On light microscopy, a renal biopsy from a patient with membranoproliferative glomerulonephritis shows a 'tram-track' or double-contour appearance of the glomerular basement membrane on silver stain. What is the mechanism responsible for this finding?
- A Subepithelial immune deposits lifting the outer surface of the basement membrane
- B Splitting of the lamina densa by intramembranous dense deposits
- C Mesangial cell interposition into the subendothelial space with new basement membrane material laid down around it ✓
- D Duplication of the endothelial layer lining the capillary lumen
Correct answer: C. Mesangial cell interposition into the subendothelial space with new basement membrane material laid down around it
Explanation
In MPGN, mesangial cells extend their cytoplasm between the endothelium and the original GBM, and a second layer of new basement membrane material is synthesized around the interposed cell. Silver stain outlines both membranes, producing the split or double contour. Subepithelial deposits produce spikes in membranous nephropathy, intramembranous dense bands define dense deposit disease, and endothelial duplication does not occur. The interposition mechanism explains why tram-tracking is the signature light microscopic feature of MPGN.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.