The histogenetic base of renal capsular-derived tumors.

Capsulomas comprise a category of very rare benign tumors derived from the renal capsule, the most encountered being myxomas and leiomyomas. To get more information on the histogenetic origin of these tumors, a comprehensive ultrastructural investigation on the human renal capsule has been done on kidney biopsy samples performed for nephropathologic diagnosis. The human renal capsule ultrastructure is similar to that of the mammalian renal capsule. There are two cellular layers: an inner layer made up of particular (immature) smooth muscle cells, and a second outer layer consisting of fibroblasts, collagen fibers, extracellular matrix, and telocyte-like cells. Two cases of leiomyomas of microscopic dimensions, situated beneath the capsule have been described. Data from the literature presenting the ultrastructure and perirenal location of myxomas support the affiliation of these capsulomas with the resident renal capsular cells. Based on ultrastructural studies, the authors demonstrate the presence of telocyte-like cells in the outer layer of the human renal capsule and propose distinct histogeneses for leiomyomas and for capsular myxomas as derived from the inner and outer capsular layers, respectively.