The concept of onychodermis containing onychofibroblasts has histological (microanatomical), immunohistochemical as well as molecular basis

The terms “onychofibroblast” (nail-specific fibroblast) and onychodermis (nail-specific dermis) were first introduced in 2006 and 2012, respectively, based on distinctive histologic and immunohistochemical features from the dermis of the surrounding skin and have been demonstrated in multiple studies. Recently, based on molecular research, the definition of onychodermis containing onychofibroblasts has been expanded to encompass the area located between the nail matrix and bed epithelium and periosteum. Single-cell RNA sequencing and in situ hybridization demonstrated that onychofibroblasts within the onychodermis express the genes including RSPO4, MSX1, WIF-1, and BMP5, which are implicated in nail formation and/or in disorders with nail phenotype. A mutation in RSPO4, a component of the Wnt signaling pathway, causes anonychia congenita. Nail matrix onychodermis and nail bed onychodermis share many similar characteristics which differ from the surrounding normal dermis of the skin. Comparative spatial transcriptomic and single-cell analyses of human nail units and hair follicles suggest that onychodermis is the counterpart of follicular dermal papilla, which plays a key role in hair follicle growth and morphogenesis. Onychomatricoma, as a nail-specific tumor, has been demonstrated to be a mesenchymal tumor that originates from onychofibroblasts and is associated with the upregulation of Wnt signaling. Collectively, the onychodermis and onychofibroblasts play crucial roles in nail development and these specialized nail mesenchymal elements are key components in the pathogenesis of onychomatricoma. The concept of onychodermis containing onychofibroblasts is very important for nail biology and pathology.