The GBM lines the outer surface of the endothelial cells and is continuous with the basement membrane of Bowman’s capsule. It is synthesized by both endothelial cells and podocytes, and it consists of three layers: a thin lamina rara interna, a thick central lamina densa, and a thin lamina rara externa. Together, these layers measure approximately 300 to 350 nm across, being somewhat thicker in males than in females. The GBM consists primarily of type IV collagen and other proteins, such as laminin and nidogen (also known as entactin). The tight arrangement of these proteins contributes to the size-based filtration barrier. In addition, the GBM contains negatively charged proteoglycans that contribute to the charge-based filtration barrier. The potential space between the endothelial cells and GBM is known as the subendothelial space, while the potential space between the GBM and the podocytes is known as the subepithelial space.