Role of the precorneal tear film

The precorneal tear film has several important functions that are essential to the health of the eye. It:

  • maintains a moist environment on the surface of the eye, preventing the epithelial cells from becoming dry and damaged
  • lubricates the surface of the eye, facilitating the movement of the eyelids
  • maintains a smooth refracting surface on the cornea, to ensure that the eye can see correctly
  • provides the cornea, which has no blood vessels, with the nutrients and oxygen necessary for its metabolism
  • contains enzymes (such as lysozyme), antibodies and phagocytic cells that destroy bacteria and prevent the growth of microcysts of the cornea
  • removes waste products from the cornea
  • eliminates foreign bodies from the surface of the eye by blinking
  • protects the cornea and conjunctiva against atmospheric and chemical irritants, as well as temperature changes.
Any change in the equilibrium of the three-layered tear film can cause damage that may lead to disorders such as dry eye. 

 

SUMMARY

  • The lacrimal apparatus produces and drains tears (‘reflex’ and ‘basal’)
  • Tears clean, lubricate and keep the eye moist
  • The lacrimal apparatus may be divided into three different systems according to their function:
    – The secretory system produces the components of the tear fluid
    – The distribution system spreads the tear fluid over the surface of the eye during blinking
    – The excretory system drains the tear fluid out of the eye
  • The tears produced by the glands in the secretory system combine in the eye to form a thin tear film that covers the corneal epithelium
  • A stable precorneal tear film is vital for the health of the cornea as well as the optical quality of the cornea
  • The precorneal tear film consists of:
    – an underlying layer of mucus, which plays a vital role in tear film stability, acts as a wetting agent to keep the cornea moist, lubricates the eye and removes any lipid contaminants
    – a middle aqueous layer, which forms the bulk of the tear film, carries nutrients to the cornea and waste/foreign bodies away, fights infection and helps to keep the eye moist
    – an outer lipid layer, which reduces water evaporation (to prevent the eye from drying out) and ‘seals’ the tear film on the eye (to stop tears from flowing over onto the cheek)
  • During blinking, the rate of shear can be very high which, if transmitted to the epithelial surfaces, can cause cell damage
  • The viscoelastic properties of the tear film lubricate and cushion the eye during all types of eye movement
  • The non-Newtonian behaviour of tear film reduces viscosity as shear rate increases
  • Its elastic component can help absorb energy during rapid eye movements
  • In a healthy eye, the precorneal tear film begins to evaporate after 15 to 30 seconds between blinks. If the tear film isn’t regenerated, dry spots can form on the cornea, leading to problems such as dry eye

 

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