Recent advances in understanding of radiation-induced skin tissue reactions with respect to acute tissue injury and late adverse effect.

Skin is a tissue vulnerable to radiation exposure, which causes acute tissue reactions, including erythema, edema, desquamation, ulceration and late effects, such as skin cancers. As the effects of radiation exposure on the skin tissue are easily evaluated by visual examination, much information on radiation-induced skin reactions has been available from the clinical observation of people exposed to ionizing radiation, such as cancer patients receiving radiotherapy, although the mechanisms underlying skin reactions have not yet been fully understood. Recent advances in tissue biology at the molecular level have provided insights into the mechanisms of skin tissue reactions from the stem cell points of view. For example, our understanding of epidermal regeneration by epidermal stem cells as well as cells from the bulge in humans and the sebaceous gland in mouse, descriptions of the role of skin immune cells on inflammatory response and maintenance of genome integrity by epidermal stem cell competition, have greatly improved in the last decade with the identification of several key molecules. Thus, this review will provide an overview of the current status toward the comprehensive understanding of the mechanisms of adverse skin tissue reactions, with respect to mitigation of acute skin injuries as well as late carcinogenesis in response to ionizing radiation. In particular, the pleiotropic features of various types of cells consisting of skin tissue and their roles in securing skin functional homeostasis will be discussed.