The nail dermis: from microanatomy to constitutive modelling

The nail mesenchyme, as ligamentous connective tissue, is classically described as a single compartment. Some authors have even suggested the concept of the nail as a musculoskeletal appendage. Recent studies conducted on supernumerary digits surgically removed for polydactyly or on the developing nail organ have introduced into the literature a new concept: the onychodermis and its onychofibroblasts that are CD10-positive/CD34-negative. The aim of this study was to explore the nail mesenchyme more comprehensively.

A series of 10 normal adult nail units were examined with a combination of morphological and immunohistochemical analysis. This study demonstrates that the nail mesenchyme has two distinct compartments, with a complex microanatomy of matrical dermis and its hypoderm. The matrical dermis is a relatively independent substructure, and comprises two parts: a thin papillary dermis, and a relatively thick reticular dermis. The matrical hypoderm corresponds to a cushion-like layer of adipose tissue, which distally intermingles with an area of loose connective tissue. The nail bed dermis comprises a single, relatively homogeneous compartment.

The matrical nail mesenchyme is a modified dermis. The concept of onychodermis is not applicable in the normal adult nail. Underrecognized non-pathological structures may pose diagnostic problems. The chameleon matrical hypoderm is one such structure.