Seborrheic karatosis, a tumor of the infundibular and isthmus of hair follicles: Identification of hair sheath keratins and histopathological features suggesting hair follicle origin.

The study investigated seborrheic keratosis, focusing on its potential origin from the infundibular and isthmus regions of hair follicles. Researchers identified hair sheath keratins (Moll catalog No. 6, 16, 17), which are specific to the outer root sheath, in all 15 lesions of seborrheic keratosis examined. Densitometry analysis suggested that these keratins were present in amounts 25 times greater than epidermal keratin (Moll catalog No. 1). A histopathological review of 156 cases over 12 years revealed features suggesting an origin from the upper hair follicle, such as continuity between the tumor and upper follicle, squamous eddies, various intermediate types, and significant presence of clear cells. These findings strongly suggested that seborrheic keratosis originated from the infundibular and isthmus regions of hair follicles.