Germinotropic Onychocytic Matricoma: A New Histopathologic Subtype of Onychocytic Matricoma in the Light of the Microanatomy of the Normal Nail Unit, With Special Reference to Nail Mesenchyme.

Onychocytic matricoma (OCM) is a benign tumor of adnexal keratinocytes in which there is complete cytodifferentiation toward the germinative cells of the nail matrix, the keratogenous zone cells, and cells of the nail plate. Several microscopic variants of OCM were initially recognized; acanthotic type, acanthotic and papillomatous type, keratogenous type, and melanocytic as slight proliferation of dendritic melanocytes within the basaloid cells. This study reports an additional histopathologic variant of OCM. The case presented as an acquired localized longitudinal pachymelanonychia of the thumb in a 50-year-old man. The excisional biopsy, with partial nail plate avulsion, showed a matrical acanthoma composed of basaloid cells with peripheral palisading. This matrical lesion showed extension into the proximal nail bed. The boundary between the matrix and nail bed was identified by the 2 microanatomical zones of the matrical mesenchyme, and the matrix metaplasia of the proximal nail bed was identified by the coexpression of hair-related keratins and K75. This onychogenic epithelial benign tumor presented a prominent germinative compartment without spheres of prekeratogenous and keratogenous zone cells and exhibited an unusual expression of K35. The melanocyte count was high in the proximal nail bed with matrix metaplasia (22.25 melanocytes per mm). By contrast, the distal nail bed, showed only rare melanocytes. This study describes a previously uncharacterized histopathologic subtype of OCM that can microscopically mimic onychocytic carcinoma, onychomatricoma and basal cell carcinoma. Careful conventional microscopy coupled with knowledge of the fine microanatomy of the normal nail mesenchyme can facilitate a correct diagnosis to be established.