Knowledge of the architecture of the renal glomerulus is a prerequisite for understanding both glomerular function and pathology. Glomerular filtration depends on comparably high intracapillary hydrostatic pressures. These high pressures must be balanced by commensurate counterforces to maintain structural integrity. Elevated pressures can damage the glomerulus. The "window" for pressures high enough for effective filtration and yet tolerable to the glomerulus is narrow. Precise regulation is therefore necessary to keep intraglomerular pressure within an appropriate range. This study provides a comprehensive description of the glomerular arterioles, which are the vessels primarily responsible for the regulation of intraglomerular pressures. It thereby describes the structures which may be presumed to generate counterforces needed to assure structural stability. The arterioles are closely associated with the supporting system comprised of the intra- and extraglomerular mesangium. The extraglomerular mesangium forms a spider-like clamp providing a mechanical interconnection of all components of the vascular pole. At the same time, it may act as a tension receptor, i.e. as a sensor of intraglomerular pressures. Due to its intimate relationship to the macula densa, the extraglomerular mesangium receives information on the distal solute delivery and therefore may act as the integrative center within the juxtaglomerular apparatus. The close spatial association between the afferent and efferent arterioles at the vascular pole suggests a direct regulatory interaction between both arterioles. In addition, the specific wall structure of the efferent arteriole exhibiting the features of a shear stress receptor suggests new pathways for feedback regulation of glomerular hemodynamics.

Klappentext

Glomerular flltration represents one of the basic mechanisms in the function of an organism. Our understanding of this process is still quite fragmentary. Regulation of blood flow and pressure, together with regulation of the ultraflltration coefficient (which is an attribute of the flltration barrier), are the two fundamental mechanisms accounting for maintenance and adaptability of glomerular flltration. Regulation of glomerular blood flow is generally considered to result from an interplay between afferent and efferent glomerular arterioles, and much progress has been made recently in understanding this interplay (Navar et al. 1996). The present study provides a detailed structural description of the glomerular vascular pole of rat. The results of this study appear to be relevant for several open questions of glomerular function. First, the interaction between afferent and efferent arterioles in regulating glomerular blood is generally understood to occur between the preglomerular and the postglomerular portions of these vessels. As shown in the present study, the structural elaborations of these arterioles and the spatial relationships between them within the glomerular hilum strongly suggest an interplay also at this site. Moreover, the current understanding of glomerular blood flow regulation by tuning the interplay between afferent and efferent arterioles is exclusively based on signals whose regulatory loops are established in follow-up events outside the glomerulus (tubuloglomerular balance, tubuloglomerular feedback).

Inhalt

1 Introduction.- 2 Material and Methods.- 3 Results.- 3.1 The Opening in Bowman's Capsulex.- 3.1.1 Transition of the GMB into the PBM.- 3.1.2 Transition from Podocytes to Parietal Cells.- 3.2 Glomerular Arterioles.- 3.2.1 Afferent Arteriole.- 3.2.2 Efferent Arteriole.- 3.3 Extraglomerular Mesangium.- 3.3.1 EGM Cells.- 3.3.2 EGM Matrix.- 3.3.3 EGM Relationships to Neighboring Structures.- 3.3.4 Glomerular Stalk.- 4 Discussion.- 4.1 Stabilization of the Vascular Pole.- 4.2 Regulation of Glomerular Blood Flow and Filtration.- 4.3 Integration of Vascular Pole Structures into the Juxtaglomerular Apparatus.- 4.4 Fluid Leakage Through the Glomerular Stalk.- 5 Summary.- References.