Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Tear Film


Key Concepts

  • Tear film stability is essential to maintaining ocular surface health.

  • Lipids, mucins, and aqueous are the major components. There has been a shift from a strict three-layer model to understanding the tear film as a single dynamic functional unit.

  • With technologic innovation, evaluation of the tear film has become more precise, allowing for better understanding of normal and diseased ocular surfaces both in clinical and research settings.

  • Expanding knowledge of the tear film microstructure shines light on its function in health and may provide utility in diagnosing ocular and systemic disease states in the future.

Macrostructure and Function

The integrity of the ocular surface begins largely with the quality of the tear film. The tear film serves three essential functions: lubrication, protection, and refraction. The human tear film functions as the first refractile structure light traverses on its journey to the retina. Although an association between dry eye and visual compromise is poorly documented, measurement of “functional visual acuity” after prolonged periods without blinking demonstrates poorer acuity in those with dry eye. Although previously described as three distinct layers, the tear film is currently believed to behave as a single dynamic functional unit spanning 2–5 μm in thickness. Despite this, these aqueous, mucous, and lipid components are often referred to as separate layers for simplicity and convention.

The tear film’s composition allows it to adequately coat the ocular surface while maintaining a smooth and thin surface. Water has a high surface tension precluding it from forming an ultrathin film without collapsing. Hence properties of the tear film allow water tension of tears to be reduced. Interactions between transmembrane mucins attached to microplicae of corneal and conjunctival epithelial cells and the lipid component of tears facilitate the spread of tears across the ocular surface. ,

Mucoaqueous Component

Mucin is produced by conjunctival goblet cells, whereas aqueous tears originate from the lacrimal gland located in the supratemporal orbit and accessory lacrimal glands of Wolfring and Krause located within the palpebral conjunctiva and fornix, respectively. Evidence suggests that the concentration of mucin decreases from the epithelial surface toward the air-tear interface. This portion serves to bathe and provide nutrients to the ocular surface while forming a protective barrier. It contains oxygen, metabolites (including amino acids, urea, glucose, lactate), electrolytes (such as sodium, potassium, calcium, magnesium, chlorine, phosphate, bicarbonate), microbial peptides, proteins, and soluble immunoglobulins. Normal tears contain 6–10 mg/mL total proteins. Electrolyte and protein content of the tears is different than that of serum. Chloride and potassium are higher in tears (120 mEq/L and 20 mEq/L; serum, 102 mEq/L and 5 mEq/L, respectively) and glucose concentration is lower in the tears (approximately 2.5 mg/100 mL) compared with plasma (85 mg/L).

Lipid Component

Meibomian glands located along the upper and lower eyelid margins are the only known source of tear film lipid. Lipid, 90% of which is secreted as meibum, spreads over the tear film with each blink. Unlike the mucoaqueous layer, the lipid layer remains continuous over the menisci and continues to move upward over the ocular surface with each blink. The lipid layer measures approximately 40 nm thick ; however, observations using interference techniques demonstrate that the lipid layer is of variable thickness ranging from 15 to 157 nm. Lipids aid in lowering surface tension at the air interface, allowing the spread of tear film over the ocular surface. Numerous studies credit the lipid component for the tear film’s ability to resist evaporation. Evaporation is the primary factor in tear film thinning. Tear evaporation rates have measured at 0.14 ± 0.07 μL/min in healthy eyes while those with meibomian gland dysfunction have evaporations rates of 0.26 ± 0.16 μL/min. Yet, other studies suggest no correlation between clinical dry eye with lipid layer thickness and thinning rate. In addition, lipid emulsion–based eye drops have failed to decrease tear film thinning. Therefore, clinical dry eye is likely the product of a multifactorial process.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts