The endothelial glycocalyx: composition, functions, and visualization

Abstract

This review aims at presenting state-of-the-art knowledge on the composition and functions of the endothelial glycocalyx. The endothelial glycocalyx is a network of membrane-bound proteoglycans and glycoproteins, covering the endothelium luminally. Both endothelium- and plasma-derived soluble molecules integrate into this mesh. Over the past decade, insight has been gained into the role of the glycocalyx in vascular physiology and pathology, including mechanotransduction, hemostasis, signaling, and blood cell-vessel wall interactions. The contribution of the glycocalyx to diabetes, ischemia/reperfusion, and atherosclerosis is also reviewed. Experimental data from the micro- and macrocirculation alludes at a vasculoprotective role for the glycocalyx. Assessing this possible role of the endothelial glycocalyx requires reliable visualization of this delicate layer, which is a great challenge. An overview is given of the various ways in which the endothelial glycocalyx has been visualized up to now, including first data from two-photon microscopic imaging.

Fig. 1

Schematic representation of the endothelial glycocalyx, showing its main components. Left: The endothelial glycocalyx can be observed in vivo as a red blood cell exclusion zone, located on the luminal side of the vascular endothelium. It consists of membrane-bound and soluble molecules. Right: Components of the endothelial glycocalyx. Bound to the endothelial membrane are proteoglycans, with long unbranched glycosaminoglycan side-chains (GAG-chain) and glycoproteins, with short branched carbohydrate side-chains. Incorporated in and on top of this grid are plasma and endothelium-derived soluble components, including hyaluronic acid and other soluble proteoglycans (e.g., thrombomodulin) and various proteins, such as extracellular superoxide dismutase (ec-SOD) and antithrombin III (AT III). Together, these components form the endothelial glycocalyx that functions as a barrier between blood plasma and the endothelium and exerts various roles in plasma and vessel wall homeostasis. Note that this figure is not drawn to scale; its purpose is to illustrate glycocalyx composition

Fig. 2

Visualization of the endothelial glycocalyx with different microscopic techniques. a Endothelial glycocalyx of a rat left ventricular myocardial capillary stained with Alcian blue 8GX and visualized using electron microscopy. Bar represents 1 μm. Reproduced with permission from reference number [124]. b Intravital microscopic recording of the endothelial glycocalyx of a hamster cremaster muscle capillary. The anatomical diameter of 5.4 μm is larger than the red blood cell column width (left pane) or the plasma column width (right pane) labeled with fluorescent dextran (70 kD). This difference is caused by the presence of the endothelial glycocalyx. The bar in the left pane represents 5 μm. Reproduced with permission from reference number [129]. c Endothelial glycocalyx of a mouse common carotid artery. 3D-reconstruction of a series of optical slices obtained with two-photon laser scanning microscopy showing part of the vessel wall. The intact vessel was perfused with FITC-labeled lectin (WGA) to stain the endothelial glycocalyx (green) and SYTO 41 to label cell nuclei (blue). The arrows indicate the direction of the X, Y, and Z axis. The scanned volume approximates 200 × 200 × 60 μm³. For details on methodology see also reference number [67]