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.