The pathophysiology of photoaging of the skin.

Exposure to solar UV radiation is the main environmental factor that causes premature aging of the skin (photoaging). Human skin aging resulting from UV irradiation is a cumulative process that occurs based on the degree of sun exposure and the level of skin pigment. UV irradiation induces matrix metalloproteinases (MMPs) responsible for alterations in the collagenous extracellular matrix of connective tissue, resulting in impaired integrity. On a molecular level, UV radiation from the sun attacks keratinocytes and fibroblasts, resulting in the activation of cell surface receptors, which initiate signal transduction cascades. This in turn leads to a variety of molecular changes, which causes a breakdown of collagen in the extracellular matrix and a shutdown of new collagen synthesis. This article describes these molecular changes that contribute to the alterations in collagen and the clinical effects seen in photoaging.