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The tear film consists of three layers (although a contemporary model proposes a gradient model rather than distinctly layered model) (1). The outermost layer consists of lipids that are secreted by the meibomian glands. This layer is responsible for lubrication and ensuring uniform spread of the tear film, prevention of evaporation, and stabilization of a smooth ocular surface for refraction (2). The aqueous layer comprises the middle layer of the tear film and contains proteins, cytokines, growth factors, electrolytes, oxygen, and glucose and is supplied by the main and accessory lacrimal glands (2). The proteins in this layer include lysozyme, lactoferrin, transferrin, ceruloplasmin, metalloproteinases, defensins, and immunoglobulins (3). This layer has many functions including but not limited to corneal antimicrobial defense, lubrication, nutrition and supply of oxygen, mechanical clearance of debris, and regulation of cellular functions (3). Lastly, the mucinous layer, which is supplied by both the conjunctiva and the cornea (conjunctival goblet and epithelial cells and corneal epithelial cells) stabilizes the aqueous layer (by providing a viscous medium), maintains attachment of the tear film to the corneal epithelium through the use of glycoproteins, and helps uniformly distribute the tear film layer through the creation of a low surface tension environment (3).

1. McCulley, J. P. & Shine, W. A compositional based model for the tear film lipid layer. Trans Am Ophthalmol Soc 95, 79-88; discussion 88-93 (1997).

2. Holland, E. J., Mannis, M. J. & Lee, W. B. Ocular surface disease : cornea, conjunctiva and tear film. (Elsevier/Saunders, 2013).

3. Krachmer, J., Mannis, M. J. & Holland, E. J. Cornea. Third edn, Vol. 1 (Mosby, 2011).