2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 1: Structure and Function of the External Eye and Cornea
Our understanding of the structure and composition of the tear film has gradually evolved. The tear film was formerly described as a structure composed of 3 layers: lipid (expressed from the meibomian glands), aqueous (expressed from the lacrimal gland), and mucin (produced primarily by goblet cells). It is now thought of as a uniform gel consisting of soluble mucus, which is secreted by conjunctival goblet cells, mixed with fluids and proteins secreted by the lacrimal glands (Fig 1-3).
Figure 1-3 Components of the tear film—produced by the lacrimal glands, conjunctival goblet cells, and surface epithelium—lubricate (mucins), heal (epidermal growth factor [EGF]), and protect the cornea from infection (lactoferrin, defensins, immunoglobulin A [IgA]). When the tear film is inflamed, it produces interleukin-1 receptor antagonist (IL-1RA), transforming growth factor β (TGF-β), and tissue inhibitor of matrix metalloproteinase 1 (TIMP 1). MMP-9 = matrix metalloproteinase 9.
(Modified with permission from Pflugfelder SC. Tear dysfunction and the cornea: LXVIII Edward Jackson Memorial Lecture. Am J Ophthalmol. 2011;152(6):902.)
The air–tear film interface at the surface of the cornea constitutes the primary refractive element of the eye. The tear film is primarily responsible for maintaining a smooth optical surface between blinks. It also serves as a medium to remove irritants and pathogens and contains a variety of elements that control the normal ocular flora. Further, tears dilute toxins and allergens and allow for the diffusion of oxygen and other nutrients. Maintenance of the tear film is thus critical to normal corneal function.
Pflugfelder SC. Tear dysfunction and the cornea: LXVIII Edward Jackson Memorial Lecture. Am J Ophthalmol. 2011;152(6):900–909.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.