The concept of nail matrix onychodermis (onychomatricodermis) in the nail unit: Histology and elastin immunohistochemistry

Backgrounds: We previously demonstrated the presence of onychodermis below nail matrix and nail bed. Because nail matrix is a producer of nail plate, we hypothesized that onychodermis below nail matrix could be the nail counterpart of follicular dermal papilla. In this study, we sought to further characterize histologic, histochemical, and immunohistochemical features of nail matrix onychodermis. Methods and Results: Hematoxylin and eosin slides of 10 polydactyly nail units and 10 nail matrix biopsies from children and adults were reviewed. In polydactyly nail units, the onychodermis beneath nail matrix was characterized by onychofibroblasts showing abundant cytoplasm, and this area was slightly separated from the undersurface of the nail matrix. Nail matrix biopsy specimens also showed similar histology in the nail matrix onychodermis. Alcian blue stain demonstrated mucin deposition in onychofibroblasts within the nail matrix onychodermis. Immunohistochemically, elastin was rarely expressed in the nail matrix onychodermis while it was strongly expressed in the dermis of other areas of polydactyly nail units. Elastin was not expressed in follicular dermal papilla of terminal hair follicles of the scalp. Conclusion: Our findings demonstrate the presence and localization of nail matrix onychodermis (onychomatricodermis). Our study also demonstrates similar elastin expression patterns in the onychomatricodermis and follicular dermal papilla.