Onychomatricoma is a rare peculiar tumor of the nail unit. It was originally reported to be a benign tumor of the nail matrix as its name implies. However, the terminology onychomatricoma may be a misnomer. Histologically, it is a fibroepithelial tumor consisting of epithelial and mesenchymal components. Thus, the concept of epithelial onychogenic tumor with onychogenic mesenchyme was suggested1.

Recently, we found the presence of onychodermis of Dongyoun, a specialized mesenchymal cell population below the the nail matrix and proximal nail bed demonstrating CD10 expression2. Considering the components and location of onychomatricoma and its CD10 expression, we suggested the relation of onychomatricoma to onychodermis in the nail unit3. In addition, very recently, we demonstrated that stronger CD13 expression was found in the mesenchyme containing onychofibroblasts below the nail matrix compared to that below the nail bed, suggesting that CD13 may be a marker for onychofibroblasts within nail matrix onychodermis4.

In this study, to evaluate CD13 expression in onychomatricoma and further elucidate the association of onychomatricoma with onychodermis in the nail unit we performed CD 13 immunohistochemistry in three onychomatricoma cases (one case was diagnosed at Samsung Medical Center [Fig. 1]. The other two cases were kind gifts from Dr. Robert Baran and from Catholic University Hospital, Seoul, Korea). Immunohistochemical staining was performed using the monoclonal antibody to CD13 (1:50; clone 38C12; Abcam, Cambridge, UK).

H&E staining showed a fibroepithelial tumor with characteristic digitations. The tumor was lined by nail matrix-like epithelium. The tumor cells consisted of spindle shaped nuclei. Immunohistochemically, CD13 was strongly expressed diffusely in dermal portion of onychomatricoma (Fig. 2). CD13 expression was very similar in thee onychomatricoma cases.

Onychomatricoma is a dermal tumor with nail matrix-like epithelium. It is located around the nail matrix. Based on its histopathology and location, nail matrix onychodermis, which is located below the nail matrix, may be related to the development of the onychomatricoma. According to the previous report, cultured fibroblasts around nail matrix induced hard keratin expression in the non-nail-matrix keratinocytes in vitro through epithelial-mesenchymal interactions5. This finding suggests that nail matrix onychodermis containing onychofibroblasts may play an important role in nail formation in vivo through epithelial-mesenchymal interactions. Thus, nail matrix-like epithelium in onychomatricoma may be induced by mesenchymal tumor occurring in the nail matrix onychodermis. Furthermore, in our cases, CD13, which is known to be expressed at sites of epithelial-mesenchymal interactions and may be a marker of the nail matrix onychofibroblasts, was expressed in the dermal portion of the onychomatricoma. This finding also supports that onychomatricoma may derive from nail matrix onychodermis.

In conclusion, CD13 was strongly expressed in all 3 cases of onychomatricoma. Nail matrix onychodermis containing onychofibroblasts may be involved in the histogenesis of onychomatricoma.