Continuity of the mesangial channel network with the glomerular basement membrane and intraglomerular subendothelial space

The composition and exact structure of the non-cellular mesangial matrix in the glomerulus of the human kidney are a matter of debate. It may appear like a structure similar to the glomerular basement membrane (GBM), it has been described to contain microfilaments. The exact transport route of fluids, solvents and immunocomplexes in the mesangium is not well-known either. We know that in some glomerular diseases immunocomplexes can be found in the GBM and the mesangium at the same time in the same patient. A possible explanation of the above findings could be provided by our hypothesis, i.e. the existence of a well-defined mesangial channel network (MChN). This MChN would consist of intercommunicating channels, which were embedded into the spongy cytoplasm of the mesangial cells (MCs) and surrounded by the plasma membrane of the mesangial cells. The MChN would lead from the subendothelial space through deep mesangium to the vascular pole or the juxtaglomerular apparatus and may transport fluid and other materials such as immunocomplexes into the mesangium. It would be continuous with the GBM. Microfilaments of the MC would be anchored to the walls of the MChN regulating its diameter, thus mesangial fluid transport and pressure. The dilatation of these channels by mechanical obstruction could contribute to glomerular sclerosis. The hypothesis can be challenged by methods like electronmicroscopy, immunoelectronmicroscopy, confocal laser-scanning microscopy, and vital stain studies. We provide some images suggesting the existence of the channel and its connection with the GBM. If the hypothesis was true, it could contribute to understanding of mesangial transport processes, pressure regulation and pathogenesis of glomerular mesangial diseases.