ABSTRACT
The brick-and-mortar model of the outermost part of human skin, the stratum corneum or “horny layer,” was introduced by Michaels et al. [1] and later developed by Elias [2]. The model describes a tortuous structure, 2 to 10 µm thick, in which corneocyte cells (80-90 vol %) are embedded in a lipid matrix, which in turn consists of a complex mixture of different lipid species arranged in stacked bilayers parallel to the skin surface (Fig. 1) [3]. The major lipid classes are ceramides, cholesterol, and fatty acids (Fig. 2) [4]. Compared with other biomembrane lipids, the skin lipids have relatively long hydrocarbon chains, which make them crystalline at body temperature when isolated in test tubes and submerged with water.
