Glomerular Basement Membrane

The glomerular basement membrane (GBM) is the basal laminal portion of the glomerulus which performs the actual filtration through the filtration slits between the podocytes, separating the blood on the inside from the filtrate on the outside. It is a fusion of the endothelial cell and podocyte basal laminas. The glomerular basement membrane is composed of three layers: 1) lamina rara externa, which consists of heparan sulfate and lies adjacent to podocyte processes; 2) lamina densa, which is situated in dark central zone and is composed of type 4 collagen and laminin; 3) lamina rara interna, which lies adjacent to endothelial cells and also consists of heparan sulfate.

The glomerular basement membrane forms the boundary between blood and urine. Across it, water and other small molecules from the blood are filtered. The GBM is composed of a meshwork of proteins and other constituents. Type IV collagen and laminin are present in the largest quantities. Some specialized subtypes of these molecules are only found in specialised basement membranes such as the GBM.

Thin GBM disease

Thin glomerular basement membrane disease is the thinning of the basement membrane of the glomerulus. The GBM is thin, and must sometimes break, as it causes blood to appear in the urine. However it seems to repair without any ill effect, as thin GBM disease almost never causes serious trouble. The condition often runs in families, and can be a cause of benign familial haematuria.