Structure and Function of Human Skin

Abstract

Human skin is a uniquely engineered organ that permits terrestrial life by regulating heat and water loss from the body whilst preventing the ingress of noxious chemicals or microorganisms. It is also the largest organ of the human body, providing around 10% of the body mass of an average person, and it covers an average area of 1.7 m2. Whilst such a large and easily accessible organ apparently offers ideal and multiple sites to administer therapeutic agents for both local and systemic actions, human skin is a highly efficient self-repairing barrier designed to keep ‘the insides in and the outside out’. Skin membranes can be examined at various levels of complexity. In some mathematical treatments of transdermal drug delivery (see Chapter 2), the membrane can be regarded as a simple physical barrier; more complexity can be introduced by viewing skin as various barriers in series. We can then introduce barriers in parallel by considering drug transport through pores in the tissue. Degrees of complexity also exist when examining basic structures and functions of the membrane. In some extreme cases it may be that transdermal drug delivery is limited by metabolic activity within the membrane. Alternatively, immunological responses may prevent the clinical use of a formulation that has proven to be optimal during in-vitro studies. A further complication is introduced in clinical situations where topical delivery is intended to treat diseased skin states; here, the barrier nature of the membrane may be compromised and so data extrapolated from in-vitro experiments using healthy tissue may be inappropriate. This chapter provides an overview of the structure and function of healthy human skin, followed by a review of some physiological factors that can affect transdermal and topical drug delivery, such as skin disorders and age-related alterations to the membrane.