Cornea 1 - Week 2 Flashcards
What are the 3 basic ways that the cornea interacts with light and the environment?
- Transparency - optimum for vision
- Refraction - good quality focused image (with lens)
- Strength - barrier against env. protect from injury
Name the two mechanisms for maintaining corneal shape?
- Corneal thickness
2. Corneal hydration
List the 5 layers of the cornea in order.
- Epithelium
- Bowman’s layer
- Stroma
- Descement’s membrane
- Endothelium - most posterior border of cornea
How may bumps on the tear film affect vision?
any bumps on tear film can interrupt ability of light to bend properly – lead to blurry vision
Name 3 factors that affect corneal refraction?
- stable pre-ocular tear film
- corneal shape: THICCNESS and HYDRATION
What effect does ageing have on transmittance?
Decreases with age
At what wavelength does the transmittance of light increase rapidly?
300nm
Define transparency (of cornea)
How much light can get through without scatter
Are corneal nerves myelinated
Not quite. They are umyelinated from their entrance at the limbus (within the avascular region)
Does the cornea have blood vessels?
NO. (however it can if cornea is diseased, in which case you’d need a transplant)
Is the epithelium of the cornea pigmented?
NO it is not
Does the cornea have cells?
Very little. There are very few cells here (those being keratocytes and macrophages)
List the 5 anatomical factors contributing to corneal transparency
- corneal nerves unmyelinated at limbus entrance
- cornea = avascular
- cornea = few cells
- epithelium is NON-PIGMENTED
- Anterior + posterior epithelium CELL BARRIERS and ION TRANSPORT mechanisms to MAINTAIN HYDRATION
What is the key factor that controls corneal transparency?
Precise organisation and separation of stromal collagen fibres in the ECM
How is precise fibre separation in the corneal ECM maintained?
A highly specialised arrangement of inter-fibrillar matrix is used to maintain it
Do collagen fibril diameters vary in the cornea?
No, there is very little variation
List the 3 ECM factors that affect transparency
- Precise organisation + separation of stromal collagen fibres
- specialised arrangement of inter-fibrillar matrix (maintain fibre separation_
- Minimal fibril diameter variation
What is the role of the corneal epithelium?
acts as a barrier to protect the cornea, resisting the free flow of fluid from the tears, and prevents bacteria from entering the epithelium and corneal stroma
What are the 3 components involved in corneal epithelium barrier function
- Tight junctions (zonula occludens) - acts as a (FLEX) SEAL to prevent leakage between cells
- Desmosomes - (along with tight junctions), join the epithelial cells together to prevent leakage
- Gap junctions - allow communication b/w cells
Does mitosis occur in the corneal epithelium? If so, where?
Yes. Mainly in the Basal layer. (small amount also in germinal layer)
What happens to epithelial cells as they age?
They lose their attachments and slough/fall off the epithelium
How many cells THICC is the corneal epithelium?
5-6 cells thick
What is the corneal epithelium continuous with? and where?
With the conjunctival epithelium at the limbus
Describe the cell types of the corneal epithelium
Superficial cells = ‘Flattened non-keratinised squamous cells
Deepest (basal) cells = columnar
Between superficial and basal = WING CELLS
When epithelial cells fall off the epithelium, what happens next?
Underlying cells move to the surface to take their place
How long is the turnover time of the entire cornea (in terms of cells)?
Around 7 days
What shape do the corneal flattened, non-keratinised, squamous cell nuclei have?
They are horizontal
Is the cornea keratinised or non-keratinised? Why?
non-keratinised. This is because keratin is bad at letting light through
What force are corneal epithelial cells always exposed to?
Frictional force
How does the epithelium stay in place in response to frictional force?
It is anchored to a basement membrane and bowmans layer
Describe how epithelium anchoring occurs?
- hemidesmosoms anchor to lamina densa
- anchoring fibrils attach lamina densa to bowmans layer at anchoring plaques
What does the epithelium secrete?
mucins, which contribute to the mucin layer of the tear film
Describe the surface of the corneal epithelium? Why is it like this?
Has Microvilli and Microplicae - to increase surface area, which increases tear film stability
(we want to increase the potential of the tear film to stick nicely to the corneal epithelium)
How much does the stroma contribute to the thickness of the cornea?
90% of corneal thickness
What types of cells does the corneal stroma contain?
Fibroblasts (called keratocytes since they are in the cornea)
What is the connective tissue of the stroma like?
Fibrous, dense, regular
As part of the stromal connective tissue, what takes up the most amount of dry weight proportion?
A) collagen
B) keratocytes
C) proteoglycans
Collagen
How are stromal collegen fibrils arranged?
- they are evenly spaced and run parallel to each other in bundles
What are collagen bundles with the cornea called?
Lamellae
Are the collagen fibrils uniform in diameter?
Yes
How many lamellae are in the stroma?
200-300
How thicc is the bowman’s layer?
8-14um (very thin)
How does bowman’s layer appear histologically?
faint pink line beneath epithelium
Describe the fibril composition of the bowman’s layer
- dense fibrous sheet of interwoven fibrils
- fibrils have small diameters and not obviously ordered
Is Bowman’s layer cellular or acellular?
Acellular
What is the main type of collagen found within the corneal stroma? Name two more types found here
Mainly type 1. Also found are III, V, VI
How does stromal collagen fibril size relate to the wavelength of light?
collagen fibril diameter and separation are small relative to the wavelength of light (prevents scatter)
What is the fibril diameter of stromal collagen? Does it vary?
30nm. It does NOT vary. It is very consitant
How does the diameter of fibres change as you approach the sclera?
diameter starts to fluctuate
What proportion of the corneal stroma hydrated volume is keratocytes?
Around 2.5%
Are keratocytes evenly distributed throughout the stroma?
No. There is linear reduction going anterior to posterior
What is the function of keratocytes?
Active control of stromal remodelling
What happens to fibroblasts/keratocytes if trauma occurs?
They turn into myofibroblasts
Name 2 factors contributing to corneal swelling
- GAGs attracting water with their negative charge
- intraocular pressure
Name 3 factors contributing against corneal swelling
- epithelial and endothelial barriers
- endothelial pumps
- evaporation
What 4 things can corneal swelling/oedema result in?
- Increased light scatter, reduced light transmission, degraded image quality
- Further decompensation of cellular barriers
- pain
- complete corneal opacification - graft required
Describe Donnan Swelling
Major mechanism of water uptake in a charged gel is due to ionic imbalances.
- major cause of water uptake into corneal stroma
Describe volume exclusion, comparing it to donnan
Small holes within a matrix allow passage of water (but not anything bigger). Is slow.
Donnan is 600 times faster (and is major mechanism)
Describe Descemet’s membrane in terms of endothelium
It is the basement membrane of the endothelium
What produces Descemet’s membrane? How does age affect it?
Produced constantly by the endothelium and therefore thickens with age
Describe the layer thickness of descemet’s membrane
1/3rd anterior
2/3rd posterior
What collagen type forms Descemet’s membrane?
Types IV and VIII (4 and 8)
Describe Descemet’s membrane appearance (anterior vs posterior)
Anterior = more granular posterior = homogenous
Do corneal endothelial cells have gap junctions?
yes
How does corneal endothelium contribute to corneal hydration? Is it a permeable membrane?
It is a permeable membrane barrier that controls inflow of ions and solutes from aqueous into stroma
- has leaky barriers compared to superficial epithelial cells
Do endothelial cells divide?
No. They are lost with age, disease, surgery, CL wear
What happens to the corneal epithelium as you approach the limbus?
Gets thicker
What is the limbus?
Point where the cornea meets the sclera
What happens to lamellar organisation at the limbus?
Becomes irregular
What are Palisades of Vogt? What is located there?
Radial projections of limbal epithelium and stroma, and are spoke like
- stem cells are here
What nerve innervates the cornea, and how many branches per cornea?
70-80 branches of long ciliary nerve
How are the nerves that innervate the cornea oriented?
Stromal nerve fibres turn abruptly 90 degrees and proceed towards corneal surface
- they then pierce bowman’s layer, forming the subepithelial plexus (which is whorl-like)
Where are the majority of corneal nerves found?
In anterior stroma
