Many ingredients in creams and ointments do more than just act locally on the skin surface or within the ‘stratum corneum’ (the outermost layers of skin). For example, urea is a compound that is widely used as a remedy for dry skin because of its ability both to hydrate or ‘moisturize’ the skin and to help remove skin build-up or scale. But urea also penetrates through the stratum corneum into the underlying nucleated cell layers of the epidermis. We have demonstrated in both human and mouse skin that urea is taken up into the nucleated cells of the epidermis (or ‘keratinocytes’) via specific urea transporters (proteins located on the cell membranes that facilitate movement of molecules into cells). Once inside the keratinocyte, we show that urea upregulates genes (i.e. causes genes to be activated and transcribed) that result in improved competence of the skin barrier and of its antimicrobial defense.
Bottom line: Do not assume that what you put on your skin stays on the surface. Some ingredients may be absorbed into deeper layers and exert potent effects on skin function.