum lamellae is also elusive but it is theoretically possible that cholesterol esters may span adjacent bilayers and serve as additional stabilizing moieties. 

The precise mode of interaction between lipid vesicles and skin remains unclear. There is considerable doubt about the ability of whole vesicles to permeate intact stratum corneum. The majority of evidence suggests that vesicles can penetrate the outer cell layers of the stratum corneum where desmosomal linkages have become disrupted and presumably, the keratinocytes are less tightly bound and surrounded by a mixture of intercellular lipid and sebum. However, continuing diffusion of vesicles through the approximately 60 nm intercellular space of the deeper layers of the stratum corneum seems unlikely. Current thinking suggests... 

For iontophoretic treatments involving placement of the system on the skin, the therapeutic agent in the donor reservior must cross the outermost layer of the skin, known as the stratum corneum, which is the primary barrier to permeation of substances both into and out of the body. The stratum corneum s excellent barrier properties result from its unique structure approximately 10-20 layers of flattened, keratin-rich cells cemented together by lipid bilayers composed primarily of ceramides. In general, lipophilic species are capable of traversing the stratum corneum because of their ability to partition into the intercellular lipid... 

The stratum corneum is the outermost layer of the epidermis and has a thickness of 10-15 pm. It is the principal barrier for the transport of most solutes (except for very lipophilic compounds) across the skin. The stratum corneum is a continuous heterogeneous structure that consists of approximately 10-25 layers of closely packed dead keratinized cells (corneocytes) cemented together by intercellular lipids. The intercellular lipids in the stratum corneum are in the form of multiple lamellar bilayers composed mainly of ceramides, cholesterol, and fatty acids. Proteins in the stratum corneum are largely concentrated within the corneocytes as keratin fibrils. The transport of lipophilic compounds across the stratum corneum is related to the intercellular lipids (lipoidal or intercellular pathways). On the other hand, it is believed that the transport of polar and ionic compounds is related to pathways with aqueous properties (the polar or pore pathways) when the stratum corneum is under a hydrated state. ... 

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