Franco Ferrario and Maria Pia Rastaldi
Dr Franco Ferrario
Renal Immunopathology Center
San Carlo Borromeo Hospital
Membranous glomerulonephritis (MGN) is characterized by thickening of the glomerular basement membrane, due to the presence of subepithelial immune deposits.
From a morphological point of view, Idiopathic MGN is a indistinguishable from class V lupus nephritis.
The glomerular lesion is extremely variable and the basement membrane thickening changes over time. According to the classic descriptions, MGN is, in fact, divided into four stages.
By light microscopy glomeruli appear normal, although the capillary lumen is mildly dilated and a stiffness of the capillary wall can be detected.
Even at higher magnification, it is impossible to detect thickening of the capillary wall or the presence of subendothelial deposits.
Diagnosis is possible thanks to immunofluorescence and electron microscopy, otherwise differential diagnosis from minimal change disease would be difficult.
Finely granular or pseudo-linear IgG deposits are present all along the glomerular capillary wall.
By electron microscopy, small electron-dense subepithelial deposits with segmental distribution can be observed. Focal foot process effacement is a constant feature (x 10000).
This stage is characterized by glomerular enlargement and diffuse thickening of the capillary wall. Mesangial expansion and proliferation are usually absent.
Red subepithelial deposits can be demonstrated by Masson’s and AFOG trichrome stainings. The typical feature of membranous glomerulonephritis is best highlighted by silver staining: argyrophilic extensions of the glomerular basement membrane (spikes) look black, due to the lack of light transmission, whereas the interposing immune deposits appear as lucent areas.
Granular IgG deposits are present all along the glomerular capillary wall.
By electron microscopy, many subepithelial electron-dense deposits are evident and among them there are small basement membrane extensions (spikes). Foot process effacement is a diffuse phenomenon (x 4600).
Diffuse immune deposits and a more severe thickening of the basement membrane are detectable by light microscopy.
The severe thickening of the basement membrane, that covers the immune deposits, is evident at higher magnification.
Huge and diffuse granular deposits are present all along the glomerular basement membrane.
Together with subepithelial deposits totally covered by new layers of basement membrane, there are immunodeposits in less advanced stages (x 8000).
A further thickening of the glomerular basement membrane together with segmental or global glomerulosclerosis are detectable by light microscopy.
Some cases of Stage II MGN show focal and segmental sclerotic lesions, indistinguishable from idiopathic focal segmental glomerulosclerosis.
Rare cases present circumferential extracapillary lesions with rapidly progressive renal failure.
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