= Glycocalyx
The <glycocalyx> is the carbohydrate-rich coat at a cell's external surface. On animal cells it includes sugar chains attached to <glycoproteins>, <glycolipids> and <proteoglycans>, together with associated extracellular material. It is outside the <lipid bilayer>, rather than an extra membrane leaflet. Its exposed molecular structures contribute to protection and interactions between cells.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter10, Membrane Proteins, The Cell Surface Is Coated with Sugar Residues; Figures10-44 and10-45]{ref}
A useful distinction is between the anatomical boundary and the region accessible to a tracer. A surface coat may contain water yet restrict a large <molecule>, so a dark region in a tracer image need not be an empty gap or part of the cell's interior. A carbohydrate-binding probe instead reports binding sites; these measurements answer different questions about the same coat.
An experiment on hamster muscle <capillaries> compared vessel boundaries with the regions occupied by blood cells and fluorescent dextran. Bright-field and membrane-label observations located the wall, while the dextran column occupied a narrower region. Sustained light-dye treatment widened the accessible tracer column. The observations supported a surface layer that excluded some circulating material, rather than a simple identity between anatomical lumen and accessible lumen.<Identification of distinct luminal domains for macromolecules, erythrocytes, and leukocytes within mammalian capillaries by H. Vink and B. R. Duling>[Abstract: anatomical/functional diameters and light-dye perturbation]{ref}
For an original geometric illustration, a vessel of radius $3\,\mathrm{\mu m}$ with an inaccessible layer $0.5\,\mathrm{\mu m}$ thick leaves the tracer an accessible area fraction $(2.5/3)^2=25/36$, about $69\%$. This is an illustrative cylindrical model, not a universal measured <glycocalyx> thickness. Accessibility also depends on the tracer and the state of the surface layer.
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