Background: We recently discovered the presence of specialized nail mesenchyme below the nail matrix and designated it as onychomatricodermis. Objective: We did further research to characterize the histologic, histochemical, immunohistochemical and ultrastructural features of the onychomatricodermis containing onychofibroblasts in the nail unit. Methods: Ten polydactyly nail unit specimens and 8 nail matrix biopsies were included. H&E-stained slides were reviewed. We did Alcian blue staining and Masson Trichrome staining, as well as immunohistochemical staining for type Ⅰ collagen, CD10, CD13 and CD34. In addition, polydactyly nail units were examined by transmission electron microscopy. Results: In H&E staining, the specialized mesenchyme called onychomatricodermis was observed to be slightly distant from the undersurface of the nail matrix and be less eosinophilic area. Onychomatricodermal onychofibroblasts showed light purple abundant cytoplasm.Masson Trichrome staining revealed fewer collagen fibers within the onychomatricodermis. In Alcian blue staining the onychomatricodermis showed mucin deposition within the onychofibroblasts and around them. Immunohistochemically, type Ⅰ collagen was expressed much less in the onychomatricodermis while it was strongly expressed elsewhere in the nail unit. In nail matrix biopsy specimens onychomatricodermal onychofibroblasts expressed CD10 and CD13 strongly, and expressed CD34 as well. Ultrastructurally, collagen fibrils were found sparsely within the onychomatricodermis, whereas collagen fibrils were densely distributed in the dermis of other parts of the nail unit. Conclusion We demonstrated that there was less collagen expression in the onychomatricodermis containing onychofibroblasts. In addition, we found morphological and immunohistochemical features of onychomatricodermal onychofibroblasts (onychofibroblasts of Dongyoun). These findings support the presence of onychomatricodermis containing onychofibroblasts in the nail unit.