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0V015 Biology & Pathology > L43 The Outer Coat: Cornea & Sclera > Flashcards

L43 The Outer Coat: Cornea & Sclera Flashcards

1
Q

What is the function of the outer coat and why does it need to be tough?

A

It needs to be tough to resist Intra-ocular pressure (IOP) and to protect delicate internal ocular tissues.

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2
Q

What does the outer coat consist of

A

The cornea (which makes up 7% of the outer coat) and the sclera (which makes up 93% of the outer coat).

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3
Q

What is a keratometer?

A

A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea

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4
Q

What is the average radius of the cornea?

A

7.8 mm (remember though cornea is not perfectly circular)

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5
Q

Define Foramina (for-a-min-a)

A

Plural of foramen which means a small opening or hole.

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6
Q

Describe the Sclera- what is it? What is the thickest part? What is the thinest part? What does it contain a number of formaina for?

A

Largest part of the fibrous coat of the globe. Thickest posteriorly (1mm) (so at the back of the eye) Thinnest under tendons of rectus muscles (0.3mm) Contains a number of foramina for : -Optic Nerve -Short posterior ciliary arteries and nerves -Long Ciliary nerves -Vortex Veins -Anterior ciliary arteries.

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7
Q

Into what three layers is the sclera resolved via a microscope and what is each layer comprised of?

A

Episclera - upper most layer - where blood vessels are located- this layer is made up of loose connective tissue. Scleral Stroma - makes up bulk of the sclera-made up of relatively regular scleral collagen Lamina fusca - inner (towards choroid) more looser connective tissue which is pigmented.

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8
Q

What is inflammation of the episclera called?

A

Episcleritis

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9
Q

What does the episclera do and where does it lie?

A

The episclera (which is the upper loose connective tissue layer of the sclera) connects the sclera to the conjunctiva. It lies under the tenon’s capsule.

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10
Q

Is the Episclera highly vascular?

A

Yes

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11
Q

What is the Tenon’s capsule and what is it also known as?

A

Also known as the Fascia Bulbi. The Tenon’s capsule is a thin fibrous sheath that envelops the globe from the limbus to the margin of the optic nerve.

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12
Q

What are the two major roles of the cornea?

A

Protection major refractive surface (accounts for 2/3 rds of the refractive power of the eye)

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13
Q

What are the cornea dimensions? When is it thickest? Thinnest? Anterior Central radius of curvature? Posterior -central radius of curvature?

A

Thickest in peripheral - 0.52mm Thinnest in central - 0.52 Anterior central - 7.8 mm Posterior cnetral - 6.5mm

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14
Q

What are the 5 distinct layers of the cornea that can be seen under a microscope?

A

Epithelium Bowman’s membrane Stroma Descemets membrane Endothelium

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15
Q

Corneal Epithelium:

How much of the thickness of the cornea as a whole does it take up?

How many microns thick?

How many cell layers?

Histologically how would it be described?

A

Roughly 10%

50 microns thick.

5-7 cell layers

Stratified squamous, non-keratinized (obviously because keratin is opaque)

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16
Q

What are the three cell types (arranged in each layer) of the corneal epithelium?

A

Squamous apical layer (hence it histologically being referred to as stratified (bc there are different types of cells present) squamous)

middle layer - 2-3 layers of Wing cells

Bottom layer - Single layer of Basal cells

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17
Q

Which ‘non-native’ cells or non-epithelial cells are found in the corneal epithelium?

A

Lymphocytes and Langerhans (antigen-presenting cells) cells

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18
Q

What may be found on the squamous cells of the corneal epithelium?

A

Extensive surface projections (microvilli and microplicae).

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19
Q

What’s the difference between microvilli and microplicae?

A

The difference is microvilli are finger-like projections whereas microplicae are ridge-like projections.

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20
Q

Under very high magnification of the squamous layer of cells of the corneal epithelium what may be seen?

A

Tight junctions between superficial epithelium cells which form an effective permeability membrane.

Glycocalyx (mucus-like transmembrane proteins which emanate from the microvilli) provide a hydrophilic surface (allowing water to spread across the surface well, therefore) anchoring the tear film.

21
Q

Why is a wing cell called a wing cell?

A

It is polygonal with inferior (relating to position in space) indentations giving it that ‘wing-like’ appearance.

[shown in the image by second arrow/black triangle]

22
Q

How are wing cells linked together and why?

A

Adjacent cells are linked by excessive interdigitations (think finger joint in DT) and abundant desmosomes.

It is adapted to resist damage from shear forces.

23
Q

Comment on the electron microscopic structure of a wing cell and why that is the case.

A

There are very sparse organelles - this is to prevent light scatter.

There are numerous intermediate filaments in the cytoplasm.

24
Q

Describe the structure of the basal cell layer of the corneal epithelium.

A

It is a single layer of cells.

Structurally similar to wing cells.

25
Q

What is special about the basal layer of the corneal epithelium compared to the squamous and wing cell layer?

A

It is the only layer that undergoes mitosis thus the basal layer is the only layer that consists of mitotic cells.

26
Q

Under very high magnification of the basal layer of cells of the corneal epithelium what may be seen?

A

Adjacent cells linked together by desmosomes.

Attached to the underlying stroma via a complicated anchoring system

Nerve terminals can be seen between basal cels.

27
Q

Why does there need to be a strong anchoring system between the epithelium and stroma?

A

Because then you would end up lifting off the epithelium leaving behind the stroma.

28
Q

What is the Bowmans membrane?

How thick is it?

What is it composed of?

What is its function?

A

It is the anterior limiting membrane of the cornea

8-14 microns thick

A Randomly orientated array of fine collagen fibrils.

The function is unclear - thought that it is not critical to the integrity of the cornea.

29
Q

Stroma :

How much of the total thickness of the cornea does it make up?

How many microns thick is it?

What is its composition?

What is its arrangement?

What is it synthesized by?

A

Makes up 90% of the cornea

500 microns thick

78% water, 1% salts, 21% biological macromolecules

Composed of collagen fibrils embedded in a matrix of proteoglycans.

Synthesized by keratocytes

30
Q

What is a property of proteoglycans?

A

They are very hydrophilic - attract water.

31
Q

What are the similarities and differences between a fibroblast and keratocyte?

A

Similarity - they both synthesize the same thing; collagen.

Difference - due to location in the cornea, fibroblasts are called keratocytes here.

32
Q

Under a very high magnification of the stroma of the cornea what may be seen?

A

Very regular arrangement of collagen fibrils arranged in the lamellae.

collagen types I and V predominantly (fibril type) with lesser amounts of type VI and type XII (non fibril)

Electron microscopy shows that collagen fibrils are small (25-35nm) and arranged very regularly to allow for transparency.

33
Q

Describe collagen fibril orientation of the stroma.

A

Collagen fibrils have a preferred orientation corresponding to the medial-lateral and superior-inferior axes of the cornea (Basically a horizontal and vertical orientation).

This preferred arrangement tends to occur more posteriorly than anteriorly and increases towards the limbus.

34
Q

What does limbal collagen take the form of?

A

Limbal collagen takes the form of annulus- (a ring-shaped object, structure, or region).

35
Q

What are the reasons for the difference in opacity of the cornea and sclera?

A

In the cornea, collagen fibrils are arranged regularly in a uniform structure and very small - allowing transparency.

In the sclera, this is not the case opacity comes from the irregular arrangement of collagen fibrils and the irregular size (high variation of size).

36
Q

What is the Descemets membrane?

How thick is it as birth?

How thick is it in an adult?

A

Posterior limiting membrane

It is the basement membrane of the corneal endothelium.

3-4 microns thick at birth

10-12 microns thick as an adult

37
Q

Which part of the Descemet’s membrane is present from birth and which part is developed post-natally?

A
  • The anterior 1/3 of Descemets membrane represents that part produced in foetal life
  • The posterior 2/3 is formed postnatally.
38
Q

What is an important difference between the Bowmans membrane and Descemets membrane?

A

Descemets membrane is a BASEMENT membrane whereas Bowmans membrane is not.

(Important to note that Descemets membrane is unusual as it is very thick)

39
Q

What is a basement membrane?

A

a thin, delicate membrane of protein fibres and mucopolysaccharides separating an epithelium from underlying tissue.

40
Q

Describe the corneal ENDOTHELIUM under an electron microscopic view.

How are adjacent cells linked?

A

There is a monolayer (single layer) of squamous cells.

The lateral borders of endothelial cells are markedly convoluted

Large numbers of mitochondria reflect their high metabolic demand.

Adjacent cells are linked by tight junctions (barrier function)

41
Q

What can a specular microscope be used for?

A

A specular microscope enables the viewer to study the morphology of the cells to look for any abnormalities indicating trauma.

42
Q

How is the corneal endothelium affected by age?

A

There is a limited capacity for mitosis to replace damaged cells.

Thus there is a progressive reduction in endothelial cell number by age.

Variation in cell size (polymegathism) and shape (pleomorphism) occurs with age, trauma, and contact lens wear.

43
Q

What is polymegathism?

A

Variation in (cell) size

44
Q

What is pleomorphism?

A

Variation in (Cell) shape

45
Q

What type of innervation does the cornea receive?

A

Predominantly sensory (trigeminal innervation).

46
Q

True or false - the cornea is the most richly innervated surface tissue in the body.

A

It’s True!

47
Q

Corneal innervation causes what?

A

Reflex in the form of blink

and Trophic function ( to protect and support the corneal function)

(Trophic function refers to maintenance or regulation of a bodily organ or function).

48
Q
A

0V015 Biology & Pathology (57 decks)

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