Cornea Flashcards
What is the anterior segment?
Everything from the back of the lens to the cornea
What is the anterior chamber?
Fluid filled space between cornea and iris
What covers the ocular surface?
A continuous layer of epithelium which covers the cornea, limbus, anterior part of the sclera and reflects itself at conjunctival fornix onto the back of the eyelids
How does ocular epithelium changes at the gray line?
Grey line: mucocutaneous junction along eyelid margins-> epithelium becomes keratonised
How does ocular surface epithelium stay moist?
Mucin
Aqueous tears: lacrimal glands
Oil: meibomian glands
Where is the corneal epithelium replenished?
From limbal stem cells
What makes up the corneoscleral coat?
Tough collangenous outer coat
Made up of sclera (white) and cornea (clear but no BV lots of nerves. 10mm diameter 0.5mm thick)
What things attach to the sclera?
EOM Optic nerve and meninges Vortex veins Posterior ciliary nerves and arteries Conjunctiva and tenons at the limbus
What are the 3 main layers of cornea?
Epithelium (heals well)
Stroma (90% of thickness)
Endothelium (single layer of cells)
What is the main refractive surface of the eye?
Cornea (2/3 of eyes optical power, 1/3 from lens)
How much aqueous fluid is in the anterior chamber?
0.25 of a ml
Turned over every 90 minutes
How does the position of the iris relative to the cornea change with age?
With age the lens grows, causing the iris/lens surface of the anterior chamber to curve anteriorly towards the cornea (shallowing of anterior chamber)
What is the uveal coat of the eye?
Continuous layer of highly pigmented highly vascular tissue
Apposed directly to the inner surface of sclera
Makes up choroid, ciliary body and iris
What makes up the iris?
Rests on anterior surface of the lens
12mm diameter
Separates anterior chamber from posterior chamber
Continuous with ciliary body and trabecular meshwork
Made up on 2 epithelial layers, 2 muscle layers and iris stroma
What are the layers of the lens? How thick is it?
Central nucleus, peripheral cortex, and thin outer capsule
10mm diameter
Starts at 3.5mm can reach 5mm thickness in old age
Held in place by zonular fibres attached to ciliary bodies
Anterior lens epithelium (single layer of cells, migrate posteriorly and lengthen in life)
What are the 2 functions of the ciliary body?
Aqueous production and accommodation of the lens
What are the 2 parts of the ciliary body?
Pars plicata -anterior
Pars plana- posterior (intravitreal injections)
Made up of 2 layers of epithelium, stroma and muscle layer
What are the dimensions of the cornea?
Horizontal ellipse 10.5mm vertically by 11.55 horizontally
0.55 mm thickness centrally
0.65 mm thickness peripherally
Normally more curved vertically (with the rule astigmatism)
What is continuous with the corneal epithelium and endothelium?
Corneal epithelium is continuous with sclera epithelium
Corneal endothelium is continuous with trabecular meshwork
Describe corneal epithelium
10% of cornea Thicker in periphery May be pigmented in periphery Non keratonised stratified squamous epithelium Micro villi coated with proteoglycans 5-7 cells thick Pain nerve fibres between cells 7 days for cells to change over
How does the corneal epithelium stay attached to basement membrane and itself?
Hemidesmosomes lock basal cells to basement membrane
Desmosomes, gap junctions and interdigitations between wing cells form a mechanical barrier
What is loss of corneal epithelium called?
An ulcer
BM stains fluorescein
How thick is Bowman’s layer and how is it different to the stroma?
10 microns
Denser
No fibroblasts no keratocytes
Provides strength
What makes up corneal stroma?
90% of thickness
Lamellae of collagen
200 sheets at 90 degrees to each other
Type 1 collagen
Glycosanimoglycans ECM centrally: keratan sulfate, chondroitin sulfate peripherally
Contains few keratocytes connected in a syncytium
What is the descemet membrane?
10 microns thick Strong elastic membrane BM produced by endothelium Banded and non banded layers Gets thicker with age
Describe the corneal endothelium
3-5 microns Hexagons Joined by water proof tight junctions Don't divide Cell density 5000 cell per mm squared at birth as we age, drops to 2500
Function of corneal endothelium
Active transport
Out of stroma into anterior chamber (aqueous)
Maintains transparency of the stroma
Where does the corneal endothelium end?
Schwalbe’s line aka at the limbus
Joins trabecular meshwork
What is the sensory supply of the cornea? How does it get to the cornea?
Trigeminal nerve V1
Ophthalmic nerve-> long ciliary nerve
Penetrates scleral stroma just posterior to limbus
Forms annular plexus in scleral stroma
Enters corneal stroma radially and loses its myelin
Forms a subepithelial plexus and then turns superficially to form intraepithelial plexus
Nerve endings lie in superficial epithelium
What causes decreased corneal sensitivity?
Age
Herpes infections
LASIK
Contact lens wearers
What is the blood supply to the cornea?
Avascular normally
Nutrients from aqueous and limbal blood vessels and O2 dissolved in tear film
Unless: long term contact wearers, chronic inflammation
What is the limbus?
Landmark
Circular zone between cornea and sclera
1.5mm wide
What are the two different definitions of the limbus?
Anatomical: Schwalbe’s line
Surgical: Bluish area, anterior to anatomical limbus
What does bulbar conjunctiva attach itself to at the limbus?
3mm from the limbus bulbar conjunctiva and tenons fuse
1.5mm behind limbus Tenon’s fascia attaches itself to the globe
What are the Palisades of Vogt?
Radial folds at the limbus of limbal epithelium/conjunctiva involving fibrovascular tissue
Most visible at 12o’clock superior cornea
Site of limbal stem cells
What happens to Bowman’s membrane ta the limbus?
Ends
Its deep surface merges into substantia propria
What happens to Descemet’s membrane at the limbus?
Ends at Schwalbe’s line
Continuous with trabecular meshwork
Corneal endothelium is also continuous with endothelium of trabecular meshwork
What are the functions of the tear film?
Optical interface smooth surface
Health of epithelium and conjunctiva : moisture, O2 and glucose
Protective; anti-bacterial, flushing dust/chemicals.
No tear film -> exposure keratopathy
Which chemicals make up the tear film?
Ions
Antibodies (IgA, IgM, IgE)
Antibacterials (lysozyme)
Metabolites (glucose, pre-allbumin, lactate)
Inflammatory mediators (prostaglandins, histamine)
Describe the layers of the tear film and their thickness in microns
Superficial oil layer 0.2um thick. From Meibomian glands, limits evaporation
Middle aqueous layer 7um thick from main and accessory lacrimal glands
Deep mucin layer 0.5um thick, from conjunctival goblet cells and lacrimal gland. Related to corneal epithelium microvilli
What is the consequence of the poor tear film?
Punctate epithelial erosions
More common inferiorly as interpalperbral fissure is exposed for the longest period of time
Describe the embryological timeline of corneal development
5th week: surface ectoderm invaginates
6th week: corneal epithelium formed
7th week: neural crest derived mesenchyme forms stroma and endothelium (Bowman’s formed by stroma, Descemet’s is formed by endothelium)
7th month: corneal maturity
What happens to the cornel epithelium after its injured?
Rapidly regenerates (hours) Limbal stem cells divide and push daughter cells centrally to become new basal cells of stratified squamous epithelium (centripetal movement)
What happens to the corneal stroma after injury?
Depends if penetrating or non penetrating
Penetrating: leaky eye ball. Tested with Seidel’s test
Acutely: stoma swells and opacified, white cell infiltration
Later: white scar formation by fibroblasts. Epithelium will smooth over defects
What happens to corneal endothelium after injury?
No mitosis
Cells slide and grow to cover defect
Decreases overall pump function
Fewer cells on endothelium
How does age affect the cornea?
Nerve: decreased nerve ending density (except for subepithelial nerve plexus)
Tear film less stable
Decreased barrier function and healing in epithelium
Stroma: arcus senilis, deposition of lipid in anterior corneal stroma, not visually significant
Decreased elasticity due to collagen crosslinks
Astigmatism changes from with the rule to against the rule.
Endothelium: cell density decreases 0.5% per year
Faster loss after 60
What is keratoconus?
Progressive thinning and conical protusion of the cornea
Mainly stromal
Occurs teens-30s
Associated with eye rubbing
Treated with penetrating keratoplasty and deep anterior lamellar keratoplasty
What is a pentrating keratoplasty?
Full thickness corneal graft
Replace stroma and endothelium
Used for keratoconus, corneal scars, endothelial failure
Risks: rejection, glaucoma, cataract, infection, astigmatism
What is endothelial failure?
Cornea has become opaque as full of fluid
Endothelium not removing water out of corneal stroma
Bullae visible under corneal epithelium
Guttata : drop like opacities on endothelium
Decreased density of cells
How does DSEK work?
Descemet stripping and endothelial keratoplasty
Take endothelium and a bit of stroma inserted into patient’s anterior chamber
Air bubble for 24hours pushes graft on patient’s cornea and it will stick
How does LASIK work?
Blade cuts through corneal epithelium and 1/3 stroma
Lifts flap
Laser reshapes stromal bed
Flap replaced
New anterior surface of the cornea to improve refractive error
What are the 5 layers of the cornea?
Corneal epithelium Bowman's layer Corneal stroma Descemet's membrane Corneal endothelium
What is Fuch’s endothelial dystrophy?
Autosomal dominant condition results in loss of endothelial cells
Leads to a higher risk of corneal decompensation post cataract surgery
How does the number of layers vary in the corneal epithelium?
5-6 layers centrally
10 layers at the limbus
Describe the superficial epithelial cells of the cornea
Flat polygonal nucleated cells
Apical microvilli and microplicae
Covered by glycocalyx to prevent tear film evaporating`
Describe the intermediate wing cells of the corneal epithelium
Polyhedral
oval or round nuclei
How is the basal layer of the corneal epithelium attached to the basement membrane?
Hemidesmosomes
Anchoring fibrils type 7 collagen attach hemidesmosomes and stroma
Does the cornea have lymphatic drainage?
No
What is the autonomic innervation of the cornea?
Sympathetic fibres from superior cervical ganglion
Parasympathetic fibres from ciliary ganglion
How long does it take for corneal epithelium to regenerate?
7-10 days
What forms the bowman’s layer?
Superficial proliferation of stromal mesenchymal cells at day 53 gestation
Different cell lineage than that of overlying corneal epithelium
Describe Bowman’s layer
Smooth, non regenerative, accelular precipitate of the cornea
8-10 microns thick
Composed of type I, II and V collagen
What is the function of the Bowman’s layer?
No one knows
Maybe acts as a corneal ligament
May prevent spread of infection
What is the nutrition and blood supply of the Bowman’s layer?
It is acellular so does not need nutrition or blood
Avascular
Which layer of the cornea is affected in keratoconus?
Bowman’s layer (characterised by absence or disorganisation of this layer)
What are the roles of the cornea stroma?
Transmit and focus light
Mechanical support to cornea
What is the embryological origin of corneal stroma?
Mesenchymal cells of neural crest origin
What are the fibrils of the corneal stroma?
Heterodimeric Type 1 and V collagen
Embedded in GAG proteoglycan matrix
Running in parallel
Act for tensile strength and maintain transparency via electrostatic inter fibril space
What are the packaging units of corneal stroma collagen fibrils called and how are they arranged?
Lamella
Arranged perpendicular to each other
Important for maintaining transparency
Where do the keratocytes lie in corneal stroma?
Between lamella
Sparsely distributed
Transparent
Describe how the annular plexus enters the stroma
Radial patterns
Bifurcates towards epithelium into subepithelial plexus and intraepithelial plexus
Lose myelin sheath as pass through Bowman’s membrane
How does the corneal stroma get nutrients?
Aqueous humour and circumferential capillaries
What are the two layers of Descemet’s membrane?
Anterior: fetal, type IV and VIII collagen
Posterior: non banded, post natal only type IV collagen
What are Hassall-Henle Bodies and Guttata?
Wart like projections of Descemet’s membrane in the periphery
Hassal-Henle: peripherally, do not effect vision
Guttata: central, affect vision (Fuch’s)
What is the complicating factor of Descemet’s membrane in relation to DMEK?
Descemet’s membrane has a tendency to scroll up
Which is stronger, Descemets or Bowman’s?
Descemet’s is 10x stronger
Can Descemet’s layer heal?
Yes, endothelium migrates to cover any defects
Descemet’s layer increases in size over life
Is the corneal endothelium innervated?
No
When does the corneal endothelium form a monolayer?
18 weeks gestation
Nil further replication after this
What is the histological appearance of corneal endothelium?
Hexagonal single layer of cells
Held together by tight junctions and gap junctions
Attached to Descemets by hemidesmosomes
What are the functions of the corneal endothelium?
Pumping water out of the stroma
Allowing passage of nutrients in and out of stroma via active and passive transport
