Stratum corneum water content

Urea is a hydrating agent (a hydrotrope) used to treat scaling conditions such as psoriasis, ichthyosis, and other hyperkeratotic skin conditions. Applied in a water-in-oil vehicle, urea alone or in combination with ammonium lactate hydrated stratum corneum and improved barrier function when compared to the vehicle alone in human volunteers in vivo [45], Urea also has keratolytic properties, usually when combined with salicylic acid for keratolysis. The somewhat modest penetration-enhancing activity of urea probably arises from a combination of increasing stratum corneum water content (water is a valuable penetration enhancer) and through the keratolytic activity. 

Interactions between water and the polar head groups of lipid molecules are necessary for the formation of lamellar phases however, it appears that there is no free water associated with the 13 nm trilaminar units. This is supported by the observation that this periodicity does not increase with increasing stratum corneum water content.22 There is likely water hydrogen bonded to the polar regions of the lamellae. In contrast, the minor short periodicity swells from 5.8 to 6.6 nm as the water content of stratum corneum increases from 12 to 50%.23 This suggests that the lipid lamellae are simple individual bilayers and free water molecules can exist between adjacent bilayers, thus causing the increase in the lamellar spacing. 

Pruritus of the elderly is another common dermatosis associated with itch.5 One study has shown that elderly patients with generalized pruritus had a higher degree of skin dryness than in age matched control subjects.6 This study also demonstrated that skin surface conductance, a marker of stratum corneum water content, was decreased in elderly patients with generalized pruritus. The study was also successful in demonstrating an acquired abnormality in keratinization in these patients. This abnormality manifested with increased intracorneal cohesion compared with the controls. Another study showed an increase in histamine release and hypersensitivity in patients with senile pruritus.7 Paradoxically, oral anti-histamines are not very efficacious in the treatment of senile pruritus. 

With topically applied 10% urea ointment, Sasaki et al. showed an improvement of stratum corneum water content, hygroscopicity, and TEWL, in psoriasic patients.56... 

Koyama, J., Nakanishi, J., Masuda, Y., Nomura, J., Suzuki, Y., and Nakayama, Y. Effect of humectants on stratum corneum water content and digestion of desmosomes J. Soc. Cosmet. Chem. Jpn., 33, 16-26, 1999. 

A natural concentration gradient across human skin exists the inner dermal layer is essentially fully hydrated whereas the outer stratum corneum water content depends on external factors such as relative humidity and temperature typically, stratum corneum water content is around 20-30% of the tissue dry weight. Of this, approximately 25-35% is associated with some structural component of the tissue (i.e. bound ) with the remainder free (Walkley, 1972). Increasing the water content of the barrier stratum corneum is well known to... 

Glycerol is effective in enhancing hydration of the stratum corneum in dry skin.29 Recently it has been shown that the epidermal water/glycerol transporter aquaporin-3 in deficient mice leads to severely impaired stratum corneum hydration.9 In these mice, stratum corneum glycerol content was reduced threefold. And it was shown that glycerol replacement corrects the defects in these mice.30 However, it has not yet been published whether changes in the aquaporin-3 transporter occur in human dry skin or in aged dry skin. 

The intensity of the 120°-130°C endotherm increases as stratum corneum moisture content increases there is a corresponding decrease in the high temperature meltings at 194° and 210°C (Figure 20). Highly hydrated corneum displays the 120°-130°C endotherm while below 15 to 20% water content, the transition is quite small. These DSC measurements were performed in hermetically sealed pans to avoid the endothermic loss of water. The transition in the presence of water is a cooperative one in that water facilitates the magnitude of the observed transition. The heat change associated with the 120°-130°C transition... 

Emulsion components enter the stratum corneum and other epidermal layers at different rates. Most of the water evaporates, and a residue of emulsifiers, Hpids, and other nonvolatile constituents remains on the skin. Some of these materials and other product ingredients may permeate the skin others remain on the surface. If the blend of nonvolatiles materially reduces the evaporative loss of water from the skin, known as the transepidermal water loss (TEWL), the film is identified as occlusive. AppHcation of a layer of petrolatum to normal skin can reduce the TEWL, which is normally about 4—8 g/(m h), by as much as 50 to 75% for several hours. The evaporated water is to a large extent trapped under the occlusive layer hydrating or moisturizing the dead cells of the stratum corneum. The flexibiHty of isolated stratum corneum is dependent on the presence of water dry stratum corneum is britde and difficult to stretch or bend. Thus, any increase in the water content of skin is beHeved to improve the skin quaHty. 

The nails are composed of flattened, keratinized cells, fused into a dense and hard, yet slightly elastic plate. Their thickness varies from 0.5 to 1.0 mm. In contrast to the stratum corneum (10%), the total lipid content of the nails lies between 0.1% and 1%, and the keratin domain is harder, due to higher sulfur content (cystine). Moreover, the water content is only 7% to 12%, in comparison to 25% in the stratum corneum. The relative water gain may not exceed 25% at 100% relative humidity, in sharp contrast to 200-300% as found in the stratum corneum. 

Urea is used in cream or ointment to make it less greasy, it increases the water content of stratum corneum which may be because of its hygroscopic property. It also possesses keratolytic property. 

Urea increases the water content of the stratum corneum, presumably as a result of the hygroscopic characteristics of this naturally occurring molecule. Urea is also keratolytic. The mechanism of action appears to involve alterations in prekeratin and keratin, leading to increased solubilization. In addition, urea may break hydrogen bonds that keep the stratum... 

Propylene glycol is also an effective humectant and increases the water content of the stratum corneum. The hygroscopic characteristics of the agent may help it to develop an osmotic gradient through the stratum corneum, thereby increasing hydration of the outermost layers by drawing water out from the inner layers of the skin. 

Blank, I.H. Factors which influence the water content of the stratum corneum. J. Invest. Derm. 18 433, 1952. 

Werner, Y. The water content of the stratum corneum in patients with atopic dermatitis. Acfa. Derm. Venereol. (Stockh.) 66 281-284 (1986). 

Dry, scaly skin is characterized by a decrease in the water retention capacity of the stratum corneum (SC),1 with water content diminished to less than 10%. Barrier function of the SC is usually declined, and transepidermal water loss (TEWL) is increased because of an abnormality on barrier homeostasis.2 People feel tightness of their skin, and the skin surface becomes rough, scaly, and sensitive. Hyperkeratosis, abnormal scaling, and epidermal hyperplasia are usually observed in the dry skin.2 Keratinization also shows abnormal features.2 These phenomena are commonly observed in atopic dermatitis and psoriasis.3 Dermatitis induced by environmental factors such as exposure to chemicals, low humidity, and UV radiation also shows these features. Thus, many researchers have been investigating the cause and treatment of dry skin, and there is currently great interest in adequate model systems for dry skin studies. In this chapter, I will describe several model systems of dry skin for clinical research of dermatitis associated with skin surface dryness and also mention recent studies to improve the dry skin. 

Sato, J., Yanai M., and Denda, M. (2000) Water content and thickness of stratum corneum contribute to skin surface morphology. Arch. Dermatol. Res. 292 412-417. 

Katagiri, C., Sato, J., Nomura, J., and Denda, M. (2003) Changes in environmental humidity affect the water-holding property of the stratum corneum and its free amino acid content, and the expression of filaggrin in the epidermis of hairless mice. J. Dermatol. Sci. 31 29-35. 

Wilhelm, K.P., Cua, A.B., and Maibach, H.I., Skin aging effect on transepidermal water loss, stratum corneum hydration, skin surface pH, and casual sebum content, Arch. Dermatol., 127,1806-1809,1991. 

Thune, P. et al., The water barrier function of the skin in relation to the water content of stratum corneum, pH and skin lipids. The effect of alkaline soap and syndet on dry skin in elderly, non-atopic patients, Acta Derm. Venereol., 68, 277, 1988. 

Warner, R.R. and Lilly, N.A. Correlation of water content with ultrastructure in the stratum corneum, In Eisner, P., Berardesca, E. and Maibach, H.I. Eds. Bioengineering of the Skin Water and the Stratum Corneum. CRC Press Inc., Florida, 1994, pp. 3-12. 

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