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PART ONE REVISION. > ANATOMY - CORNEA > Flashcards

ANATOMY - CORNEA Flashcards

1
Q

How thick is the corneal epithelium in terms of cell layers?

A

six layers thick

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2
Q

The anterior surface of the cornea is ….. shaped whereas posterior surface of the cornea is ….. shaped

A
  1. eliptical
  2. spherical
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3
Q

What is the horizontal diameter of cornea?
What is the vertical diameter of the cornea?
What is the average diameter of cornea?

A

11.7mm
11mm
11.5mm

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4
Q

What is the radius of the curvature of cornea?
What is the radius of curvature of the sclera?

A
  1. cornea – 7.8mm
  2. sclera – 12mm.
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5
Q

What is the average central corneal thickness?
What is the average peripheral corneal thickness?

A

520 micrometers
670 micrometers

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6
Q

What is the refractive power of the cornea?
What is the refractive power of the lens?

A

+45 D
+15 D

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7
Q

What is the refractive index for these structures?
1. Air
2. Cornea
3. Anterior Chamber
4. Lens
5. Vitreous

A

Air - 1
Cornea - 1.37
AC + vitreous -1.33
Lens - 1.42

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8
Q

Where are the corneal layers derived from?
(Embryologically)

A
  • epithelium - surface ectoderm
  • stroma, descemet’s, endothelium - mesoderm
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9
Q

What type of epithelium is the cornea?

A

Stratified (5-6 layers)
Squamous (superficial cells flattened)
Non keratinised (nucleated, no keratin)

squamous superficially, columnar in deeper layers

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10
Q

How long does it take for entire cornea epithelium to be replaced?

A

7 days

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11
Q

What is the function of the microvilli in the cornea?

A

Are present with glycocalyx and help in adhesion and stability of the tear film to prevent tear film evaporation and subsequent dry eye.

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12
Q

Where are the corneal epithelium stem cells found?

A

In the limbal basal epithelium at the palisades of Vogt

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13
Q

What is the thickness of Bowman’s membrane?

What is the thickness of Descemet’s membrane?

A

Bowman’s: 8-16 micrometers

Descemets: 8-12um

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14
Q

What is Bowman’s membrane composed of?

A

Collagen type 7: randomly arranged collagen fibres generally resistant to trauma - cannot be regenerated

Hemidesmosomes to link cells to extracellular matrix

Desmosomes to link cells together with tight junctions to prevent penetration of tear electrolytes

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15
Q

What are the main components of the corneal stroma (3)

A

1.Uniform Collagen fibrils (arranged in flat bundles called laminae - 200-300 laminae present)

  1. keratocytes connected by gap junctions which secrete collagen - arranged in corkscrew pattern to produce GAGs
  2. GAGs - keratan sulphate posteriorly (most common), chondroitin sulphate and dermatan sulphate anteriorly.
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16
Q

What is responsible for the transparency of the cornea?

A

The highly uniform collagen fibres in the stroma (25-35micrometers) and distance between two corneal fibres is uniform (41.5nm)

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17
Q

What are the reasons for corneal transparency? (5)

A

1.Optically smooth tear film
2.Role of corneal epithelium
3.Arrangement of stromal fibres
4.Avascularity of the cornea
5.Absence of myelination in corneal nerves

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18
Q

What is descemets membrane composed of?

A

Type IV collagen arranged in hexagonal pattern
Laminin
Fibronectin
Heparan sulfate

There are no TIGHT junctions.

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19
Q

Descemet’s membrane is ….. attached to the stroma and is …… to enzyme degradation by phagocytes and toxins

A

weakly

resistant

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20
Q

What is the main shape of the corneal endothelium?
How are they connected together? (3)

A

hexagonal forming continuous mosaic pattern

Various junctional complexes
(1) zonula occludens (tight junctions)
(2) macula occludens
(3) macula adherens

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21
Q

What do the corneal endothelial pumps do?

A

Sets osmotic gradient that causes fluid to move from stroma to the aqueous to maintain 70% corneal hydration to keep cornea transparent

22
Q

What are the 3 different endothelial pumps?

A

Na/K –> 3 Na exit to AH, 2 K+ enter
Na/H –> 1 Na enters, 1 H+ exits
Na/HCO3 –> Na exits to AH, bicarb exits

23
Q

What endothelial cell density leads to corneal decompensation?

A

800 cells / mm2

24
Q

What is the blood supply of the cornea?

A

No blood supply

Anterior ciliary artery (ophthalmic artery) forms arcade in limbal region

25
Q

Is the corneal epithelium hydrophilic or lipophilic?
Is the corneal stroma hydrophilic or lipophilic?

A

epithelium - lipophilic
stroma - hydrophilic

26
Q

Corneal endothelium is held to its basal lamina (descemets membrane) with what strucuture?

Each endothelial cell is held together by what structure?

A

Endothelium to basal lamina - hemidesmosomes
Endothelium to endothelium -desmosomes.

27
Q

What is the structure of Bowman’s layer?

A

structure posterior to the corneal epithelium basement membrane - it is the condensation of stroma made up of acellular type 1 collagen

28
Q

How are the lamellae in the stroma organised?

A

At right angles to one another.

29
Q

What are the main age-related changes in the cornea? (4)

A
  1. Decreased translucency
  2. Increased thickness of Bowman’s membrane
  3. Increased thickness of Descemet’s membrane
  4. Reduced endothelial density
30
Q

What are the layers of Descemet’s membrane?

A
  1. Anterior fibrillar layer (present at birth)
  2. Posterior layer - deposits after birth by endothelium
31
Q

What is the peripheral termination of Descemet’s membrane?

A

Schwalbe’s line - anterior limit of the trabecular meshwork and anatomical limbus

32
Q

Which MMP is found in uninjured cornea, which MMP is found after injury?

A

MMP-2 - normal cornea

MMP-1 produced by stroma
MMP-3 produced by stroma
MMP-9 produced by epithelium - involved in corneal remodelling

33
Q

What happens to the keratan sulphate dermatan sulphate ratio in corneal scars?

A

Decreased keratan sulphate leading to irregularly arranged fibrils.

34
Q

Where is the main concentration of dendritic cells in the cornea? Periphearlly or centrally?

A

Higher in periphery than centrally

35
Q

What are the host risk factors for corneal graft rejection?

What are the mechanical risk factors for corneal graft rejection?

A

Host: young age, pre-op inflammation, vascularisation, previous ocular surgery

Mechancal: Large graft, eccentric graft, loose suture, suture infiltrate, suture vascularisation, suture removal

36
Q

Which layer does copper deposit in Wilson’s disease?

A

Descemet’s membrane

37
Q

What are integrins? What is their role in the eye

A

Transmembrane glycoproteins present in almost all cells

  1. Promote cell-ECM and cell-cell attachments
  2. Convey biochemical signals

Disruption of normal integrin-ECM interactions prevents normal eye development

38
Q

What is the function of laminin in the cornea?

A

Located in the basal lamina and are re-synthesised within 48 hours of corneal trauma under migrating cells

39
Q

What are the main functions of fibronectin? Where are they found in the eye?

A

Found on the stromal side of Descemet’s membrane and are stimulated by EGF and TGF.

They deposit on bare stromal surface within moments of epithelial injury to act as temporary scaffold

They promote adhesion between cells via integrins

40
Q

What are the components of the extracellular matrix? (4)

A
  1. Collagen
  2. Laminin
  3. Fibronectin
  4. Integrins
41
Q

What is the role of platelet derived growth factor?

A

Limbal stem cell proliferation

42
Q

What do limbal stem cells differentiate into?

A

Corneal epithelial cells

43
Q

What is the role of epidermal growth factor? What is the role of fibroblast growth factor?

A

Epidermal: stimulates control of corneal epithelial migration and proliferation

Fibroblast: Epithelial and fibroblast proliferation

44
Q

What are the effects of contact lens wear on the cornea? (4)

A
  1. Thickened corneal stroma
  2. Rise in lactate concentrations
  3. Disturbance to tear mucin layer
  4. Oxygen transfer from tear film to epithelium is reduced
45
Q

What is the thickness of each corneal layer?
1. Epithelium
2. Bowman’s
3. Stroma
4. Descemet’s
5. Endothelium

A

Epithelium: 50μm
Bowman’s membrane: 10μm
Stroma: 400μm
Descemet’s membrane: 10μm
Endothelium: 15μm

46
Q

What are the contiunation structures of the layers of the cornea?
1. Epithelium
2. Bowman’s
3. Stroma
4. Descemet’s
5. Endothelium

A

Epithelium - bulbar conjunctival epithelium
Bowman’s - lamina propia of conjunctiva and Tenon’s
Stroma - sclera
Descemets - Schwalbe’s line towards trabecular meshwork
Endothelium - endothelial trabecular meshwork and then to anterior surface of iris.

47
Q

What is the innervation of the cornea? What is its route?
When does the nerve lose myelin sheaths?

A

Long ciliary nerve from V1 - enters sclera from perichoroidal space and divides posterior to limbus to form the annular plexus and branches forward to enter anterior stroma.

Lose myelin sheaths after entering the stroma to preserve corneal clarity.

48
Q

Where are the langerhan cells located in the cornea? What is the precursor of langerhan cells?

A

Located in peripheral cornea and limbus.

Derivative of bone marrow macrophage precursors

49
Q

What is the pathway of the corneal reflex?

A

Corneal touch –> Long ciliary nerve –> Nasociliary nerve –> Ophthalmic nerve –>Trigeminal nerve –> Spinal nucleus of trigeminal nerve –> Facial nucleus –> Facial nerve –> Contraction of orbicularis oculi.

50
Q

What at the two layers of the basal lamina of the corneal epithelium?

A
  1. Lamina lucida (superficial)
  2. Lamina densa (deep)

PART ONE REVISION. (78 decks)

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