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Cornea Flashcards

(106 cards)

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1
Q

What is the anterior horizontal diameter of the cornea?

A
  • 12mm
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2
Q

What is the anterior vertical diameter of the cornea?

A

11mm

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3
Q

How does the curvature of the cornea vary?

A
  • curvature steeper in the centre
  • flatter at periphery
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4
Q

What is the radius of curvature of the cornea a the posterior surface?

A

6.5mm

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5
Q

What is the radius of curvature of the central cornea at the anterior surface?

A

7.8mm

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6
Q

What is the central corneal thickness?

A

0.53mm

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7
Q

What is the corneal periphery thickness?

A

0.71mm

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8
Q

How is the cornea optimum for light transmittance?

A
  • transparency and avascularity
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9
Q

What covers the anterior surface of the cornea?

A
  • tear film
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10
Q

What posteriorly borders the cornea?

A

anterior chamber

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11
Q

What is the cornea continuous with at the periphery?

A
  • conjunctiva and sclera
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12
Q

What are the five layers of the cornea?

A
  • epithelium
  • Bowmans layer
  • stroma
  • Descemets membrane
  • endothelium
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13
Q

What is the outermost layer of the cornea epithelium?

A
  • stratified corneal epithelium
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14
Q

What’s the amount of cells and thickness of the stratified corneal epithlium?

A
  • five to seven cells thick
  • 50um
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15
Q

Where is the corneal epithelium thickest?

A
  • periphery
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16
Q

What is the corneal epithelium continuous with at the limbus?

A

conjunctival epithelium

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17
Q

What’s the thickness of the surface layer epithelium?

A
  • two cells thick
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18
Q

What does the surface layer of corneal epithelium consist of?

A
  • nonkeratinized squamous cells
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19
Q

What are the features of nonkeratinized squamous cells?

A
  • contain flattened nucleus
  • fewer organelles than deeper cells
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20
Q

What the cell size of the superficial cells of the surface layer corneal epithelium?

A

-50um diameter
- 5um height

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21
Q

What does the plasma membrane of the surface epithelial cells secrete?

A
  • glycocalyx
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22
Q

What is the function of glycocalyx?

A
  • adjoins the mucin layer of the tear film
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23
Q

What are the many projections on the apical surface on the outermost cells of the corneal epithelium?

A
  • microvilli
  • microplicae
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24
Q

What’s the functions of the microvilli and microplicae on the corneal epithelium?

A
  • increases surface area
  • enhances tear film stability
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25
What's the function of the Zonula occludens?
- barrier to intercellular movement of substances from the tear layer - prevent excess fluid uptake from the tear film - semipermeable membrane allows passage of fluid and molecules through the cells, but not between them
26
What joins the surface layer corneal epithelium cells?
- zonula occludens - tight junctions - additional adhesion between cells from desmosomes
27
What happens as the corneal surface epithelium cells age?
- they degenerate - cytoskeleton disassembles - cytoplasm condenses - cells lose attachments, sloughed off - constantly replaced by cells below
28
What does the corneal surface look like when viewed through an electron microscope?
- variously sized cells - lighter cells are newly replaced - darker cells those that are degenerating
29
What's the middle layer of the corneal epithelium made up of?
- two to three layers of wing cells
30
What's the structure of the corneal epithelium wing cells?
- wing like lateral processes - polyhedral - convex anterior surface - concave posterior surface - 20um in diameter
31
What joins wing cells together?
- desmosomes - gap junctions
32
What joins wing cells to surface and basal cells?
- desmosomes
33
What is the innermost layer of corneal epithelium?
- basal cell layer - single layer of columnar cells
34
What's the diameter of the cells of the basal cell layer?
8 to 10um
35
What do the basal cells contain?
- oval shaped nuclei displaced towards the apex
36
Where do basal cells lie?
- rounded apical surface of each cell lies adjacent to the wing cells - basal surface attaches to underlying basement membrane
37
What do the basal cells secrete?
- basement membrane to attach the cells to the underlying tissue through hemidesmosomes
38
Where does epithelial cell proliferation take place?
- basal layer
39
How do new cells reach the epithelial cell surface?
- basal cells move up to become wing cells - wing cells move up to become surface cells
40
What are the only epithelial cells that are able to divide?
- cells in contact with the basement membrane(basal cells)
41
What is the source for renewal of the corneal basal cell layer?
- stem cells
42
Where are the stem cells that renew the basal layer located?
- a 0.5 to 1mm band around the cornea periphery
43
How do new basal cells reach the centre cornea?
- slow migration of basal cells from the periphery stem cells
44
What is the turnover time for the entire corneal epithelium?
7 days
45
How does the epithelial healing differ to damage to the basement membrane?
- repair to corneal epithelium heals quickly - basement membrane takes much longer to heal
46
How is the corneal epithelial barrier function maintained despite constantly being sloughed?
- the cell below moves into the position to be replaced
47
What is the second layer of the cornea?
Bowmans layer
48
What's the thickness of the Bowmans layer?
- 8 to 14um thick
49
What's the structure of the Bowmans layer?
- dense, fibrous sheet of interwoven collagen fibrils randomly arranged in a mucoprotein ground substance
50
What's the diameter of the bowman's layer fibrils?
- 20 to 25um
51
How are the Bowmans layer fibrils arranged?
- run in various directions - not arranged into bundles
52
What happens to corneal nerves when they pass through the Bowmans membrane?
- lose their Schwann cell covering and pass into the epithelium as naked nerves
53
What kind of layer is the Bowmans layer?
- a transition layer to the stroma
54
How does the Bowmans layer differ to the stroma?
- its acelluar - contains collagen fibrils of a smaller diameter
55
How do the anterior and posterior surfaces of the Bowmans layer differ?
- anterior surface pattern irregular and reflects the contour of the bases of the basal cells epithelium - posteriorly the fibrils adopts a more orderly arrangement and merge into bundles that intermingle with those of the stroma - posterior surface not clearly defined
56
What happens if the Bowmans layer is injured?
- layer usually replaced by epithelial cells or stromal scar tissue
57
What's the thickness of the corneal stroma?
500um
58
How much of the corneal thickness does the stroma take up?
- 90%
59
What is the corneal stroma composed of?
- collagen fibrils - keratocytes - extracellular ground substances
60
What is the stroma collagen Fibrils diameter?
- 25 to 35 nm
61
What do the stroma collagen fibrils form?
- run parallel to one another forming lamellae
62
How many lamellae lie within the corneal stroma?
- 200 to 300
63
How are the lamellae and collagen fibrils arranged?
- Fibrils and lamellae parallel to corneal surface - adjacent lamellae lie at angles to one another - all fibrils within lamellae run in same direction - each lamellae extends across entire cornea - interweaving occurs between lamellae
64
How are the lamellae arranged in the anterior one third of the stroma?
- lamellae are thin - branch and interweave more than deeper layers
65
How are the lamellae arranged in the posterior two thirds of the stroma?
- arrangement more regular - lamellae larger
66
Why does the anterior cornea have a high incidence of lamellae cross linking?
- makes it more rigid to help maintain cornea curvature
67
What happens to the fibrils in the innermost layer of the corneal stroma?
- the fibrils interlace to form a thin collagenous sheet that contributes to the binding between the stroma and Descemet's membrane
68
What are the Keratocytes?
- flattened cells that lie between and within the lamellae
69
How are the Keratocytes distributed?
- a corkscrew pattern from the anterior to posterior - density higher in anterior stroma
70
How do the keratocytes maintain the stroma?
- synthesize collagen and extracellular matrix components
71
What fills the areas between stroma fibrils, lamellae and cells?
- ground substance
72
What contributes to stromal transparency?
- regular arrangement of stromal components - small diameter of fibrils
73
What's the index of refraction of the stromal fibrils and extracellular matrix?
- fibrils is 1.411 - extracellular matrix is 1.365
74
How does the distance between areas of different refractive indices affect transparency?
- If the change in the index of refraction occurs across a distance that is less than one half the wavelength of visible light (400 to 700 nm), destructive interference occurs, and light scattering is reduced significantly
75
How much of the light that enters the cornea is scattered?
1%
76
What's the Descemet's membrane considered to be?
- basement membrane of the endothelium
77
How does the Descemets membrane change with age?
- thickens throughout life - doubled by age 40 - children 5um thick to 15um over lifetime
78
What are the two lamellae the Descemets membrane consists of?
- anterior lamellae - posterior lamellae
79
What's the difference between the anterior and posterior Descemets membrane lamellae?
- anterior lamina, 3um thick, banded apperance, latticewrok of collagen fibrils - posterior lamina, non banded, homogenous, the portion secreted by the endothelium throughout life
80
How does the Descemets membrane have elastic properties?
- no elastic fibers present - collagen fibrils arranged in such a way
81
What happens if Descemets membranes torn?
- curls into the anterior chamber
82
is the Descemets membrane resistant?
yes
83
can Descemets membrane be regenerated?
yes
84
What is the Schwalbes line?
- thickened area of collagenous connective tissue at the membranes termination in the limbus - circular
85
What's the innermost layer of the cornea?
- endothelium
86
What is the endothelium composed of?
- single layer of flattened cells
87
What's the thickness of the endothlium?
- 5um
88
What does the basal part of each endothelium cell rest on?
Descemets membrane
89
What does the apical part of the endothlium rest on?
- the anterior chamber
90
What shape are the endothelial cells?
- polyhedral - 70 to 80% of cells hexagonal
91
Why are the endothelial cells hexagonal?
- hexagon the most efficacious shape to provide coverage without gaps
92
What is the regular arrangement of the endothelial cells called?
- endothelial mosaic
93
What joins the lateral walls of the endothelial cells?
- interdigitations
94
What do the endothelial gap junctions provide?
- intercellular communication
95
What are the two primary functions of the cornea?
- refract light - transmit light
96
What is the total refractive power of the eye?
60 to 65 diopters
97
What is the refractive power of the cornea?
43 to 48 diopters
98
How is scattering of incident light minimized in the cornea?
- smooth optical surface formed by corneal epithelium and tear film - regular arrangement of surface epithelial cells - tear film fills in slight irregularities between cells - absence of blood vessels - maintenance of correct spatial arrangement of components - endothelial and epithelial cell cytoplasm contain large amounts of water soluble proteins so its homogenous
99
What are the cell cytoplasm water soluble proteins called?
- corneal crystallin's
100
How is the precise arrangement of the collagen fibrils in the cornea stroma maintained?
- negatively charged molecules located around each collagen fibril maintain their precise bonds with water molecules
101
What are the two primary functions of the cornea?
- refract light - transmit light
102
factors that affect amount of corneal refraction?
- curvature of anterior surface - change in refractive surface from air to cornea - corneal thickness - curvature of posterior cornea surface - change in refractive index from cornea to aqueous humor
103
total refractive power of eye focused at infinity?
- 60 to 65 diopters
104
refractive power of cornea?
- 43 to 48 diopters
105
What is the corneal deturgescence required precise control of stromal extracellular water content dependent on?
- barrier function of the epithelium and endothelium - anionic characteristics of molecules within the stromal matrix - water and ion transport through the epithelial and endothelial cell membranes.
106
How is fluid continuously entering the cornea?
- enters through leaky barrier formed by the junctions joining the endothelial cells

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