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Sem1revcards > Cornea 1 - Week 2 > Flashcards

Cornea 1 - Week 2 Flashcards

1
Q

What are the 3 basic ways that the cornea interacts with light and the environment?

A
  1. Transparency - optimum for vision
  2. Refraction - good quality focused image (with lens)
  3. Strength - barrier against env. protect from injury
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2
Q

Name the two mechanisms for maintaining corneal shape?

A
  1. Corneal thickness

2. Corneal hydration

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3
Q

List the 5 layers of the cornea in order.

A
  1. Epithelium
  2. Bowman’s layer
  3. Stroma
  4. Descement’s membrane
  5. Endothelium - most posterior border of cornea
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4
Q

How may bumps on the tear film affect vision?

A

any bumps on tear film can interrupt ability of light to bend properly – lead to blurry vision

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5
Q

Name 3 factors that affect corneal refraction?

A
  • stable pre-ocular tear film

- corneal shape: THICCNESS and HYDRATION

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6
Q

What effect does ageing have on transmittance?

A

Decreases with age

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7
Q

At what wavelength does the transmittance of light increase rapidly?

A

300nm

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8
Q

Define transparency (of cornea)

A

How much light can get through without scatter

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9
Q

Are corneal nerves myelinated

A

Not quite. They are umyelinated from their entrance at the limbus (within the avascular region)

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10
Q

Does the cornea have blood vessels?

A

NO. (however it can if cornea is diseased, in which case you’d need a transplant)

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11
Q

Is the epithelium of the cornea pigmented?

A

NO it is not

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12
Q

Does the cornea have cells?

A

Very little. There are very few cells here (those being keratocytes and macrophages)

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13
Q

List the 5 anatomical factors contributing to corneal transparency

A
  • 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
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14
Q

What is the key factor that controls corneal transparency?

A

Precise organisation and separation of stromal collagen fibres in the ECM

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15
Q

How is precise fibre separation in the corneal ECM maintained?

A

A highly specialised arrangement of inter-fibrillar matrix is used to maintain it

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16
Q

Do collagen fibril diameters vary in the cornea?

A

No, there is very little variation

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17
Q

List the 3 ECM factors that affect transparency

A
  • Precise organisation + separation of stromal collagen fibres
  • specialised arrangement of inter-fibrillar matrix (maintain fibre separation_
  • Minimal fibril diameter variation
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18
Q

What is the role of the corneal epithelium?

A

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

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19
Q

What are the 3 components involved in corneal epithelium barrier function

A
  1. Tight junctions (zonula occludens) - acts as a (FLEX) SEAL to prevent leakage between cells
  2. Desmosomes - (along with tight junctions), join the epithelial cells together to prevent leakage
  3. Gap junctions - allow communication b/w cells
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20
Q

Does mitosis occur in the corneal epithelium? If so, where?

A

Yes. Mainly in the Basal layer. (small amount also in germinal layer)

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21
Q

What happens to epithelial cells as they age?

A

They lose their attachments and slough/fall off the epithelium

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22
Q

How many cells THICC is the corneal epithelium?

A

5-6 cells thick

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23
Q

What is the corneal epithelium continuous with? and where?

A

With the conjunctival epithelium at the limbus

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24
Q

Describe the cell types of the corneal epithelium

A

Superficial cells = ‘Flattened non-keratinised squamous cells
Deepest (basal) cells = columnar
Between superficial and basal = WING CELLS

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25
Q

When epithelial cells fall off the epithelium, what happens next?

A

Underlying cells move to the surface to take their place

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26
Q

How long is the turnover time of the entire cornea (in terms of cells)?

A

Around 7 days

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27
Q

What shape do the corneal flattened, non-keratinised, squamous cell nuclei have?

A

They are horizontal

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28
Q

Is the cornea keratinised or non-keratinised? Why?

A

non-keratinised. This is because keratin is bad at letting light through

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29
Q

What force are corneal epithelial cells always exposed to?

A

Frictional force

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30
Q

How does the epithelium stay in place in response to frictional force?

A

It is anchored to a basement membrane and bowmans layer

31
Q

Describe how epithelium anchoring occurs?

A
  • hemidesmosoms anchor to lamina densa

- anchoring fibrils attach lamina densa to bowmans layer at anchoring plaques

32
Q

What does the epithelium secrete?

A

mucins, which contribute to the mucin layer of the tear film

33
Q

Describe the surface of the corneal epithelium? Why is it like this?

A

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)

34
Q

How much does the stroma contribute to the thickness of the cornea?

A

90% of corneal thickness

35
Q

What types of cells does the corneal stroma contain?

A

Fibroblasts (called keratocytes since they are in the cornea)

36
Q

What is the connective tissue of the stroma like?

A

Fibrous, dense, regular

37
Q

As part of the stromal connective tissue, what takes up the most amount of dry weight proportion?
A) collagen
B) keratocytes
C) proteoglycans

A

Collagen

38
Q

How are stromal collegen fibrils arranged?

A
  • they are evenly spaced and run parallel to each other in bundles
39
Q

What are collagen bundles with the cornea called?

A

Lamellae

40
Q

Are the collagen fibrils uniform in diameter?

A

Yes

41
Q

How many lamellae are in the stroma?

A

200-300

42
Q

How thicc is the bowman’s layer?

A

8-14um (very thin)

43
Q

How does bowman’s layer appear histologically?

A

faint pink line beneath epithelium

44
Q

Describe the fibril composition of the bowman’s layer

A
  • dense fibrous sheet of interwoven fibrils

- fibrils have small diameters and not obviously ordered

45
Q

Is Bowman’s layer cellular or acellular?

A

Acellular

46
Q

What is the main type of collagen found within the corneal stroma? Name two more types found here

A

Mainly type 1. Also found are III, V, VI

47
Q

How does stromal collagen fibril size relate to the wavelength of light?

A

collagen fibril diameter and separation are small relative to the wavelength of light (prevents scatter)

48
Q

What is the fibril diameter of stromal collagen? Does it vary?

A

30nm. It does NOT vary. It is very consitant

49
Q

How does the diameter of fibres change as you approach the sclera?

A

diameter starts to fluctuate

50
Q

What proportion of the corneal stroma hydrated volume is keratocytes?

A

Around 2.5%

51
Q

Are keratocytes evenly distributed throughout the stroma?

A

No. There is linear reduction going anterior to posterior

52
Q

What is the function of keratocytes?

A

Active control of stromal remodelling

53
Q

What happens to fibroblasts/keratocytes if trauma occurs?

A

They turn into myofibroblasts

54
Q

Name 2 factors contributing to corneal swelling

A
  • GAGs attracting water with their negative charge

- intraocular pressure

55
Q

Name 3 factors contributing against corneal swelling

A
  • epithelial and endothelial barriers
  • endothelial pumps
  • evaporation
56
Q

What 4 things can corneal swelling/oedema result in?

A
  1. Increased light scatter, reduced light transmission, degraded image quality
  2. Further decompensation of cellular barriers
  3. pain
  4. complete corneal opacification - graft required
57
Q

Describe Donnan Swelling

A

Major mechanism of water uptake in a charged gel is due to ionic imbalances.
- major cause of water uptake into corneal stroma

58
Q

Describe volume exclusion, comparing it to donnan

A

Small holes within a matrix allow passage of water (but not anything bigger). Is slow.

Donnan is 600 times faster (and is major mechanism)

59
Q

Describe Descemet’s membrane in terms of endothelium

A

It is the basement membrane of the endothelium

60
Q

What produces Descemet’s membrane? How does age affect it?

A

Produced constantly by the endothelium and therefore thickens with age

61
Q

Describe the layer thickness of descemet’s membrane

A

1/3rd anterior

2/3rd posterior

62
Q

What collagen type forms Descemet’s membrane?

A

Types IV and VIII (4 and 8)

63
Q

Describe Descemet’s membrane appearance (anterior vs posterior)

A 
Anterior = more granular
posterior = homogenous
64
Q

Do corneal endothelial cells have gap junctions?

A

yes

65
Q

How does corneal endothelium contribute to corneal hydration? Is it a permeable membrane?

A

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

66
Q

Do endothelial cells divide?

A

No. They are lost with age, disease, surgery, CL wear

67
Q

What happens to the corneal epithelium as you approach the limbus?

A

Gets thicker

68
Q

What is the limbus?

A

Point where the cornea meets the sclera

69
Q

What happens to lamellar organisation at the limbus?

A

Becomes irregular

70
Q

What are Palisades of Vogt? What is located there?

A

Radial projections of limbal epithelium and stroma, and are spoke like
- stem cells are here

71
Q

What nerve innervates the cornea, and how many branches per cornea?

A

70-80 branches of long ciliary nerve

72
Q

How are the nerves that innervate the cornea oriented?

A

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)

73
Q

Where are the majority of corneal nerves found?

A

In anterior stroma

Sem1revcards (19 decks)

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