The Vitreous

Question 1

The vitreous is what % water?

Answer 1

98%

Question 2

What makes up the framework of the vitreous and provides its plasticity (propensity of a material to undergo permanent deformation under load)?

Answer 2

collagen

Question 3

What fills the spaces between the framework and gives the vitreous its viscoelasticity (possess both viscous and elastic properties when deformed)?

Answer 3

hyaluronic acid and water

Question 4

An increase/decrease in what will make the vitreous more gel-like/fluid?

Answer 4

amount of collagen

Question 5

What accounts for species variation in vitreal consistency?

Answer 5

collagen content (cortical areas generally contain more collagen)

Question 6

How does embryology vitreous differs from postnatal vitreous?

Answer 6

much more dense and opaque

Question 7

What happens to the vitreous after birth that makes the vitreous less dense and clear?

Answer 7

collagen content stays relatively the same, however hyaluronic acid concentration increase fourfold, dispersing the collagen fibrils and increasing the clarity of the vitreous (HA and water fill more spaces between the vitreous)

Question 8

What happens to the vitreous as it ages?

Answer 8

continue production of HA and dispersal of the collagen fibrils causes the vitreous to become liquefied with age.

Question 9

What happens to collagen bundles and hyaluronic acid as liquefaction of the vitreous progresses with age?

Answer 9

collagen bundles are packed into the remaining gel fraction, and HA molecules are redistributed to the liquid fraction

Question 10

What is a common complication of progressive virtual liquefaction, and what condition can this predispose to?

Answer 10

As the vitreous continue to liquify the posterior virtual cortex can separate from the internal limiting membrane. This predisposed to rhegmetogenous retinal detachment as forces along the vitreoretinal interface are altered.

Question 11

At what age in humans do rheologic changes in the central vitreous occur?

Answer 11

5 year

Question 12

In elderly human patients, how much of the vitreous is eventually liquefied?

Answer 12

more than 50%

Question 13

What % of the globe does the vitreous occupy?

Answer 13

80%

Question 14

How does the vitreous play a role in growth of the eye?

Answer 14

• expansion of the vitreous aids in growth of the globe (removal of the vitreous prematurely lowers intravitreal pressure and stops growth of the eye)
• goldfish use this to their advantage to help increase the refractive power of an eye submerged in water

Question 15

How does virtual elongation effect the refractive power of the eye?

Answer 15

• vitreal elongation causes an increase in the axial length of the globe, which lengthens the path of incoming light, thus providing for greater light refraction
• an increased axial length of the globe can also lead to axial myopia (has been demonstrated through visual deprivation in nonhuman primates, chickens, and the cat)

Question 16

What effects elongation of the vitreous?

Answer 16

• the synthesis of collagen

- synthesis, molecular reconfiguration and hydration of HA molecules

Question 17

In humans, how might the vitreous aid in accommodation?

Answer 17

there is evidence that the vitreous moves anteriorly against the lens, changing the lens diameter and axial length

Question 18

What anatomic factors contribute to the optical clarity of the vitreous?

Answer 18

• small diameter of virtual collagen fibers
• wide dispersion of collagen fibers
• relative acellularity
• blood vitreous barrier

Question 19

What accounts for the blood vitreous barrier, and in what circumstances might it be disrupted?

Answer 19

• HA molecules are though to act as a barrier preventing diffusion of macromolecules and cell into the vitreous
• may be disrupted in trauma or cortex disruption

Question 20

The vitreous transmits 90% of light through what wavelengths?

Answer 20

300 and 1400 nm

Question 21

True or False: inflammatory responses, neovascularization, and collagenase activity are suppressed in the vitreous (likely as a function of the blood vitreous barrier).

Answer 21

true

Question 22

What are the two primary factors affecting diffusion of molecules through the vitreous?

Answer 22

hyaluronic acid concentration and molecule size

Question 23

What are some ways the vireos plays a role in ocular metabolism?

Answer 23

• serves as a storage site for retinal metabolites such as glycogen, amino acids and potassium
• acts as a depository for retinal and lenticular waste products such as lactic acid and free radicles (help protect the lens and retina from toxic compounds)

Question 24

In the cow, what size are pores in the vitreous?

Answer 24

400nm in diameter

Question 25

The vitreous provides both mechanics and structure support for what structures?

Answer 25

lens and retina

Question 26

The viscoelastic properties of the vitreous protect the internal eye structures during what activity, and what components of the vitreous contribute to its viscoelasticity?

Answer 26

• protect the internal structures of the eye from trauma and stress, especially during rapid eye movement
• the concentrations of collage and HA, as well as their cross linking, contribute to viscoelasticity

Question 27

How does the concentration of vitreal HA and collagen differ between the human and pig, and what property of the vitreous does this correspond to?

Answer 27

• the concentration of vitreal HA and collagen is twice as high in the human eye compared to the pig
• this corresponds to a 60% increase in the Spring constant (measure of elasticity) in the human vs pig vitreous