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Ocular Anatomy > Lecture 4: Cornea > Flashcards

Lecture 4: Cornea Flashcards

1
Q

(corneal dimensions) Anterior horizontal diameter:

A

12 mm

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2
Q

(corneal dimensions) Anterior vertical diameter:

A

11 mm

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3
Q

(corneal dimensions) Posterior view:

A

cornea appears circular

H and V diameter: 11.7 mm

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4
Q

is the cornea flatter or steeper in the center?

A

steeper

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5
Q

is the cornea flatter or steeper near the periphery?

A

flatter

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6
Q

(Radius of curvature) Anterior central cornea:

A

7.8 mm

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7
Q

(Radius of curvature) Posterior central cornea:

A

6.5 mm

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8
Q

Central corneal thickness (CCT):

A

0.53 mm or 530 micron

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9
Q

Peripheral corneal thickness :

A

0.71 mm or 710 micron

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10
Q

why are corneal dimensions clinically relevant?

A

Astigmatism: Variation in corneal curvature in various meridians
contributes to astigmatism

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11
Q

Regular astigmatism:

A

steepest and flattest meridians lie

90 degree apart

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12
Q

Irregular astigmatism:

A

steepest and flattest meridian are

not 90 degrees apart (uncommon)

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13
Q

With the rule astigmatism:

A

the steepest curvature lies in V- meridian and the flattest curvature lies in H-meridian

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14
Q

Against the rule astigmatism:

A

the steepest curvature is in H- meridian (not common, ‘WTR to ATR’ in old age due to lens change)

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15
Q

what structure is the principal refracting surface of the eye?

A

cornea

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16
Q

why is the cornea transparent?

A
  1. Due to smooth and regular epithelium
  2. Regular arrangement of stromal fibrils and small
    fibril diameter
  3. Destructive interference of light minimizes light
    scattering (cornea scatter less than 1% of light)
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17
Q

is the cornea avascular or vascular?

A

avascular

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18
Q

the peripheral cornea is continuous with what?

A

The peripheral cornea is continuous with conjunctiva and sclera

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19
Q

Corneal haze:

A

the spacing of stromal fibrils is altered due to edema or due to activated fibroblast, myofibroblast cells (especially after PRK)

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20
Q

how many layers does the cornea have? what are they?

A
  1. Epithelium
  2. Bowman’s layer
  3. Stroma
  4. Descemet’s membrane
  5. Endothelium
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21
Q

how many cell layers thick is the epithelium layer of the cornea? microns?

A

5-7 cell layers thick, approx. 50 micron

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22
Q

the apical surface of the epithelium layer is the superficial layer. How many layers compose this layer and what is the tissue type?

A

2-3 layers of flattened,

squamous

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23
Q

In the epithelium layer of the cornea what layer comes after the apical layer? And how many layers compose that layer?

A

2-3 layers of “wing” cells

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24
Q

how are wing cells attached to each other?

A

Attach to each other by desmosomes and gap junctions.

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25
Q

what layer of cells in the epithelium are the only cells to undergo mitotic division?

A

basal layer cells

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26
Q

The basal surfaces of these cells hold to the underlying Bowman’s layer by what?

A

hemidesmosome*

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27
Q

what 3 layers are in the first corneal layer: epithelium?

A

apical, wing, and basal

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28
Q

how thick is bowman’s layer?

A

8-14 micron thick

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29
Q

is bowman’s layer a true membrane?

A
  • no it is transition layer to stroma – not a true membrane (passive)
  • It’s an acellular membrane
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30
Q

what corneal layer is rigid and provides shape to the cornea?

A

bowman’s layer

31
Q

T or F: bowman’s layer is resistant to damage by penetrating injury or infection

A

T

32
Q

is bowman’s layer regenerative or non-regenerative?

A

Non-regenerative: if injured it is replaced by epithelium

(PRK) or stromal scar tissue

33
Q

If the corneal injury goes into Bowman’s layer or deeper, it will heal with what?

A

a scar

34
Q

what is Keratoconus?

A

corneal dystrophy due to localized disruptions of Bowman’s layer, stroma thins and weakens and intraocular pressure causes cornea to bulge and take shape of a cone

35
Q

what corneal layer is the thickest?

A

Stroma: 500 micron thick or 90% of corneal thickness

36
Q

what is the stroma composed of?

A

Composed of collagen fibrils, keratocytes, extracellular matrix

37
Q

what maintains stroma by synthesizing collagen and extracellular matrix?

A

Keratocytes

38
Q

are WBC, Macrophage, lymphocytes, PMNs in the stroma normally active or dormant?

A

dormant - they may become active if the corneal injury is severe

39
Q

Fibril bundles form lamellae, which are regularly spaced, runs where to where?

A

limbus to limbus, parallel to the corneal surface.

40
Q

Glycosaminoglycans (GAGs):

A

carbohydrate molecules, hydrophilic, attract and bind water (cornea has 78% water)

41
Q

what is corneal edema?

A

Over-hydration of cornea results in corneal edema

loss of transparency = corneal haze

42
Q

how thick is Descemet’s membrane (4th layer)?

A

5-15 micron thick, in children 5 microns and thickens to 15 microns

43
Q

is Descemet’s membrane a true membrane with the ability to regenerate?

A

yes

44
Q

where does Descemet’s membrane terminate? and what does this form?

A

It terminates at the limbus and forms an important circular structure in angles called “Schwalbe’s line

45
Q

is the Descemet’s membrane firmly or weakly attached?

A

Weak attachments to the neighboring structures (stroma and endothelium)
• Can be detached easily

46
Q

how thick is the endothelium layer of the cornea?

A

5 micron thick

47
Q

what layer is considered the Innermost layer of the cornea?

A

endothelium

48
Q

what type of cells are covering the entire posterior cornea?

A

Hexagonal cells

49
Q

do endothelial cells divide?

A

Endothelial cells do not divide or

replicate, stretch to cover a defect.

50
Q

why does the endothelium layer receive nutrients from the aqueous humor?

A

The barrier is slightly leaky: allows
nutrient flow from the aqueous humor

e.g. glucose, amino acids as well as water

51
Q

how is the excess water removed in the endothelium?

A

Excess water needs to be moved- out by an active metabolic endothelial pump (Na-K-ATPase pump)

52
Q

what are the 3 Endothelial cells densities?

A
  • 1000-2000 cells/mm sq at 80 years
  • 3000-4000 cells/mm sq in children
  • 2500-3000 cells/mm sq in adult
53
Q

what is the minimum cell density per mm sq needed for function?

A

400-700 cells/mm sq minimum needed to function

54
Q

Pleomorphism:

A

change in cell shape

55
Q

Polymegathism:

A

change in cell size

56
Q

what are 3 clinical relevances regarding the endothelium?

A

Hassall-Henle bodies

Guttata

Contact lens effect

57
Q

Hassall-Henle bodies:

A

thickening of Descemet’s membrane (DM) bulging into the anterior chamber near the corneal periphery (common finding increases with age)

58
Q

Guttata:

A

thickening of DM in the central cornea is called guttata (indicative of endothelial dysfunction)

59
Q

Contact lens effect:

A

pleomorphism and polymegathism were documented after 6-years of GP lens or soft lens wear (cell density remain normal).

60
Q

The cornea is richly innervated by corneal nerves that send what type of signal/ impulse and where?

A

afferent impulses through the long ciliary nerves (CN V Ophthalmic division).

61
Q

within the cornea are nerves myelinated or unmyelinated?

A

unmyelinated

62
Q

the cornea acquires a myelin sheath where?

A

at the limbus

63
Q

does the corneal nerve innervate Descemet’s membrane and the endothelium

A

Descemet’s membrane and the endothelium are not innervated.

64
Q

what 3 things does the corneal nerve play an important role in?

A
  • Reflex tearing
  • Blink response
  • Maintaining healthy epithelium
65
Q

T/F: the corneal nerve is an important consideration for making LASIK flap

A

T

66
Q

is the corneal nerve damaged during PRK and LASIK

A

yes

67
Q

how long does it take corneal nerves to regenerate after refractive surgery?

A

3-9 months

68
Q

T/F: Diabetes patients may have reduced corneal nerves

A

T

69
Q

what are Palisades of Vogt?

A
  • Radial projections of fibrovascular tissue in spoke-like fashion around the corneal periphery
70
Q

what does the interpalisade region of Palisades of Vogt contain?

A

The interpalisade region contain thickened epithelium, limbal stem cells reside here

71
Q

(Palisades of Vogt) Limbal stem cell deficiency:

A

Loss of stem cells leading to corneal neovascularization and corneal opacity

72
Q

Pinguecula:

A

Slightly elevated mass of
modified conjunctival tissue

  • Usually at 3 and 9 o’clock
  • Change in the submucosal layer
  • Epithelium unchanged
73
Q

Pterygium:

A

Fibrovascular, triangular over-
growth of bulbar conjunctiva

  • onto cornea usually progressive
  • Induces astigmatism and vision
    change (drop-in UCVA, BCVA)
  • As it progresses onto the cornea, It damages the corneal epithelium, as well as bowman’s layer, is compromised

Ocular Anatomy (15 decks)

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