Actinic Keratosis: The Key Event in the Evolution from Photoaged Skin to Squamous Cell Carcinoma

Skin aging is the result of intrinsic and extrinsic factors. Extrinsic aging, also called photoaging, is mainly caused by ultraviolet radiation from the sun and leads to a state which has been termed dermatoheliosis. The aim of this article is to provide an overview of photoaged skin addressing actinic keratoses (AKs) in particular. This review will describe the clinical features of photoaged skin and briefly summarize the underlying histological, photobiochemical and molecular mechanisms responsible for photoaging. The concept of the disease continuum from AK to squamous cell carcinoma will also be presented. A special focus will be on established and new therapeutic approaches to undo photoinduced skin damage.