tear film

1. TEAR FILM
Vishal Prasad
B. OPTOMETRY

2. TEAR FILM
 The main role of lacrimal system is to establish & maintain a continuous
tear film over the ocular surface.
 Pre-corneal tear film was 1st demonstrated by Fischer in 1928,by using
light reflected from the corneal surface on a photographic plate
(reflectography).
 Rollet described it as the most superficial 6th layer of cornea.

3. FUNCTION OF TEAR FILM
 Makes cornea a smooth optical surface.
 Wets cornea and conjunctiva and prevent from drying.
 Flushes out debris and noxious irritants.
 Presence of antibacterial substances such as lysozyme, betalysin,
lactoferrin and other protein.
 Defend the eye against infections.
 Lubricating action.
 It provides a pathway for white blood cells in case of injury.
 Provides epithelial cells with O2,Glucose and Growth factors.

4. STRUCTURE OF TEAR FILM
 Wolff (1946) was the first to describe in detail the structure of tear film.
 Coined the term “Pre-Corneal Film”.
 Tear film consists of three layer:-
1. Lipid layer
2. Aqueous layer
3. mucin layer

5. STRUCTURE OF THE TEAR FILM

6. LIPID LAYER
 Outermost superficial oily layer.
 Formed from the secretions of Meibomian, Zeiss and moll glands.
 Marginal tear strip.
 0.1-0.2 mm thick.
 Low polarity, lipids-wax & cholesterol esters.
 High polarity lipid-triglycerides(TG),free fatty acids(FFA),phospholipids
are present in negligible amount

7. CONTROL OF LIPID SECRETION
Androgen sex hormones regulate lipid synthesis and secretion.
Neurotransmitters from the nerves surrounding the acini can alter
lipid synthesis or alveolar cell rupture.

8. FUNCTION OF LIPID LAYER
Prevents the overflow of tears.
Prevent evaporation.
Prevents migration of skin lipids onto the ocular surface.
Provides a clear medium and smooth surface for refraction of light.
Acts as a barrier of preventing contamination of tear film.

9. AQUEOUS LAYER
 Middle layer.
 It is secreted by accessory gland and lacrimal glands of Krause and
Wolfring .
 Constitutes the main bulk of tear film.
 Comprises 60% of the tear film.
 Thickness over the cornea 7𝝁𝒎.
 Film covering the cornea is thinner than over the conjunctiva.

10. COMPOSITION OF AQUEOUS LAYER
 Inorganic salts , glucose , urea , enzymes , proteins & glycoprotein’s.
 Buffering capacity of the tear fluid is due to bicarbonate ions and proteins.
FUNCTIONS
1) Provides oxygen to corneal epithelium.
2) Washes away debris and noxious irritants.
3) Contains antibacterial substance like lysozyme & betalysin.

11. MUCUS LAYER
 Innermost layer.
 Secreted mainly by the conjunctival goblet cells.
 50𝝁𝒎 (approx.)thick can be demo in living eye by alician blue drops.
 Plays a vital role in the stability of the tear film.
 Semi solid state.

12. FUNCTIONS OF MUCUS LAYER
1. Converts the hydrophobic corneal epithelium to a hydrophilic one.
2. Lubricates the ocular and palpebral surface’s.
3. Provides a slippery coating over the foreign bodies; thereby protecting
the cornea & conjunctiva against the abrasive effects of such particles
as they move about with blinking.
4. Absorbs various organic compounds in tears.

13. NEW TEAR FILM MODEL
 Mucin exist as a network distributed in the
aqueous body of the tear film.
 Glycocalyx emanate as transmembrane
molecules into the aqueous and are
anchored at the cell membrane.
 MUC5AC is the main tear mucin which is
produced along with the trefail protein
TFF1 and 3.

14. PHYSICAL PROPERTIES OF TEAR FILM
Properties
1) Thickness 7-8 𝝁𝒎
2) Volume 4-13𝝁𝒍
3) Rate of tear secretion 1.2 𝝁𝒍/min.
4) Turn over rate 18% per min.
5) Refractive index 1.357
6) pH 7.3-7.7
7) Osmotic pressure 0.90-0.95% NaCl solution
8) Temperature 35℃ 𝒂𝒕 𝒕𝒉𝒆 𝒍𝒊𝒎𝒃𝒖𝒔 ;
30℃ 𝒂𝒕 𝒕𝒉𝒆 𝒄𝒆𝒏𝒕𝒓𝒆.
9) O2 tension 40-160 mmHg

15. CHEMICAL COMPSITION OF HUMAN TEARS
AND PLASMA
Tears Plasma
Water 98.2g% 94gm/100ml
Solids ,total 1.8% 6gm/100ml
Na+ 142mEq/1 137-142 mEq/1
K+ 15-29 mEq/1 5 mEq/1
𝐂𝐥−
120-135 mEq/1 102 mEq/1
𝐇𝐂𝐎𝟑
−
26 mEq/1 24.3 mEq/1
𝑪𝒂++
2.29mg/100ml
Total proteins 0.6-2gm/100ml 6.78gm/100ml
Amino acids 8mg/100ml
Urea 0.04mg/100ml 20-40mg/100ml
Glucose 3-10mg/100ml 80-90mg/100ml

16. TEAR FILM DYNAMICS
 Secretion of tears
 Formation of tear film
 Retention and redistribution of tear film
 Displacement phenomenon
 Evaporation from the tear film
 Drying and break-up of tear film
 Dynamic events during blinking
 Elimination of tears

17. SECRETION OF TEARS
(Accessory
lacrimal gland)
(Main lacrimal gland)

18.  Concept of basal secretion is presently thought to be redundant one.
 Reflex secretion occurs response to sensations from the cornea and
conjunctiva ,probably produced by evaporation and break up of tear film.
 Even minimal tear production in the undisturbed eye is thought to be
secondary to light or temperature stimulation or both
 Afferent pathway of this secretion is formed by fifth nerve and efferent by
parasympathetic (secretomotor) supply of lacrimal gland.
 82% of the full term new born babies secrete tear within 24 hours and 95%
by 1st week.
 Abnormal tearing starts only after an infant is 4 months old.

19. FORMATION OF PREOCULAR TEAR
FILM
 Corneal epithelium is a relatively hydrophobic surface.
 Lemp and Holly have indicated that principal constituent of tears mucin
responsible for the wetting of corneal surface by converting the corneal
surface from hydrophobic to hydrophilic one .
 Sequence of events in formation of tear film
 Lids surfacing cornea with a thin layer of mucus .
 On this new surface , the aqueous component of tears now spreads
spontaneously.
 The superficial lipid layer spreads over the aqueous film, probably
contributing to its stability and retarding evaporation between blinks .

20. RETENTION AND REDISTRIBUTION OF
TEAR FILM
The tear film is retained at a uniform thickness over the corneal surface
against a gravitational force.
The absence of downward flow of the precorneal tear film was suggested
by Wolff in 1954.
The outermost layer of corneal epithelium, along with
mucopolysaccharides play an important role in tear film.
The fluid in the precorneal tear film is stagnant , unless it is mixed by
blinking and eye movement with the tear fluid in marginal strip.
Redistribution occurs in the form of bringing of new tear fluid by way of
marginal strip where there is constant flow of tears.

21. DISPLACEMENT PHENOMENON
 This properties is responsible for movement of particle in the when
lower eyelid is carefully displaced upwards .
 Surface of cornea is covered by a film possessing a certain stability,
compressibility and elasticity and that it is more or less unaffected by
gravity .
 This phenomenon is possible due to presence of thin monomolecular
layer on the surface of cornea .

22. EVAPORATION FROM THE TEAR
FILM
 All lipid films including those of wax esters and cholesterol esters retard
the evaporation of water.
 Important in low humidity and turbulent air flow near the cornea such as
exists in a windy and arid climate .
 Evaporation from the tear film is estimated to be about 10% of the
production rate .( The tear production rate is 1.2₶l/min.)
 There is little effect of air motion on the evaporation rate because most of
the resistance to evaporation is the oily layer on the tear film .

23. STABILITY, DRYING AND
RUPTURE OF TEAR FILM
 The tear film covers the entire preocular surface .
 It is re-established quickly and completely blink. The precorneal tear
film has a short-lived stability.
 A brief time interval of 15-40 sec.,the tear film ruptures and dry spots
appear on various part of cornea .
 The drying of the corneal surface cannot be a result of evaporation of
water alone ,because at least ten min. would be required to eliminate
the whole tear film by drying alone.

24. MACHANISM OF TEAR FILM BREAK
UP
 Holly (1973) has described a mechanism of tear film
rupture.( also known as Holly and Lemp’s mechanism)
 First of all tear film thins uniformly by evaporation.
 When thinned out to some critical thickness ,some lipid
molecules attracted by the mucin layer and migrate down
to this layer.
 When the mucin layer on the epithelium is sufficiently
contaminated by lipid migrating down from top surface of
the tear film, the mucin becomes hydrophobic and the tear
film ruptures.
 Blinking repair this and restore aqueous layer .

25. MECHANISM OF TEAR FILM BREAK UP

26. DYNAMIC EVENTS DURING BLINKING
 Holly(1980) has given a brief account of events during blinking.
 As the upper lid moves downwards , the superficial layer is compressed
between the lid edges.
 The lipid contaminated mucus is rolled up in a thread –like shape and
dragged into lower fornix.
 When the eye opens, at first the lipid spreads in the form of a monolayer
against the upper eyelid.
 The spreading of the excess lipid follows and in about 1 second
multimolecular layer of lipid is formed.
 The spreading lipid drags some aqueous tears with it , thereby thickening
the tear film.
 The magnitude of this effect is controlled by the size and shape of tear
meniscus adjacent to lid edge.

27. DYNAMIC EVENTS DURING BLINKING

28. MECHANISM OF REMOVAL OF LIPID
CONTAMINATED MUCUS

29. ELIMINATION OF TEARS
The lacrimal fluid flows over the preocular surface and reaches the
ciliary marginal of each eyelid and collect lacus lacrimalis in the
inner canthus.
The lacrimal fluid is then drained by the lacrimal passages into
nasal cavity .

30. ELIMINATION OF TEARS
TEAR FLUID
MARGINAL TEAR STRIP
PUNCTUM
INNER CANTHUS
NASAL CAVITY

31. THE TEAR IS MADE OF
1% Water
99% Feeling
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