- TEARS are produced by the lacrimal glands. Each upper eyelid has one main lacrimal gland and several "accessory" glands responsible for producing tears. (LACRIMATION)
- EYELIDS BLINK to smooth the tear film over the cornea, providing a clear surface for optimal vision and comfort. The secondary benefit of blinking is when the lids press together, they squeeze meibum (a thin oil) out of the tiny openings near the eyelash follicles. This oil coats the outer layer of the tears protecting the tear film from evaporating too quickly. Each eyelid has about 20-40 vertical columns of glands that open near the base of the lashes (at the "water line" of the lid margin). (MEIBOMIAN GLANDS)
- MUCIN is the inner layer of the tear film. GOBLET CELLS help the tear film adhere to the front surface of the eyes by producing mucin. This layer can be affected by inflammation and allergies. Glaucoma drops and seasonal allergies can trigger inflammation on the surface of the eyes leading to reduced goblet cell function and dry eyes.
- The lids blink on average every 8-10 seconds. If the tears evaporate quicker than 8 seconds, or the lids blink less often due to concentration, or the lacrimal glands don't make enough tears to coat the cornea, the eyes become dry.
The layers of the tear film have always been explained as three layers:
- Mucin Layer- the inner layer of the tear film that helps the tears adhere to the cornea. Mucin is made by goblet cells on the cornea and conjunctiva. When the goblet cells on the cornea are damaged by inflammation, eye drops and stem cells with amniotic membrane bandage lens treatments can improve this layer significantly.
- Aqueous Layer- the middle layer of the tear film is made up of the bulk of the tears produced by the lacrimal gland.
- Oil Layer- the outer layer is made up of oils from the meibomian glands in the eyelids. This layer depends on blinking to secrete the oils. When the meibomian glands become inflamed, the meibum thickens to a toothpaste type texture and the oil layer fails. Tears evaporate too quickly causing dryness even when the lacrimal gland produces enough tears for the aqueous layer.
As we learn more about the tear film, it is understood that the three layers are not as distinct as once believed. The three layers intermingle to create a complex coating for the front surface of the eye. Any disruption in any of the layers can create dry eye symptoms.