Structure of the interendothelial cell cleft

Since the work of Bundgaard it is known that the interendothelial cell cleft is a narrow, but open, sometimes highly convoluted gap linking the blood vessel lumen with the subendothelial space. A remarkable feature of this cleft is its constant width. It is here suggested that electrostatic repulsions, and the presence of a system of posts between the walls act to maintain cleft width. Contrary to intuition the presence of posts would not constitute an intolerable increase in hydrodynamic resistance, since no massive posts are necessary. Links between the walls involving only a single macromolecular chain each, which would be very difficult to detect ultrastructurally, would, nevertheless, particularly when charged, act as extremely stiff springs of essentially fixed length. A detailed calculation is presented to show how this idea could work and it is proposed that similar such posts support the subendothelial layers. Recent ultrastructural evidence in support of single chain links is quoted.