The Lipids of Skin and Other Surfaces

All other mammal macrolides are isolated or detected from the skin surface lipid of horse and other Equus spp. (donkey, zebra, ass, and mule) and named... 

Skin lipids of humans differ markedly from those of other animals (Nicolaides etaL, 1968). Comments in this section relate to humans. Lipogenesis stems from two skin cell types - sebocytes in the sebaceous glands and the keratinocytes in the keratinizing epidermis. The lipids of the two cell types are quite different and may fulfil separate physiological functions (Nicolaides, 1965). In humans total surface lipids are, therefore, derived from the sebaceous glands, the keratinizing epidermis and external sources such as soaps, cosmetics or atmospheric pollutants (Burton, 1979). 

There is today a consensus about penetration pathways through the skin barrier (Bodde et al. 1990). Under normal conditions, the corneocytes are permeable essentially only to water, which implies that the transport route for hydrophilic and hydrophobic substances is via the extracellular space of the stratum corneum. The lipids of this space are organised in bilayers stacked on top of each other. The corneocyte envelopes have long-chain ceramides covalently bound to their surfaces... 

Some evidence indicates that long-term use of topical antimicrobial agents may alter skin flora. Water content, humidity, pH, intracellular lipids, and rates of shedding help retain the protective barrier properties of the skin. When the barrier is compromised (e.g., by hand hygiene practices such as scrubbing), skin dryness, irritation, cracking, and other problems may result. Although the palmar surface of the hand has twice as many cell layers and the cells are >30 times thicker than on the rest of the skin, palms are quite permeable to water. 

The other major route of chemical hazards to plant workers is penetration through the skin, which has a surface area of about 1.8 m for a male adult, and is lined with a tough horny layer of densely packed cells without nuclei. So the skin is reasonably impervious to the penetration of chemical substances, and penetration depends on the lipid solubility of the compound, where the diffusion speed is inversely related to molecular weight. Skin penetration hazard becomes relatively more important than breathing for high boiling point liquids, which have lower vapor pressures. 

There is substantial history regarding the application of conventional vibrational spectroscopy methods to study the intact surface of skin, the extracted stratum corneum and the ceramide-cholesterol-fatty acid mixtures that constitute the primary lipid components of the barrier. The complexity of the barrier and the multiple phases formed by the interactions of the barrier components have begun to reveal the role of each of these substances in barrier structure and stability. The use of bulk phase IR to monitor lipid phase behavior and protein secondary structures in the epidermis, as well as in stratum corneum models, is also well established 24-28 In addition, in vivo and ex vivo attenuated total reflectance (ATR) techniques have examined the outer layers of skin to probe hydration levels, drug delivery and percutaneous absorption at a macroscopic level.29-32 Both mid-IR and near-IR spectroscopy have been used to differentiate pathological skin samples.33,34 The above studies, and many others too numerous to mention, lend confidence to the fact that the extension to IR imaging will produce useful results. 

Desquamatory proteases and other enzymes mediate their action in the lipid-rich intercellular space and need free water to be active.3 Disturbed SC lipid structure results in reduced SC hydration and retention of corneocytes on the skin surface. Subsequently skin xerosis becomes evident due to reduced desmosome degradation.5 The occurrence of dry skin associated with cold, dry weather may result from an extensive, elevated level of skin lipids in the solid state. Thereby, the material that maintains a higher proportion of lipid in the liquid crystalline state may be an effective moisturizer.6... 

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