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Sensory Neuroscience 316 > Molecular physiology of transparency - The Cornea. Lecture 3 > Flashcards

Molecular physiology of transparency - The Cornea. Lecture 3 Flashcards

1
Q

What is the function of the cornea?

A

1) Major physical barrier

2) Major refractive (optical) element (40diop)

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2
Q

What are the layers of the cornea?

A 
1) Epithelium (Ant)
Bowmans layer
2) Stroma (90%)
Descements layer
3) Endothelium (Post)
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3
Q

What covers the cornea and what is its role?

A

A tear film covers the cornea forming a smooth optical element. However the epithelium secretes a mucin layer to prevent tear (water) lipid interaction.

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4
Q

Describe the cellular structure of the epithelium?

A

Stratified, non-keratanized, squamous epithelium 5-7 layers thick.

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5
Q

What are the cellular layers within the epithelium?

A

1) Superficial
2) Wing cell
3) Basal

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6
Q

What allows the epithelium to act as a physical barrier>

A

The high expression of high resistance tight junctions.

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7
Q

Describe the Basal cellular layer;

A

Single layer of cuboidal cells.
Capable of dividing into wing cells.
Sits on the bowman layer.

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8
Q

Describe the Wing cellular layer

A

1-3 cells thick
Intermediate stage of differentiation.
Gives rise to superficial cells

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9
Q

Describe the superficial cellular layer.

A

Terminally differentiated squamous cells that are sloughed off into the tears.

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10
Q

Whats the function of the epithelium?

A

Act as a physical barrier (high resistance tight junctions)

To secrete fluids into the tear film to maintain steady state stroma volume.

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11
Q

What does damage to the corneal epithelium result in?

A

Rapid wound healing (as cell turnover is every 7 days and damage accelerates this process)

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12
Q

How is the corneal epithelium maintained?

A
  • The corneal epithelium maintains a constant thickness.
  • Epithelium turned over every 7 days.
  • XYZ hypothesis for cell turnover
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13
Q

Whats the XYZ hypothesis?

A 
X = Proliferation basal cells
Y = Migration of stem cells
Z = Cells lost from the surface.
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14
Q

What percentage of the cornea is the stroma?

A

90%

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15
Q

What cells are found in the stroma?

A
  • Keratocytes (fibroblasts of the eye)

- ECM (PG’s, water, Collagen fibrils)

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16
Q

What collagen types are found in the stroma?

A
  • Collagen fibrils are made up of type 1

- lesser of type 5 and 6

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17
Q

What is the function of type 5 collagen?

A

Required to initiate the assembly of collagen type one into fibrils.

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18
Q

Describe the histological appearance of keratocytes:

A

Long, thing, stellate cells which run PARALLEL to the corneal surface.

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19
Q

Whats the function of keratocytes?

A

They secrete collagen and its associated ECM.

20
Q

What structures does collagen form in the cornea?

A

Collagen fibrils form 200-250 lamellae bundles.

21
Q

What are the properties of collagen?

A

Add to corneal strength, elasticity and shape.

22
Q

What is the arrangement of collagen fibrils/ lamellae?

A

Uniform spacing <200nm to create transparency.

23
Q

How are PG’s arranged in the stroma?

A

PG’s form ladder like attachments between collagen fibrils and along them.

24
Q

What determines the arrangement of collagen fibrils?

A

Their size and charge

25
Q

What modulates the diameter of collagen fibril?

A

Modulated and maintained by the PG core protein.

26
Q

What are the core proteins of PG’s?

A

Decorin
Lumican
Keratocan
Biglycan

27
Q

What are the types of gag side chains in the retina?

A

1) Chondroitin + Dermatan sulphate
2) Keratin Sulphate
3) Herapan Sulphate

28
Q

What determines the spacing between collagen fibrils?

A

The hydration properties of the PG sulphate side changes determines the fibril spacing.

i.e the side chains charges draw in external ions, which in turn cause osmosis.

29
Q

Where does water flow into the cornea from?

A

The tear film and the aqueous humor.

30
Q

What problem does the hydration properties of PG side chains present?

A

The flow of water into the stroma can disrupt the charges of the PG side chains, therefore this process must be managed.

31
Q

Whats the function of the stroma?

A

To be transparent and be a optical element.

32
Q

How much light does the stroma scatter?

A

Less than 10%

33
Q

How does the stroma scatter less than 10% of light?

A

The fibril/lamellae lattice structure acts as a diffraction grating, allowing light scattering fibrils to be cancelled by destructive interference (as the lamellae are arrange at 1/2 the wavelength of light)

34
Q

What specific PG’s control collagen fibril diameter?

A

Decorin and lumican

35
Q

What happens if osmosis of the stroma is not controlled?

A

The stroma would swell interfering with lamellae spacing therefore light would be scattered. Decreasing transparency.

36
Q

How is excess osmosis in the stroma countered?

A

Active transport of ions by the epithelium and endothelium

37
Q

Describe the structure of the endothelium:

A

Single layer of Hexagonal cuboidal cells attached to the desemonts membrane

38
Q

Whats the function of the endothelium?

A

Maintenance of stroma hydration

39
Q

Whats unique about the corneal endothelium?

A

High density of mitochondria to pump ions.

40
Q

What removes majority of fluid from the stroma?

A

The endothelium removes the majority of fluid from the stroma.

41
Q

How does the endothelium balance fluid into and from the stroma?

A

Active outward directed transport of HCO3- and Na+ drive fluid secretion that balances passive ion and water leakage into the stroma.

42
Q

Where does the HCO3 come from?

A

CO2 water interaction in the stroma. CO2 from aerobic respiration.

43
Q

What does the endothelium require to maintain stroma hydration?

A

O2 and energy. (to perform aerobic respiration)

44
Q

Where does the cornea obtain nutrients from?

A

Aqueous humour - Glucose, AA, Vitamins
Tears - Glucose, O2

These are carried into the stroma by the the passive diffusion of water.

45
Q

What happens to the ability of the cornea to obtain O2 when eyes are open, closed or have contacts on.

A

Eyes open. Large diffusion gradient for O2. Lesser for eyes shut and significantly impaired by contacts. Tennis profiles reduced.

O2 obtained from the tear layer.

Sensory Neuroscience 316 (32 decks)

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