Cytokeratin expression in trichoblastic fibroma (small nodular type trichoblastoma), trichoepithelioma and basal cell carcinoma.

Classical trichoblastic fibroma or small nodular type trichoblastoma (Ackerman) is a rare tumour. This tumour, trichoepithelioma and basal cell carcinoma (BCC) have some overlapping histopathological features. There are only a few reports on immunohistochemical studies in large series of these three neoplasms. We investigated immunostaining patterns of 10 different anticytokeratin (CK) antibodies and several other markers in these neoplasms, comparing them with the patterns in normal adult and fetal skin. In trichoblastic fibroma (three cases), CK1/5/10/14, CK7, CK8/18, CK10/11, CK14, CK17 and CK19 were expressed in the basaloid nests, and CK6 and involucrin were detected in the inner layers of keratinous cysts. Trichoepithelioma (seven cases) expressed CK1/5/10/14, CK8/18, CK14, CK17 and CK19 in the basaloid nests, and CK6, CK10, CK10/11 and involucrin were positive in the keratinous cysts. However, no CK7 expression was observed. Solid and keratotic types of BCC (29 cases) expressed CK1/5/10/14, CK7, CK8/18, CK14, CK17 and CK19 in the basaloid nests. The keratinous cysts in BCC were stained with anti-CK6, CK10, CK10/11 and involucrin antibodies. Coupled with the expression of CK8/18, CK17 and CK19 in the outer root sheath of the adult hair follicle, these three neoplasms shared a keratin phenotype characteristic of the outer root sheath. Judging from our immunohistochemical results, trichoblastic fibroma and BCC cannot be differentiated by their patterns of CK expression. The expression of CK7, which is noted in fetal hair follicles, trichoblastic fibroma and BCC, suggests the presence of subpopulations that retain fetal phenotypic characteristics in these two neoplasms. Although the current concept regards trichoepithelioma and trichoblastic fibroma as a single tumour group, the lack of CK7 expression in trichoepithelioma supports the notion that the two are different.