Dry Eye Information

The Tear Film Structure

Tear Film

Traditionally, the tear film has been viewed as having three distinct layers:

  • an outermost lipid (oily) layer
  • an aqueous layer that makes up 90% of the tear film volume; and
  • a mucin layer that coats the corneal surface.

However, it is now recognized that rather than being three distinct layers, the tear film is actually a lipid boundary layer with aqueous phases incorporating differing concentrations of lipid and mucins throughout.

In addition, mucins in the tear film play a much more active role in maintaining tear film stability than was once thought.

The Role of the Epithelial Glycocalyx

Glycocalyx are long chain molecules formed by corneal cells that help anchor mucin to the epithelial surface. Mucin creates a hydrophilic ocular surface, as well as a barrier to bacterial pathogens.1

Glycocalyx

Since the corneal surface is naturally water repellent, damage to the glycocalyx and corneal surface cells may be caused by insufficient mucin. This can cause the tear film to destabilize and break up before resurfacing from the blink can occur, exposing the injured cornea to the air and bacterial pathogens.

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  1. Korb DR, Craig J, Doughty M, Guillon J, Smith G, Tomlinson A. The Tear Film: Structure, Function and Clinical Examination. BCLA, 2002.

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† Survey of Ophthalmologists and Optometrists, Harris Interactive®, December 2008.