Chronological changes in trichofolliculoma: Folliculosebaceous cystic hamartoma is not a very-late-stage trichofolliculoma.

Some authors have reported that the secondary hair follicles in trichofolliculomas (TF) undergo regressive changes and are subsequently replaced by the developed sebaceous elements, and that folliculosebaceous cystic hamartoma (FSCH) is a TF at a very late stage. In the present study, we revaluated the histopathological features of 40 TF lesions, focusing on their chronological changes. The results of the present study indicate that while the secondary follicles in the TF exhibited the hair cycle, the normal hair cycle was out of control, and tertiary hair follicles randomly developed from the involuting secondary follicles. The repeated development of hair follicles in this disordered hair cycle caused the development of chains of several continuous hair follicles in late-stage TF. In the TF lesions, no features indicating the replacement of the regressing secondary hair follicles by any sebaceous elements were observed, thereby suggesting that FSCH is not a very-late-stage TF.