Relationship between covalently bound ceramides and transepidermal water loss (TEWL)

The stratum corneum, which is the outermost layer of the skin, functions as an important barrier to maintain biological homeostasis. The multilamellar structures formed by intercellular lipids present in the stratum corneum a re considered to play an important role in barrier function. Most intercell ular lipids are unbound and can be extracted by organic solvents, but some intercellular lipids are covalently bound to cornified envelope proteins. D ecreases in unbound lipid levels reduce the barrier function of the stratum corneum, but the relationship between bound lipid and the barrier function of the stratum corneum is not well understood. In this study, we examined the relationship between the amount of covalently bound ceramide, the main bound lipid, and the barrier function of the stratum corneum, A single dose of UVB irradiation (2 x MED), or continuous UVB irradiation (0.5 x MED/day for 14 days) to the back, or feeding with an essential fatty acid-deficien t (EFAD) diet for 8 weeks caused a significant elevation of TEWL and a sign ificant reduction in covalently bound ceramides in hairless rats. Transmiss ion electron microscopy revealed that the intercellular multilamellar struc tures in the stratum corneum of treated rats were incomplete (folding, defe cts, unclear images) compared to the structures seen in the stratum corneum of non-UVB-irradiated and non-EFAD rats. These results suggest that the am ount of covalently bound ceramides is highly correlated with the barrier fu nction of the skin, and that covalently bound ceramides play an important r ole in the formation of lamellar structures, and are involved in the mainte nance of the barrier function of the skin.