S15C7P2-3: This field is representative of the floor of the crater. Again, the lining is rather uniform in thickness. In areas showing rete ridge-like patterns, the epithelial projections are thin, and elongated. The epithelium is supported by a zone of inflamed, reactive fibrous tissue.

S15C7P2-4: In a keratoacanthoma, showing advanced senescence to this degree manifested in this lesion, the diagnosis may be difficult. Generally, in a lesion such as this, there is some preservation of a keratin-filled defect along the surface of the skin. In addition, it is common to find foci in which invasive growth continues, even in the face of advanced senescence. In this field, there is a “clonal” interface to the right side of the field. It is defined by the zone in which the more acidophilic epithelium to the far right abuts upon the hyperplastic, pale epithelium. The hyperplastic, pale epithelium provides evidence of invasive growth; the rounded, pale defects in the component of pale epithelium, near its interface with the inflamed fibrous tissue, contain elastotic material. The pale elastotic material in the epithelium is a marker for material that is found in the reticular dermis of sun-damaged skin. The presence of this material in the interstitium of the pale epithelium is a distinctive marker for invasion of the domain of actinically damaged reticular dermis. The material becomes entrapped in the epithelium as the epithelium invades the dermis. Invasion of unprepared dermis, with entrapment of connective tissue fibers, is a characteristic of actinic keratoacanthomas; it is seen to greatest degree in early stages in the evolution of such lesions.

S15C7P2-5: The pale, bluish-grey material in defects in the hyperplastic squamous epithelium is elastotic material. Its presence in the epithelium, in combination with the cytoplasmic qualities of the epithelial cells, are characteristic features of keratoacanthoma. There is minimal atypia in this example, but this is a lesion late in its evolution. The clonal pattern as seen in S15C7P1-4 might be cited as a feature in favor of a neoplastic process, even in the absence of significant cytologic atypia; it is possible that immunostimulation alone (in the absence of a neoplastic clone of keratinocytes) might produce a similar pattern.