The Tear Film

Anatomy, Structure and Function

Tear Film Anatomy and Physiology

The healthy ocular surface comprises a functional unit that utilizes a variety of structures, all of which remain intertwined in relation to anatomy, composition, and physiological function. These structures include the tear film, corneal and conjunctival epithelium, meibomian and lacrimal glands, and eyelids. A normally functioning tear film is required to maintain clarity of vision and ocular health. The tear film serves to provide ocular surface comfort, mechanical, environmental, and immune protection, maintain epithelial cellular health, and provide a smooth and very powerful refracting surface for clear vision.

One of the primary functions of the tear film includes providing ocular surface comfort through continuous lubrication. Tears are continually replenished from the inferior tear meniscus by blinking.¹ This counters the forces of gravity and evaporation on the volume of the precorneal tear film and protects corneal and conjunctival epithelial cells from the shear forces exerted by the eyelids during blinking. Tear production is approximately 1.2 microliters per minute, with a total volume of 6 microliters and a turnover rate of 16% per minute.² Tear film thickness, as measured by interferometry, is 6.0 µm ± 2.4 µm in normal subjects and is significantly thinner in dry eye patients with measured values as low as 2.0 µm ± 1.5 µm (Fig. 3.1).³

Figure 3.1 Slit lamp photographs with fluorescein staining of a representative dry eye patient and a normal subject. (A) Twenty-six-year-old male normal subject. Estimated tear film thickness was 6.4 µm. (B) Thirty-six-year-old female dry eye patient with Sjögren syndrome. Estimated tear film thickness was 2.4 µm. (Reprinted with permission from Hosaka E, Kawamorita T, Ogasawara Y, et al. Interferometry in the evaluation of precorneal tear film thickness in dry eye. Am J Ophthalmol 2011;151:18–23.e1.)

The ocular surface is the most environmentally exposed mucosal surface, and the tear film serves to protect against irritants, allergens, environmental extremes of dryness and temperature, potential pathogens and pollutants. Reflex tearing can help flush pathogens and irritants from the ocular surface. Antimicrobial components of the tear film include peroxidase, lactoferrin, lysozyme, and immunoglobulin A, among others. The superficial lipid component of the tear film helps prevent evaporation.⁴

Because the cornea is an avascular structure, the epithelium relies on the tear film to supply glucose, electrolytes, and growth factors, as well as the elimination of waste and free radicals. The tear film is a dilute protein solution that shares similar components to serum, although in different concentrations. Glucose concentration is much lower than in plasma (25 mg/L compared to 85 mg/L), and chlorine and potassium are higher. Other electrolyte components include calcium, magnesium, bicarbonate, nitrate, phosphate, and sulfate. Antioxidants, such as Vitamin C, tyrosine, and glutathione scavenge free radicals to help minimize cellular oxidative damage. The tear film also provides a large number of growth factors, neuropeptides, and protease inhibitors, important in maintaining corneal health and stimulating wound healing (Fig. 3.2, Table 3.1).

Table 3.1

Growth factors, neuropeptides, and protease inhibitors in the tear film.

Transforming growth factor (TGF-α,β1,β2)	Mitogenic, inhibits corneal epithelial cell proliferation, pro-fibrotic
Tear hepatocyte growth factor (HGF), keratocyte growth factor	Stimulates corneal epithelial cells, promotes wound healing
Basic fibroblast growth factor (FGFβ, FGF2), Epidermal growth factor	Mitogenic
Substance P	Neuropeptide; stimulates epithelial growth, wound healing
Plasminogen, plasmic, plasminogen activator	Proteases, matrix degradation/wound healing
Matrix metalloproteinases (MMP-2,3,8,9)	Matrix degradation/wound healing
Tryptase, α1-antichymotrypsin, α1-protease inhibitor, α2-macroglobulin	Protease inhibitors

(Adapted with permission from Beuerman R. Tear Film. In: Krachmer JH, Mannis MJ, Holland EJ, editor. Cornea. 2nd ed. Philadelphia, PA: Elsevier Mosby; 2005. p. 45–52.)