Corneum In this section, structures of the stratum corneum are mentioned at first and then an evaluation of their barrier function is reviewed.

2.2.1 Structures of the Stratum CorneumThe stratum corneum of human skin is a thin accumulated membrane of 10-20 μm thickness, which consists of 12-16 cell layers [4]. The thickness varies depending on the site of body, for example, typically 9 layers at the forehead and 50 layers at the palm. Components of the stratum corneum are mainly cell envelops, keratin fibers, and lipids, which are filling gaps between the keratin fibers. Thus, the structure of the stratum corneum is compared to “bricks and mortar” (Fig. 2.1) [1]. The “bricks” are the keratin fivers, and the “mortar” is the lipids. The keratin constitutes 60 wt% of the stratum corneum (per dry weight) [5].