An Immunohistochemical Study on Cell Differentiation in the Outer Root Sheath of Normal Human Anagen Hair Follicles with Antikeratin Monoclonal Antibodies

The study investigated the expression of various cytokeratins in the outer root sheath (ORS) of human anagen hair follicles using 11 antikeratin monoclonal antibodies on 10 scalp specimens. It was found that CKs 1, 10, and 11, which are markers for differentiating keratinocytes, were present in the intermediate and granular cells at the infundibulum, with a clear demarcation between the infundibulum and isthmus. Trichilemmal keratinization was observed to progress in an inner-upward direction toward the hair canal. CK 19, a marker for undifferentiated stem cells, was located in the outermost cells of the ORS at the isthmus and some lower ORS cells. CK 16, associated with hyperproliferative keratin, was found in the outermost cells of the infundibulum and all ORS cells below the isthmus. The findings suggested that keratinocytes at the infundibulum differentiate similarly to interfollicular epidermal keratinocytes, and ORS cells below the isthmus move upward along the hair axis as they differentiate.