tearfilm physiology and biochemistry.pptx

tearfilm physiology

1. Tear film physiology and biochemistry

2. The 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 1 st demonstrated by Fischer in 1928.

• Rollet described it as the most superficial 6 th layer of cornea.

3. STRUCTURE OF THE TEAR FILM ‘PRE-CORNEAL FILM’.

• Tear film consists of 3 layers.

1)Outer Lipid layer

2) Intermediate Aqueous layer &

3)Inner mucin layer

4. LIPID LAYER • Outer most layer.
0.1um thick
• Formed from the secretions of Meibomian , Zeiss & Moll glands. Contents 1)low polarity lipids-wax & cholesterol
esters 2)high polarity lipids-TG, FFA, phospholipids. Functions 1) prevents the overflow of tears.
2)prevents evaporation.

5. AQUEOUS LAYER • Middle layer.
• Formed by secretions from the main & accessory lacrimal glands of Krause & Wolfring .
• Constitutes the main bulk of tear film.
• thickness over the cornea 10um.
• Film covering the cornea is thinner than over the conjunctiva.
• Contents inorganic salts, glucose, urea, enzymes, proteins & glycoproteins.
• Buffering capacity of the tear film is d/t HCO3 ions & protein Functions 1)provides O2 to corneal epithelium.
2)washes away debris & irritants.
3)contains antibacterial sub- lysozyme & betalysin .

6. MUCOUS LAYER • Innermost layer.
• Secreted mainly by the conjunctival goblet cells
• 30um thick
• Functions 1)plays a vital role in the stability of the tear film.
2)converts the hydrophobic corneal epithelium to a hydrophilic one.
3) lubricates the ocular & palpebral surfaces.
4)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.
5) absorps various organic compounds in tears

7. NEW TEAR FILM MODEL • Recent observation– mucins exist as a network distributed in the aqueous body of the tear film.

• Glycocalyx emanate as transmembrane molecules into the aqueous & are anchored at the cell membrane.

• Membrane associated proteins-MUC1,4&16 as well as secretory mucins - MUC5AC &MUC7 have been identified at the ocular surface.

8. TEAR FULID COMPOSITION • Mainly composed of 3 protein factors-albumin, globulin and lysozyme

• IgA is the most prominent Ig.

• IgE levels increase in patients with allergic conjunctivities .

• IgM increases in patients with acute infections.

• Lysozyme constitutes 20% all tear protiens , highest in conc among all body fluids

• Electrolytes Na,K and Cl higher concetrations in tears than in blood

9. FUNCTIONS OF TEAR FILM • Makes corea a smooth optical surface
• Wets cornea and conjunctiva and prevent from drying
• Flushes out debris and organisms
• Bactericidal properties (presence of lusozyme , lactoferrin and betalysin )
• Ig’s and specific Ab in tears defend the eye against infections
• Lubricating action
• Enables anti inflammatory cells to reach injured cornea and conjunctiva
• Provides epithelial cells with 02, Glucose and Growth Factors

10. NEURAL ASPECTS • The trigeminal v1 (fifth cranial) nerve bears the sensory pathway(afferent) of the tear reflexes.

• The motor pathway is autonomic (involuntary), &, in general, uses the pathway of the facial (seventh) nerve in the parasympathetic division via pterygopalatine palatine ganglion, as efferent pathway.

• Applied A newborn infant has insufficient development of nervous control, so she/he “cries without weeping”.

11. APPLIED ASPECTS
• Crocodile tears syndrome/ Bogorad’s syndrome”

uncommon consequence of nerve regeneration subsequent to Bell’s palsy or other damage to the facial nerve in which efferent fibers from the superior salivary nucleus become improperly connected to nerve axons projecting to the lacrimal glands (tear ducts), causing one to shed tears ( lacrimate ) during salivation while smelling foods or eating.

• Presumed that one would also salivate while crying due to the inverse improper connection of the lacrimal nucleus to the salivary glands, but less noticeable.

12. TEAR FILM DYNAMICS • Secretion of tears
• Formation of tear film
•Retention & redistribution of tear film
• Displacement phenomenon
• Evaporation from the tear film
• Drying & break up of tear film
• Dynamic events during blinking
• Elimination of tears

13. 1.SECRETION OF TEARS
basal
reflex

14. BASAL SECRETION
In the human eyes the cornea is continually kept moist & nourished by basal tears.

• They lubricate the eye & help to keep it clear of dust.

• Secreted by accessory lacrimal glands

15. REFLEX SECRETION

•Results from irritation of the eye by foreign particles, bright light, hot & peppery stimuli to the tongue.

•These reflex tears attempt wash out irritants that may have come into contact with the eye.

•Secreted by main lacrimal gland

16. • Applied If lacrimal gland is damaged in surgery or other failure of lacrimal function occur, it is not a serious matter, for the accessory glands are enough for general secretion

17. 2.FORMATION OF PRE OCULAR TEAR FILM • Conjunctival mucus spreads on to the cornea by the action of the lids.

On this new surface- aqueous layer is spread spontaneously.

Over this the superficial lipid layer spreads; probably contributing to its stability & retarding evaporation b/w blinks.

18. 3. RETENTION & REDISTRIBUTION The outer most layer of corneal epithelium+mucopolysaccharides leads to retention.

• Precorneal film is stagnant. Redistribution occurs in the form of bringing of new tear fluid by way of marginal strip where there is constant flow of tears

19. 4. DISPLACEMENT PHENOMENON • Demo that cornea is covered by a film which has stability, compressibility, elasticity & unaffected by gravity

• Demo by upward movement of particles in the film on displacing lower eyelid upwards over eyeball

• This phenomenon is possible due to presence of thin monomolecular layer on the surface of cornea

20. 5.EVAPORATION • All lipid films including wax esters & cholesterol esters retard evaporation of water

• Important in low humidity & turbulent air flow near cornea, such as exists in a windy & arid climate

• Evaporation from tear film = 10% of production rate, so, evaporation = 0.12ul/min (as tear production = 1.2ul/min)

21. 6.STABILITY, DRYING & RUPTURE OF TEAR FILM • Tears has to cover entire preocular surface to function properly
• It is re-established completely after a blink, but has short lived stability
• It takes 15-40 secs for tear film to rupture & dry spots to appear, when blinking is prevented
• Drying of corneal surface cannot be a result of evaporation of water alone, as it takes at least 10 mins to eliminate whole tear film by drying alone.

22. 7.ELIMINATION OF TEAR FILM HOLLY & LEMP’S MECHANISM

• Initially all the tear film thins uniformly by evaporation.
• When thinned out to critical thickness, some lipid molecules attracted by the mucin layer & migrate down to this layer.
• When the mucin layer is sufficiently contaminated by lipid from the top, the mucin becomes hydrophobic & the tear film ruptures
• Blinking repair this and restore aqueous layer