Dead but highly dynamic--the stratum corneum is divided into three hydration zones

Topically applied water exerts mechanical stress on individual corneocytes as well as on the whole stratum corneum (SC), resulting in an alteration of barrier function. In this study we used complete skin biopsies and showed that the SC reacts to water stress as a highly optimized and well-regulated structure against osmotic changes. Following a relatively new cryo-processing protocol for cryo-SEM, it is possible to reliably maintain and investigate the hydrated state of the SC and individual corneocytes after treatment with solutions of different ionic strength. Treatment with distilled water results in swelling of SC cells together with formation of massive water inclusions between adjacent cell layers. Treatment with 5-20% NaCl reveals three different hydration zones within the SC: Corneocytes near the live-dead transition zone can swell to nearly double their thickness. The second zone is the most compact, as the corneocytes here show the smallest thickness variation with all treatments. Within the outermost zone, again a massive swelling and loosening of intracellular filament packing can be observed. We therefore conclude that the SC itself is subdivided into three functional zones with individual water penetration and binding potentials. Since the second zone remains nearly unaffected by water stress, we propose that this zone hosts the functional SC barrier.