NEW ASPECTS OF THE PATHOMORPHOLOGIC SEQUENCE OF NEPHRON LOSS IN DIABETIC NEPHROPATHY

Abstract Background and Aims Diabetic nephropathy (DN) is the leading cause of end-stage-renal disease in western countries. Despite of innumerable studies undertaken to elucidate the pathogenesis of DN the underlying morphologic alterations have been insufficiently analyzed. Method Re-evaluation of more than 800 biopsies was done showing several unknown features. Results: 1. Matrix accumulation in the mesangium: Thickening of the glomerular basement membrane (GBM) and expansion of the mesangial matrix are hallmarks of DN, generally considered to emerge from different sites of overproduction: GBM components from podocytes and mesangial matrix from mesangial cells. We show, that the accumulation of matrix in the mesangium emerges from an overproduction of GBM material by podocytes and endothelial cells and an impaired degradation by mesangial cells. The progressing deposition of worn-out GBM material into the mesangium accounts for the advancement from diffuse mesangial sclerosis (DMS) to nodular sclerosis (NS) and to the herniation of the tuft through the glomerular vascular pole to the outside; the latter is associated with the outgrowth of glomerular capillaries into the peri-glomerular space leading to the destruction of the juxtaglomerular apparatus. 2.The role of podocytes Podocytes have frequently been accused to play a central role in DN. This is correct, but in another way than generally assumed. Damage to podocytes cannot be seen in DMS. The albuminuria regularly seen during this stage derives, as previously suggested by others, from an increased leakiness of the glomerular capillary endothelium based on a deranged glycocalyx. Podocyte detachments start at the transition from DMS to NS, based on the loss of cross talk signals with the capillary endothelium: the increasing deposition of matrix leads to the collapse of many capillaries. These podocytes contribute little to the further progression of the damage: they are lost into primary urine or they undergo cell lysis.In addition to their role in increased matrix production, podocytes take an active role in the formation of tuft adhesions to Bowman’s capsule (BC), starting the progression to NS. Expansion of the matrix within the mesangium has led to expansion of the tuft (frequently associated with nodules) towards Bowman’s capsule (BC) or towards the urinary orifice. Podocytes on the surface of these expansions are in their majority structurally intact, exhibiting an intact pattern of foot processes. These podocytes come into contact with parietal epithelial cells and initiate DN-specific tuft adhesions to BC allowing the proliferation of glomerular capillaries into BC. There they deliver an exudate into BC that spreads around the entire circumference of the glomerulus presenting as giant insudative spaces. Moreover, this process encroaches via the glomerulo-tubular junction onto the tubule constituting the major pathway of glomerular damage extending to the tubulointerstitium. 3. Tubulointerstitial fibrosisIt is current opinion that the tubulointerstitial fibrosis may start from tubular damage resulting in an own, glomerular-independent pathway to nephron loss. However, there is scant evidence for such a mechanism. Studying 162 glomerulo-tubular transitions, we did not see a tubular epithelial or interstitial damage in those biopsies without any evidence of a glomerulo-tubular damage transfer. The only exception consists of the well-known prominent thickening of the tubular basement membrane, which may result in functional loss but does not lead to structural epithelial damage. Conclusion We consistently found that tubulo-interstitial damage develops after encroachment of the glomerular damage onto the tubule, leading first to a gradual degeneration of tubules which subsequently initiate the process of interstitial fibrosis.