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Ocular Anatomy > Cornea and Sclera > Flashcards

Cornea and Sclera Flashcards

1
Q

if the cornea is steeper in the vertical meridian, it is:

A

WTR

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2
Q

if the cornea is steeper in the horizontal meridian, it is:

A

ATR

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3
Q

what is the average corneal thickness in the center and the periphery?

A

center: 550 microns
periphery: 670 microns up to 710 microns

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4
Q

what is the refractive index of the cornea

A

1.376

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5
Q

what is the central radius of curvature of the cornea, anterior and posterior surfaces?

A

anterior surface: 7.8mm curvature

posterior surface: 6.5mm curavture

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6
Q

prolate

A

steeper in center and flatter in periphery, normal cornea

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7
Q

oblate

A

flatter in center and steeper in periphery, ex. post-LASIK, ortho-K corneas

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8
Q

average anterior horizontal and vertical diameters of the cornea

A

anterior horizontal diameter: 11.7mm

anterior vertical diameter: 10.6mm

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9
Q

average posterior horizontal and vertical diameter of the cornea

A

posterior both horizontal and vertical are 11.7mm (spherical from behind)

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10
Q

what are the corneal layers?

A

epithelium (surface, wing, basal cells), basement membrane, Bowman’s layer, stroma, Descemet’s membrane, endothelium

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11
Q

what is the corneal epithelium thickness?

A

52 microns, 5 to 6 cell layers

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12
Q

what is Bowman’s layer thickness?

A

8-14 microns, acellular, random type 1 and type 5 collagen fibrils

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13
Q

what is the thickness of corneal stroma?

A

450 microns

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14
Q

Descemet’s membrane

A

5-15 microns, the basement membrane of the endothelium, composed of type 4 collagen, very resistant to trauma and damage, becomes Schwalbe’s line at the limbus

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15
Q

what is the thickness of the corneal endothelium?

A

5 microns

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16
Q

the 3 factors behind RCEs

A
  1. poor hemidesmosome attachments
  2. epithelial basement membrane dystrophy
  3. age-related thickening of the basement membrane
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17
Q

Reis-Buckler’s dystrophy

A

rare corneal epithelial dystrophy that appears early in life and is secondary to damage to Bowmans

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18
Q

corneal stroma is made up of

A

dense, regular connective tissue composed of keratocytes (fibroblasts), collagen fibrils, ground substance, and water (75-80% of the stroma)

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19
Q

keratocytes

A

fibroblasts of the cornea that produce collagen fibrils and the extracellular matrix

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20
Q

hydrops

A

occurs in keratoconus when Descemet’s membrane ruptures

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21
Q

Haab’s striae

A

folds in Descemet’s membrane that occur in congenital glaucoma

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22
Q

Hassall-Henle bodies

A

small areas of thickened Descemet’s membrane in the corneal periphery that protrude towards the AC, increase with age and have no visual significance

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23
Q

pleomorphism

A

corneal endothelial cells no longer being hexagonal, change shape
(HEX marker, 70-80% is normal)

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24
Q

polymegathism

A

when endothelial cells die, neighbouring cells enlarge to cover the empty space, to compensate for the decrease in cell density

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25
Q

stromal edema

A
  • loss of endothelial cells results in a decrease of Na+/K+ ATP pumps and leads to edema in the stroma
  • hypoxia of cornea leads to increase of lactate and acidity
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26
Q

deturgescence

A

the physiological process in which the stroma is kept relatively dehydrated to maintain normal corneal clarity and transparency, depends on GAGs, cell barriers, and ion transport
78%

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27
Q

how does the avascular cornea obtain nutrients?

A

diffusion from the aqueous humor, limbal conjunctival and episcleral capillary networks, palpebral conjunctival networks
O2 under open-eye conditions - comes from tear film, O2 under closed-eye conditions - comes from palpebral conjunctival blood vessels

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28
Q

corneal neovascularization

A

defense response to oxygen deprivation, the new vessels arise from endothelial cells of the limbal capillary network in response to cytokines and growth factors (including VEGF)

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29
Q

how does the cornea get sensory innervation

A

sensory innervation comes from V1, the LPCNs and SPCNs enter the mid-stroma of the cornea. inside the stroma the nerves lose their myelin sheaths and become highly sensitive with nocireceptors (pain)

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30
Q

neurotrophic keratitis

A

characterized by poor corneal sensitivity and poor wound healing, secondary to damage of V1 (ex. HSV, HZO, CVA, DM)

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31
Q

guttata

A

protrusions of Descemet’s membrane in central cornea (Hassall-Henle bodies are in periphery in young pts)

32
Q

bullous keratopathy

A

low cell density causes swollen cornea because the endothelial cells aren’t pumping fluid as efficiently due to low count

33
Q

corneal plana

A

flat cornea due to mutated corneal keratocan, also can cause sclerocornea

34
Q

which layers of the cornea have nerves running through?

A

nerves anter mid-stroma and go anteriorly, through to the epithelium but terminate in the wing cell layer

35
Q

how does the anterior third of the corneal stroma differ from the posterior third?

A

in the anterior third of the corneal stroma, the lamellae are thin, have more crosslinking, more rigid/harder, more homogenous, more abundant fibroblasts (keratocytes), and have nerves running through. in the posterior third collagen lamellae are larger

36
Q

lumican

A

a protein that is essential for maintaining collagen sizing, especially in the posterior stroma

37
Q

UVR absorption and the cornea

A

epithelium and Bowmans layer are the primary sites for UV-C absorption, some UV-B

38
Q

corneal hypoxia

A

cornea switches to anaerobic glycolysis for energy production in the absence of O2, produces lactate which is hard to remove and changes concentration gradients -> leads to edema (stromal) also increasing H+ means loss of K+ causing cells to shrink and function poorly

39
Q

what is the average eccentricity of the normal cornea?

A

e-value is usually around 0.50 for a normal cornea

40
Q

sclera

A

forms the posterior 5/6th of the protective connective tissue coat of the eye and helps to maintain the shape of the globe and to protect intraocular structures

41
Q

What is the mean radius of curvature for the sclera?

A

11.5mm

42
Q

Where is the thickest area of the sclera?

A

1.0mm at the posterior pole

43
Q

Where is the thinnest area of the sclera?

A

0.30mm under the recti tendon insertions

44
Q

What is the weakest area of the sclera?

A

the lamina cribrosa (within the optic nerve)

45
Q

How does the sclera receive blood flow?

A

Sclera is considered avascular, it receives minimal blood supply from episcleral vessels, choroidal vessels, and branches of the LPCAs

46
Q

What innervates the sclera?

A

minimally innervated by the LPCNs and SPCNs

47
Q

episclera

A

loose connective tissue layer that contains a capillary network (from ACAs) that surrounds the cornea

48
Q

What causes characteristic ciliary flush/circumlimbal injection?

A

inflammation of the ciliary body or iris will cause dilation of ACAs causing the circumlimbal flush

49
Q

sclera proper

A

thick, dense, avascular connective tissue that is continuous with the cornea stroma

50
Q

What is the sclera proper composed of?

A

composed of irregular collagen bundles that provide strength but no transparency. contrains less fibroblasts and GAGs but similar ground substance to the corneal stroma

51
Q

lamina fusca

A

the innermost layer of the sclera adjacent to the choroid that contains elastin fibers and numerous melanocytes

52
Q

What can make the sclera look blue?

A

blue sclera can be seen in osteogensis imperfecta or Ehlers-Danlos syndrome
also in infants (seeing underlying uvea) as the sclera is sitll thin

53
Q

What can make the sclera look yellow?

A

lipids become trapped in the sclera proper connective tissue in elderly patients, or can also be a sign of liver disease

54
Q

Name the layers of the eye from anterior to posterior (sclera part not corneal)

A

conjunctival epithelium, conjunctival stroma, Tenon’s capsule, episclera, sclera proper, and lamina fusca

55
Q

Which layers of the cornea are hydrophilic and which are hydrophobic?

A

epithelium and endothelium are hydrophobic

stroma is highly hydrophilic

56
Q

Which corneal layers absorb shorter wavelengths of UV light (UV-C and UV-B) to protect the inner layers of the eye?

A

Corneal epithelium and Bowmans layer

57
Q

What light wavelengths does the cornea transmit?

A

300nm (UV) to 2500nm (infrared)

58
Q

corneal crystallins

A

located in the cytoplasm of epithelial and endothelial cells and help maintain corneal transparency by limiting light scattering

59
Q

Vitamin C/ ascorbate and glutathione

A

located within the epithelial cells and help to protect the cornea from UV rays and free radical scavengers

60
Q

What features help the cornea to transmit light?

A
  • proteoglycans, corneal crystallins, ascorbate
  • presicely spaced lamellae (destructive interference)
  • being avascular
  • high water content helps transparency
61
Q

the major proteoglycan in the corneal stroma is

A

keratin sulfate

62
Q

partial pressure of O2 in tear film during open eye conditions

A

155 mmHg

63
Q

partial pressure of O2 in closed eye conditions

A

55 mmHg

64
Q

critical partial pressure of O2 for the cornea is

A

10-20 mmHg

65
Q

the entire corneal epithelium replaces itself every…

A

7-14 days

66
Q

Which layers of the cornea regenerate and which do not?

A

Epithelium and Descemets membrane can regenerate

Bowmans and the endothelium CANNOT regenerate

67
Q

Aging of lid tension leads to what change in corneal cyl in older patients?

A

increase in ATR

68
Q

What is the only mitotic layer in the corneal epithelium?

A

the basal layer, (secretes its own BM that attaches with hemidesmosomes)

69
Q

Which layers of the cornea thickens with age?

A

basement membrane of epithelium doubles with age,
descemets triples in size

70
Q

Bowmans function?

A

resistance to damage/injury, also may maintain the correct curvature of the cornea

71
Q

Clinical conditions related to Bowmans

A

band keratopathy, pterygia, crocodile shagreen, Reis-Bucklers, keratoconus starts here, refractive sx

72
Q

Clinical conditions related to Descemets

A

hydrops, Haab’s striae (horizontal), Hassall-Henle bodies (normal, insignificant, peripheral guttata)

73
Q

min amount of endo cells before cornea swells

A

400-700 cell/mm2

74
Q

average amount of corneal endo cells

A

1,000-2,000

75
Q

What germ cell layers do the corneal layers derive from?

A

Surface ectoderm - corneal and conjunctiva epithelium
Neural crest cells - cornea endothelium, stroma, Bowmans, Descemets

76
Q

Which layers of sclera/conj are vascular and which are not?

A

vascular: conj and episclera
avascular: Tenons, sclera proper

Ocular Anatomy (12 decks)

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