This is a typical example of an actinic keratosis, forming on sun-exposed skin. These are dysplastic, or pre-cancerous, lesions related to ultraviolet light exposure. There is a spectrum from actinic keratosis to hypertrophic actinic keratosis to squamous cell carcinoma in situ (keratotic type) to invasive squamous cell carcinoma. Although this spectrum admittedly exists, it is best not to lump all actinic keratoses into in situ carcinomas because only a small percentage of the former lead to invasive carcinoma. The dysplasia of an actinic keratosis is different from that of Bowen's disease, which likely derives from adnexal units and shows a different, more aggressive, biologic behavior.
At slightly higher power, the keratinocytic dysplasia is more evident. There is nuclear crowding and hyperchromasia. It is largely confined to the lower layers of the epidermis.