Biochemistry and Metabolism- Vitreous

Question 1

What volume of the eye does vitreous represent?

Answer 1

80%

Question 2

What is vitreous liquefaction associated with

Answer 2

loss of vitreous ascorbate and the
development of posterior vitreous detachment

Question 3

Why does pars plana vitrectomy accelarate formation of cataract

Answer 3

increases the diffusion of oxygen in the posterior segment
of the eye. The resultant increase in oxidative stress has been implicated in the ac- celeration of cataract formation after vitrectomy

Question 4

What does the primary vitreous contribute

Answer 4

hyaloid artery which nourishes the developing anterior segment and lens

Question 5

What does failure of regression of primary vitreous result in

Answer 5

persistent hyperplastic primary vitreous

Question 6

What does the tertiary vitreous give rise to

Answer 6

zonular fibers

Question 7

What does the secondary vitreous give rise to

Answer 7

consists of a gel matrix representing the largest structure of the eye and is routinely seen on clinical examination

Question 8

What is the index of refraction of vitreous

Answer 8

1.334- similar to aqueous humour

Question 9

What is the basic physical structure of the vitreous

Answer 9

a gel composed of a collagen framework interspersed with molecules of hydrated hyaluronan, also known as hyaluronic acid. he hyaluronan contributes to the viscosity of the vitreous humor and is thought to help stabilize the collagen network

Question 10

Where is the vitreous gel thickest

Answer 10

The rigidity of the gel is greatest in regions of highest collagen concentration: the peripheral (cortical) vitreous and the vitreous base

Question 11

What is the vitreous composed of

Answer 11

primarily of water (≈98%) and macromolecules (0.15%), including collagen, hyaluronan, and soluble proteins. There are very few resident cells in the vitreous; these are called hyalocytes

Question 12

What is the smallest molecular unit of various collagen types

Answer 12

tropocollagen

Question 13

Which collagen types are found in the vitreous

Answer 13

Type 2 (75% of vitreous collagen)- also found in cartilage
Type 9- found on surface of fibres acting to shield type 2 fibrils and prevent them from fusing together which can lead to condensation of vitreous collagen
Type 5/11- located in the core of collagen fibres particpating in initial stages of fibre formation

Question 14

How thick is the collagen fibre layer in the peripheral vitreous

Answer 14

100–300 μm

Question 15

What mediates interaction of vitreous and ILM

Answer 15

mediated by laminin, fibronectin, and the proteoglycan chondroitin sulfate

Question 16

Is hyaluronan positively or negatively charged

Answer 16

Negatively charged hence attracts sodium and therefore water which make up vitreous

Question 17

What is the predominant form of chondroitin in the vitreous

Answer 17

Versican

Question 18

Wagner syndrome and vitreous

Answer 18

Mutations in the VCAN gene, which encodes versican, have been implicated in Wagner syndrome. affected patients have an optically empty vitreous with peripheral condensation and retinal degeneration

Question 19

What is the major structural protein of the zonular fibres

Answer 19

Fibrillin

Question 20

Marfan syndrome implications on the lens

Answer 20

Defects in fibrillin are present in patients with Marfan syndrome, some of whom experience spontaneous lens subluxation and premature vitreous liquefaction, which can lead to retinal detachment.

Question 21

What structures constitute blood ocular barrier

Answer 21

• vascular endothelium of iris vessels
• nonpigmented epithelium of the ciliary body
• inner wall endothelium of the Schlemm canal
• vascular endothelium of retinal vessels
• retinal pigment epithelium (RPE)

Question 22

Electrolyte constituents of the vitreous

Answer 22

The concentrations of sodium (Na+) and chloride (Cl−) in the vitreous are similar to those in plasma, but the concentration of potassium (K+) is higher than that in plasma, as is that of ascorbate

Question 23

Predominant cell type in the vitreous

Answer 23

Hyalocytes. The highest concentration of these cells occurs at the vitreous base and in the posterior cortical vitreous

Question 24

How do epiretinal membranes form

Answer 24

In specimens obtained after PVD, hyalocytes have been found on the surface of the retina, where they contribute to formation of idiopathic epiretinal membranes (also known as macular pucker or cellophane maculopathy.)

Question 25

Myopia and PVD

Answer 25

Myopia is associated with faster liquefaction and earlier development of PVD. Vitreous samples taken from myopic eyes exhibit a higher concentration of MMP-2. MMPs are proteases involved in remodeling extracellular matrices, such as the vitreous

Question 26

What inhibitors of angiogenesis are present in the vitreous

Answer 26

thrombospondin 1 and pigment epithelium–derived factor. The vitreous protein opticin also suppresses angiogenesis in mouse models of retinal neovascularization

Question 27

What gene mutation is found in Stickler syndrome

Answer 27

COL2A1- Which codes for type 2 collagen- a major component of vitreous fibres.Affected patients have an optically empty vitreous due to premature liquefaction with peripheral condensation, which may induce retinal detachment. Mutations in collagen 2 and 11 are shown to be responsible in this syndrome

Question 28

Role of ascorbate in the vitreous cavity

Answer 28

he vitreous acts as a barrier to the diffusion of oxygen within the posterior segment. The available ascorbate binds with oxygen, forming dehydroascorbate, which is taken up by surrounding cells
Ascorbate + Oxygen
Dehydroascorbate + H2O. In postvitrectomized eyes, the amount of oxygen exceeds the capacity for clearance, leading to the production of reactive compounds that create oxidative stress in the lens, which in turn accelerates cataract formation