The lipid layer is the thin outermost layer that is exposed to the air. Its thickness (0.1 to 0.2 mm) varies during the day, attaining its maximum on awakening. The precise composition of the lipid layer may differ from one individual to the next, but is generally composed of triglycerides, free fatty acids, waxy and cholesterol esters.
| Main components of the lipid layer |
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These components are secreted mainly by the Meibomian glands, but also by the glands of Zeis and Moll. Secretion is influenced by several factors:
- mechanical (blinking reflex)
- nervous (as shown after trigeminal nerve sectioning)
- hormonal (stimulatory action of androgens)
- physical (feedback regulation according to surface tension)
Role of the lipid layer
The lipid layer is important as it reduces water evaporation from the aqueous layer, preventing cooling and drying of the eye. Without this layer, water would be lost from the tear film 20 times faster. The lipid layer also maintains surface tension and stabilises the tear film, ‘sealing’ it on the eye and preventing tears from overflowing onto the cheeks.
| The tear film is a complex, multilayered barrier - not just a watery solution. |
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