The stratum corneum of the epidermis is responsible for the most important function of the skin, namely its protective barrier function. The mechanism by which such protection is conferred is complex and involves the function of intracellular fibrous proteins, dense cellular membranes, and intercellular lipid constituents. The intercellular lipid portion is now believed to be the crucial component responsible for barrier function. Keratinocytes also play a role in immune responses through the production of a variety of cytokines. The skin is not completely impermeable.
Compounds can pass through intact skin.
This permeability of the skin is used for the delivery of topical therapeutic agents used in the treatment of certain skin diseases, and also for delivery of medication used in the treatment of systemic diseases. Examples of percutaneous delivery include scopolamine, used for treatment of motion sickness, nitroglycerine for angina pectoris and estrogen for postmenopausal symptoms. Melanocytes are cells derived from the neural crest. They migrate early during embryonic development and pass through the dermal mesenchyme to reside in the basal layer of the epidermis. There is approximately one melanocyte for every 10 basal keratinocytes.