Two intercellular lamellar structures in stratum corneum and their functional regulation mechanism

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The outermost layer of skin, stratum corneum (SC), is composed of corneocytes and intercellular lipid matrix. The matrix works as not only the main barrier but also the pathway of water, drug, etc. In mammalian SCs, for the longitudinal arrangement of the lipid molecules, long and short lamellar structures with a repeat distance of about 13 nm and about 6 nm, respectively, have been observed by small-angle X-ray diffraction. With increasing water content in SC, the diffraction peak position for the diffraction of long lamellar structure (LLS) is almost unchanged. On the other hand, the position of the diffraction of short lamellar structure (SLS) markedly shifts towards lower angle, suggesting that SLS exhibits swelling with the increase of the water content. Furthermore, we found that the widths of the diffraction peaks of both LLS and SLS become narrow simultaneously at a water content of 20-30 wt%. The present results indicate that LLS and SLS interact with each other, swelling of SLS takes place, and as a result at the water content of 20-30 wt% the both LLS and SLS are stabilized simultaneously. Besides, for the lateral arrangement of the lipid molecules, hexagonal and orthorhombic hydrocarbon chain packings have been observed by wide-angle X-ray diffraction. From the temperature dependence of the simultaneous small- and wide-angle X-ray diffraction, we revealed that the intercellular lipid matrix forms two domains which, at room temperature, are composed of the long lamellar structure with the hexagonal hydrocarbon-chain packing and the short lamellar structure with the orthorhombic hydrocarbon-chain packing.