A tridimensional view of keratoacanthomas.

: Human keratoacanthomas are benign, hyperkeratotic skin tumors which, until now, have not been studied by the combined techniques of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Correlation of the results obtained by these two methods revealed the tridimensional anatomy of the tumor and illustrated the origin of keratin plugs within the tumor. Tumor cells were modified keratinocytes which differed from normal keratinocytes in that they were larger and sometimes contained vacuoles and intracellular desmosomes. the presence of vacuoles among the tonofilaments and tonofibrils indicated that the keratinization process was altered in the tumor cells. Keratin plugs arose from hair follicles and extrafollicular tumor cells. Hairs usually degenerated following histogenesis of the plugs. Extrafollicular plugs originated about foci of degenerating tumor cells and leukocytes. Keratin plugs, regardless of their mode of origin, were surrounded by concentric lamellae of tumor cells. Keratinized tumor cells in the plugs and at the free surface of the lesion had a "honeycomb" appearance. This resulted from prominent microridges and microgrooves which interdigitated extensively and produced a "tongue and groove" effect. As reported for several animal models of keratoacanthomas, there was no evidence that sweat ducts, sebaceous glands, or Langerhans cells were involved in the pathogenesis of the human keratoacanthoma. The findings supported the view that the human keratoacanthoma arises from hair follicles.