CD13 is a marker for onychofibroblasts within nail matrix onychodermis: Comparison of its expression patterns in the nail unit and in the hair follicle

Background: We previously demonstrated the presence of onychodermis, a specialized mesenchymal cell population beneath the nail matrix and proximal nail bed demonstrating CD10 expression. We hypothesize that the onychodermis could be the nail analog of the follicular dermal papilla, which is known to express CD13. We compare CD13 expression patterns between specialized mesenchymes of nail and hair, and compare these findings with CD10 expression patterns. Methods: CD10 and CD13 immunohistochemistry was performed on polydactyly and adult cadaveric nail units, and on hair follicles in scalp nevus sebaceus excision specimens. Results: CD10 and CD13 were expressed in the mesenchyme below the nail matrix and nail bed. Stronger CD13 expression was observed in the mesenchyme containing onychofibroblasts below the nail matrix compared with that below the nail bed. CD10 was expressed in the dermal sheath of terminal hair follicles, but it was expressed in the dermal sheath and follicular dermal papilla of primitive hair follicles within nevus sebaceus lesions. CD13 was expressed in the dermal sheath and dermal papilla of terminal and primitive hair follicles. Conclusion: CD13 may be a marker for onychofibroblasts within nail matrix onychodermis. We demonstrate CD13 expression in the specialized mesenchymes of both nail and hair.