A podocyte is a specialized epithelial cell covering the outer surface of a glomerular capillary within the renal corpuscle. It belongs to the visceral portion of the glomerular capsule. Its branching extensions end in podocyte foot process structures, with a slit diaphragm spanning the gaps between neighboring processes. These structures cooperate with the basement membrane and capillary endothelium in the glomerular filtration barrier.
A 1999 experiment located nephrin more precisely than ordinary tissue staining could. Antibodies recognized the protein in glomerular extracts, and immunoelectron microscopy placed small gold labels in the slits between podocyte processes. The protein was therefore localized to a particular junctional region, rather than uniformly over the cell body.[ref]
Nephrin is specifically located at the slit diaphragm of glomerular podocytes by V. Ruotsalainen and others. Abstract;Figures1–3,antibody tests and immunogold localization
This provides an illuminating scale comparison: a light-microscope image can identify a glomerular region, while a molecular label in an electron-microscope section can assign a protein to one component of its filtration surface. Localization supports a structural role but does not, by itself, measure how rapidly water or each solute crosses the complete barrier.
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