The onychomatricoma: additional histologic criteria and immunohistochemical study.

Onychomatricoma (OM) is a tumor of the nail matrix typified histologically by multiple distal fibroepithelial projections and a thick keratogenous zone forming multiple V-shaped invaginations at the level of epithelial ridges, with the formation of a thick nail plate. In its proximal portion, the thickness of the nail looks like a spur originating from the ventral part of the nail plate. In its distal part, beyond the lunula, the nail plate is globally thickened and filled with cavities containing serous fluid. Often, however, the pathologist is not provided with the nail plate. The diagnosis then rests on the presence of a fibroepithelial tumor. In this article the histologic criteria of OM without nail plate are refined and OM is characterized immunohistochemically using three tumors fixed in liquid nitrogen and examined separately from the nail plate. On longitudinal section OM without nail plate appears as a unique pedunculated fibroepithelial tumor i.e., the multiple distal epithelial digitations arranged along a transversal plane are not seen. The feature is reminiscent of fibrokeratoma. When OM is visualized in longitudinal section, 3 main criteria differentiate OM from fibrokeratoma: the presence of epithelial-lined invaginations around optical cavities, a stroma organized in 2 layers, and the absence of horny corn. Patterns of expression of cytokeratins and integrins in OM are identical to that observed in the normal nail matrix. Involucrin finds expression from the basal layer through to the top of the epithelium, where it is more marked and where transglutaminase 1 is restricted. Merkel cells detected by CK 20 are increased in number and sometimes disposed in clusters. The fibrous component of OM is composed of 2 layers: a superficial stroma made of numerous fines fibrils of collagen IV intermingled with collagen I, and deep stroma made principally of collagen I. Antibody AE13, specific to trichocytic keratins Ha 1-4, represent a good potential marker of OM. Its V-shaped expression in epithelium ridges offers early identification of the keratogenous zone of OM, on tumors separated from their nail plates and limited to their fibroepithelial components.