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.
Histologic sections of skin show irregular epidermal acanthosis with elongated bulbous rete, and overlying mounds of hyperortho- and parakeratosis. The columns of orthokeratosis alternate with the parakeratosis. Cellular changes shadow the changes in the stratum corneum. Subjacent to the orthokeratosis, in areas near the follicles, the keratinocytes appear normal. Under the parakeratotic mounds, the keratinocytes show nuclear enlargement and appear more eosinophilic at this power.