The external capillary basement membrane is the original native basement membrane. The internal capillary basement membrane is a newly formed basement membrane. Between the two basement membranes is cytoplasm of mesangial cells that have extended beyond the confines of the mesangium. The newly formed internal basement membrane is a product of the mesangial cell and the endothelial cell. 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.