Black skin dryness: lack of ceramides

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Michael T. Longaker

aHagey Laboratory for Pediatric Regenerative Medicine, Stanford University Medical Center, Stanford, California;

bInstitute for Stem Cell Biology and Regenerative Medicine, Stanford University Medical Center, Stanford, California;Find articles by

aHagey Laboratory for Pediatric Regenerative Medicine, Stanford University Medical Center, Stanford, California;

bInstitute for Stem Cell Biology and Regenerative Medicine, Stanford University Medical Center, Stanford, California;Find articles by

dDepartment of Plastic and Reconstructive Surgery, Chang Gung Memorial Hospital, Kuei-Shan, Taoyuan, Taiwan R.O.C.Find articles by

Corresponding author.

The skin is a complex organ involved in thermoregulation, gas exchange, protection against pathogens, and barrier function to maintain proper hydration. When dry, the ability for skin to execute these tasks becomes impaired. Dry skin affects almost everyone as we age, but it is also dependent on external factors, such as dry climate, colder temperatures, and repeated washing. In addition, increasing evidence has shown racial variability in the physiological properties of skin, which directly impacts water content of the stratum corneum and sensitivity to exogenously applied agents. A multitude of products have been developed to treat dry skin, and as a group, moisturizers have been designed to either impart or restore hydration in the stratum corneum. Given the large number of moisturizers presently available, depending on individual components, several different mechanisms may be employed to promote skin hydration. As there exists dramatic racial variability in skin properties, certain moisturizers may thus be more effective in some and less effective in others to treat the common condition of dry skin.

Dry skin occurs worldwide, with increasing frequency as we age.1,2 Common sites of involvement include the shins, lower flanks, and posterior axillary line, but in more severe cases, the face and neck may also be affected. Dry skin, also known as xerosis cutis or asteatosis, can result in a rough, scaly quality to the skin. When mild, it can be relatively asymptomatic, but when more pronounced, it may be associated with inflammation and superficial cracking causing unpleasant itching, stinging, and general discomfort.3 In theory, any break in the skin barrier can potentially predispose patients to cutaneous infections. Dry skin has long been noted to be linked to various exogenous factors, such as dry climates, colder winter months, and excessive showering or bathing.4,5 Other factors, such as exposure to various alkali and detergents, particularly in patients with dispositional skin irritability, can serve as a trigger or worsen the ability for skin to maintain moisture.

While ambient and lifestyle factors have been strongly associated with severity of dry skin, various endogenous factors have also gained attention. Malnutrition, renal insufficiency, and hereditary conditions, such as ichthyosis vulgaris and atopy, may also contribute to loss of skin moisture.1 Along these lines, racial differences in skin properties may also explain some of the disparity in dry skin and may provide insight into differences in management for this dermatological disorder. Recent studies have now elucidated multiple racial variations in anatomical and physiological qualities, which, not surprisingly, may alter a disease process or treatment approach.6 With respect to dry skin, racial differences in water loss, water content, lipid content, and skin reactivity have been correlated with disease severity.6

Treatment of dry skin involves the elimination of aggravating factors, and when unrelenting, may sometimes require a course of topical corticosteroid treatment.1 More commonly, though, attention to proper skin care with regular use of moisturizers is stressed.3,4 However, the marketplace is flooded with a great number of products, each containing different components, and selection of the most effective moisturizer may be difficult. Furthermore, given the physiological variations in skin among different races, an ideal moisturizer for one patient may be less useful for another. As billions of dollars are spent yearly on these products, a better understanding of how moisturizers work is thus critical.4 This review will therefore focus on the pathogenesis of dry skin, racial differences in skin properties, and how these may impact selection of an optimal strategy to maintain skin moisture.

PATHOGENESIS OF DRY SKIN

Dry skin occurs worldwide and may affect patients of all ages, but it can probably be found in almost all patients over the age of 60.1,7,8 Skin barrier function resides primarily within the top layer of the epidermis, also known as the stratum corneum or “horny layer.” While important for thermoregulation, gas exchange, and protection against pathogens, this external-most layer also serves to maintain proper hydration.911 At birth, the water-handling properties of infant skin are unique and distinct from adults. Though considerably drier during the first month of life, over the next two years, water content in the skin significantly increases, and the stratum corneum that is initially rough becomes far smoother.1215 This occurs despite lower production levels of sebum and natural moisturizing factor (NMF), a hydroscopic mixture of amino acids, urea, and other compounds.1,14

In adults, though deficient production of sebum may contribute to the development of xerosis, a more complex dysfunction of the horny layer is likely responsible for dry skin. By evaluating skin from young and aged patients, Ghadially et al16 found marked abnormalities in barrier integrity and barrier repair associated with dry skin. While the relative distribution of ceramides, cholesterol, and free fatty acids were similar, functional skin changes were attributed to global deficiency in all key stratum corneum lipids, resulting in decreased lamellar bilayers in the stratum corneum interstices with age.16 Studies employing high-performance gas chromatography have also shown altered ratios of fatty acids esterified to ceramide 1, which may increase susceptibility of skin to moisture loss, particularly during drier months.5 In addition, Engelke et al2 reported decreased epidermal prolif-eration in aged, dry skin relative to young, normal skin.2 Significant changes in expression of various keratins were also noted, with decrease in keratins Kl and K10 and increase in basal keratins K5 and K14 detected in dry skin samples.2 Finally, in patients with dry skin, investigations have shown premature expression of involucrin, a protein component that becomes cross-linked to corneo-cyte (the cell type in the stratum corneum) cell membranes, thus increasing cellular stiffness.2,17,18 Collectively, these findings are believed to promote corneo-cyte retention and impair barrier recovery of the horny layer. Furthermore, these physiological changes contribute to reduced water-binding capacity of the skin.

As the stratum corneum dries out, loss of pliability occurs along with formation of small cracks.1 This leaves the skin surface dull, rough, and scaly in appearance. Fine bran-like scales may also be released as powdery clouds during removal of clothing.3 When more advanced, a criss-cross pattern of superficial cracks may be seen appearing like a “dried river bed.” In the most advanced stages, oozing, crusting, and abundant scratch marks can be noted.3 Dry skin may frequently be associated with itching and stinging, both of which may be caused by direct stimulation of cutaneous nerve fibers.1 Symptoms can also be exacerbated by release of histamines and other proinflammatory cytokines secondary to barrier perturbation, mechanical scratching and rubbing, or by the application of irritating or sensitizing substances ( ).1 While mild xerosis may clear within a few days of treatment with moisturizers, more severe cases, such as those seen in atopic patients, may require prolonged therapy with topical corticosteroids. However, in either situation, regular use of occlusives, humectants, emollients, and/or rejuvenators is necessary to minimize relapses.4,19 In addition, use of bath oils and the elimination of factors aggravating dry skin are critical for the treatment of any patient with xerosis.1,20

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