Figure 2.

Schematic representation of the human tear film and its layers and sublayers.

The inner aqueous layer is shown in blue. The layer is formed of a complex mixture of proteins, salts, and other low-molecular weight compounds. The aqueous layer covers the epithelial cells of cornea and conjunctiva, many of which express soluble (secreted, gel-forming) and bound (surface) mucins (Argueso, 2013; Hodges and Dartt, 2013; Mantelli and Argueso, 2008). The opposite side of the aqueous layer is covered with the tear film lipid layer (shown in green and yellow). The inner part of the lipid layer is enriched with amphiphilic lipids (anionic OAHFA and free fatty acids, and, possibly, zwitter-ionic species such as SM, PC, and lyso-PC). This layer is called amphiphilic (or "polar") lipid sublayer and is shown in green. The outer part of the lipid layer is formed mostly of nonpolar lipids and is called nonpolar lipid sublayer (shown in yellow). The actual tear film and tear film lipid layer are dynamic structures whose thickness and geometrical features change with time, blinking, aqueous tears in- and outflow, secretion of meibum, and tear evaporation.