Tear film

The tear film is one of the most important components in the tear system

There are several roles that the tear film (tear film) fulfils:

• Providing an optimal smooth surface between the eye and the air.
• Clearing away remnants of dirt from the surface of the eye
• Protection of the outer surface of the eye
• Supply of oxygen, growth factors and more to the corneal epithelium
• Contains antimicrobial substances (antibacterial)
• Lubrication of the cornea and the eyelid

A further article that may interest you: The tear system – ophthalmology

Layers of the tear film

1. The lipid layer (lipid layer) – the outermost layer
2. The aqueous layer (aqueous layer) – the middle layer
3. The mucin layer (mucin layer) – the back layer

The thickness of the tear film is about 3 microns and at any given moment it has a volume of about 7 microlitres without local anaesthetic substances, and after the placement of a local anaesthetic substance the volume drops to a third of the original volume. In addition, the volume of the tear film decreases with age.

The tear film is composed of 98% water and every 6-8 minutes it is completely renewed/replaced

The lipid layer (Lipid Layer)

The lipid layer is the outermost layer and it is secreted mainly by the meibomian glands (Meibomian Glands) which are located in the tarsal plate (Tarsal Plate) of the eyelids. There are about 40 such glands in the upper eyelid and about 20 in the lower eyelid. The Zeis gland (Zeis Sebaceous Gland) also secretes lipid substances that form part of this layer. One of the most important roles of this layer is to slow the rate at which the tear film evaporates. In addition, this layer prevents leakage of the tears by increasing the surface tension.

The aqueous layer (Aqueous Layer)

The aqueous layer is the middle layer and it is secreted by the main lacrimal gland and the accessory glands. It contains electrolytes, water, proteins and more. The central gland is composed of two anatomical parts, the orbital part and the palpebral part. The main accessory gland, which constitutes 2/3 of the accessory glands, is the glands of Krause (Glands of Krause); there are about 40 such glands in the lateral upper eyelid and about 5-10 such glands in the lateral lower eyelid. In addition, there are the lacrimal glands of Wolfring (Wolfring Glands) which are larger and are located at the edges of the eyelids.

Most of the electrolytes in this layer are found in a concentration similar to that of the blood or rise accordingly with a rise in the concentration in the blood, except for potassium which is 5-10 times higher in this layer. 96% of diabetic patients who receive one hundred mg of sugar by mouth will afterwards have sugar levels above a threshold of 11 mg per ml.

Among the other proteins in this layer one can find IgA and sIgA (types of immunoglobulins) which play a role in the immune system. Increased levels of IgE can be found in patients with Vernal Conjunctivitis (inflammation of the conjunctiva with an allergic mechanism). Further proteins with antimicrobial substances, growth factors and more. The main roles of this layer are to supply oxygen and electrolytes to the corneal epithelium, antimicrobial and antiviral substances and to wash away remnants of dirt.

The mucin layer (Mucin Layer)

Among other things, this layer enables uniform distribution of the tear film over the corneal epithelium. The main cell that secretes this layer is the goblet cell (Goblet Cell). A deficiency of mucins, an excess of mucins and a change in the structure of the mucin can lead to damage to this layer.