Precorneal Tear Film
The exposed surfaces of the cornea and bulbar conjunctiva are covered by the precorneal tear film, which was formerly described as having 3 layers: lipid (from meibomian glands), aqueous (from the lacrimal gland), and mucin (primarily from goblet cells). It is now thought of as a lipid layer with underlying uniform gel consisting of soluble mucus (secreted by conjunctival goblet cells), mixed with fluids and proteins (secreted by the lacrimal glands). A glycocalyx mediates the interaction of the mucoaqueous layer with surface epithelial cells of the cornea.
Maintenance of the precorneal tear film is vital for normal corneal function. The tear film does the following:
provides lubrication for the cornea and conjunctiva
facilitates the exchange of solutes, including oxygen
contributes to the antimicrobial defense of the ocular surface
serves as a medium to remove debris
Further, the air–tear film interface at the surface of the cornea constitutes a major refractive element of the eye, because of the difference in the refractive index of air and that of the tear film. Aberrations in the tear film result from a variety of diseases (eg, dry eye, blepharitis) that can profoundly affect the integrity of the ocular surface and consequently the patient’s vision. See Chapter 7 for in-depth discussion of the tear film.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.