The ultrastructure of the capillary fenestrae in the adrenal medulla of the rat

A single-layered membrane that appears to be structurally highly organized has been observed to bridge capillary fenestrae in the rat adrenal medulla. The membrane measures about 20–30 Å in thickness and is suspended from the peripheral opaque component of the triple-layered plasma membrane of the endothelial cell. In the central area of the membrane is a dense ring-like structure, the periphery of which contains some highly stained particles and which has a center similar in density to the rest of the membrane. In cross sections the innermost region can be seen to contain some stained material extending out perpendicularly to the plane of the membrane on both sides of the fenestrae. The diameter of the fenestra is about 500 Å, the overall width of the central region being 150–200 Å, and the light center being 50–100 Å in diameter. The membrane structure is observed in the fenestrae, where it separates the capillary lumen from the pericapillary space. It is also observed in the walls of endothelial vesicles. The single-layered membrane is interpreted to be chemically different from the plasma membrane of the endothelial cell and to be nonlipid.