Meibomian gland and tear film lipids: structure, function and control.

Current understanding of the interactions between composition and function in the tear film is inadequate or misleading, especially as it pertains to the tear film lipid layer. This inadequacy begins as the primary source of the lipids, the meibomian gland orifice, releases its aliquot of meibum onto the aqueous-mucin layer. The polar and nonpolar lipid composition of meibum and the ionic composition of the aqueous-mucin layer result in formation of a functional tear film lipid monolayer. Many external parameters constrain lipid composition, including initial formation of a composite monolayer with polar and nonpolar phases, adequate fluidity near body temperature, ability to undergo compression and expansion without extensive hysteresis (as during a blink) and resistance to enzymatic degradation. Lipid type, as well as fatty acid and alcohol composition, affects each process.

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