Skin

The skin is the body's largest organ, a layered barrier that protects against water loss, microbes and injury while sensing touch, temperature and pain.

It has three layers: the epidermis, a thin outer sheet of keratinocytes that renews itself about every month and ends in the dead, flattened cells of the stratum corneum; the dermis, a thicker layer of collagen, blood vessels, nerves, hair follicles and sweat and sebaceous glands; and the subcutis, a fat layer that insulates and cushions. Immune cells, nerve endings and the resident microbiome all live within these layers. In dogs and cats the structure is similar, but the epidermis is thinner and the hair coat does much of the protective work.

The barrier limits what can pass inward, which is why most substances applied to the skin act locally rather than reaching the bloodstream. Skin cells and nerves express cannabinoid receptors and TRP channels, and cannabinoids have been studied in cell and animal models of skin conditions; human evidence is early and no such product has an approved use.