N. Otberg, H. Richter, U. Jacobi, U. Blume-Peytavi, H. Schaefer, W. Sterry, J. Lademann

Center of Experimental and Applied Cutaneous Physiology, Department of Dermatology and Allergology, Charité - Universitätsmedizin, Berlin, Germany

Laser-scanning microscopy in combination with cyanoacrylate skin surface biopsy was used to observe follicular penetration of topically applied dyes into the hair follicle. The follicle status (active/inactive) was checked by penetration experiments. The fluorescent food dye curcumin (C₂₁H₂₀O₆) in an O/W formulation (2% oil in water emulsion) was applied onto the investigated skin areas. After an application time of half an hour, a cyanoacrylate follicular biopsy was removed from the treated skin area and measured using laser-scanning microscopy. The penetration behavior of the applied dye into the follicles could be detected analyzing the cross-section of the removed follicle contents by fluorescence measurements using laser scanning microscopy. The dye was found in most of the hair follicles (ca. 70%), although some follicles were obviously closed for penetration. Reasons for this phenomenon are found in the specific follicular properties. A combination of different tape stripping and staining methods made it possible to measure hair growth and sebum excretion of every single hair follicle in the defined skin area. A correlation between the penetration properties, hair growth activity and sebum production was found. Telogen hair follicles that were not excreting sebum were closed for the penetration process. This means that only in the case of a movement from inside out of the hair follicle, a topically applied substance can penetrate into the follicle. It was proposed that the inactive follicles should be closed by a cover, which prevents the penetration. This effect was investigated by the in vivo analysis of the follicle infundibula cross-section by optical coherence tomography (OCT). A type of plug was found on the top of all closed follicles, which was determined in the penetration experiments.