Anatomy Of The Cornea

Question 1

What makes up the fibrous tunic?

Answer 1

Cornea
Sclera

Question 2

What is the principle refracting component of the eye? What characteristic of this structure allows it to have this property?

Answer 2

Cornea
Transparency

Question 3

What are the functions of the sclera?

Answer 3

Maintains shape of the globe
Protects against internal and external factors
Provides attachment for EOMs

Question 4

What properties of the cornea allow it to remain transparent?

Answer 4

Smooth surface
Avascularity
Energy required

Question 5

What does the cornea protect against?

Answer 5

External penetration
UV light

Question 6

What are the dimensions of the cornea?

Answer 6

12mm horizontal diameter
11mm vertical diameter

Question 7

What are the 7 important corneal properties?

Answer 7

Power
Curvature
Topography
Sphericity
Induced aberrations
Thickness
Viscoelasticity

Question 8

What is the radius of curvature of the cornea and sclera? Which is more curved (steeper)?

Answer 8

Cornea R= 7.8mm (more power, more curved)
Sclera R= 11.5mm

Question 9

How does the refractive index of the cornea compare to the aqueous?

Answer 9

Cornea n= 1.376
Aqueous n= 1.336

Question 10

What is the refracting power of the cornea? Is the front and back - or +?

Answer 10

Refracting Power= +42D
Front +
Back -

Question 11

What is keratometry? What does it measure?

Answer 11

Measurement of the anterior surface of the cornea curvature in a small region.
Gives power of the cornea.

Question 12

What is Topography? What does it measure? What does it reveal about the power of the cornea?

Answer 12

Contour of the corneal surface.
Cornea does not have consistent power throughout.

Question 13

What is Sphericity/eccentricity? What term can be used to describe the shape of the cornea?

Answer 13

Cornea is not a perfect sphere.
It is steeper centrally then peripherally (prolate)

Question 14

What is Pachymetry? How does it differ throughout the cornea? How does pressure relate?

Answer 14

Measure corneal thickness.
Peripheral cornea is thicker than central.
Central is steeper and thinner.
Thicker cornea=higher pressure

Question 15

What is the average central corneal thickness (CCT)? How is it measured?

Answer 15

544.5 microns (varies from person to person)
Ultrasonic pachymetry

Question 16

What is Corneal Hysteresis? What can it help diagnose?

Answer 16

Measurement of the ability of the cornea to withstand change
Glaucoma

Question 17

What are the layers of the cornea from front to back? How many microns thick are each?

Answer 17

Epithelium 50
Bowman’s Layer 10
Stroma 500
Descemet’s Membrane 5-15
Endothelium 5
(Even Bama State Did Estrogen)

Question 18

What are tight junctions? What are they called in the cornea? Where are they located?

Answer 18

Closely associated areas of two cells whose membrane join together forming a virtually impermeable barrier to fluid.
Zonula Occuludens
Epithelium and Endothelium (leaky)

Question 19

What are desmosomes? What are they called in the cornea? Where are they located?

Answer 19

Spot-like adhesions randomly arranged on the lateral sides of cell membranes specialized for cell-to-cell adhesion.
Macula adherens
Epithelium

Question 20

What are Hemidesmosomes? Where are they located?

Answer 20

Rivet-like structures that link cells to the ECM.
Appear similar to desmosomes with electron microscopy.
Epithelium

Question 21

What are gap junctions? Where are they located?

Answer 21

Allow communication between cells by linking the cytoplasm of the cells. Allows passage of ions, molecules, and electrical impulses between cells.
Epithelium and endothelium

Question 22

Describe the corneal epithelium. Thickness, layers, what is it continuous with?

Answer 22

50 microns thick
5-7 cell layers thick
Thickens in periphery where it becomes continuous with the conjunctival epithelium at the limbus.

Question 23

Describe the surface epithelium (apical layer). Thickness, cell types, surface interactions?

Answer 23

2 cells thick
Nonkeratinized stratified squamous cells
Cells are flattened
Surface glycocalyx interacts with mucin layer of tear film Surface has microvilli and microplicae to increase surface
area
Constantly being abraded with blinking

Question 24

Layers of cornel epithelium cell layer (top to bottom)?

Answer 24

Superficial cells (tear film, glycocalyx layer, apical microvilli)
Wing cells
Basal cells
Basement membrane (Hemidesmosomes, tight junctions)
(Socks with bouncing balls)

Question 25

Describe the wing cell layer of the corneal epithelium.

Answer 25

2-3 cell layers
Winglike lateral processes
Polyhedral in shape with convex anterior surfaces and concave posterior surfaces to fit over basal cells
Joined to each other by desmosomes and gap junctions
Joined to superficial and basal cells by desmosomes

Question 26

Describe the basal cell layer of the corneal epithelium.

Answer 26

Single layer of columnar cells
12 microns wide
Desmosomes and gap junctions
Attached to basement membrane by hemidesmosomes

Question 27

What structures holds cells in place during blinking?

Answer 27

Hemidesmosomes

Question 28

Describe bowman’s layer.

Answer 28

8-14 microns thick
Transition to stroma
Dense, fibrous sheet of collagen fibrils randomly arranged
Very resistant to damage
Does not regenerate (replaced by epithelial cells or stromal scar if injured)

Question 29

Describe the stroma (substantia propria).

Answer 29

500 microns thick
Composed of collagen fibrils (uniform), keratocytes, extracellular ground substance (maintains clarity of stroma)

Question 30

Describe the layout of collagen fibrils. Where are they located? What is their diameter?

Answer 30

Uniform arrangement
Corneal Stroma
25-35 nm in diameter

Question 31

What is the arrangement of fibrils in the corneal stroma?

Answer 31

Fibrils are organized into bundles called lamellae
Regular spacing (latticework)
Parallel to the corneal surface and run in the same direction
Extend from limbus to limbus and become circumferential at limbus
200-300 lamellae per cornea
Lamellae are 2 microns thick

Question 32

Describe the arrangement of the lamellae in the anterior portions of the stroma.

Answer 32

The lamella are thinner in the anterior 1/3 of the cornea (0.5 to 30microns wide and 0.2 to 1.2 microns thick)
More branching and interweaving

Question 33

Describe the arrangement of the lamellae in the posterior portions of the stroma.

Answer 33

In the posterior 2/3, the arrangement is more regular (parallel)
Lamellae are larger (100 to 200 microns wide and 1 to 2.5 microns thick)

Question 34

Describe the arrangement of the fibrils in the innermost portion of the stroma.

Answer 34

In the innermost stroma, the fibrils interlace to form a collagenous sheet
Contributes to the binding between stroma and Descemet’s

Question 35

Describe the location, main function, and activity of keratocytes (corneal fibroblasts).

Answer 35

lie between and occasionally within lamellae.
Corkscrew patterns from anterior to posterior.
Extensive branching processes laterally and anterior and posterior.
Synthesize collagen and other ECM components.
Very active with lots of mitochondria.

Question 36

What are the functions of keratocytes?

Answer 36

Synthesize collagen and ECM components
Matrix turnover
Intracorneal communication
Reservoir for glycogen
Interlamellar tethering
Wound healing

Question 37

What is ground substance? What is it composed of?

Answer 37

Fills in areas between lamellae and cells
Composed of proteoglycans
- Core protein + GAGs (glycosaminoglycans)
- GAGs (Hydrophilic, negative charge, maintains spatial relationship between fibrils)

Question 38

How much light does the cornea scatter? How does it do this?

Answer 38

Cornea scatter less than 1% of light entering it.
Destructive interference due to regular arrangement of fibrils
Small diameter of fibrils

Question 39

Describe Descemet’s membrane. How does its thickness change overtime?

Answer 39

Modified basement membrane that lies between the endothelium and the stroma
Maintains cornel structure, homeostasis and transparency.
Thickens throughout life
- doubles by age 40
- triples by late in life

Question 40

Describe the corneal endothelium. Thickness? Location? Do they regenerate?

Answer 40

Single layer of flattened cells (simple squamous) (mosaic)
5 microns thick x 20 microns wide
Basal portion rests on Descemet
Apical portion into AC
Cells do not regenerate

Question 41

Describe the endothelial barrier.

Answer 41

Barrier is leaky, allowing large molecule to enter.
- Glucose and AA from AC
- Water

Question 42

What is Deturgesced? What maintains it?

Answer 42

State of relative dehydration maintained by the normal cornea
Necessary for transparency
Maintained by the epithelium, which to a large extent is impermeable to water
Metabolic transport system in the endothelium.

Question 43

What happens to the endothelium as a person ages?

Answer 43

BM material produced throughout life
Descemet’s thickens
- Peripheral: Hassall-Henle bodies bulge into AC
- Central: corneal guttata (indicates endothelial dysfunction

Question 44

Define polymegathism and pleomorphism.

Answer 44

Variation in cell size (poly)

Variation in cell shape (pleo)

Question 45

Describe specular microscopy.

Answer 45

Non invasive tool that allows for in vivo evaluation of corneal endothelium to access health

Question 46

How does endothelial cell density change with age?

Answer 46

Decreases with age, can get too low

Question 47

Describe the corneal blood supply.

Answer 47

Normal cornea is avascular
Receive nutrients from diffusion from air, AC, Limbal conjunctival and episcleral capillaries

Question 48

Describe the sensory innervation of the cornea. What nerve innervates the cornea?

Answer 48

Richly innervated with sensory nerve (V1)
70-80 large nerve enter peripheral stroma
Lose myelin sheath in from limbus

3 sensory nerve networks formed
• Stromal: below anterior 1/3 of stroma
• Subepithelial: between Bowman’s and basal epithelium
• Epithelial: naked nerve endings between cells (PAIN!)