The epithelial cell surface has numerous fine extensions (micro-projections) that form a carpet-like structure
The surface of the conjunctiva and of the cornea is not completely smooth but it shows many undulations, termed micro-projections. These are membrane bound cytoplasmic protrusions that extend for a small distance of about 0,5 to 1 micrometer outwards.
Some of the micro-projections resemble finger-like structures and they are called microvilli whereas others have more the shape of folds and are termed microplicae. The multitude of micro-projections results in a vast enlargement of the surface area and thus of the water-binding capacity that the cell membrane exerts by their attached mucins – see below.
Cell-bound mucins of the glycocalyx make the ocular surface wettable
The micro-projections are covered by a fuzzy coat that is termed Gycocalyx. It consists of membrane-bound glycoproteins – polysaccharide sugars linked to a central protein backbone. They project from the cell membrane into the surface compartment, which is typically aqueous and forms the bottom of the Tear Film.
The potential functions of a glycocalyx can be diverse:
Such a membrane-bound Glycocalyx can perform different functions either for specific binding, as receptors for cell recognition. One well known example are membane-bound polysaccharides on blood cells that determine the blood groups. Due to its high hydrophilic water binding capacity, the ocular glycocylyx is very suitable for the binding of water.
Another function is the formation of a mucus barrier that is intended to block binding of other structures, particularly of microbes. The mucin layer is a chemical barrrier and one of the main protective barriers of the ocular surface epithelium and it belongs to the non-specific, innate, defense systems.
On the ocular surface epithelium the glycocalyx consists of particularly large molecules with certain structural features that are termed as mucins. They are so large that they can be observed in electron microscopy as a fuzzy cell coat of about 100 nm thickness in the human (see images at top of this page).
The mucins radiate into the overlying tear fluid and can thus bind large amounts of water to the cell surface of the ocular surface epithelium. In early research on the ocular surface it was reported by Frank Holly and Mike Lemp that the ocular surface appeared to be naturally ´un-wettable´ by aqueous solutions – but could be made ´wettable´ by (re-) addition of mucins. `Wettablity and Wetting´ was thus a big issue in the early days of systematic ocular surface and tear film research in the 1970s.
By the ´trick´of nature, to use water-binding surface molecules, the ocular surface can bind and hold the aqueous tear solution, that is needed here for functional reasons, on the surface. Question that will concern us later are ... how the the aqueous tear fluid gets to the front of the cornea before it can be bound there... and how long it stays there. These questions are answered in the section for ´Tears and Tear Film´ – for details please see there.
Micro-projections with mucin coating reduce frictional forces
It may be surprising that the surfaces of the cornea and the conjunctiva, that glide over each other during every eye blink, are not completely flat and smooth. Conventional wisdom would probably suggest that a smooth surface produces the least amount of friction during eye movements during reading and particularly during the frequent blink movements when the upper eyelid wipes over the bulbar surface.
Other means of reducing frictional forces at the ocular surface are lubricative protein within the tear film such as Lucricin that is explained in the ´conjunctiva´ section.
Apparently, such a slightly ´rough´ surface results in a lowering of friction as explained elsewhere (Heiko Pult).