Michael A Lemp

Disclosures

Curr Opin Allergy Clin Immunol. 2008;8(5):457-460. 

In This Article

Structure and Function of the Tear Film

Our understanding of the structure and function of the tear film and its breakdown in disease has undergone extensive revision over the last decade. This understanding is being further refined and is essential to clinical decision-making in the area under consideration. Let us briefly review this.

The tear film is a dynamic structure that serves multiple functions to maintain the health of the ocular surface, protect it against noxious influences, repair damage and create an optically clear and stable anterior refracting surface of the eye to subserve clear vision. These concepts and the following aspects of tear physiology and the effects of contact lenses on the tear film are discussed in a comprehensive yet succinct manner in recent reviews.[1••,2••]

The structure of the tear film is thought to be that of a meta-stable film between blinks consisting of three major components, an innermost membrane-spanning mucin layer contributing to the structure of the epithelial cell surface and anchoring the overlying aqueous gel. The membrane-spanning mucins are the product of the corneal and conjunctival epithelial cells; the gel-forming mucins of the overlying aqueous gel are produced by the goblet cells of the conjunctiva and stabilize the tear-film, interact with the outermost lipid layer and serve to cleanse the surface of the wiping away debris, exfoliating cells and microbial contaminants. The lipid layer produced by the meibomian glands of the eyelids retards evaporative tear loss and with the gel-forming mucins provides lubrication between the lids and the ocular surface.

The aqueous component of the tears produced by the main and accessory lacrimal glands contains all the water-soluble elements of the tears including electrolytes and hundreds of proteins and peptides. Although there is a resident population of a few relatively stable proteins, for example, albumin, lipocalin, lysozyme, lactoferrin and others, many represent cytokines, growth factors and other groups of agents normally present in only nanogram quantities or absent and can increase in response to disease, injury or environmental stress, for example, IgE in ocular allergy. This exquisitely balanced and responsive system is referred to as the lacrimal functional unit and is linked by a neural network consisting of sensory receptors, afferent nerve fibers to the central nervous system, efferent fibers to the lacrimal and meibomian glands. The communication between the structure of the ocular surface and glandular structures regulate secretory activity in response to disease, injury and environment stress. Breakdown in one or more of these elements can lead to mild-to-severe dysfunction of the tear film and ocular surface, which is the hallmark of dry eye disease. For readers interested in a more detailed discussion of this area, please refer to [1••,2••].

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