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Ocular Physiology > Cornea and Sclera part 1 > Flashcards

Cornea and Sclera part 1 Flashcards

1
Q

what are the horizontal and vertical dimensions of the cornea

A

horizontal is 12.6mm and vertical 11.7mm

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2
Q

what is the refractive component of the cornea

A

48 Diopters of plus power

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3
Q

what shape is the central part of the cornea

A

spherical or toroidal contour

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4
Q

what is the radius of curvature of the outer aspect of the cornea

A

7.8mm

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5
Q

what is the thickness of the central and peripheral portions of the cornea

A

the central is 0.52mm and peripheral is 0.65mm

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6
Q

99% of which part of the spectrum gets transmitted through the cornea

A

about 400nm

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7
Q

how many layers are there in the corneal epithelium

A

5-7 stratified, nonkeratinized, squamous epithelium

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8
Q

how many cells thick is the surface/apical layer

A

3-4

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9
Q

how many cells thick is the wing cell layer

A

1-3

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10
Q

how many cells thick is the basal cell layer

A

1 cell thick

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11
Q

which layer of the corneal epithelium has mitosis

A

the basal cell layer

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12
Q

which layer of the corneal epithelium is adhered to the basement membrane

A

the basal cell layer

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13
Q

what happens to the basal cells as cell division begins to occur

A

the daughter cells move towards the surface and begin to differentiate forming the wing layer

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14
Q

how long does it take for epithelial cell turnover

A

7 days

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15
Q

where do the basal cells originate from

A

stem cells in the limbal epithelium (palisades of vogt)

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16
Q

because basal cells have high metabolic activity, what structures are prominent here

A

mitochondria, endoplasmic reticulum, and golgi apparatus (high glycogen storage)

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17
Q

what is the mitotic rate of the epithelium per day

A

10-15%

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18
Q

does the cornea have a constant or intermittent cycle of shedding superficial cells and proliferating basal layer cells

A

constant cycle

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19
Q

which direction do the cells migrate in the corneal epithelium

A

centripetal migration

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20
Q

what are the smaller, light cells on the surface of the corneal epithelium

A

younger cells that have recently reached the cornea

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21
Q

what are the larger, darker cells on the surface of the cornea

A

mature cells that will be sloughed off

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22
Q

what are the holes that you see on the surface of the cornea

A

exfoliation holes or breaks in epithelium (cells in process of peeling of the surface)

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23
Q

when looking at the cornea with a scanning electron microscope, what does the cornea look like

A

an irregular array of polygonal cells, larger, smaller, and exfoliation holes

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24
Q

how are the basal cells adhered to the basement membrane and stroma

A

by hemidesmosomes

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25
Q

what are connected to hemidesmosomes in the basement membrane that pass through bowmans layer

A

anchoring fibrils

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26
Q

what type of collagen are anchoring fibrils

A

type 7 collagen

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27
Q

how deep do the anchoring fibrils penetrate into the strom

A

2 mirons deep

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28
Q

what is located at the ends of the anchoring fibrils

A

anchoring plaques (end plates)

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29
Q

what is destroyed in photorefractive keratectomy (PRK) and must be reassembled during the healing process

A

the connection between the anchoring fibrils and the hemidesmosomes

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30
Q

what is located in the superficial cells of the epithelium (apical cells) that completely encircle the cells to form a barrier

A

zonula occludens

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31
Q

where are gap junctions found

A

in all layers but more numerous in the basal layers than the superficial layers

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32
Q

what type of membrane do the zonula occludens form

A

highly effective semipermeable membrane

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33
Q

which ion is transported into the stroma and which is pumped out

A

Na+ is pumped from the tears into the stroma and Cl- is transported from the stroma into tears

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34
Q

what is EBMD

A

epithelial basement membrane dystrophy

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35
Q

what is a symptom of EBMD

A

painful recurrent epithelial erosions, susceptible to edema and infection

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36
Q

what causes recurrent erosions in EBMD

A

the basal cells have decreased hemidesmosomes and have an abnormal adhesion

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37
Q

what happens to the basement membrane in aging and diabetic patients

A

thickening of basement membrane

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38
Q

why do aging and diabetic patients have an increased risk of epithelial erosions

A

the anchoring fibrils cannot penetrate as deep through the thickened basement membrane into the stroma

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39
Q

why would the erosions in diabetic patients not heal as quickly as they should

A

if they have nerve damage the brain has a delay in receiving the information, the healing process is delayed

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40
Q

how thick is the basement membrane (basal lamina)

A

40-60 nm thick

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41
Q

what is the basement membrane made of

A

type 4 collagen, laminin, proteoglycan perlecan, fibronectin, and fibrin

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42
Q

does a corneal epithelium abrasion demand a slow or fast healing response

A

fast- must recover the exposed basement membrane with cells

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43
Q

what happens to mitosis after an abrasion occurs

A

mitosis stops and the attachment to the basement membrane is lost to conserve energy

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44
Q

after mitosis stops, what else happens to the cells

A

the cells enlarge and the epithelial sheet migrates by ameboid movement to cover the defect

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45
Q

what is the primary function of the corneal epithelium

A

form a barrier to invasion of the eye by pathogens and to uptake excess fluid by the stroma

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46
Q

what happens to the epithelium after the wound closes

A

mitosis resumes, protein synthesis by epithelial cells increases during cell migration and growth factors increase

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47
Q

how fast does a 6mm epithelial wound close

A

within 48 hours

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48
Q

what is the rate that an epithelial wound heals

A

60-80 microns per hour

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49
Q

is there an increase or decrease in glycogen levels in migrating cells

A

decrease in levels

50
Q

is there an increase or decrease in glycolytic activity during cell mirgration

A

increase

51
Q

are cells dependent on anaerobic or aerobic glycolysis

A

anaerobic- it is not efficient but fast

52
Q

what has an increased expression to supply support for the healing process

A

glucose transporter GLUT1 mRNA and GLUT1 protein levels

53
Q

what are 3 growth factors that increase during healing

A

EGF (epidermal growth factor) KGF and HGF (keratocyte and hepatocyte growth factor)

54
Q

which growth factor is expressed by the corneal epithelium

A

the epidermal growth factor

55
Q

which growth factor is expressed by stromal keratocytes

A

keratocyte and hepatocyte growth factors

56
Q

which conditions have delayed healing or no normal epithelium adhesion is established

A

basement membrane dystrophy, diabetes, persistent or recurrent epithelial defects and severe injuries (alkali burns)

57
Q

why is the peripheral epithelium faster to heal than the central epithelium

A

there is a greater blood supply in the limbus

58
Q

if the basement membrane is NOT damaged, is there a delay in reestablishing formation of new hemisesmosomes

A

no it is formed quickly

59
Q

if the basement membrane is damaged are the formation of new adhesion complexes formed quickly or slowly

A

slowly, there is a delay

60
Q

how long is the new development of adhesion complexes delayed for

A

more than 12 months

61
Q

how thick is bowman’s layer

A

12 microns thick

62
Q

what type of collagen is in Bowman’s layer

A

type 1 collagen

63
Q

is the collagen in Bowman’s layer randomly or regularly arranged

A

randomly arranged

64
Q

True or False: bowman’s layer acellular

A

True

65
Q

what does the stroma consist of

A

an extracellular matrix comprised of lamellar arrangement of collagen fibrils

66
Q

are the collagen in the stroma parallel or perpendicular to the corneal surface

A

parallel

67
Q

what separates the collagen fibrils in the stroma

A

a matrix of proteoglycans

68
Q

what 2 things produce and maintain the extracellular matrix

A

keratocytes and fibroblasts

69
Q

how many lamellas of collagen fibers are in the stroma

A

200-250

70
Q

which direction do the collagen fibers lie in the anterior stroma

A

obliquely

71
Q

which direction do the collagen fibers lie in the posterior stroma

A

orthogonally

72
Q

at the limbus, the collagen fibers run circumferentially around the cornea, how wide is the annulus formed

A

1.5-2.0 mm

73
Q

what types of collagen are in the stroma

A

mostly type 1, also type 5 and 6 collagen

74
Q

what is the refractive index for the collagen fibers in the stroma

A

1.411

75
Q

what is the refractive index for the extrafibrillar matrix

A

1.365

76
Q

how much light is scattered because of the highly uniform size and spacing of the collagen fibers

A

only 10%

77
Q

what is Maurice’s proposal

A

the corneal transparency is a consequence of a crystalline lattice arrangement and the light scattered by individual fibrils os uniform diameter is canceled by destructive interference with scattered light from adjacent fibers

78
Q

do the requirements for Maurice’s proposal apply when talking about the cornea

A

no

79
Q

do the sizes of the collagen vary in size in the stroma

A

yes but within a small range

80
Q

why is there weak scatter of light in the stroma

A

the collagen have a diameter of a small fraction of the wavelength of visible light

81
Q

how far apart must the collagen fibers be to remain transparent

A

less than 1/2 the wavelength of visible light

82
Q

if the cornea swells from damage to epithelial or endothelial barriers, what happens to the transparency

A

it is lost

83
Q

if there is a “lake” of water in the stroma where there aren’t collagen fibers, what 3 things can it cause

A
  1. increase divergence of refractive index
  2. increase in distance between collagen fibrils
  3. all resulting in a loss of light transmittance
84
Q

is the collagen fibril diameter larger in the anterior or posterior cornea

A

the anterior

85
Q

is the density of the fibrils larger in the anterior or posterior cornea

A

the posterior

86
Q

why is there a twofold increase in light scatter by the anterior cornea compared to the posterior cornea

A

because the fibril diameter is larger and the density is smaller

87
Q

what is the rate of evaporation on the corneal surface

A

2.5 microliters per cm^2 per hour

88
Q

what percentage of the cornea thins during the day compared to at night

A

5%

89
Q

what disorder causes a compromised endothelial pump function and worse edema in the morning

A

Fuch’s dystrophy

90
Q

what is dellen

A

localized area of corneal drying and evaporation

91
Q

what may persistant dellen reflect

A

a decrease in stromal fluid flow when stroma hydration is abnormal or minimal flow of water in the cornea

92
Q

what 3 conditions may disrupt tears to spread evenly

A

coloboma, pterygium, or a pingeula

93
Q

if IOP is high but remains under 50mm Hg, will the cornea remain a constant thickness

A

yes because the stromal swelling pressure is in a similar range

94
Q

if IOP is higher than 50mm Hg or there is abnormal endothelial function, will the cornea remain a constant thickness

A

no there is epithelial edema and increased stromal thickness

95
Q

what is the relationship between IOP and stromal swelling pressure

A

IP (stromal imbibitions pressure) = IOP - SP

96
Q

as there is an increase in corneal thickness, what happens to stromal swelling pressure

A

it decreases

97
Q

how would you restore tensile strength after a stromal wound

A

re-synthesis and cross linking of collagen, alterations in proteoglycan synthesis, and gradual wound remodeling

98
Q

what cells show up within hours around areas of cellular necrosis in a penetrating corneal wound

A

polymorphonuclear cells - granulocytes and then monocytes (white blood cells)

99
Q

how many years can tensile strength gradually increase up to

A

the 4th year post-op

100
Q

why do incisions in avascular cornea far from the limbus heal more slowly

A

they are farther from the blood supply than a peripheral incision

101
Q

why would you want to prescribe a steroid for corneal wound healing

A

to control the speed of recovery or control the amount of scaring

102
Q

what is a disadvantage of prescribing steroids after a corneal wound or surgery

A

the wound heals slower

103
Q

how thick is the corneals endothelium, Descemet’s membrane

A

10-15 microns thick

104
Q

what happens to Descemet’s membrane as we age

A

it thickens

105
Q

what fibers is descemet’s membrane composed of

A

type 4 collagen, laminin, and fibronectin

106
Q

what secretes Descemet’s membrane

A

the endothelial cells

107
Q

what is a descementocele

A

herniation of descemet’s membrane (the membrane stays intact after severe corneal ulcerations and it pushes decemet’s membrane up after destruction of the epithelium and stroma)

108
Q

what is Descemet’s membrane highly resistant to

A

proteolytic enzymes

109
Q

what happens when Descemet’s membrane is lost

A

because it doesn’t regenerate, the cells enlarge to compensate for loss, they change shape and spread over the missing areas

110
Q

what is Fuch’s dystrophy

A

a disease of the endothelial cells and an abnormal Descemet’s membrane is secreted

111
Q

what happens to the endothelial cells as Fuch’s Dystrophy progresses

A

the endothelial cell function decreases and cells are lost

112
Q

what is Guttata

A

collagenous material formed on the posterior surface of Descemet’s membrane

113
Q

what happens to the endothelial cells in Guttata

A

thinning and enlargement of endothelial cells

114
Q

what happens to endothelial cells as we age

A

the cell density decreases

115
Q

what is the newborn endothelial cell density

A

greater than 5500 cells/mm^2

116
Q

what is the adult endothelial cell density

A

2500-3000 cells/mm^2

117
Q

what is the minimum endothelial cell density

A

400-700 cells/mm^2

118
Q

what do endothelial cells contain

A

large nucleus, mitochondria, endoplasmic reticulum , and golgi apparatus

119
Q

endothelial cells are metabolically active in what 3 things

A

transport, synthesis, and secretory

120
Q

what junctions are in the endothelial cells layer

A

tight junctions- macula occludens and gap junctions

121
Q

what is the purpose of the macula occludens

A

they do not completely circle the cells and provide a leaky barrier between aqueous and stroma

122
Q

where are the gap junctions in the endothelial cells

A

between lateral membranes for intercellular communication

Ocular Physiology (6 decks)

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