ABSTRACT
This chapter describes the structure and function of healthy human skin, and considers some skin-related factors that can affect transdermal and topical drug delivery, such as body site, the overlying skin microbiome, and age-related alterations to the membrane. Human skin is a remarkable barrier between the body and the environment, providing protection against ingress of allergens, chemicals, and microorganisms, regulating the loss of water and nutrients from the body and responding to mitigate the effects of ultraviolet (UV) radiation. The anatomy of human skin is complex, but for the purpose of transdermal and topical drug delivery, we can examine its structure and function in four main layers: the innermost subcutaneous fat layer, the overlying dermis, the viable epidermis and the outermost layer of the tissue the stratum corneum. Three main appendages found on the surface of human skin originate in the dermis; the pilosebaceous unit, eccrine glands, and apocrine glands.
