Multiphoton STED and FRET in Human Skin: Resolving the Skin Barrier

Abstract

Understanding the penetration properties of substances across biological barriers and membranes is vital for many areas of research. In the case of human skin, the barrier is primarily found in the stratum corneum and consists of protein-enriched cells surrounded by a lipid membrane -enriched intercellular spaces. Characterization of the structural and dynamical processes occurring across the skin barrier is essential for understanding healthy and diseased skin and for designing successful transdermal drug delivery strategies. In this study we use Stimulated emission depletion (STED), two photon excited STED and Forster Resonance Energy Transfer (FRET) microscopy to probe the structure of human skin. Super resolution optical microscopy enables resolving structures in the skin below to 60 nm allowing visualization of the stratum corneum intercellular lipid matrix and individual proteins such as tight junction (TJ) proteins and corneodesmosomes. To further probe the nanoscopic structure of the intercellular lipids and the nanoscopic diffusion routes of hydrophilic and hydrophobic particles through the skin barrier we use FRET measurements of lipophilic and hydrophilic dye pairs. The measurements are carried out in artificial membranes of extracted human skin lipids and in skin tissue. They show FRET distance which agree with membrane models derived from small angle x-ray measurements of human skin lipids.