Cold atmospheric temperatures lead to lower humidity. In such conditions, water is more likely to evaporate from the skin, particularly in individuals with an impaired skin barrier. With the arrival of winter, a discussion of the importance of the skin barrier and how to repair it is appropriate. Notably, cosmeceutical barrier repair products have an important role to play.
The Skin Barrier
Several important functions are served by the skin barrier: preventing transepidermal water loss (TEWL), shielding the skin from allergens and irritants, and protecting against infections. This defensive role depends largely on corneocyte function and the surrounding extracellular matrix (J. Invest. Dermatol. 2005;125:183-200).
The cornified cell envelope that encases the corneocyte is a 10-nm-wide, insoluble layer composed of various highly crossed proteins, particularly loricrin, the principal component, and involucrin, desmoplakin, and periplakin (J. Cell Sci. 2001;114:3069-70). The envelope structure is formed via cross-linking by the calcium (Ca2+)-dependent transglutaminase-1 (TG-1) enzyme.
The water barrier function of the skin is largely attributed to the lipids in the extracellular matrix that surrounds the corneocytes (Adv. Lipid Res. 1991;24:1-26). (Of note, TEWL is considered insensible water loss through the skin, which differs from active perspiration.) This lipid mixture is composed of approximately 50% ceramides, 25% cholesterol, and 15% fatty acids (J. Lipid Res. 2007;48:2531-46). Changes in any of these three components of the extracellular matrix can lead to a disruption in skin barrier function.
Ceramides
Ceramides constitute 40% of the lipids in the human stratum corneum (SC) (J. Invest. Dermatol. 1987;88:2S-6S), but they are not present in significant amounts in the stratum granulosum or basal layer. Consequently, terminal differentiation is likely an important factor in ceramide synthesis. The basic structure of ceramides consists of a fatty acid covalently bound to a sphingoid base.
In a study conducted by Unilever, ceramide levels were shown to increase in keratinocytes after the exogenous application of sphingoid precursors (specifically tetra-acetyl phytosphingosine or TAPS) (J. Invest. Dermatol. 1996;106:871). In another study by Unilever, TAPS, combined with the fatty acids 1% linoleic acid and 1% juniperic acid, also increased ceramide levels (J. Invest. Dermatol. 1996;106:918). In the latter study, researchers found that barrier integrity was improved in patients treated with TAPS, and the improvement was even greater when TAPS was combined with linoleic and juniperic acids. These findings imply that topically applied lipid precursors integrate into ceramide biosynthetic pathways in the epidermis, augmenting SC ceramide levels and thus ameliorating barrier integrity.
Cholesterol
Most cholesterol is synthesized from acetate in cells such as the keratinocytes, although basal cells can also absorb cholesterol from the circulation. Cholesterol production increases when the epidermal barrier is impaired (“Skin Barrier.” New York: Taylor and Francis, 2006, pp. 33–42).
Both peroxisome proliferator-activated receptors and retinoid X receptors play a role in transporting cholesterol across keratinocyte cell membranes by augmenting expression of ABCA1, a membrane transporter that regulates cholesterol flow (“Fitzpatrick's Dermatology in General Medicine,” 7th ed., New York: McGraw-Hill, 2007, pp. 386-7).
Fatty Acids
Free fatty acids and fatty acids in the skin are bound in triglycerides, glycosylceramides, ceramides, and phospholipids. The free fatty acids in the SC are mainly straight chained (“Skin Barrier.” New York, Taylor and Francis, 2006, pp. 33–42). Essential fatty acids such as linoleic acid can be obtained only through the diet or topical application.
Currently, it is thought that no single lipid alone mediates barrier function, and that normal levels of ceramides, cholesterol, and fatty acids, in the correct ratio, are crucial for maintaining barrier integrity.
Interestingly, Man et al. evaluated barrier recovery by altering the barrier with acetone, then applying ceramides or fatty acids alone, or a combination of ceramides and fatty acids, and found that normal barrier recovery was achieved only with the application of all three extracellular matrix components—ceramides, fatty acids, and cholesterol (Arch. Dermatol. 1993;129:728-38).
Skin Barrier Repair
Occlusives. Occlusive ingredients, which are oily compounds often used in cosmetics because of their capacity to dissolve fats, coat the SC and inhibit TEWL. Occlusives also impart an emollient effect.
Petrolatum and mineral oil are two of the best occlusive ingredients available. Used as a skin care product since 1872 and considered one of the optimal moisturizing agents, petrolatum displays a water vapor loss resistance 170 times that of olive oil and is well known for being noncomedogenic (Dermatologica 1971;142:89-92; J. Am. Acad. Dermatol. 1989;20:272-7). By virtue of its long-standing status as the most effective occlusive moisturizing agent, petrolatum is typically thought of as the accepted standard to which other occlusive ingredients are measured (“Dry Skin and Moisturizers,” Boca Raton, Fla.:CRC Press, 2000, p. 251).