Biochemistry and Metabolism- Vitreous Flashcards
What volume of the eye does vitreous represent?
80%
What is vitreous liquefaction associated with
loss of vitreous ascorbate and the
development of posterior vitreous detachment
Why does pars plana vitrectomy accelarate formation of cataract
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
What does the primary vitreous contribute
hyaloid artery which nourishes the developing anterior segment and lens
What does failure of regression of primary vitreous result in
persistent hyperplastic primary vitreous
What does the tertiary vitreous give rise to
zonular fibers
What does the secondary vitreous give rise to
consists of a gel matrix representing the largest structure of the eye and is routinely seen on clinical examination
What is the index of refraction of vitreous
1.334- similar to aqueous humour
What is the basic physical structure of the vitreous
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
Where is the vitreous gel thickest
The rigidity of the gel is greatest in regions of highest collagen concentration: the peripheral (cortical) vitreous and the vitreous base
What is the vitreous composed of
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
What is the smallest molecular unit of various collagen types
tropocollagen
Which collagen types are found in the vitreous
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
How thick is the collagen fibre layer in the peripheral vitreous
100–300 μm
What mediates interaction of vitreous and ILM
mediated by laminin, fibronectin, and the proteoglycan chondroitin sulfate
Is hyaluronan positively or negatively charged
Negatively charged hence attracts sodium and therefore water which make up vitreous
What is the predominant form of chondroitin in the vitreous
Versican
Wagner syndrome and vitreous
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
What is the major structural protein of the zonular fibres
Fibrillin
Marfan syndrome implications on the lens
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.
What structures constitute blood ocular barrier
- 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)
Electrolyte constituents of the vitreous
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
Predominant cell type in the vitreous
Hyalocytes. The highest concentration of these cells occurs at the vitreous base and in the posterior cortical vitreous
How do epiretinal membranes form
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.)
Myopia and PVD
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
What inhibitors of angiogenesis are present in the vitreous
thrombospondin 1 and pigment epithelium–derived factor. The vitreous protein opticin also suppresses angiogenesis in mouse models of retinal neovascularization
What gene mutation is found in Stickler syndrome
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
Role of ascorbate in the vitreous cavity
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
