Outer Root Sheath Keratinization in Anagen and Catagen of the Mammalian Hair Follicle: A Seventh Distinct Type of Keratinization in the Hair Follicle - Trichilemmal Keratinization

The study described trichilemmal keratinization as a distinct type of keratinization in the hair follicle, which was not derived from the hair matrix. This process converted the stratified epithelium of the outer root sheath into anuclear keratin without an intervening keratohyalin layer. It occurred in the anagen phase in the zone of sloughing below the sebaceous duct opening and in the catagen phase in the trichilemmal sac surrounding the lower end of the dying hair shaft, forming the club of the telogen hair. The study used electron microscopy on dog hairs to reveal intricate infoldings of non-keratinized and keratinized cells and identified unique ladder-like membrane coating granules in anagen, which were acid phosphatase-positive and suggested to be involved in the disintegration of the inner root sheath.