Which skin model is the most appropriate for the investigation of topically applied substances into the hair follicles?

Abstract

It has recently been demonstrated by the utilization of different techniques, such as differential stripping and laser scanning microscopy, that the hair follicles represent an efficient long-term reservoir for topically applied substances. In the present paper, the different in vivo techniques are compared to those of in vitro analysis. It was found that in vitro measurements on pig ear skin are highly superior for the analysis of follicular penetration, as compared to in vitro investigations on excised human skin, mainly due to the fact that the human skin contracts after removal. Restretching of the skin to its original size mainly stretches the interfollicular fibres, whereas the fibres around the hair follicles remain contracted. In contrast to excised human skin, pig ear tissue does not contract when the cartilage is not removed. Moreover, it has an intact barrier on both sides of the ear. Regardless of the fact that the hair follicles on pig ear skin are larger than those of humans, the porcine ear skin represents a more suitable in vitro model for the analysis of the penetration and storage of topically applied substances in the hair follicles than excised human skin.