Cornea and Light Transmission

Question 1

What is the order of tissues that scatter light the most to scatter the least?

Answer 1

1. Endothelial cells
2. epithelial cells
3. nerve cells
4. keratocytes
5. collagen fibrils or extracellular matrix

Question 2

Which scatter more: smaller wavelengths or larger wavelengths?

Answer 2

smaller wavelengths

Question 3

What does the current theory of corneal transparency say must be true?

Answer 3

1. Destructive interference of scattered light must occur
2. The changes in refractive index occur with sufficiently small spacing and regularity
3. When the distance between scattering structures is less than ½ the wavelength of visible light the media is transparent

Question 4

What are the properties of the cornea that influence transparency?

Answer 4

1. thin nature of cornea epithelia

2. organized layering of collagen/ lamellae

Question 5

How are collagen fibers arranged in the cornea?

Answer 5

Regular spacing creates the lattice structure

Question 6

What are the steps of collagen formation in the cornea?

Answer 6

1) Prolyl hydroxylase and lysyl hydroxylase modify AA’s
2) glycosylation
3) Sulfide bonding/triple helix formation
4) procollagen peptidase activity
5) Lysyl oxidase catalyzes procollagen linkage

Question 7

What are the types of collagen fibers found in the corneal stroma (and what are their functions)?

Answer 7

1. Type I (banded fibrils)
2. Type VI (bridges interfibrillar and interlamellar spaces and binds lamellae; NONBANDED)
3. Type V (limits diameter of collagen fibers, found in banded fibrils)

Question 8

What are the stromal proteoglycans?

Answer 8

1. decorin
2. lumican
3. keratocan
4. mimecan

Question 9

What are the stromal GAGs?

Answer 9

1. dermatan sulfate

2. keratan sulfate

Question 10

What are the functions of stromal proteoglycans?

Answer 10

1. help determine spacing of collagen fibrils.
2. control fibril diameter growth and assembly
3. provide tissue volume, resist compressive forces, and give viscoelastic properties

Question 11

Which GAG is mostly in the anterior cornea? And why?

Answer 11

dermatan sulfate (decorin proteo) because binds water better and high O2 environment

Question 12

Which GAG is mostly in the posterior cornea?

Answer 12

keratan sulfate (lumican, keratocan, and mimecan proteos) because low O2 environment

Question 13

How is transparency affected in disease?

Answer 13

1. excess water content
2. disordered fibril distribution
3. enlarged fibrils

Question 14

Why is the sclera white and not transparent?

Answer 14

non-uniform fibril size and spacing

Question 15

How do keratocytes contribute to transparency?

Answer 15

Express large quantitites of crystallin protein in the thin cytoplasm

Question 16

What secondary factors can contribute to corneal transparency problems?

Answer 16

1. Prematurity
2. Avascularity
3. Non-myelinated nerves
4. Non-keratinization of thin epithelium
5. Corneal epithelium

Question 17

In the cornea what absorbs UVC radiation? 1. UVB radiation? 2. UVR radiation? 3

Answer 17

1. high ascorbic acid
2. tryptophan
3. ALDH3A1 (crystallin proteins)