4. Becoming after cutaneous administration

4.1 Skin physiology refresher

The skin consists of three superimposed layers (Figure 7 ). The outermost layer, the epidermis, is formed by the stratum corneum, a stratified keratinized squamous epithelium that is constantly renewing itself, and the deeper stratum germinativum, which contains the cells that will eventually form the stratum corneum. The epidermis provides mechanical and biochemical protection and is the first barrier to be crossed by drugs administered to the skin. The second, looser layer is the dermis. This tissue contains blood vessels and connective tissue, and facilitates the diffusion of molecules. The deepest layer, the...