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OPTHT801 > Cornea > Flashcards

Cornea Flashcards

(81 cards)

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1
Q

What is the anterior segment?

A

Everything from the back of the lens to the cornea

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2
Q

What is the anterior chamber?

A

Fluid filled space between cornea and iris

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3
Q

What covers the ocular surface?

A

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

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4
Q

How does ocular epithelium changes at the gray line?

A

Grey line: mucocutaneous junction along eyelid margins-> epithelium becomes keratonised

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5
Q

How does ocular surface epithelium stay moist?

A

Mucin
Aqueous tears: lacrimal glands
Oil: meibomian glands

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6
Q

Where is the corneal epithelium replenished?

A

From limbal stem cells

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7
Q

What makes up the corneoscleral coat?

A

Tough collangenous outer coat

Made up of sclera (white) and cornea (clear but no BV lots of nerves. 10mm diameter 0.5mm thick)

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8
Q

What things attach to the sclera?

A 
EOM
Optic nerve and meninges
Vortex veins
Posterior ciliary nerves and arteries
Conjunctiva and tenons at the limbus
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9
Q

What are the 3 main layers of cornea?

A

Epithelium (heals well)
Stroma (90% of thickness)
Endothelium (single layer of cells)

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10
Q

What is the main refractive surface of the eye?

A

Cornea (2/3 of eyes optical power, 1/3 from lens)

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11
Q

How much aqueous fluid is in the anterior chamber?

A

0.25 of a ml

Turned over every 90 minutes

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12
Q

How does the position of the iris relative to the cornea change with age?

A

With age the lens grows, causing the iris/lens surface of the anterior chamber to curve anteriorly towards the cornea (shallowing of anterior chamber)

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13
Q

What is the uveal coat of the eye?

A

Continuous layer of highly pigmented highly vascular tissue
Apposed directly to the inner surface of sclera
Makes up choroid, ciliary body and iris

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14
Q

What makes up the iris?

A

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

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15
Q

What are the layers of the lens? How thick is it?

A

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)

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16
Q

What are the 2 functions of the ciliary body?

A

Aqueous production and accommodation of the lens

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17
Q

What are the 2 parts of the ciliary body?

A

Pars plicata -anterior
Pars plana- posterior (intravitreal injections)
Made up of 2 layers of epithelium, stroma and muscle layer

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18
Q

What are the dimensions of the cornea?

A

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)

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19
Q

What is continuous with the corneal epithelium and endothelium?

A

Corneal epithelium is continuous with sclera epithelium

Corneal endothelium is continuous with trabecular meshwork

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20
Q

Describe corneal epithelium

A 
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
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21
Q

How does the corneal epithelium stay attached to basement membrane and itself?

A

Hemidesmosomes lock basal cells to basement membrane

Desmosomes, gap junctions and interdigitations between wing cells form a mechanical barrier

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22
Q

What is loss of corneal epithelium called?

A

An ulcer

BM stains fluorescein

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23
Q

How thick is Bowman’s layer and how is it different to the stroma?

A

10 microns
Denser
No fibroblasts no keratocytes
Provides strength

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24
Q

What makes up corneal stroma?

A

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

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25
What is the descemet membrane?
``` 10 microns thick Strong elastic membrane BM produced by endothelium Banded and non banded layers Gets thicker with age ```
26
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 ```
27
Function of corneal endothelium
Active transport Out of stroma into anterior chamber (aqueous) Maintains transparency of the stroma
28
Where does the corneal endothelium end?
Schwalbe's line aka at the limbus | Joins trabecular meshwork
29
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
30
What causes decreased corneal sensitivity?
Age Herpes infections LASIK Contact lens wearers
31
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
32
What is the limbus?
Landmark Circular zone between cornea and sclera 1.5mm wide
33
What are the two different definitions of the limbus?
Anatomical: Schwalbe's line Surgical: Bluish area, anterior to anatomical limbus
34
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
35
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
36
What happens to Bowman's membrane ta the limbus?
Ends | Its deep surface merges into substantia propria
37
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
38
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
39
Which chemicals make up the tear film?
Ions Antibodies (IgA, IgM, IgE) Antibacterials (lysozyme) Metabolites (glucose, pre-allbumin, lactate) Inflammatory mediators (prostaglandins, histamine)
40
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
41
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
42
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
43
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) ```
44
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
45
What happens to corneal endothelium after injury?
No mitosis Cells slide and grow to cover defect Decreases overall pump function Fewer cells on endothelium
46
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
47
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
48
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
49
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
50
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
51
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
52
What are the 5 layers of the cornea?
``` Corneal epithelium Bowman's layer Corneal stroma Descemet's membrane Corneal endothelium ```
53
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
54
How does the number of layers vary in the corneal epithelium?
5-6 layers centrally | 10 layers at the limbus
55
Describe the superficial epithelial cells of the cornea
Flat polygonal nucleated cells Apical microvilli and microplicae Covered by glycocalyx to prevent tear film evaporating`
56
Describe the intermediate wing cells of the corneal epithelium
Polyhedral | oval or round nuclei
57
How is the basal layer of the corneal epithelium attached to the basement membrane?
Hemidesmosomes | Anchoring fibrils type 7 collagen attach hemidesmosomes and stroma
58
Does the cornea have lymphatic drainage?
No
59
What is the autonomic innervation of the cornea?
Sympathetic fibres from superior cervical ganglion | Parasympathetic fibres from ciliary ganglion
60
How long does it take for corneal epithelium to regenerate?
7-10 days
61
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
62
Describe Bowman's layer
Smooth, non regenerative, accelular precipitate of the cornea 8-10 microns thick Composed of type I, II and V collagen
63
What is the function of the Bowman's layer?
No one knows Maybe acts as a corneal ligament May prevent spread of infection
64
What is the nutrition and blood supply of the Bowman's layer?
It is acellular so does not need nutrition or blood | Avascular
65
Which layer of the cornea is affected in keratoconus?
Bowman's layer (characterised by absence or disorganisation of this layer)
66
What are the roles of the cornea stroma?
Transmit and focus light | Mechanical support to cornea
67
What is the embryological origin of corneal stroma?
Mesenchymal cells of neural crest origin
68
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
69
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
70
Where do the keratocytes lie in corneal stroma?
Between lamella Sparsely distributed Transparent
71
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
72
How does the corneal stroma get nutrients?
Aqueous humour and circumferential capillaries
73
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
74
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)
75
What is the complicating factor of Descemet's membrane in relation to DMEK?
Descemet's membrane has a tendency to scroll up
76
Which is stronger, Descemets or Bowman's?
Descemet's is 10x stronger
77
Can Descemet's layer heal?
Yes, endothelium migrates to cover any defects | Descemet's layer increases in size over life
78
Is the corneal endothelium innervated?
No
79
When does the corneal endothelium form a monolayer?
18 weeks gestation | Nil further replication after this
80
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
81
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

OPTHT801 (20 decks)

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