Cytokeratins as markers of follicular differentiation - An immunohistochemical study of trichoblastoma and basal cell carcinoma

Trichoblastoma(s) (TB) are benign neoplasms of follicular differentiation f requently found in nevus sebaceus. Many morphologic features are shared wit h nodular basal cell carcinoma(s) (BCC), sometimes rendering the differenti al diagnosis difficult. Because both neoplasms can simulate components of m ature hair follicles histologically, we attempted to corroborate this by im munohistochemical examination of cytokeratins and hair keratins differentia lly expressed in the hair follicle. Trichoblastoma(s) and BCC showed homoge nous expression of CK14 and CK17 The innermost cells of the tumor nodules i n all TB and in 72% of BCC were positive for CK6hf. Using a specific CK15 a ntibody, 38% of TB showed a focal labeling and all BCC remained negative; 7 0% of TB and 22% of BCC expressed CK19. CK8 was expressed by numerous Merke l cells present in all TB but in none of the BCC examined. All type I and I I hair keratins tested, (especially hHa1, hHa5, and hHa8) remained negative in all tumors examined. Trichoblastoma(s) and BCC show consistent expressi on of CK6hf, CK14, and CK17: variable expression of CK15 and CK19 and absen ce of hair keratins. This indicates a differentiation toward the outer root sheath epithelium or the companion layer and not toward the inner root she ath, matrix, or cortex.