Pathogenesis of hair infection and black dots in tinea capitis caused by Trichophyton violaceum: a histopathological study.

The majority of tinea capitis in southern Taiwan occur in adult women and are caused by Trichophyton violaceum. We report the histopathological findings of a series of 10 cases of tinea capitis caused by T. violaceum, the largest such study to date. Our study provides new information regarding the process of hair infection, mechanism of black dot formation, and chronicity of infection caused by this fungus. The cuticle remains intact. The fungi enter the proximal cortex where the cuticle is immature. They then colonize the proximal keratinized cortex and generate septate hyphae which transform gradually into arthrospores as they are carried upwards by the growing hair. At the infundibular level, the hair cortex is almost completely replaced by spores and swells, impeding further exit of the growing hair and causing the already weakened hair to coil up inside the infundibulum, forming a black dot. In one patient who had infection for more than 20 years, there were changes suggestive of cyclic reinfection of the same follicles which might contribute to the chronicity of the infection.