Outermost lipid layer

The outermost skin layer, the stratum corneum (SC), consists of corneocytes embedded in lipid layers and represents the main barrier for skin penetration of xenobiotics. Its thickness in healthy adults may vary from 5 to 20 /um, except in the palm and sole where it is much thicker. SC can be removed sequentially by repeated application of appropriate adhesive tapes.1 This technique, commonly known as SC tape stripping, is a relatively noninvasive method to investigate the structure, properties, and functions of SC in vivo2 and is the most frequently used for such purposes. Other techniques to remove SC are skin surface biopsy using cyanoacrylate strips and skin scraping. 

Recently, a unique anteiso methyl-branched saturated fatty acid of 21 carbons, 18-methyl eicosanoic acid or 18-MEA, was identified in the outermost portion of the epicuticle, which is part of the CMC [104,106-110], 18-MEA is the predominant fatty acid in the epicuticle. It makes up approximately 40% of the surface lipid layer of wool and human hair [106,107,109], In addition to 18-MEA, other fatty acids have been isolated in smaller amounts from the epicuticle including... 

It is the nature of the stratum corneum, the outermost skin layer, that is responsible for the ability of terrestrial animals to exist in a non-aquatic environment without desiccation. The ability to control both the loss of water and the influx of potentially harmful chemicals and micro-organisms is the result of the evolution of a unique mixture of protein and lipid materials which collectively form this coherent membrane composed of distinct domains. These domains are principally protein, associated with the keratinocytes, and lipid, largely contained within the intercellular spaces. 

The endosperm cells are thin-walled and packed with amyloplasts containing compound starch granules. The endosperm is divided into two layers sub-aleurone and starchy endosperm. The two outermost cell layers (the sub-aleurone layer) are rich in protein and lipid they contain smaller amyloplasts and compound starch granules than the inner endosperm. The starch granules are mostly 3-9 pm in size. 

Because of the low permeability of the skin to many drugs, trans-dermal delivery has limited applications. The low permeability is attributed primarily to the stratum comeum, the outermost skin layer which consists of flat, dead cells filled with keratin fibers surrounded by lipid bilayers. One common method of increasing the passive transdermal diffusional drug flux involves pretreating the skin with a skin permeation enhancer. 

Mammalian skin is perhaps the most formidable transport barrier found in nature. The lipids of the stratum comeum, the outermost skin layer, form the primary barrier to transport of many compounds of therapeutic interest (Scheuplein, 1965,1978 Potts and Guy, 1992 Blank and Scheuplein, 1969 Elias, 1983,1987,1991). AsshownschematicallyinFig. 2, these lipids form broad multilamellar arrays in the extracellular space surrounding the remains of epidermal cells known as comeocytes. The lipids have a unique composition (fatty acids, cholesterol, and ceramides no phospholipids are present) and form the only continuous domain within the stratum comeum (Elias, 1983, 1987, 1991). Despite profound differences between stratum comeum lipids and those of the phospholipid bilayers more commonly found in other biomembranes, direct comparison of passive transport through each suggests a common mechanism involving Ifee-volume fluctuations in the lipid alkyl chains. Transport within the lipid hydrocarbon domain substantially restricts the permeability of large mol-... 

It is important to realize that the detailed analysis of the biomolecular composition and protein secondary structure of the SC has important implications for understanding the barrier function of the outermost skin layer.On average, human SC is 10 to 20 pm in thickness, and consists of flattened, anucleated and protein-rich cells embedded in a multi-lamellar lipid matrix composed mainly of ceramides. It is believed that the penetration pathway is... 

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

In addition, data obtained from infrared, thermal, and fluorescence spectroscopic studies of the outermost layer of skin, stratum corneum (SC), and its components imply enhancer-improved permeation of solutes through the SC is associated with alterations involving the hydrocarbon chains of the SC lipid components. Data obtained from electron microscopy and x-ray diffraction reveals that the disordering of the lamellar packing is also an important mechanism for increased permeation of drugs induced by penetration enhancers (for a recent review, see Ref. 206). 

Alpha hydroxy acids (AHAs) are water-soluble substances and thereby penetrate the outermost epidermal skin layers. In contrast, beta hydroxy acids (BHAs) are lipid (fat) soluble and are capable of penetrating to the underlying layers of skin (the dermis) located 1-5 mm below the surface of the skinJ2 Most AHAs are derived from plant materials and marine sources. Commonly used AHAs include malic acid (found in apples), ascorbic acid (a common ingredient in numerous fruits), glycolic acid (a constituent of sugar cane), lactic acid (a component of milk), citric acid (naturally abundant in citrus fruits), and tartatic acid (found in red wine). A common BHA is salicylic acid (an ingredient in aspirin). 

The skin barrier properties and effect of hand hygiene practices are known to be important in protecting the body. The average adult has a skin area of about 1.75 m2. The superficial part of the skin, the epidermis, has five layers. The stratum corneum, the outermost layer, is composed of flattened dead cells (comeocytes or squames) attached to each other to form a tough, homy layer of keratin mixed with several lipids, which help maintain the hydration, pliability, and barrier effectiveness of the skin. This part of skin has been compared to a wall of bricks (comeocytes) and mortar (lipids) and serves as the primary protective barrier. Approximately 15 layers make up the stratum corneum, which is completely replaced every 2 weeks a new layer is formed almost daily. From healthy skin, approximately 107 particles are disseminated into the air each day, and 10% of these skin squames contain viable bacteria. This is a source of major dirt inside the house and contributes to many interactions. 

The outermost layer of the skin, the cornified layer or stratum corneum, has been identified as the principal diffusion barrier for substances, including water [2,3]. It is approximately 10 to 20 pm thick when dry but swells to several times this thickness when fully hydrated [17], It contains 10 to 25 layers lying parallel to the skin surface of nonviable cells, the corneocytes, which are surrounded by a cell envelope and imbedded in a lipid matrix. This architecture is often modeled as a wall-like structure, with the corneocytes as protein bricks embedded in a lipid mortar [18]. Similarly to the viable epidermis, desmosomes (corneodesmosomes) contribute to the cell cohesion. 

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. 

This chapter focuses on the lipid structure found in the outermost layers of the SC in humans. We present a modified TEM technique to investigate this structure, attempt to systematize and understand the variability in lipid structure observed in the outer SC, and explore the effect of moisturizers on the outer SC at microscopic and macroscopic levels. 

From a materials science perspective, the SC is a laminated composite membrane comprised of two distinct domains, specifically, proteins (corneocyte cells with embedded keratin bundles) and lipid bilayers. Corneocyte cells have covalently attached lipids, which makes them compatible with the surrounding lipid matrix. In addition, corneocytes in different layers are held together by protein staples called desmosomes. SC has been designed to exfoliate dead cells in an orderly fashion where the upper layers come off in a layer-by-layer fashion. For this to happen, the desmosomes have to be cleaved by proteolytic enzymes in the SC as the cells approach the outermost layers. 

The second diffusion problem, desorption from oil-water multilaminates, is considered as a model for (a) controlled release from liposomes and lipid multilayers and (b) for transport through biological laminates such as stratum corneum. In contrast to nonsteady-state transport across multilaminates, desorption from laminates depends only on the outermost layers. 

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. 

---