The diagram in slide 53 illustrates the dense transformation of the basement membrane that causes the thickening. The electron micrograph in slide 54 shows the urinary space, an expanded mesangial region with a little bit of dense material in the increased matrix, and capillary basement membrane with stretches of normal lamina densa and zones of dense transformation. Slide 54a shows GBM as well as mesangial deposits. These dense deposits are not subepithelial or subendothelial, but rather are within the basement membrane.
By immunofluorescence microscopy (slide 55) there is intense staining for C3, typically with almost no staining for immunoglobulin. The capillary wall staining is usually linear or bilinear. There often are spherical or ring-shaped mesangial deposits that correspond to the mesangial dense deposits observed by electron microscopy.
Type II MPGN has a higher frequency of hypocomplementemia and C3 nephritic factor than type I MPGN.
In summary, a pathologic diagnosis of MPGN requires not merely light microscopic recognition of an appropriate pattern of glomerular injury, but, more importantly, specific ultrastructural changes that are the diagnostic features of these diseases. Many types of glomerular disease can produce light microscopic patterns of glomerular injury that mimic MPGN, but the diagnostic term MPGN should be reserved for the specific types of disease just described, and should be further qualified as MPGN type I, MPGN type II or MPGN type III. Otherwise, a diagnosis of MPGN would be of no value for predicting prognosis, identifying possible causes and directing therapy.
