 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.^[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.^[ref]

For an original geometric illustration, a vessel of radius $3\mu \mathrm{m}$ with an inaccessible layer $0.5\mu \mathrm{m}$ 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.