Original StudyA Revaluation of Trichofolliculoma: The Histopathological and Immunohistochemical FeaturesMisago, Noriyuki MD*; Kimura, Tetsunori MD†; Toda, Shuji MD‡; Mori, Toru MD*; Narisawa, Yutaka MD*Author Information From the *Division of Dermatology, Department of Internal Medicine, Faculty of Medicine, Saga University, Saga, Japan; †Sapporo Institute for Dermatopathology, Sapporo, Japan; and ‡Department of Pathology, Faculty of Medicine, Saga University, Saga, Japan. Funding sources: none. Conflicts of interest: none declared. Reprints: Noriyuki Misago, MD, Division of Dermatology, Department of Internal Medicine, Faculty of Medicine, Saga University, Nabeshima 5-1-1, Saga 849-8501, Japan (e-mail: firstname.lastname@example.org). The American Journal of Dermatopathology: February 2010 - Volume 32 - Issue 1 - p 35-43 doi: 10.1097/DAD.0b013e3181a77414 Buy SDC Metrics Abstract Few investigations on the histopathology of trichofolliculoma (TF) have so far included an immunohistochemical study. To seek new insight into TF with a revaluation of the histopathological features and an investigation of the immunohistochemical profile, 14 TFs were revaluated for the histopathology and the immunohistochemical profile of various cytokeratins (CKs), hair follicle stem cell markers, and others. The CK15 expression was upregulated in the basal cells from the primary cystic structures beyond to secondary follicles without expression of CK19. CK16 and CK17 were positive in the suprabasal cells of the primary cystic structures and the immature secondary hair follicles. No exact isthmus/bulge region was seen in the anagen secondary hair follicles, and newly developed (tertiary) hair follicles arose randomly from the involuting secondary follicles. Ber EP4 expression was generally weakened in the secondary or tertiary hair germ-like structures. The size of secondary hair follicles varied from vellus hair follicles to terminal hair follicles, even though no lesions located on the regions where the terminal hairs develop were included in this study. S-100 protein-positive wavy spindle cells were accidentally found in the surrounding connective tissue of the secondary follicles in 2 TF lesions. TFs were characterized by the proliferation of abnormal CK15-positive hair follicle stem cells, which basically differentiated toward the outer root sheath and attempting to make hair but losing the proper differentiation. The control of the size of the anagen hair follicles and the regular hair cycle were also disordered. © 2010 Lippincott Williams & Wilkins, Inc.