7 Corneal Light Transmission

Smaller wavelengths transmit (more/less) and scatter (more/less)

transmit less, scatter more

What color scatters most?

violet (short wavelength)

What color transmits most?

red (long wavelength)

Why is light scattering highest in cellular tissue?

organelles and nuclei

Rank parts of cornea from greatest transmission (least light scattering) to least (most light scattering)?

Endothelial > epithelial cells > nerve cells > keratocytes > collagen firbils/ECM

Requirement of stromal transparency theory related to wavelength:

Transparency is maintained as long as distance between fibrils is less than ½ wavelength of visible light

Requirement of stromal transparency theory related to fibril diameter:

collagen fibril diameters uniform, ~25 nm

Requirement of stromal transparency theory related to spacing:

Distances between fibrils should be ~60nm

What factors are part of keeping the stroma transparent?

wavelength, collagen fibril diameter and spacing

Destructive interference cancels out all scattering in any direction except the

forward direction

What would irregular fibril size cause with light waves?

destructive interference unable to dissipate all back scattered light

T or F, in a normal cornea with regular fibril size and spacing, destructive interference dissipates all back scattered light

T

What could cause back light scattering in the cornea?

non-uniform fibril size, spacing (← lack of negative charge from GAGs on proteoglycans)

What happens to fibril diameter from cornea through limbus to sclera?

diameter increases

What happens to fibril density from cornea through limbus to sclera?

Density decreases

What happens withouth the ability to break down stromal proteoglycans?

More GAGs → increased negative charge repels fibrils (and attracts water) → increase spacing → transparency decreased

What is mucopolysaccharidoses?

Group of inherited metabolic diseases that affect breakdown of GAGs

Why is the cornea cloudy in mucopolysaccharidoses?

Genes of enzyme that breaks down GAGs disrupted → GAGs accumulate and disrupt fibril lattice structure → negative charge repels fibrils and attracts water

What would breakdown of endothelial pumps cause?

fibrils more spaced apart → less transparent cornea

Premature infants have more or less transparent corneas than full term? Why?

less, incomplete development of stromal fiber organization

Maturation of stroma occurs in what direction? So a premature baby…

Posterior to anterior (so, water would be anterior in premature infant)

Keratocytes are light scatterers. How do they also promote transparency?

Express large quantities of crystallin protein in its thin cytoplasm that demonstrate destructive interference

Crystallin

water soluble proteins that facilitate minimal light scatter through destructive interference

Why does corneal wounding cause reduced transparency?

Keratocytes become highly reflective myofibroblasts, don’t produce crystallin protein

T or F, there is active inhibition of blood and lymphatic vasculature in the cornea

T

When does vascularization of the cornea occur?

Corneal damage and/or lack of oxygen

T or F, there are no pro-angiogenic factors in the cornea

F, a balance of pro- and anti-angiogenic factors result in normal avascularity/promote neovascularization in the cornea

What inhibits vascularization of the cornea?

sFlt-1

What is the main pro-angiogenic molecule that triggers cascade in the cells to make more vessels?

VEGF-A

What is sFlt-1? (Soluble Flt-1)

Anti-angiogenic receptor that does not trigger vascularization when VEGF binds (because no intracellular portion to cause signal)

T or F, the manatee has vascular growth on the cornea, so it must express sFlt-1

F, it does NOT express sFlt-1 (anti-angiogenic)

How does myelination relate to corneal transparency?

nerve bundles lose perineurium and myelin sheaths as they enter central cornea

What about the corneal epithelium assist transparency?

• also express crystallin proteins (like keratocytes)

• Relatively thin

• Organelles (like mitochondria) are sparse to minimize scatter

  • NON KERATINZED

What might cause keratinization of thin epithelium of cornea?

injury/acid burn, autoimmune, Keratinizing Squamous Metaplasia

3 categories of UV light from smallest to largest wavelengths (and thus amount of absorption)

UVC, UVB, UVA

Which UV light is 100% absorbed (0% transmitted) by the cornea?

UVC

Which UV light is closest to visible light spectrum?

UVA

Which has longer wavelength, visible light or infrared?

Infrared

In visible light, 85-99% is (absorbed or transmitted) by the cornea?

transmitted

Infrared light is transmitted but depends on hydration, thickness, and __

wavelength

Why is it good that the cornea absorbs most of UV light?

Prevent transmission of high energy UV light to the sensory cells in the retina

Is there more or less transmission of visible light in thicker cornea?

less

Smaller wavelengths (violet) scatter more or less than larger ones (red)?

more

Why is cornea more susceptible to UV damage than skin?

no melanin

2 ways UV overexposure damages epithelium

• inducing apoptosis pathways

• Generating ROS (reactive oxygen species)

Photokeratitis

massive loss of corneal epithelial cells from UV exposure

Is photokeratitis acute or chronic injury from UV light?

acute

What is a chronic injury from UV light exposure?

pterygium (overgrowth of conj into cornea)

__ acid absorbs UV light and is found in large amounts in corneal epithelium and stroma

ascorbic acid

Ascorbic acid

absorbs UV light, mitigates ROS by functioning as electron donor

How does ascorbate reduce ROS?

donates electron to damaged molecule so the unpaired electron does not cause damage

What protein/enzyme is produced by cornea and is one of the crystallins?

aldehyde dehydrogenase

3 roles of aldehyde dehydrogenase

1) absorbs UV light

2) Mitigates ROS by preventing damaging protein modification

3) mitigates ROS by preventing protein unfolding

What has tryptophan/tyrosine residues and accounts for 50% of UVB absorption in the cornea?

aldehyde dehydrogenase