Structure-stabilizing forces in the glomerular tuft.

The glomerular tuft is constantly exposed to considerable expansile forces resulting from high capillary pressures. Counterforces must be generated in order to maintain structural stability. This review analyzes those structures of the glomerular tuft capable of developing such stabilizing forces. Two systems are described. A basic system consists of the glomerular basement membrane (GBM) and the mesangium. The GBM represents the main skeletal element of the glomerular tuft. In general, opposing portions of the GBM are bridged by contractile mesangial cell processes, generating inwardly directed forces that balance the expansile forces resulting from pressure gradients across the GBM. A second structure-stabilizing role of the podocytes appears to be superimposed on this system. Podocytes are attached to the GBM by numerous foot processes that contain a contractile system. The foot process attachments probably stabilize small patches of the underlying GBM, counteracting local elastic distension. In addition, podocytes may contribute to the stabilization of the folding pattern of the tuft by linking neighboring capillary loops to each other.