Epithelial Stem Cells of the Skin Contribute to the Histopathologic Umbrella-like Appearance in Actinic Keratosis.

Although the histopathologic umbrella-like appearance of the epidermis in actinic keratosis (AK) is well known, its histopathogenesis has remained unknown. The author suggests that the penetration depth of ultraviolet B (UVB) radiation can account for such a histopathologic feature by affecting specific epidermal stem cell pools, as the stem cells in the epidermis and the bulge stem cells and sebocyte stem cells in the mid-dermis reside at different depths.