The Molecular Structure of the Endothelial Glycocalyx Layer (EGL) and Surface Layers (ESL) Modulation of Transvascular Exchange.

There has been rapid progress over the past decade to extend the concept that a quasiperiodic inner endothelial glycocalyx layer (EGL, <300 nm thick, with key components associated with the endothelial cell membrane) forms the primary molecular filter between circulating blood and the body tissues. The EGL is common to both continuous and fenestrated microvessels. The revised Starling Principle describing steady-state fluid exchange across the EGL describes new ways to understand transvascular exchange of water and plasma proteins in microvessels in both normal and disturbed states such as hemorrhage and fluid replacement during surgery. At the same time, direct optical observations describe endothelial surface layers (ESLs) with porous outer layers that extend 1-2 μm beyond the EGL. Preliminary analyses of water and plasma protein transport through barriers formed by a thick ESL in series with the EGL indicate that such two-layer structures can have permeability properties that are not consistent with measured water and plasma exchange in microvessels. Such multilayer models provide a basis for future detailed evaluations of both transports across endothelial surface layers and the methods to image components of both the EGL and the ESL. Furthermore changes in the thickness and distribution of thick ESLs in vessels with diameters larger than 50 μm may not reflect functional changes in the inner glycocalyx layer.