The histopathology of actinic keratosis is epidermal dysplasia with alterations in cell polarity and mild nuclear atypia. Alternating hyperkeratosis and parakeratosis, irregular acantho-sis and thinning of the granular layer are commonly seen, with buds of atypical keratinocytes extending downward into the papillary dermis. Epithelium of the hair follicles is shielded from actinic damage and maintains normal structure, reaching the epidermis and extending over adjacent atypical cells to produce the characteristic umbrella sign. The dermis reveals solar elastosis and there is often a mild chronic inflammatory infiltrate.
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