Monday, January 02, 2006

Structure and Compositon of the Tear Film

Detailed Information About Structure and Composition of the Tear Film.
The human tear film serves to protect and lubricate the cornea and conjunctiva (10). Although several models of the tear film have been proposed recent observations suggest that the mucins exist as a network distributed in the aqueous body of the tear film and covered by 2 layers of lipid (Chen 1997, Gipson 1998).


Model of the tear film. Lipid is shown in yellow and contains both polar and non-polar lipids. The aqueous layer is shown in blue and contain soluble mucins and proteins in addtions to water and ions. The glyocalyx is a speckled darker blue is shown emanating as transmembrane molecules into the aqueous but anchored at the cell membrane. Numerous microvilli from the grey undulating surface of the cornea.


Nichols in 1985 described a mucin layer that measures 2-7 microns above the corneal surface and intimately associated with corneal microvilli and presumably anchored to the glycocalyx of the conjunctiva. Three membrane associated mucins, MUC 1, 4, and 16, as well as two secretory mucins, MUC5AC and MUC 7, have been identified at the ocular surface. Prydal in 1992 showed by laser confocal microscopy that the human aqueous-mucin layer may be much thicker than first estimated, 41-46 microns. In the current model of the tear film, the aqueous-mucin layer is covered by 2 thin layers of lipid. A series of studies showed that polar lipids such as phospholipids and fatty acids lie adjacent to the aqueous-mucin layer and non-polar lipids such as cholesterol esters, and triglycerides are present at the tear-air interface (Greiner 1996, Schindler 1989, Somerharju 1999, Shine and McCulley 1997,1998,2004). The thickness of the lipid layer, estimated by observation of interference patterns, measures between 0.06-0.18 microns in the open human eye (McDonald 1969, Korb 1998). The aqueous-mucin layer of tears contains many proteins but there are 3 major protein components: lysozyme (24-47%), lactoferrin (23-29%) and tear lipocalin (15-33%) (Fullard 1988, Gachon 1982). IgA becomes the predominant protein when the lids are closed for prolonged periods (Sack 1992, 2000). Other components play a larger role in the exposed open eye (Sitaramma 1998). Any concept of tear film structure requires an understanding of the individual components and their specific interactions with other components. Tear lipocalin interacts with the lipid components of the tear film (Glasgow 1995,1999, Gasymov 1999). The detailed mechanisms of this interaction are not known.

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