Basement membrane changes in membranoproliferative glomerulonephritis

Summary

In a previous study on membranoproliferative glomerulonephritis (MPGN) we reported the preliminary result that a basement membrane (b.m.) lesion incompatible with the criteria of subendothelial deposits (type I) or intramembranous dense “deposits” (type II) can be recognized by silver impregnation (s.i.) of ultra-thin sections. This technique has been further evaluated and applied firstly to control cases with normal b.m., perimembranous GN, diffuse proliferative (“MPGN-like”) lupus nephritis and, secondly to additional cases of idiopathic MPGN comprising 10 biopsies with doubtful findings as judged by electron microscopy with conventional impregnation. S.i. of ultra-thin sections proved to be a reliable method, of particular value in the visualization of fine structural details of b.m. changes in the field of MPGN. The light microscopic (l.m.) equivalent of the new lesion is defined. Accordingly, the series of 31 patients with idiopathic MPGN has been subdivided into three groups: Type I (19), type II (3), type III (9 patients). Type III is understood to be an intermediate type lesion, distinguished from type I by true membranous changes (discontinuity of the lamina densa) and from type II by the lack of the intramembranous electrondense (argyrophilic) material. It resembles in part, however, perimembranous GN due to segmental spike formation and little proliferation.

The clinical course of the patients with the type III lesion did not significantly differ from that of the other groups. The details are given in short case reports. Serum C3 was persistently depressed in 6, initially depressed in 2 patients and normal in one. As in type II, a predominant or isolated presence of C3 can be seen by immunofluorescence microscopy. Therefore, type III is likely to be mistaken for type II on the basis of immunological and l.m. data, and for type I on the basis of e.m. with conventional impregnation. The resultant inconsistencies so far inherent in the dual subclassification concept of MPGN can probably be solved — at least in part — by the acceptance of the type III lesion as defined by its appearance in silver impregnated ultra-thin sections.

Both lesions, type II and type III, are understood to be conditions in which the notional difference between “deposits” and a substantial alteration of the b.m. is poorly defined.