The many clinico-pathologic faces of actinic keratosis: an atlas.

Actinic keratoses (AKs) are intraepithelial neoplasms formed by atypical keratinocyte proliferation. Histopathologically, typical AKs are characterized by a slightly thickened epidermis, usually with irregular downward buds, orthokeratosis alternating with parakeratosis ('pink and blue') with loss of the underlying granular layer and a disarrangement of the epidermis with atypical keratinocytes at the basal layer. There are several histopathological variants of AK according to the different degrees of keratinocytic atypia, epidermal hyperplasia/atrophy, inflammation and pigmentation. Some authors consider AK an early in situ squamous cell carcinoma and propose a classification of AK based on the extent of atypical keratinocytes in the epidermis.