A slit diaphragm is a specialized junctional structure spanning the filtration slit between adjacent podocyte foot process structures. It forms one component of the glomerular filtration barrier, alongside the glomerular basement membrane and capillary endothelium. The slit diaphragm bridges a gap; it is distinct from the cell membrane of either neighboring podocyte.
Nephrin provides a molecular example. Antibodies against its extracellular region, followed by gold-coupled secondary antibodies, localized the recognized protein to podocyte slits in the 1999 study. A model of how neighboring nephrin molecules might assemble was then proposed. The observed label location and the proposed molecular arrangement are different levels of evidence.[ref]
Nephrin is specifically located at the slit diaphragm of glomerular podocytes by V. Ruotsalainen and others. Figure3: localization; Figure5: hypothetical assembly
For a useful comparison, imagine two barriers with the same total slit length but different junctional organization. Equal length does not establish equal passage of water or proteins: molecular organization and the other barrier layers also matter. A localization image therefore helps identify components but cannot alone supply the glomerular filtration rate.
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