The glomerular tufts are diffusely hypercellular. There is lobular accentuation, meaning that the segments or lobules are discernible because of cleft-like spaces between adjacent lobules. The cellularity is essentially confined to the tuft and there is no significant cellular proliferation in the urinary space. I is more a morphologic pattern of injury that a single disease.
Many cases are idiopathic.
I is an immune complex mediated disorder. Immune complexes are deposited in the mesangium and subendothelial spaces. Mesangial cells proliferate in response to the immune complex deposition and extend beyond the confines of the mesangium to mesangialize the capillary loops. A second internal basement membrane is formed by the mesangial cell and the undermined endothelial cells. Idiopathic membranoproliferative glomerulonephritis type I tend to be seen in older children and in adults.