Lecture 11 - Vitreous Physiology

Where is the anterior chamber located in the eye?

Between the cornea and iris.

Where is the posterior chamber located in the eye?

Between the iris and lens.

Where is the vitreous chamber located in the eye?

Between the lens and retina.

What is the vitreous chamber?

The largest ocular chamber that contains vitreous humor.

What percentage of ocular volume does the vitreous chamber make up?

Approximately 80% of ocular volume.

What is the approximate volume of the vitreous chamber?

Approximately 4 mL.

What fills the vitreous chamber?

Vitreous humor (vitreous body)

How is vitreous humor described physically?

A transparent, gel-like substance.

What structures form the peripheral and posterior boundaries of the vitreous chamber?

Pars plana of the ciliary body, retina, and optic disc.

What structures form the anterior boundaries of the vitreous chamber?

Posterior lens surface and retrozonular portion of the posterior chamber.

What are the five vitreous attachment sites from strongest to weakest?

Vitreous base, posterior lens, optic disc, macula, and retinal vessels.

Which vitreous attachment site is the strongest?

Vitreous base.

Which vitreous attachment site is the weakest?

Retinal vessels.

What is the ora serrata?

The anterior boundary of the retina associated with the vitreous base attachment.

What are the three vitreous zones?

Vitreous cortex, intermediate zone, and Cloquet’s canal.

Which vitreous zone is the outermost?

Vitreous cortex.

Which vitreous zone is the innermost?

Cloquet’s canal.

What is the intermediate zone of the vitreous?

The region between the vitreous cortex and Cloquet’s canal.

Is the vitreous transparent or opaque?

Transparent.

What is the viscosity of the vitreous?

Approximately 3.00–6.00.

What is the refractive index of the vitreous?

1.335.

What percentage of the vitreous is water?

98%.

What gives the vitreous its gel-like composition?

Interactions/complexes between collagen and proteoglycans.

What percentage of vitreous collagen is type II?

75%.

What is the role of type IX collagen in the vitreous?

Modulates spatial arrangement of collagen.

What is the role of type V/XI collagen complexes?

May initiate collagen fibril formation and assembly.

What is the role of type VI collagen?

Binds type II collagen and hyaluronic acid and helps organize vitreous structure.

What is the role of type VII collagen?

Found at the vitreoretinal interface and mediates vitreoretinal adhesion.

How are collagen fibrils arranged in the vitreous?

Regular and widely spaced.

Why is the spacing of collagen fibrils important?

It aids in transparency.

What provides mechanical strength to vitreous collagen?

Cross-linkages between fibrils.

Where is collagen density highest in the vitreous?

Vitreous base.

Rank collagen density from highest to lowest.

Vitreous base > posterior cortex > anterior cortex > center.

What cells synthesize collagen in the vitreous?

Fibroblasts.

Where are fibroblasts located in the vitreous?

Vitreous base near the ciliary body and near the optic disc.

What other cells may contribute to vitreous collagen synthesis?

Retinal Müller cells and ciliary body epithelial cells.

When is most vitreous collagen synthesized?

During childhood.

Is vitreous collagen synthesized physiologically in adults?

No, only pathologic synthesis occurs.

What is a proteoglycan?

A core protein with glycosaminoglycan (GAG) side chains.

What is the main function of proteoglycans in the vitreous?

To regulate hydration and collagen fibril spacing/arrangement.

What are common vitreal proteoglycans?

Opticin and versican.

What are common vitreal glycosaminoglycans (GAGs)?

Hyaluronic acid, chondroitin sulfate, and heparan sulfate.

How do proteoglycans interact with collagen?

They bind to collagen to regulate hydration and fibril organization/spacing.

What are glycosaminoglycans (GAGs)?

Long, unbranched polysaccharides.

What structural feature do GAGs have?

Repeating disaccharide units that vary depending on the specific GAG.

What is the charge of GAGs?

Highly negatively charged and highly polar.

Why are GAGs negatively charged?

Due to sulfate and/or carboxyl groups.

What is a key property of GAGs related to water?

They attract and bind water.

What can GAGs bind to in the vitreous?

Core proteins and/or collagen.

What are the functions of GAGs in the vitreous?

Extracellular matrix assembly, cell-cell interactions, and immune and inflammatory response.

What type of molecule is hyaluronic acid?

A glycosaminoglycan (GAG).

What is the structure of hyaluronic acid?

A linear polymer that can form large, open coils.

What determines the conformation of hyaluronic acid?

Surrounding environment (amount of water, ionic charges, etc.).

How does hyaluronic acid respond to high ionic charges?

It can contract.

Where is hyaluronic acid most concentrated in the vitreous?

Posterior vitreous cortex.

What cells synthesize hyaluronic acid in the vitreous?

Hyalocytes in the vitreous cortex near the vitreal surface.

What makes hyaluronic acid unique among GAGs?

It is not sulfated.

Where does hyaluronic acid exist in the extracellular matrix?

Freely (not bound initially).

What type of collagen does hyaluronic acid bind to?

Type II collagen.

What structural role does hyaluronic acid play in the vitreous?

Cross-linking creates the bulk of the vitreous gel structure.

What are the biological properties of hyaluronic acid?

Anti-inflammatory and anti-scarring.

What pathological process does hyaluronic acid inhibit?

Pathological angiogenesis.

What is the function of chondroitin sulfate in the vitreous?

Prevents collagen degradation and vitreous liquefaction

What is the function of heparan sulfate in the vitreous?

Maintains regular spacing between collagen fibrils.

Is versican large or small?

Large.

What type of side chains does versican have?

Many chondroitin sulfate side chains.

What is the function of versican in the vitreous?

Maintains hydration of the extracellular matrix surrounding collagen fibrils.

What additional role does versican have?

Regulates immune and inflammatory responses.

Is opticin small or large?

Small.

How many GAG side chains does opticin have?

Few.

What is the role of opticin in collagen structure?

Prevents aggregation of collagen fibrils for structure and transparency.

How does opticin contribute to vitreoretinal attachment?

Links vitreous to the internal limiting membrane (“molecular glue”).

What pathological process does opticin inhibit?

Pathological angiogenesis.

What hormone does opticin bind?

Growth hormone.

What types of antioxidants are present in the vitreous?

Ascorbate (vitamin C), glutathione, riboflavin.

How does vitamin C concentration in the vitreous compare to blood plasma?

Higher in the vitreous.

What organic/inorganic substances are found in the vitreous?

Oxygen, lactate, glucose, Na+, K+, Cl-, Ca2+.

How do lactate levels in the vitreous compare to blood plasma?

Much higher in the vitreous.

Where is glucose concentration highest in the vitreous?

Central vitreous.

What is a primary structural function of the vitreous?

Maintains spherical ocular shape.

How does the vitreous support the retina structurally?

Holds the retina against the choroid.

How does the vitreous contribute to eye growth?

Regulates growth of the eye/emmetropization via internal pressure.

What is a mechanical function of the vitreous?

Shock absorption.

What property allows the vitreous to absorb shock?

Viscoelastic properties.

What types of forces does the vitreous protect against?

Rapid eye movements, strenuous activity, and mechanical impact.

What type of barrier does the vitreous serve between ocular segments?

Diffusion barrier between anterior and posterior segments.

Why does the vitreous slow diffusion of substances?

Due to its gel-like consistency.

Is the vitreous a selective diffusion barrier?

Yes.

How does diffusion differ between cations and anions in the vitreous?

Cations diffuse more slowly than anions.

Why do topical medications poorly reach the posterior segment?

Because diffusion through the vitreous is inefficient.

How does diffusion into vitreous compare to aqueous humor?

Diffusion into vitreous humor is more difficult than into aqueous humor.

Why do systemic medications poorly reach the central vitreous?

Due to diffusion limitations of the vitreous.

Are intravitreal injections fully unaffected by diffusion?

No, their efficacy is still impacted by diffusion characteristics.

What metabolic support role does the vitreous provide?

Metabolite and nutrient storage and transport.

Which structures benefit from vitreous nutrient support?

Retina and lens.

Under what condition is vitreous support especially important?

Anoxic conditions.

What optical functions does the vitreous perform?

Absorbs, transmits, and refracts light.

Where does most UV filtering (<400 nm) occur?

Before light enters the vitreous (cornea and lens).

How does the vitreous interact with visible light?

It is optically clear and transmits visible light (~400–700 nm).

What wavelengths of light does the vitreous absorb?

Light longer than 1400 nm.