Nephroid metaplasia of the urinary tract. A survey of the literature, with the contribution of 5 new immunohistochemically studied cases, including one case examined by electron microscopy.

Nephroid metaplasia is an unusual lesion confined to the lamina propria of the lower urinary tract. It is defined by a characteristic histologic picture of tubular structures, formed by a single layer of cuboidal cells, surrounded by a thick basement membrane. Two main theories concerning the histogenesis of the condition have been proposed: embryonic origin, or metaplasia. Five cases of nephroid metaplasia were studied light-microscopically and by immunohistochemistry for the content of Tamm Horsfall's uromucoprotein. In addition, one case was examined by electron microscopy. The results were compared to current knowledge of the lesion obtained from a survey of the literature, with special reference to histogenesis. Morphologically, one case of nephroid metaplasia was associated with mesonephroid adenocarcinoma. It is concluded that nephroid metaplasia arises as a metaplastic lesion, nearly always in previously traumatized urothelial mucosa. The natural history of the typical nephroid metaplasia is benign, but a possible relationship to mesonephroid adenocarcinoma, representing the malignant counterpart of the lesion, is discussed in relation to the histological findings, suggesting a rare but possible malignant potential of nephroid metaplasia. The diverse nomenclature used for this histologic entity needs re-evaluation and should be changed to: nephroid metaplasia.