On histologic examination, actinic keratoses usually show a collection of atypical keratinocytes with hyperpigmented or pleomorphic nuclei, extending to the basal layer of the epidermis. A "flag sign" is often described, referring to alternating areas of orthokeratosis and parakeratosis. Epidermal thickening and surrounding areas of sun-damaged skin are often seen. The normal ordered maturation of the keratinocytes is disordered to varying degrees: there may be widening of the intracellular spaces, cytologic atypia such as abnormally large nuclei, and a mild chronic inflammatory infiltrate.