Ultrastructure of Glomerulus and Juxtaglomerular Apparatus

Three-Dimensional Reconstruction of Glomeruli by Electron Microscopy Reveals a Distinct Restrictive Urinary Subpodocyte Space

For more than 150 years, the only urinary space that has been recognized in the glomerulus conducting primary filtrate to the proximal convoluted tubule has been Bowman's space (BS) (1). Here it is shown that ultrastructural reconstructions of the podocyte and the glomerulus reveal BS to be formed from three distinct urinary spaces through which filtrate must pass before reaching the proximal convoluted tubule. The most restricted region, the subpodocyte space (SPS; first described by Gautier in 1950), was found to cover 58 to 65% of the glomerular filtration barrier. It is morphologically distinct from the rest of BS and has a highly significant restriction to flow based on morphometric measurements. This SPS was altered during increased renal perfusion pressure, consistent with the podocyte dynamically reacting to the increase in filtration. A second anastomosing branching region draining the glomerular center, which has been termed the interpodocyte space, has fewer restrictions to flow into the final region--the shell-like peripheral urinary space. The physiologic role of the restrictive SPS is yet to be determined but likely possibilities include regulation of glomerular filtration and cleaning of the glomerular filtration barrier.

Hindered transport of macromolecules through a single row of cylinders: application to glomerular filtration

The slit diaphragms of renal glomerular capillaries form an ultrafiltration barrier which may be approximated as a row of cylindrical fibers of macromolecular dimensions. To describe the hindered transport of plasma proteins and other macromolecules through this barrier, we developed an approximate hydrodynamic model for spherical, Brownian particles passing through a row of infinitely long cylinders. The selectivity of the slit diaphragm was assessed by computing concentration profiles for a wide range of molecular sizes for Pe < or = 1, where Pe is a Peclet number based on the cylinder radius. The sieving coefficient for the slit diaphragm was computed as theta SD = CB/CO, where CO was the average concentration at a specified distance upstream from the cylinders (corresponding to the location of the basement membrane), and CB was the concentration far downstream (corresponding to Bowman's space). The results of previous experimental sieving studies using rats could be accounted for approximately by postulating a wide distribution of spacings between the fibers of the slit diaphragm. Comparing the results for theta SD with calculations for a model of the glomerular basement membrane suggests that the slit diaphragm is by far the more size-restrictive part of the overall barrier.

Maturation of the developing renal glomerulus with respect to basement membrane proteoglycans

Maturation of extracellular matrices in relation to heparan sulfate proteoglycans was investigated during glomerular development. Antibodies directed against the core-protein (Mr = 18,000) of the basement membrane heparan sulfate-proteoglycan were utilized. The IgG fraction of the antibodies was conjugated with either fluorescein or rhodamine or 125I-iodine. The fluoresceinated and radioiodinated antibodies were given intravenously to three-day-old rats; their kidneys were obtained 6, 48, and 120 hours later and processed for immunohistochemical and tissue autoradiographic studies. At six hours (day 0), matrices of all the stages were labeled. The labeling was lowest in the vesicle stage and highest in the capillary stage. The autoradiographic grain-densities of vesicle, S-shaped, precapillary, and capillary stages were 10.62 +/- 0.54, 7.44 +/- 0.65, 11.37 +/- 1.03, and 18.35 +/- 1.97, respectively. After 48 hours (day 5), the grain-density for S-shaped body decreased from 7.44 +/- 0.65 to 4.38 +/- 0.61, while no appreciable change in the precapillary and capillary stages was observed. At this time interval, the fluorescein-labeled glomeruli were seen in the deeper cortex only. After 120 hours (day 8), the grain-density for the capillary stage decreased from 18.35 +/- 1.97 to 2.82 +/- 0.92. The double-labeling experiments included a second injection of rhodaminated antibody at 48 hours (day 5). In these experiments, newly developed glomeruli of varying stages were seen in the superficial cortex. With the use of double filters, the immature glomeruli in the superficial cortex and mature ones in the deeper cortex were visualized.(ABSTRACT TRUNCATED AT 250 WORDS)