Microscopic sections revealed an exoendophytic tumor composed of mature keratinizing squamous epithelium. Papillomatosis, hyperkeratosis, and deep downward growth of irregular, fingerlike squamous projections were seen. Crypts containing keratotic debris were present between adjacent papillae. A brisk, focally bandlike, mixed inflammatory infiltrate composed of lymphocytes, plasma cells, and eosinophils abutted the base of the tumor. Fibrosis and prominent blood vessels were located subjacent to the tumor. The tumor cells were large but cytologically bland. A few basally located dyskeratotic cells were present. Cytologic atypia consisting of an elevated nuclear-cytoplasmic ratio, nuclear hyperchromasia, and prominence of nucleoli was found but was confined to the lower epithelial layers. Occasional mitotic figures were seen in the basal layers. An immunohistochemical stain was negative for human papillomavirus.
A Slowly Enlarging Umbilical Nodule—Diagnosis. Arch Dermatol. 2005;141(6):779-784. doi:10.1001/archderm.141.6.779-h