| The stratum corneum lipids, mainly ceramides, cholesterol and fatty acids, play the predominant role in maintaining the water barrier of the skin. In order to understand the biological variation in the composition of the barrier lipids, the alterations observed in winter xerosis, with the seasons, following UV damage and across the different bodysites were investigated. Marked seasonal changes in stratum corneum lipid levels were observed, being at their lowest in winter, when susceptibility to dryness is increased. In particular, a reduction in ceramide 1 linoleate was observed. In winter skin xerosis, dryness was associated with a decline in the ceramide: fatty acid ratio. This suggests that both the overall stratum corneum lipid levels and the relative proportions of the different lipids may impact on barrier performance and susceptibility to dryness. Although the stratum corneum lipids may contribute to corneocyte cohesion, the main cohesive force ensuring the integrity in the stratum corneum is provided by modified desmosomes called corneodesmosomes. These proteinaceous linkages are shown to be degraded during stratum corneum maturation by proteases such as 'stratum corneum chymotryptic-like enzyme' (SCCE). The role of lipids in modulating desquamation through influencing protease activity was investigated. Both cholesterol sulphate (elevated in recessive x-linked ichthyosis and following UV irradiation) and fatty acids (elevated in cosmetic dryness) were found to inhibit SCCE activity. This highlights one mechanism through which the stratum corneum lipids can influence the desquamatory process. Stratum corneum maturation includes changes in cornified cell envelope (CE) morphology, the highly crosslinked protein shell that encases the corneocytes. Transglutaminase 1 and 3 activity were identified within the stratum corneum, the levels of which increase during stratum corneum transit, alongside CE maturation. These effects on CE morphology may have an important structural role in maintaining the skins barrier. |
