This is a typical histologic example of an actinic keratosis, found on the hand of a 63-year-old woman. It shows the "flag sign", characterized by an alternating nature of the keratin. Columns of orthokeratosis correspond to uninvolved underlying adnexal structures. The altered fluorescent pink stratum corneum overlies the actinic changes. If examining the tissue en face, the dysplastic epidermis would have a honeycomb pattern, sparing the adnexal structures. This is in contrast to Bowen disease, which many authors believe arises from the follicular epithelium. We would sign this particular case out as 'hypertrophic actinic keratosis' due to atypical keratinocytes ascending above the basal layer of the epidermis. The atypia falls short of full-thickness, or in situ carcinoma. Actinic keratoses are considered "pre-malignant", as they will progress to squamous cell carcinoma given enough time.
At higher power of the same lesion, one can appreciate the alternating changes in the stratum corneum corresponding to changes in the nature of the keratinocytes. Underlying the fluorescent pink keratin, keratinocytes show some disorganization with overlapping nuclei, nuclear enlargement, and nuclear hyperchromasia. The rete extend down with bulbous tips in these areas. The keratinocytes in association with adnexal structures appear normal.