OVERVIEW of ...
Tears and Tear FILM
... in more Detail
The MYSTERIOUS TRANSFORMATION from TEARS into a Tear FILM
Tears occur normally as DROPS
The most natural shape for small amounts of water is a SPHERE because this has the most favorable relation between surface tension and inner volume.
When the sphere drips along the tissue or through the air atmosphere it is formed into a DROP.
We can thus imagine, how much of physico-chemical ´persuasion´ it affords to form water from the natural sphere or drop into a thin FILM with a relatively very large surface in relation to the contained minute volume.
These considerations bring us to the topic of the mysterious TRANSFORMATION from TEARS into a Tear FILM.
Mucin Coating is the secret behind the binding of Water drops - termed wetting
The secret behind this is rather physical than metaphysical - because what is needed is some substance that is spread out in the desired plane of the film ... AND that is so hygroscopic by its large water-binding capacity that its attraction of the water is larger than the previous equilibrium between volume and surface tension in the sphere.
This magic substance is MUCIN, composed of large glycoprotein polymer molecules with an enormous water binding capacity. Mucins have a two-fold source at the Ocular Surface - (1) the integral glycocalyx in the membranes of all epithelial cells and (2) the secreted mucins from the conjunctival goblet cells.
Without mucin even the cellular surface of the ocular epithelium would be non-wettable as reported in the mid 1970s by HOLLY and LEMP ... and since no contradictory evidence has been presented so far, this is still one of the foundations of ocular surface wisdom ... which has also practically proven right in terms of the analysis of Tear Deficiency and Dry Eye Disease in clinical practice.
Mucins are just one of the secretions that make up the tears
Mucins are just one component of tears that is produced by the ocular glands - in fact source or mucins are single cell glands and ordinary surface cells. The sources of the other components of the tears come from larger accumulations of secretory cells that form organs such as the lacrimal gland and the Meibomian glands. Altogether they make the liquid that we just discussing which eventually forms the tears. For more details please see the sections on ´Fine structure of the Ocular Surface´ and ´Ocular Glands´
The mere presence of Tears & ´WettinG´ is necessary but not sufficient for perfect Vision
The mere presence of ´wetting´ by the moisture of the tear fluid is a necessary component to keep the Ocular Surface system healthy - but it is not sufficient to produce perfect vision ...
... as we can see from visual disturbance/ unstable visual acuity by a lack of tears in Dry Eye Disease and an overflow of tears in emotional tearing (crying). In both cases the volume is not adequate.
What we need is a tear FILM
More important than the volume of tears is actually the homogeneity of tears in front of our optical medium of the translucent cornea - What we additionally need is: A very homogeneous and very thin Tear FILM for perfect refraction.
This leads us to the question HOW we come from simple ´wetting´ to the tear FILM.
FORMATION of the tear FILM is achieved by the wiping ACTION of the Eye Lids over the bulbar Surface
The ocular tear fluid as such does not provide any clear and stable visual acuity. Perfect vision only occurs when the regular wiping action of the intact eye lids transforms the tear fluid into a thin FILM in front of the cornea.
This film has a certain stratification of basically tree layers, the observation of which dates back to early studies by Eugene WOLFF in the 1940s and still appears as a valid concept.
Other ideas e.g. of the thickness of the tear film in total or of the thickness and composition of the individual layers have changed over time with new advances in experimental designs and imaging tools.
At present, it is assumed from the available data that the tear film has a thickness in the dimensions of only about one cell layer or less if compared to the cuboidal conjunctival cells and more than one cell layer if compared to the squamous cells of the cornea.
The regular eye lid blinking transforms the tears into a thin tear FILM
The main movement in blinking occurs due to the wiping of the upper eye lid over the bulbar surface of the cornea and the surrounding conjunctiva.
The lower eyelid performs only a minor movement to the nasal side. There is also an upward movement of the eye ball upon lid closure with the cornea that is known as Bell´s phenomenon.
During the faster down-phase of blinking the upper lid margin compresses the tear film in the palpebral fissure between the both lids. In the slower up-phase the upper eye lid extends and spreads the tears into a new fresh tear film.
VIEW INTO the Tear FILM
What makes the difference between tears and the tear FILM ?
Blinking leads to the formation of a film of tears - this provides moisture for ocular surface INTEGRITY - everywhere and everytime - which means:
- even at the exposed palpebral fissure
- even in the inter-blink period
It is therefore no surprise that LID and BLINKING DEFICIENCY (LBD) is a typical reason for the onset of Dry Eye Symptoms ... which may lead to Dry Eye Disease when the condition becomes chronic. LBD concerns e.g.: (1) the absence of blinking e.g. due to nerve palsy or (2) rare blinking in concentrated visual tasks or (3) morphological alterations of the eye lids or their congruity with the eye ball ... etc.
More important for VISUAL ACUITY is that the tear film:
- is very thin and
- is reasonably homogeneous and
- has a smooth outer surface at the air to-tear interface
A perfect smooth air to-tear interface is constituted by the very thin outer lipid layer that is the first and main refractive surface of the eye and accounts for about three quarters of the eye´s refractive power.
The lens inside the eye only has the function to modulate and adjust the refractive power of the eye for different focus distances – a process termed accommodation.
Too much and too little moisture both impair visual acuity
The importance of a very thin and homogeneous tear film can be appreciated when the visual acuity decreases in situations with INhomogeneity of the tear film:
- As we produce a lot of tears while crying. The tear drops change the homogeneous refraction of a smooth regular surface into higher order aberration with reduced visual acuity.
- The other extreme, i.e. in dry eye disease, is when tear volume is typically diminished due to insufficient gland secretion or due to increased evaporation. Then, blinking still produces a thin tear film but this becomes easily in-homogeneous and ruptures resulting in dry spot that represent a hole in the even tear film surface and thus lead to aberrant refraction with blurred vision.
When the location of dry spots, or of other impairments of regular refraction, is moving due to the eye lid blinking this leads to unstable visual acuity. This explains why unstable visual acuity is a typical symptom of dry eye disease.
The tear film is stable ... only for a relatively short time - then it breaks-up and must be renewed by a blink
The tear film is only stable for about 10 to 20 seconds on average – under 10 seconds is considered as pathological.
Irritation of sensory nerves from an exposed dry spot or from a spot of local hyper-osmolarity on the corneal surface induces a blink that spreads a new tear film.
The induction of a blink is an example of auto-regulation by a neural reflex arc of the afferent sensory cranial nerve V and the efferent cranial motor nerve VII. The normal blink rate is around 10 to 12 blinks per minute but it can change distinctly depending on the visual task.
Every Blink gradually renews the components inside the film
It may be assumed, and some evidence seems to support this, that the tear film components are gradually renewed with every blink. This means that some fresh secreted mucin, aqueous fluid, and lipids are added and spread with every blink whereas some of the “used” components are shed out of the bulbar surface, either over the anterior lid border onto the lid skin or via the nasal lacrimal puncta into the nose.
For the basal tear layer of mucins it is shown that mucins are continuously shed into a nasal mucus strip.
The turnover of lipids in the superficial tear film LIPID LAYER (TFLL) is of particular interest because it may provide some hints to the composition and arrangement of the lipid layer as such and may thus give deeper insight into its function and dysfunction. There is some evidence that the Lipid Layer may be folded like a pleated sheet by the closing upper lid and is (re-) unfolded in much of the same appearance during the up-phase of the blink. (please see animation).
If we keep in mind that, according to the results of the 2011 TFOS Workshop Report on Meibomian Gland Dysfunction (MGD), lipid deficiency is conceivably the main causative factor for Dry Eye Disease, deeper insight into lipid layer dynamics may also provide deeper insight into Dry Eye Disease.