Keratinocytes represent the major cell type of the epidermis, the outermost of the layers of the skin, making up about 90 percent of the cells there. They originate in the deepest layer of the epidermis, the stratum basale and move up to the final barrier layer of the skin, the stratum corneum.
A pagetoid pattern of isolated atypical keratinocytes or clusters are common. The atypical keratinocytes typically have irregular, often hyperchromatic, enlarged nuclei. These cells are sometimes vacuolated, reminiscent of the cells in Paget’s disease. Multinucleated keratinocytes containing clusters of nuclei are often present. Mitoses and apoptotic bodies are often identified in the spinous cell layer and some mitotic figures may be atypical. Individually keratinized cells that have homogeneous, strongly eosinophilic cytoplasm are commonly found.