Onychomatricoma in the light of the microanatomy of the normal nail unit.

Onychomatricoma (OM) is an uncommon benign tumor of the nail thought to exhibit differentiation limited toward the nail matrix. Four recent articles from our laboratory have shown, in some respect, a morphological and immunohistochemical homology between the nail unit and the hair follicle at the level of the matrix and isthmus. The purposes of this article are as follows: to investigate whether the sequential pattern of hair keratin expression in the normal nail matrix is maintained in OM, to compare and contrast follicular tumors with matrix differentiation in OM, and to furnish morphological and immunohistochemical markers of the onychogenic capacity of OM. Formalin-fixed paraffin sections from 6 OM were examined using specific keratin (K) antibodies for the matrix, nail bed, and nail isthmus. Hair keratins were expressed in a sequential pattern similar to normal nail matrix. In 3 cases where the cavities were completely lined by the fibroepithelial projections, the morphological aspect and the pattern of expression of K5, K17, K6, K16, and K75 suggested a differentiation toward the nail bed and the nail isthmus. This study shows for the first time that OM can recapitulate the entire nail unit with differentiation toward the nail bed and the nail isthmus. We have identified new histopathological and immunohistochemical features in OM, and we have abridged the diversity of its histological presentation in 2 main patterns: a lobulated or foliated pattern, observed principally on transverse section, and a "glove-finger" mono- or multidigitate pattern, observed mainly on longitudinal section. We have also concluded that OM is not a nail variant of trichoblastoma, pilomatricoma, or other pilar tumors. The concept of epithelial onychogenic tumor with onychogenic mesenchyme could shed more light about the true nature of this peculiar mixed tumor. However, the term OM is short and sanctioned by usage, which justifies keeping it.