The role of follicular penetration. A differential view.

Textbooks describe percutaneous absorption through normal skin as diffusion through the stratum corneum barrier, i.e., in a simplified view, as a physiochemical interaction of a penetrating compound with the horizontal multilayer lipid structures and the underlying corneocytes within the horny layer. Hair follicles and sweat glands are vertical structures and are considered of little importance since 'they represent a small and insignificant percentage of the skin surface'. To this integral view, a differentiated hypothesis is proposed: the more perfect the barrier function of the horny layer, the greater the relative role of the imperfections within the skin as a defence organ. Three kinds of imperfections should be taken into account: (1) the follicular apparatus of hair follicles; (2) the sweat glands, and (3) microlesions in the interfollicular horny layer. Each of these can theoretically represent a vertical pathway for percutaneous absorption, i.e. a 'shunt'.