ORBIT & OCULAR ADNEXAE - Tear Film and blinking

Question 1

What are the three layers of the tear film?

Answer 1

AIR/ENVIRONMENT

1. Lipid layer
2. Aqueous layer
3. Mucous layer

CORNEAL EPITHELIUM

Question 2

What is the lipid layer formed by?

Answer 2

Meibomian glands
Glands of zeiss and Moll

Question 3

What is the aqueous component formed by?

Answer 3

Lacrimal gland
Accesory lacrimal gland

Question 4

What is the rate of basal tear secretion?
What is the rate of reflex tear secretion?

Answer 4

Basal - 1.2 microlitres/minute
Reflex - 100 fold increase

Question 5

Basal tears are produced by which glands?
Reflex tears are produced by which glands?

Answer 5

Basal - lacrimal and accessory lacrimal
Reflex - lacrimal glands

Question 6

How is production of the lipid layer activated/stimulated?

Answer 6

By blinking (not neurally controlled)

Question 7

Final tear secreted has higher or lower potassium vs plasma?

Answer 7

higher potassium

Question 8

What are the components of the mucin layer? Where are they located?

Answer 8

1. Goblet cells - epithlium of bulbar conjunctiva and fornix (inferonasal part of conjunctiva most populated)
2. Crypts of Henlte - tarsal conjunctiva
3. Galnds of manz - ring of limbal conjunctiva

Question 9

What is the function of the lipid layer?

Answer 9

Prevents evaporation by increasing surface tension and ensures optic clarity

Question 10

How thick is the lipid layer?
How thick is the aqueous layer?

Answer 10

1. Lipid - 0.1 microns
2. Aqueous - 2-6 microns

Question 11

How much tears can the ocular surface hold at any time?

Answer 11

8 microlitres

Question 13

Where are the tear compartments of the eye? (3)

Answer 13

1. forniceal compartment
2. tear menisci
3. preocular tear film (precorneal tear film + prebulbar tear film)

Question 14

What percentage of tears are reabsorbed through the nasolacrimal duct mucosa?

What percentage of tears are drained into the floor of nasal cavity?

Answer 14

1. reabsorbed - 90%
2. drained - 10%

Question 15

What is the lid wiper zone?

Answer 15

The marginal zone between the subtarsal fold to mucocutaneous junction on the lid margin

Question 16

What is the difference between the precorneal tear film and prebulbar tear film?

Answer 16

precorneal - 3mm and highly stable
prebulbar - thickness is variable and unstable.

Question 17

What is mucin? How is it secreted?

Answer 17

glycoprotein synthesised by endoplasmic reticulum (protein) of the goblet cell and released at the apical surface (via microvilli)

Question 18

What are the two different types of mucins?

Answer 18

1. membrane bound mucins
2. secretory mucins.

Question 19

What are the different types of membrane bound mucins? What is their function?

Answer 19

MUC1, MUC4 and MUC16 with galectin-3

Question 20

What is the function of membrane bound mucins? (3)

Answer 20

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

Question 21

What are the different types of secreted mucins? What is their function?

Answer 21

MUC7 and MUC5AC

MUC7 - antifungal/antimicrobial activity
MUC5AC (main mucin)- trap contaminants to clear them from mucosal surface

Question 22

What is the function of the mucous layer?

Answer 22

1. Forms glycocalyx over ocular epithelium that prevents pathogen adhesion
2. Binds water to hydrate and lubricate the surface - is hydrophilic.
3. Reduce friction during blinking
4. Clear pathogens/debris
5. Contribute to tear stability

Question 23

What are the main components of the aqeous layer? (5)

Answer 23

1. lysozymes
2. lactoferrin
3. betalysin
4. immunoglobulins (mostly IgA)
5. VEGF

Question 23

How thick is the aqueous part of tear film?

Answer 23

2-6 microns

Question 24

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?

Answer 24

open - 155mmHg
closed - 55mmHg

oxygen saturations drops when closed to 30mmHg

Question 25

What is the function of the aqueous layer? (7)

Answer 25

1. forms 90% of tear film
2. lubricates ocular surface
3. washes away foreign bodies/contaminants
4. nourishes avascular cornea (oxygen permeability)
5. antimicrobial activity
6. growth factors/vitamins for corneal healing/health
7. smooths irregularities of the corneal surface to provide optical function

Question 26

What is a meibomian gland?

Answer 26

modified sebaceous holocrine glands whose acini discharge their entire contents in the process of secretion

Question 27

What is secreted by the meibomian gland?
How is it spread across the tear film?

Answer 27

1. meibum
2. delivered into a shallow reservoir on the lid margin and spread onto preocular tear film with each blink

Question 28

What is the capacity of the lid reservoir of the meibomian gland?

Answer 28

30 x amount of lipid present on surface of tear film.

Question 29

What is the composition of the meibum? (2)

Answer 29

95% non-polar lipids - wax esters, sterol esters, free fatty acids
5% polar molecules - phospholipids

Question 30

What is the function of the lipid layer? (6)

Answer 30

1. Forms outer layer of tear film
2. Minimise evaporation of water from eye surface
3. Improves stability of tear film
4. Prevents tear contimination by skin lipids
5. Limits aqueous layer surface tension
6. Provides smooth refractive surface

Question 31

What are the electrolyte differences between aqueous tears and plasma?
1. Sodium
2. Potassium
3. Chloride
4. Osmolarity
5. Protein

Answer 31

1. Na - similar to serum
2. K+ 5-7 times greater than serum
3. Cl - greater than serum
4. osmolarity - similar to serum

Question 32

Which conditions are associated with mucin deficiency? (3)

Answer 32

Those with goblet cell damage - ocular pemphigoid, steven-johnsons, vitamin A deficiency