On a simple basis, we can think of the tear film as a three-layer structure (Figure 1).
The top layer of the three, which meets the air, is a thin oily (lipid) layer, where the lipids have a hydrophilic head orientated towards the aqueous below and a hydrophobic tail pointing towards the air. It is this vital layer, secreted primarily by the meibomian glands within the lids, that inhibits evaporation of the main body of the tears – the aqueous layer.
The hydrophilic heads of the lipid layer, pointing down into the aqueous layer, are the interface between that vital evaporative retarding layer and the middle layer of the tear film ‘sandwich’.
This middle aqueous layer, as well as being the thickest layer, contains antimicrobial proteins; lysozyme and lactoferrin etc; growth factors and inflammatory modulators (cytokines) which carry inflammatory signals; soluble mucins secreted by goblet cells and electrolytes; and much more.
The aqueous layer originates from the lacrimal gland and the accessory lacrimal glands within the conjunctiva.