Physiology of Cornea and Tear Film

Question 1

What are the layers of the cornea starting most anterior?

Answer 1

Epithelium (55um), Bowmans layer (5um), Stroma (485um), Descemets membrane (10um), Endothelium (5um)

Question 2

Focusing system of eye?

Answer 2

cornea, aqueous, iris, crystalline lens, vitreous

Question 3

Imaging system of eye

Answer 3

retina, optic nerve/tract, visual cortex

Question 4

How does cornea anatomically stay transparent

Answer 4

avascular, non pigmented cells, un-myelinated nerves, optical surface (tear film)

*collagen fibril orientation: equal diameter and separation. Specific arrangement generating constructive interface

Question 5

Epithelium for transparency

Answer 5

semi permeable membrane

tears/epithelium:

• Na+ movement by passive diffusion
• Cl- actively pumped to tears, creates water gradient towards tears

epithelium/stroma:
- Requires energy Na+K+ATPase transporter

Question 6

Endothelium for transparency

Answer 6

permeable membrane

aqueous humour to stroma:
transports glucose, amino acids and nutrients driven by IOP and stroma pressure

Stroma to Aqueous Humour:

• pumps fluid into aqueous humour
• Bicarbonate pump
• Carbonic anhydrase within cell
• Na+K+ATPase pump
• lateral cell walla

Question 7

Corneal Function - Metabolism

Answer 7

energy needed to maintain corneal hydration

epithelium –> tears passive (osmosis)

Stroma –> epithelium active

Question 8

what is glucose stored as?

Answer 8

glycogen in epithelial cells

Question 9

what is glycolysis?

Answer 9

enzymatic breakdown of glucose to pyruvate and energy

oxygen needed for aerobic metabolism

Question 10

Epithelial function - nutrient source

Answer 10

nutrient supply via tear film + aqueous

glucose –> epithelial glucose from AqHumour (diffusion through stroma and endothelium)

Oxygen –> no blood vessels so mostly atmospheric diffusion from tears. When eyelids closed palpebral conjunctiva is source

Question 11

Aerobic Epithelial Function

Answer 11

glycolysis via two mechanisms:

Krebs cycle –> conversion of pyruvate by mitochondria lead to generating ATP (energy)

Pentose phosphate pathway –> oxidises glucose to generate NADPH, fatty acid synthesis, glycolysis

Question 12

Anaerobic epithelial function

Answer 12

glycolysis via two anaerobic mechanisms:
- pentose phosphate pathway. Oxidises glucose to generate NADPH, fatty acid synthesis and glycolysis

Anaerobic pathway produces pyruvate and lactate that breaks down CO2 and H20. Increased acidity, cellular damage, dysfunction

Question 13

Endothelial function -metabolism

Answer 13

oxygen delivery via AqHumour. Independent of eyelid closure

Glucose vis AqHumour

Glycolysis and ATP generated mostly from Krebs cycle. High density of mitochondria

Uses alot of energy due to high metabolic demand for ion transport

Question 14

Corneal function - Defence

Answer 14

epithelium major defence mechanism:

• absence of vascular system
• tight junctions and tear film

Question 15

Epithelium turnover

Answer 15

rapid wound healing. 24-48 hours

epithelial cell migration, monolayer slides over wound within 5 hours

cell proliferation at the limbus to support migration

cell adhesion, rapid from plaque adhesion to hemidesmosomal complexes

keratocytes (fibroblats) dormant until compromise (injury, inflammation)

Question 16

endothelial repair

Answer 16

Cell density: 
Birth 5000 (cells/mm2)
Age 5 - 3500
Teenager - 3000
Adult - 2500

Coverage compensation:
Lost cells are replaced by expansion and thinning of neighbouring cells
Leads to variation in cell shapes (polymorphism) and sizes (polymegathism) with age

Question 17

how do tears stick to corneal surface?

Answer 17

• corneal microvilli
• membrane spanning mucus
• secreted mucus

sticky surface with water soluble glue

Question 18

Tear film - function

Answer 18

• maintains health of ocular surface; proteins, nutrients
• acts as a lubricant for lid and corneal interaction
• disposal system to wash debris away and dilute toxins
• optically smooth surface

Question 19

Tear film - composition:

Answer 19

Three components:

• inner mucin layer
• aqueous phase (bulk)
• outer lipid layer (most anterior surface)

A mucin gradient exists between inner mucin layer and aqueous layer

Question 20

What is mucin layer

Answer 20

product of corneal and conjunctival cells

sticks tear film down, anchors aqueous phase to corneal surface

lubricant

bounding of debris/ bacteria

Question 21

what is aqueous layer

Answer 21

liquid layer

volume of tears from lacrimal gland

source of electrolytes, proteins, peptides and oxygen

removal of debris, bacteria

Question 22

what is lipid layer

Answer 22

stops tearms from evaporating

acts as lubricant

produced by meibomium glands (eyelid margins)

Question 23

source of tear film

Answer 23

mucin –> conjunctiva, squamous and goblet cells

aqueous –> lacrimal glands, acinus sacs

Lipids –>
Meibomium glands, acinus sacs

Question 24

tear film mucins

Answer 24

Combination of secreted:

• Goblet cells (MUC5Ac)
• Lacrimal gland (MUC7)
• Mixes in aqueous phase
• Cleaning up debris

Membrane spanning:
- Epithelium, cornea, conjunctiva
- Forms glycocalyx (glue)
MUC1, MUC4 and MUC16

Question 25

4 major proteins found in tear film aqueous

Answer 25

Lysozyme: (2.07 mg/ml), a potent antibacterial enzyme, by hydrolysing the bonds in the outer wall of the bacteria (G+ve, streptococcus and staphylococcus) Lipocalin: (1.55mg/ml), plays a lipid binding role, maintains the surface tension of the tears. Binds with fatty acids which enhances function of Lysozyme Lactoferrin: (1.65mg/ml), antimicrobial (G+ve and G-ve Pseudomonas, Eschericha coli, Haemophillis influenza) binds to free iron preventing use by bacteria. Part of innate defence mechanism Secretory IgA: (1.93mg/ml), part of the adaptive response system, adheres to bacteria thus a target for phagocytosis Other --> (0.1mg/ml) Aqueous largest component of tears. little glucose, also of K+ and Cl-

Question 26

tear film lipid layer

Answer 26

Produced by meibomian glands: - Modified sebaceous glands that secrete meibum - Mixture of lipids and proteins - Differs to seibum (more viscous) - Very thin complex oily layer (15-160nm) - Mixture of polar, non polar and neutral lipids, >236 lipids isolated - Melting point 19.5-32.9 degrees - always fluid on ocular surface ``` Optical finishing touch Retard evaporation Evaporation - aqueous layers thin Surface destabilises Non-wetting patches appear Poor optical performance ``` Observation easier: Instillation of water soluble dye Sodium fluorescein Clinically known as Tear Break Up time TBUT Temperature gradient: Ocular surface 37 degrees (body temp) Air --> ambient 24 degrees Tear film must evaporate. Limited by lipid layer Tear film needs: - Replacing - Refreshing Reforming

Question 27

four phases of blink action

Answer 27

downstroke - upper lid to lower lid turning point - lid stops before reversing motion upstroke - upper lid moves away from lower lid interblink - pause before downstroke

Question 28

function of blink

Answer 28

downstroke --> regenerates aqueous phase, sweeps mucin wrapped debris nasally turning point --> forms new lipid layer upstroke --> spreads lipid layer over tear film interblink --> pause before downstroke. Vision processing

Question 29

types of tears

Answer 29

basal - mediated by Na+K+ATPase reflex - parasympathetic control. sensory stimulation, retinal stimulation, pain, emotions primarily through increased aqueous volume orbital/palpebral lobes of lacrimal glands

Question 30

corneal dysfunciton

Answer 30

opacification: corneal dystrophy, opacification due to infection, cornea filled with blood vessels, stroma increasing thickness odema (milky cornea), plaques

Question 31

tear dysfunction:

Answer 31

Stability: - Aqueous dysfunction - Lipid dysfunction - Mucin dysfunction Ectropion TBUT Dry Eye

Question 32

what is osmolarity

Answer 32

The concentration of an osmotic solution, measured in litres, the total number of particles dissolve in a solvent ``` For tears: - As aqueous reduces - Salt concentration increases - Osmolarity increases (hyperosmolarity) Symptoms of dry eye increase ```