Ultrastructural study of normal rat glomeruli by the quick-freezing and deep-etching method

The ultrastructural features of isolated normal rat glomeruli were investigated using a new splitting technique and the quick-freezing and deep-etching (QF-DE) method. Examinations were also made of in vivo normal rat glomeruli by the QF-DE method to visualize the glomeruli under near natural conditions. Freeze-fractured capillary loops were observed from the capillary lumen or the urinary space. Foot processes were found to be freeze-fractured horizontally, obliquely or vertically to the glomerular basement membrane (GBM), and a glomerular slit diaphragm, which consisted of sheet-like, ladder-like and zipper-like substructures, was clearly identified. The GBM was classified into three zones, which might correspond to those seen in ultrathin sections, including the lamina densa and laminae rarae externa and interna. The lamina densa was composed of a meshwork filled with fine particles, which might represent products of insoluble proteins. In the laminae rarae, filamentous structures consisting of irregular networks could be observed connecting glomerular endothelial or epithelial cells with the lamina densa. Furthermore, the above findings were obtained from investigations at various consecutive levels of freeze-fractured capillary loops from various angles. The three-dimensional ultrastructure of the glomeruli could be demonstrated at high resolution by the QF-DE method.