Heterogeneity of the podocyte membrane in rat kidney as revealed by ethanol dehydration of unosmicated specimens

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Structure and function of podocytes: an update

Glomerular visceral epithelial cells, also termed podocytes, are highly specialized epithelial cells that cover the outer aspect of the glomerular basement membrane. Recent studies point to an important role of podocytes in the physiology and pathophysiology of the glomerulus. This review summarizes the structure-function relationships of podocytes. Following a description of the general morphology of podocytes, the technical problems associated with studying these cells are discussed. A survey of podocyte function forms the center of this review. Finally, selected aspects of podocyte development and cell division are discussed.

Quick-freeze, deep-etch studies of renal basement membranes

The fine structure of the renal (i.e., glomerular, tubular, and capillary) basement membranes was re-evaluated with the aid of a deep-etch replica method. The structure of the laminae rarae interna and externa of the rat glomerular basement membrane (GBM) and laminae lucida of other basement membranes were basically identical in that 6 to 8 nm fibrils were interconnected to form a three-dimensional, polygonal network. By contrast, all of the laminae densa examined were composed of closely packed granules, and a filamentous substructure was identified only in a limited area. These granular components were demonstrated to be an integral component of the lamina densa. From additional observations on the trypsinized bovine GBM, it appeared that the basic structure of renal basement membranes was almost identical, namely, that a three-dimensional fibrillar meshwork existed throughout the individual layers to form a structural framework upon which fine particles were variably attached. In addition, we observed some of the fine structure of the pars fibroreticularis; the laminae densa of the tubular and capillary basement membranes continued to the fibrillar meshwork resembling the structural backbone of the glomerular basement membrane. The network was sometimes directly connected to the extracellular matrix, but more often changed into a rough fibrillar framework and connected to the extracellular matrix.

Fine structure of the glomerular basement membrane of the rat kidney visualized by high-resolution scanning electron microscopy

The fine structure of the glomerular basement membrane (GBM) of the rat kidney was studied by means of high resolution scanning electron microscopy. Specimens were taken from kidneys perfused with paraformaldehyde, freeze-fractured and then processed with conductive staining. The fractured surface of glomerular tufts exhibited the inner and outer surface of the GBM uncovered by endothelial and epithelial cells. The lamina densa was composed of densely packed granular material together with scattered fibrils. The laminae rarae interna and externa were composed of a meshwork that showed some structural heterogeneities. The meshwork composing the lamina rara interna contained 5- to 9-nm-thick fibrils, had pores 11-30 nm wide, and was associated with granular material except in those places that corresponded with endothelial fenestrae. The meshwork of the lamina rara externa was made up of 6- to 11-nm-thick fibrils, and had smaller pores under the foot processes (10-24 nm wide) than those near the filtration slits (16-32 nm wide). In addition to the meshwork, the lamina rara interna contained microfibrils that were arranged differently depending on the topography of the capillary wall: scattered fibrils had no predominant orientation at the convex side, circumferential bundles lay at the concave side of the peripheral capillary wall, and had a circumferential arrangement in the paramesangial wall.

Microfibrils are a major component of the mesangial matrix in the glomerulus of the rat kidney

The mesangial matrix of the rat glomerulus was studied by transmission electron microscopy in specimens preserved by a modified technique, which avoids osmium tetroxide but uses tannic acid as a contrasting agent. It can be demonstrated that microfibrils are a major component of the normal glomerular mesangial matrix. They are non-branching tubular structures with a hollow centre, an undefined length and a thickness of approximately 15 nm. Microfibrils make up a dense fibrillar network interconnecting mesangial cells and glomerular basement membrane.

The ultrastructural organization of the basement membrane of Bowman's capsule in the rat renal corpuscle

The basement membrane of Bowman's capsule (BCBM) of the rat was studied by means of a modified tissue-preservation technique for transmission electron microscopy, which avoids the usual thorough fixation in OsO4 and applies tannic acid and uranyl acetate for staining (Sakai et al. 1986). At most sites the BCBM is multilayered, consisting of one to seven dense layers separated by electron-lucent layers. The latter, which can be termed laminae rarae, contain fine filaments which connect the dense layers to each other and the innermost dense layer to the basal cell membrane of the parietal epithelium. The laminae densae are basically composed of fine filaments arranged in an anastomosing pattern. Individual filaments ranging from 5 to 15 nm in diameter, combine to form filament bundles up to 100 nm in thickness and 1 to 2 micron in length. Within a dense layer, filaments and filamentous bundles are oriented mainly in the same direction. Often the inner dense layers do not form a continuous sheet, and the filamentous bundles are arranged in anastomosing or spiral patterns to form a ribbon-like structure that we call a "microligament". These microligaments are often embedded in basal furrows of the parietal epithelium and are best developed around the vascular pole. Intracellular actin bundles of the parietal cells are regularly associated with these extracellular ribbon-like structures of the basement membrane. In conclusion, the BCBM has an unusual structure: the laminae densae are characterized by their filamentous nature and are arranged in different patterns, i.e. as a multilayered mat and as microligaments.

The structural relationship between mesangial cells and basement membrane of the renal glomerulus

It has been shown by many studies that mesangial cell contraction exerts considerable influences on glomerular filtration dynamics. However, experimental findings about the geometrical changes within the glomerular tuft going along with mesangial cell contractions are lacking. This study analyzes the geometry of mesangial cells and their relationship to glomerular capillaries, especially to the glomerular basement membrane (GBM). By applying a new staining technique of unosmicated specimens for TEM, the cellular outlines of glomerular cells (mesangial, endothelial and epithelial) and the distribution of extracellular matrices can be more easily studied than in conventionally osmicated specimens. It became obvious that mesangial cells and the GBM are extensively connected with each other, either by direct attachments or indirectly by microfibrils. These connections are especially prominent at mesangial angles, i.e. at sites where the GBM deviates from its pericapillary course and covers the mesangium. Thereby, the GBM is not only coupled to the mesangium but--via mesangial cell processes--also to the GBM at the opposing mesangial angle. It seems possible that contraction of mesangial cells can bring the GBM from opposing mesangial angles closer together. Therefore we conclude that the GBM and the contractile mesangial cells together establish a biomechanical unit capable of developing wall tension in glomerular capillaries and of changing the geometry of glomerular capillaries following mesangial contraction or relaxation.