[Tumor of the follicular infundibulum. Study of determining follicular differentiation].

Tumor of the follicular infundibulum is a rare proliferation of cells and its histogenesis or differentiation at the morphological level has been the subject of some controversy. In recent years cytokeratins have been recognized as important markers of epithelial differentiation, and of late new retrieval methods have meant it is possible to detect them in formalin-fixed and paraffin-embedded tissue. Four patients were studied, the ages ranging between the 2nd and 7th decade. The tumors were all located in the head and neck and in one case the lesion developed in a pre-existing sebaceous nevus. The morphological investigation revealed a flat, proliferation of polygonal, pale eosinophilic cells connected to epidermis or follicular infundibulum and with a centrally located nucleus. In addition, a few ductal structures resembling sebaceous ducts were seen and in one case a hair germ papilla and a follicular papilla was noted. Immunohistochemical investigations with antibodies against cytokeratins revealed differentiation comparable to that in the fetal follicular isthmus and, in one case, also differentiation in keeping with the fetal follicular infundibulum.