Figure 3.

Photomicrographs of histological changes of regressing and almost regressed keratoacanthomas. A: The entire tumor from a 20-day-old transgenic rabbit at ×1.5 original magnification. Notice the characteristic necrotic central core (c) and the wall of epithelial cells that extend laterally under the epidermis (arrow). Typically the tumors terminated at the panniculus muscle (arrowhead), which on section gave the tumors a flattened side that was evident grossly as well as histologically. B: The same tumor at ×30 original magnification. Notice the multiple cords and clusters of neoplastic epithelial cells that have invaded deeper areas of dermis. The neoplastic cells shown squamous differentiation, including keratin production (arrow), dyskeratosis, and intercellular bridges, which are not seen at this magnification. Small clusters of cells at the leading edge of the tumor were associated with a fibrous connective tissue response. The neoplastic cells were not found to invade past the layer of panniculus muscle (m) in any of the specimens examined. C: An almost totally regressed tumor from a 60-day-old transgenic rabbit at ×50 original magnification. Notice the papillary projections of epidermal cells and hyperkeratosis that resembles papilloma. H&E stain.