Tear film layers explained: lipid, aqueous and mucin

The tear film is usually taught as three layers: an oily lipid layer on the outside, a watery aqueous layer in the middle, and a mucin layer gripping the surface of the eye. It is a useful picture, and it is the one most people are given. It is also, on the current evidence, a simplification.

The TFOS DEWS II tear film report describes the mucin concentration as falling away gradually from the eye's surface towards the water above it, and notes that the aqueous and mucin layers are commonly considered a single layer of mucoaqueous gel. The modern account is closer to two functional zones than three shelves: a mucoaqueous gel with a lipid film on top. The report goes further and describes the whole precorneal tear film as behaving as a single dynamic functional unit with different compartments.

The whole thing is extraordinarily thin. Ultrahigh-resolution optical coherence tomography puts it at 2 to 5.5 micrometres over the cornea, thin enough that the corneal surface's own roughness of about half a micrometre cannot be ignored.

Why the three-layer model changed

The three-layer model came from Wolff, and the DEWS II report is frank about why it stuck: it is simple and logical. A mucin layer lowering the supposed water-repellence of the surface cells, an aqueous layer nursing the epithelium with lubrication, nutrients, antimicrobial proteins and appropriate osmolarity, and a lipid layer to stop the aqueous being lost to the air.

The report also points out that Doane described that three-layered structure as "a considerable simplification of reality" more than twenty years before DEWS II was written, and that returning to it anyway has limited the perspectives that might explain what goes wrong in dry eye.

None of this makes the layers wrong as a teaching device. Lipid, water and mucin are all really there and they fail in different ways. It is the tidy stacking that does not survive close measurement.

The lipid layer, and where it comes from

The lipid layer is derived from meibum secreted at the lid margins, and it is spread across the tear film with each blink, driven by surface tension. Measured by interferometry, its reported thickness runs from 15 to 157 nanometres, with a mean of around 42 nanometres. For scale, that is roughly a thousandth of the thickness of the tear film beneath it.

The meibum comes from the meibomian glands. The MGD Workshop's anatomy report describes them as large sebaceous glands sitting in the tarsal plates of the eyelids, arranged in a single row along the length of each lid and, unlike the sebaceous glands of the skin, not associated with hairs. Counts differ between studies: the upper lid has been reported as having 25 glands in one study and 40 in another, with a median around 31, and the lower lid 20 or 30, with a median around 26.

That report describes the lipids these glands produce as the main component of the superficial lipid layer, which protects the tear film against evaporation of the aqueous phase and is believed also to stabilise it by lowering surface tension. If those glands block or shrink, the outer film thins, and everything downstream of it changes. That process is meibomian gland dysfunction, and it is the single most common reason a tear film stops behaving.

The mucoaqueous layer underneath

The bulk of tear volume and flow comes from the main lacrimal gland, with a smaller portion secreted by the conjunctiva.

Mucins come from two places. Gel-forming mucins are produced by the goblet cells of the conjunctiva, MUC5AC most abundantly, and they hydrate the surface and help clear debris. Transmembrane mucins sit on the microscopic folds of the corneal and conjunctival surface cells and extend up to 500 nanometres into the film, increasing the adhesion tension for water so tears spread rather than bead. They also defend the surface cells against infection and injury.

Water is not this cooperative on its own. Its surface tension is high enough that a two-micrometre film would bead rather than spread, so the mucins are what make a film that thin possible at all. Modern proteomics has identified more than 1,500 proteins in tears alongside them.

What goes wrong, and in which direction

The TFOS DEWS II pathophysiology report puts the central mechanism plainly: evaporative water loss leading to hyperosmolar tissue damage. That damage, directly or by provoking inflammation, causes loss of both epithelial and goblet cells. Losing them reduces how wettable the surface is, which brings the film to breakup sooner, which concentrates the tears further. The report calls this a vicious circle, and it is why dry eye tends to entrench rather than simply persist. Which end of the film failed first is the practical question.

These are not two separate diseases. The DEWS II pathophysiology report describes hybrid dry eye, with features of both aqueous deficiency and increased evaporation, as common, and says the effort should go into working out how much each contributes. Our fuller comparison is at evaporative versus aqueous-deficient dry eye.

An honest note about the lipid layer

Here the evidence is thinner than the textbooks suggest, and it is worth saying so. The DEWS II tear film report says the lipid layer plays an important role in stabilising the tear film and that it "in the past has been thought" to play a key role in retarding evaporation — the past tense is deliberate.

The report summarises work by King-Smith and colleagues finding a lack of correspondence between dry eye and both lipid layer thickness and tear film thinning rate, and that thinning rate was not affected by apparently thickening the lipid layer with lipid emulsion eye drops, which suggests lipid is a poor barrier to evaporation on its own. The report's own suggestion is that perhaps it is the whole healthy tear film, mucins and proteins included, that resists evaporation.

So a thicker lipid layer is a reasonable marker that the glands are delivering oil. Treating it as a measure of how protected your eye is goes beyond the evidence.

Measuring the lipid layer, and warming it

Lipid layer thickness is read by interferometry, from the colour and brightness of interference patterns on the film surface. It is one of several tests covered in our guide to dry eye tests.

Warming is the reason the measurement gets used. Olson and colleagues studied twenty patients with dry eye associated with meibomian gland dysfunction and a baseline lipid layer of 90 nanometres or less. One eye had the closed lids treated with a compress saturated with warm water at 40.0 ± 2.0 °C, the other with room-temperature water at 24 °C. In the treated eyes mean lipid layer thickness rose from 57.8 nanometres at baseline to 105.8 after five minutes and 121.5 after thirty. The control eyes did not change at any time point. The authors concluded that warm, moist compress therapy increases lipid layer thickness in meibomian gland dysfunction by more than 80% five minutes after starting, and by a further 20% after fifteen minutes.

Heat helps because meibum is wax-like rather than liquid at room temperature, and softening it lets the glands deliver it. That is why eyelid warming sits among the first-line measures for meibomian gland dysfunction. Our own Meibocare E-Heated Eye Mask is designed to bring the eyelids to about 42 °C on the recommended setting and timer, using a graphene heating element with a flaxseed filling. Because it targets the lipid end of the tear film, it is aimed at the evaporative side of dry eye and is not the right answer for someone whose problem is tear production.

If you have an active eye infection, have had eye surgery recently, have glaucoma, or notice any change in your vision, speak to your optometrist or ophthalmologist before starting heat therapy or continuing with it.

Frequently asked questions

What are the three layers of the tear film?

Classically an outer lipid layer from the meibomian glands, a middle aqueous layer from the lacrimal gland, and an inner mucin layer from conjunctival goblet cells. The TFOS DEWS II tear film report notes that the mucin concentration falls away gradually and that the aqueous and mucin layers are commonly treated as a single mucoaqueous gel, so two functional zones describes it better.

How thick is the tear film?

Ultrahigh-resolution optical coherence tomography measures the tear film over the cornea at 2 to 5.5 micrometres, and interferometry estimates concur. The lipid layer on top is far thinner again, reported between 15 and 157 nanometres with a mean near 42 nanometres. For comparison, the roughness of the corneal surface itself is around 0.5 micrometres, so the film is only a few times deeper than the surface it covers.

Does the lipid layer stop tears evaporating?

Partly, and less cleanly than commonly claimed. The DEWS II tear film report says the lipid layer has been thought to retard evaporation, but cites work finding no clear correspondence between dry eye and lipid layer thickness or thinning rate, and thickening the layer with lipid emulsion drops did not slow thinning. The report suggests the whole tear film may resist evaporation together.

Do warm compresses thicken the tear film lipid layer?

In one controlled study of twenty patients with meibomian gland dysfunction, yes. A warm moist compress at about 40 °C applied to the skin of the closed lids raised mean lipid layer thickness from 57.8 to 105.8 nanometres within five minutes, while the fellow eye treated at room temperature did not change. The authors reported an increase of more than 80% at five minutes.

Educational information only, not medical advice. References: Willcox MDP, et al. Ocul Surf. 2017;15:366–403; Knop E, et al. Invest Ophthalmol Vis Sci. 2011;52:1938–1978; Bron AJ, et al. Ocul Surf. 2017;15:438–510; Olson MC, et al. Eye Contact Lens. 2003;29:96–99; Craig JP, et al. Ocul Surf. 2017;15:276–283; Jones L, et al. Ocul Surf. 2017;15:575–628.