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 plasmaderived 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 cellvessel 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.

A thick endothelial surface coat consisting of the glycocalyx and associated plasma proteins has been hypothesized to reduce functional capillary volume available for flowing plasma macromolecules and blood cells. The purpose of this study was to compare anatomic and functional capillary diameters available for macromolecules, RBCs, and WBCs in hamster cremaster muscle capillaries. Bright-field and fluorescence microscopy provided similar estimates (mean±SE) of the anatomic capillary diameter: 5.1±0.1 μm (bright field, 39 capillaries in 10 animals) and 5.1±0.2 μm (membrane dye PKH26, 18 capillaries in 2 animals). Estimates of functional diameters were obtained by measuring the width of RBCs and WBCs and the intracapillary distribution of systemically injected fluorescein isothiocyanate (FITC)–dextran 70. WBCs (5.1±0.2 μm) fully occupied the anatomic capillary cross section. In contrast, the widths of RBCs (3.9±0.2 μm, 21 capillaries in 8 animals) and FITC-dextran (4.3±0.2 μm, 21 capillaries in 8 animals) were significantly smaller than the anatomic capillary diameter. Continuous (1- to 5-minute) excitation of fluorochromes in the capillary lumen (light-dye treatment) increased the width of RBCs passing the treated site from 3.6±0.3 to 4.4±0.3 μm (6 capillaries in 4 animals) and the width of the FITC-dextran column from 4.1±0.2 to 4.6±0.3 μm (10 capillaries in 7 animals). Furthermore, light-dye treatment increased capillary tube hematocrit by 60% in 40-μm-long capillary segments compared with untreated sites in the same capillaries. It is concluded that the wall of skeletal muscle capillaries is decorated with a 0.4- to 0.5-μm-thick endothelial surface coat, which may represent the true active interface between blood and the capillary wall.

In this inaugural paper, we shall provide an overview of the endothelial surface layer or glycocalyx in several roles: as a transport barrier, as a porous hydrodynamic interface in the motion of red and white cells in microvessels, and as a mechanotransducer of fluid shearing stresses to the actin cortical cytoskeleton of the endothelial cell. These functions will be examined from a new perspective, the quasiperiodic ultrastructural model proposed in Squire et al. Biol. 136, 239 -255] for the 3D organization of the endothelial surface layer and its linkage to the submembranous scaffold. We shall show that the core proteins in the bush-like structures comprising the matrix have a flexural rigidity, EI, that is sufficiently stiff to serve as a molecular filter for plasma proteins and as an exquisitely designed transducer of fluid shearing stresses. However, EI is inadequate to prevent the buckling of these protein structures during the intermittent motion of red cells or the penetration of white cell microvilli. In these cellular interactions, the viscous draining resistance of the matrix is essential for preventing adhesive molecular interactions between proteins in the endothelial membrane and circulating cellular components.A lthough the endothelial surface glycocalyx was first identified by special electron microscopic staining techniques nearly 40 years ago (1), it is only relatively recently that this surface layer has been observed in vivo (2) and the importance of its multifaceted physiological functions recognized. Key among these functions are its role as a molecular sieve in determining the oncotic forces that are established across microvessel endothelium (3-6), its role as a hydrodynamic exclusion layer preventing the interaction of proteins in the red cell and endothelial cell membranes (7-9), its function in modulating leukocyte attachment and rolling (10) and as a transducer of mechanical forces to the intracellular cytoskeleton in the initiation of intracellular signaling, as proposed herein.It is widely recognized that fluid shearing forces acting on endothelial cells (ECs) have a profound effect on EC morphology, structure, and function (11,12). It is now also clear from theoretical considerations (7,9,13,14) that the shear stress at the edge of the endothelial surface layer is greatly attenuated by the extracellular matrix of proteoglycans and glycoproteins in the glycocalyx, with the result that fluid velocities, except near the edge of the layer, are vanishingly small. Thus, the shear stress due to the fluid flow acting on the apical membrane of the EC itself is negligible. This paradoxical prediction has raised a fundamental question as to how hydrodynamic and mechanical forces, more generally, are transmitted across the structural components of the glycocalyx. How do these components deform under the action of these forces, and how are these forces and deformations communicated to the underlying cortical cytoskeleton (CC)?Little was known about the specific proteins or generalized structure...