Glycocalyx layer: Function

The glycocalyx is a complex layer between the endothelium and flowing blood. It is composed mostly of glycoproteins and proteoglycans. Proteoglycans have a protein core which is attached to negatively charged GAG (glycosaminoglycan) side chains. There are 5 types of GAG side chains, of which heparin sulphate makes up 50-90% of the chains. The glycocalyx layer regulates interactions between endothelial cells and blood cells, vascular permeability, coagulation and thrombosis, and vascular tone.

• The glycocalyx modulates blood viscosity and hematocrit in the microcirculation. It repels red blood cells from the endothelium and shields it from leukocyte and platelet interactions.
• The glycocalyx also plays a large role in hydrostatic and oncotic pressure gradients between the blood vessel lumen and interstitial space. Disrupting the glycocalyx layer results in increased hydraulic conductivity, anionic protein flux, and fractional filtration of albumin.
• The glycocalyx facilitates an antithrombotic surface through the binding of AT III. The anticoagulant activity of ATIII is also enhanced by the GAG side chain heparan sulphate.
• The glycocalyx also regulates vascular tone in response to shear stress. Movement of the glycocalyx cytoskeleton transduces mechanical stimuli into intracellular signals that results in vasodilation mediated by endothelial nitric oxide release.
• The glycocalyx layer can be damaged by hypervolemia, hyperglycemia, ischemia-reperfusion, inflammation, sepsis, or bypass surgery. Damage to this layer can result in platelets aggregation, leukocytes adhesion, and a prothrombotic state. Damage will also cause increased vascular permeability leading to interstitial edema and diminished vascular responses to shear stresses.