Outer root sheath acanthoma

The term “outer root sheath acanthoma” is proposed to specify a distinctive keratosis exhibiting outer root sheath differentiation. This lesion features anastamosing columns of keratinocytes emanating from the undersurface of the epidermis that respectively demonstrate, with progressive descent into the upper dermis, outer root sheath differentiation characteristic of the infundibulum, isthmus, stem and anagen bulb. In our patient, the lesion was solitary, and no associated systemic disease was evident. Outer root sheath acanthoma is a unique keratosis displaying all four types of tricholemmal differentiation, thus representing a true neoplasm of the entire outer root sheath.