A healthy glomerulus: the glomeruli are the filtration units of the kidney. This diagram shows a loop of tiny blood vessels called capillaries surrounded by a capsule called the Bowman’s capsule.

Two blood vessels are pass through the capsule taking blood into and out of the loop and as it passes through the loop the blood is filtered. The urine collects between the capillary loop and the Bowman’s capsule and is eventually taken away to the bladder.

Zooming in, we can see the capillaries are lined with porous endothelial cells surround by a membrane (the glomerular basement membrane or GBM) made of collagen four alpha 3,4 and 5. On top of this are the foot processes of specialised cells called podocytes. These podocytes contact each other via their foot processes and the gap between them is filled by the slit diaphragm, a very thin membrane.

The endothelial cells with their pores, the GBM and the podocytes with their slit diaphragms make up the filtration barrier. In healthy tissue the GBM looks like a smooth thin flat line with the endothelial cells underneath and the discrete foot processes sat on top, like fingers around a tube.

This is an image of a diseased glomerulus. The Bowman’s capsule around the capillaries and the blood vessels coming in and going out of the capsule look much like the healthy glomerulus. The significant changes are in the basement membrane, the GBM, surrounding the capillaries and the specialised podocytes that sit on top of them.

We see here that the porous endothelial cells look the same, but the GBM that sits on top becomes thickened, split and irregular and has protrusions extending from the podocyte foot processes pushing into it.

The foot processes become enlarged and flattened and lose the slit diaphragms they normally have between them. The thickened and irregular GBM, flattened foot processes and loss of the slit diaphragm stop the filtration of blood occurring as normal.