The mucus layer
The mucin layer is the deepest layer of the tear film and adheres firmly to the underlying epithelial cells of the cornea and conjunctiva. Its thickness varies from 0.8 mm over the cornea to 1.4 mm over the conjunctiva.
|Main components of the mucin layer
The mucin layer is mainly composed of high molecular weight glycoproteins called mucins. However, it also contains mucopolysaccharides and sialic acid.
Under normal conditions, mucus is secreted by three different ocular cell types:
- Goblet cells which lie in the crypts of Henlé and the glands of Manz on the bulbar conjunctiva. These produce 2–3 ml of free mucin strands per eye, per day
- Conjunctival epithelial cells, which produce the membrane-associated mucins
- To a lesser degree, the main and accessory lacrimal glands
The mucus layer is partly mixed with the overlying aqueous layer, allowing the aqueous layer to adhere to the hydrophobic membrane of the conjunctival and corneal epithelial cells. When the lids close, the majority of the mucus adheres to the epithelial cells by their microvilli, so that they are not flushed out during tear drainage. However, free mucus strands within the aqueous layer are flushed out in the same process.
Role of the mucus layer
The mucus layer plays a vital role in tear film stability and allows the aqueous layer to adhere to the epithelial cells of the cornea. By lowering the surface tension between these two layers, it acts as a wetting agent as well as a stabilising agent for the thin
precorneal tear film between blinks.
The mucus strands trap desquamated epithelial cells, excessive lipid contamination and debris (especially microorganisms). These are then removed from the eye in the aqueous layer during the blinking process.
Mucus displays non-Newtonian and elastic behaviour (viscoelasticity). This means that it is relatively thick and clings to the surface of the eye when the eye is at rest; however, when the eye blinks the mucus layer becomes much thinner and spreads easily over the surface, providing lubrication. The elastic properties of the mucus help this layer to absorb energy during the blinking process.
|Rate of shear = the relative velocity of the lid and eyeball, divided by the thickness of the fluid layer.
During blinking, the rate of shear can be very high. If shearing forces are transmitted to the epithelial surfaces, cell damage and painful dragging sensations may occur. The non-Newtonian behaviour of tear film can help by reducing viscosity as the shear rate increases.
The elastic component of the tear film helps it to absorb energy during blinking.
In combination, the viscoelastic properties of the mucus strands (in the aqueous layer) and the mucus gels (close to the epithelial surface) cushion and lubricate the eye during all types of eye movement.
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