Comparative Anatomy of Polydactyly and the Adult Nail Unit and Its Implications in the Onychofibroblasts/Onychodermis Theory.

Recent studies have proposed the use of the polydactyly nail unit model as a tool for the study of the molecular basis of onychogenesis. However, the histological and immunohistochemical description of this model is still a matter of debate in the literature, and the validity of the concept of the onychodermis used in the histological interpretation of both polydactyly and the adult nail unit has been challenged on the basis of detailed histological studies of adult fingernail units. The aim of this study was to provide new histological and immunohistochemical data on both the adult toenail unit and polydactyly nail unit to test the onychodermis/onychofibroblasts hypothesis. A histological study was performed on a series of 20 normal adult toenails from 10 cadavers and 10 polydactyly nail units. This study supported a strong analogy between the finger and toe dermis of the adult nail. The telocyte population is relatively sparse compared with the fibroblast (fibrocyte) population of the nail mesenchyme as a whole. The dynamic immunohistochemical profile of the nail telocytes was present in both fingers and toes, with the CD34+/CD10+ profile correlating with the onychogenic epithelial region, and the CD34-/CD10+ profile correlating with the spatial rearrangement of the epidermal ridges of the nail bed that are aligned longitudinally and almost parallel to each other. The main differences resulted from more consistently well-formed adipose tissue, a more discrete mucinous component at the level of the distal hypodermis of the hallux, and a thicker nail bed dermis. Previous studies of polydactyly have not taken into account the stage of development of the distal phalanx. Morphological variations of the developing phalanx have been classified into 2 types with 4 subtypes. In all but 1 type, the nail mesenchyme had 2 compartments: a matrical dermis devoid of onychofibroblasts and a CD10+ loose connective tissue with onychofibroblasts. In all cases, the onychofibroblasts interact closely with the thick periosteal/perichondrial layer of the developing bone. By contrast, in the adult nail unit, the matrix apex is independent of the distal interphalangeal (DIP) joint synovioentheseal complex and has only focal and lateral connection to the periosteum through a lateral fenestrated fascia. Moving proximally to distally the quiescent periosteum is separated from the matrix epithelium by a thick matrical dermis and its hypodermis, whereas the nail bed dermis shows a definite and stable anchorage to the periosteum. The oversimplified concept of onychodermis does not allow us to analyze the complex microanatomy of the adult nail unit with major misunderstanding of the matrical hypodermis and the hypodermal channel of the nail bed. The polydactyly nail unit is a good model for studying early nail morphogenesis and hereditary and congenital nail disorders, but not a histological model for analyzing the normal nail unit, in particular the role of telocytes versus onychofibroblasts as nail mesenchymal stem cells. The classification of morphological variations of the polydactyly nail unit is essential for the future interpretation of molecular studies using this model.