OP: Corneal Light Transmission

What % of light is transmitted thru the cornea, what is the visible range of light, which wavelengths transmit/scatter the most, and why does light scatter so much in cellular tissue?

85-99%

380-740 nm

Smaller wavelengths (violet) transmit less and scatter more while longer wavelengths scatter less and transmit more

D/t organelles and nuclei

What are the 4 contributors of light scattering in the central and temporal cornea in order of most scattering to least scattering?

1.) Epithelial cells

2.) Nerve plexus

3.) Anterior-most keratocytes

4.) Endothelium

What are the 3 requirements for corneal stromal transparency? What phenomenon allows for transmission of light forward while reducing peripheral scattering? Lastly, how is back scattering of light affected by irregular fibril size?

1. Distance between scattering structures is less than ½ the wavelength of visible light

2. Uniformity of collagen fibril diameter

3. Restriction in the range of distances between fibrils

Light is scattered upon interaction with all fibrils and destructive interference cancels out the scattering light in all directions but forward.

Irregular fibril size causes incomplete dissipation of back scattered light via destructive interference.

What do proteoglycans do in the cornea? How is this possible? How does mucopolysaccharidoses affect this?

GAGs provide repulsion force that maintains the spacing between fibrils. They can do this via negative charge repulsion and the average length of of GAG duplexes.

MPS = too many GAGs = too much water/negative charges = water disrupts spacing and negative charges repel to disrupt spacing more.

What does the fibril density and diameter look like in the cornea vs the sclera?

Cornea = smaller diameter/larger fibril density

Sclera = larger diameter/lower fibril density.

How is mucopolysaccharidoses spread, what function does it impede, and what is the pathogenesis/presentation of it?

Group of inherited metabolic diseases

Affect the breakdown of GAGs

Disruption of genes encoding one of 11 different enzymes → GAGs accumulate in stromal tissue → fibril lattice disrupted and cornea becomes cloudy

How does the transparency of the cornea of premature infants compare to that of full term infants? What is the reason for this difference?

Premature infants have less transparent corneas compared to full term. This is due to incomplete development of stromal fiber organization.

What are the 2 ways that keratocytes minimize light scatter?

1. Thin cytoplasm

2. Express large quantities of crystallin protein in the cytoplasm

What are crystallins soluble in, what is their function, and what do these proteins form + how does this help light transmission? Lastly, what is this function similar to?

Soluble in water

Confer transparency of the lens and keratocytes

Individual proteins will form higher order complexes + facilitates minimal light scatter

Occurs similarly to collagen fibril induced transparency

Why do corneal wounds affect transparency?

Keratocytes transform into reflective myofibroblasts

What is special about the cornea’s vascularity that contributes to its transparency? When can vascularity within the cornea diminish transparency?

• Active inhibition of the blood and lymphatic vasculature is necessary for formation, thus it is avascular and has less light scattering

• Vascularization can occur in response to corneal damage or a lack of oxygen

How does the cornea develop its avascularity? What does the disruption of this system lead to?

• Cornea has a balance of pro and anti-angiogenic factors

• Disruption leads to neovascularization of the cornea

What factor inhibits the vascularization of the cornea? How does it inhibit vascularization?

• sFlt-1

• Inhibits vascularization by binding to pro-angiogenic factor VEGF and preventing its function

What do nerve bundles lose upon entering the central cornea? Why?

• Lose perineurium & myelin sheathes

• Transparency

What protein does corneal epithelium express a lot of? What does its relative thickness look like, its amount of cytoplasmic organelles, and is it keratinized?

• Express a large amount of crystallin protein

• Thin

• Sparse cytoplasmic organelles

• Non-keratinized

What is squamous metaplasia, what causes it, and where does it begin/migrate to?

• Abnormal mucosal epithelial differentiation

• Conversion of NK epi with goblet cells to a metaplastic, nonsecretory, SSKE in response to chronic stress

• Can migrate to cornea

Rank the following in terms of relative absorption by the cornea from most to least:

• UVA

• UVC

• UVB

• Visible light

• Infrared

• UVC

• UVB

• UVA

• Infrared (depends on hydration, thickness, and wavelength, of which 2.5k nm has less)

• Visible

How does the transmission of visible light change as you move towards peripheral or thicker cornea? Which wavelengths scatter the most?

• Less transmission in thicker cornea = peripheral cornea

• Short wavelengths scatter the most whereas short ones scatter the least

Why is the cornea more susceptible to UV damage than skin? Furthermore, what are the 2 effects of UV overexposure on corneal epithelium? Make sure to name an acute and a chronic condition.

• Cornea is more susceptible to UV damage than skin d/t lack of melanin

• Acute UV overexposure does the following to epithelium:

  • Induces apoptosis pathways

  • Generates reactive oxygen species

  • Photokeratitis (acute): massive loss of corneal epithelial cells

  • Pterygium (chronic): overgrowth of conjunctiva into the cornea

What vitamin is critical for corneal health in the presence of UV light? What does it do, where is it found in large amounts, what does it mitigate, and lastly how does it affect the wavelengths of light absorbed by the epithelium vs bowman’s and the anterior stroma?

• Ascorbic acid

  • Absorbs UV light

  • Large amounts in corneal epithelium and stroma

    • Epithelium absorbs UVC

    • Bowman’s/ant stroma absorb UVB

  • Mitigates ROS

    • Functions as electron donor

    • Reduces oxygen radicals

What water soluble enzyme protects the cornea from UV damage? What type of protein is it, what proportion of water soluble protein of the cornea does it comprise roughly, what are its 2 functions, what benefit does its abundance have for other biomolecules, what two residues does it contain a lot of, and what wavelength does it absorb 50% of? Lastly, where does it prevent damaging protein modification and how does it protect proteins?

• Aldehyde dehydrogenase

  • Corneal crystallin

  • Large proportion of water soluble protein in cornea

  • Absorbs UV light

    • Abundance is protective to other biomolecules

    • Tryptophan/tyrosine residues

    • Accounts for 50% of UVB absorption

  • Mitigates ROS

    • Prevents damaging protein modification @ aldehyde groups

    • Prevents protein unfolding