ORBIT & OCULAR ADNEXAE - Tear Film and blinking Flashcards
What are the three layers of the tear film?
AIR/ENVIRONMENT
- Lipid layer
- Aqueous layer
- Mucous layer
CORNEAL EPITHELIUM
What is the lipid layer formed by?
Meibomian glands
Glands of zeiss and Moll
What is the aqueous component formed by?
Lacrimal gland
Accesory lacrimal gland
What is the rate of basal tear secretion?
What is the rate of reflex tear secretion?
Basal - 1.2 microlitres/minute
Reflex - 100 fold increase
Basal tears are produced by which glands?
Reflex tears are produced by which glands?
Basal - lacrimal and accessory lacrimal
Reflex - lacrimal glands
How is production of the lipid layer activated/stimulated?
By blinking (not neurally controlled)
Final tear secreted has higher or lower potassium vs plasma?
higher potassium
What are the components of the mucin layer? Where are they located?
- Goblet cells - epithlium of bulbar conjunctiva and fornix (inferonasal part of conjunctiva most populated)
- Crypts of Henlte - tarsal conjunctiva
- Glands of manz - ring of limbal conjunctiva
What is the function of the lipid layer?
Prevents evaporation by increasing surface tension and ensures optic clarity
How thick is the lipid layer?
How thick is the aqueous layer?
How thick is the mucin layer?
- Lipid - 0.1 microns
- Aqueous - 2-6 microns
- Mucin layer - 0.05 microns.
How much tears can the ocular surface hold at any time?
8 microlitres
Where are the tear compartments of the eye? (3)
- forniceal compartment
- tear menisci
- preocular tear film (precorneal tear film + prebulbar tear film)
What percentage of tears are reabsorbed through the nasolacrimal duct mucosa?
What percentage of tears are drained into the floor of nasal cavity?
- reabsorbed - 90%
- drained - 10%
What is the lid wiper zone?
The marginal zone between the subtarsal fold to mucocutaneous junction on the lid margin
What is the difference between the precorneal tear film and prebulbar tear film?
precorneal - 3mm and highly stable
prebulbar - thickness is variable and unstable.
What is mucin? How is it secreted?
glycoprotein synthesised by endoplasmic reticulum (protein) of the goblet cell and released at the apical surface (via microvilli)
What are the two different types of mucins?
- membrane bound mucins
- secretory mucins.
What are the different types of membrane bound mucins? What is their function?
MUC1, MUC4 and MUC16 with galectin-3
What is the function of membrane bound mucins? (3)
1) allow boundary lubrication and prevent adhesion of corneal epithelium and tarsal conjunctiva during blinking and sleeping
2) mucosal barrier integrity to prevent stuff entering ocular surface epithelium
3) osmosensors - osmotic concentration of tear film
What are the different types of secreted mucins? What is their function?
MUC7 and MUC5AC
MUC7 - antifungal/antimicrobial activity
MUC5AC (main mucin)- trap contaminants to clear them from mucosal surface
What is the function of the mucous layer?
- Forms glycocalyx over ocular epithelium that prevents pathogen adhesion
- Binds water to hydrate and lubricate the surface - is hydrophilic and negatively charged.
- Reduce friction during blinking
- Clear pathogens/debris
- Contribute to tear stability
What are the main components of the aqeous layer? (5)
- lysozymes
- lactoferrin
- betalysin
- immunoglobulins (mostly IgA)
- VEGF
How thick is the aqueous part of tear film?
2-6 microns
What is the oxygen saturation of the tear film when the eyes are open?
What is the oxygen saturation of the tear film when eyes are closed?
What happens when you wear contact lenses?
open - 155mmHg
closed - 55mmHg
oxygen saturations drops when closed to 30mmHg
What is the function of the aqueous layer? (7)
- forms 90% of tear film
- lubricates ocular surface
- washes away foreign bodies/contaminants
- nourishes avascular cornea (oxygen permeability)
- antimicrobial activity
- growth factors/vitamins for corneal healing/health
- smooths irregularities of the corneal surface to provide optical function
What is a meibomian gland?
modified sebaceous holocrine glands whose acini discharge their entire contents in the process of secretion
What is secreted by the meibomian gland?
How is it spread across the tear film?
- meibum
- delivered into a shallow reservoir on the lid margin and spread onto preocular tear film with each blink
What is the capacity of the lid reservoir of the meibomian gland?
30 x amount of lipid present on surface of tear film.
What is the composition of the meibum? (2)
95% non-polar lipids - wax esters, sterol esters, free fatty acids
5% polar molecules - phospholipids
What is the function of the lipid layer? (6)
- Forms outer layer of tear film
- Minimise evaporation of water from eye surface
- Improves stability of tear film
- Prevents tear contimination by skin lipids
- Limits aqueous layer surface tension
- Provides smooth refractive surface
What are the electrolyte differences between aqueous tears and plasma?
1. Sodium
2. Potassium
3. Chloride
4. Osmolarity
5. Protein
- Na - similar to serum
- K+ 5-7 times greater than serum
- Cl - greater than serum
- osmolarity - similar to serum
Which conditions are associated with mucin deficiency? (3)
Those with goblet cell damage - ocular pemphigoid, steven-johnsons, vitamin A deficiency
Where is the highest concentration of goblet cells?
Goblet cells - epithlium of bulbar conjunctiva and fornix (inferonasal part of conjunctiva most populated)
How thick is the tear film?
3-5 microns.
