Vitreous physiology and Biochem

vitreous fxns

1. storage area/ avenue for metabolites

2. shock absorber

3. light transmission/refraction

4. source of ascorbic acid - vitamin C

5. physical support

whats the insoluble part of hte vitreous

collagen (type 2)

describe the collagen of the vitreous

1. type 2

2. varying density and orientation of collagen fibers

3. provides firmness, rigidity, stiffness, resistance to tensile forces, and transparency due to think fibrils

4. synthesized by the granular endoplasmic reticulum of vitreous cells (hyalocytes found in cortex)

where is the highest conc of collagen

cortex

where is the collagen the densest

vitreous base

where is collagen the least dense

central medullary vitreous

where are hyalocytes found

cortex

whats the soluble portion of the vitreous

hyaluronic acid

describe hyalouronic acid

1. synthesized by hyalocytes in the vitreous cortex

2. long unbranched MPS chain assuming sphenoid shape that forms a 3D sponge like network coiled wi the collagen matrix

3. high negative charge

4. reinforces and stabilizes collagen network

5. retains water for rigidity

6. provides viscosity

7. controls diffusion

8. steric hinderance

9. hydrophillic

what gives the viscoelastic properties of hte vitreous

collagen + HA structure

describe ascorbic acid (Vit C) in the vitreous

1. free radical scavenger from hyalocytes or aqueous

2. keeps a hypoxic env

3. absorbs UV light

4. main viscosity reducing agent

sugars in teh vitreous

1. aid in metabolism of hyalocytes and neigboring structures

   1. metabolism occurs in vitreous cortex

2. diffuse from retinal/choroidal circulation or posterior aq

electrolytes in the vitreous

1. ionic diffusion

2. Na

3. K

4. Cl

5. HCO3

6. Ca

7. phosphates

8. lactic acid

what happens to the vitreous as we age

the vitreous turns from a gel to more of a liquid

it liquifies

where does vitreous liquification start

in the medulla where theres less collagen conc

• no hyalocytes or dense collagen

• acellular

at what age does vitreous liquification start

age 4

adult eye is 20% liquid

by 80-90 its >50% liquid

what structural changes happen to collagen/HA

1. theres an inc in MW

   1. this happens bc of the formation of new covalent cross links btw peptide chains

      1. collagen fibrils are visible as bundles instead of spaced out fibrils

synchysis

liquification creating vacuoles

syneresis

collagen fibrils contract and shrink and pull off (separation)

together, synchysis and syneresis aer

liquefaction and the formation of lacunae

posterior vitreous detatchment

1. increased liquid volume of vitreous results in a collapse of the vitreous centrally

   1. this pulls the posterior vitreous forward

2. ILM thickens w age and does not adequately hold inserted collagen fibers

3. collagen fibrils collapse and aggregate into sheets of collagen fibers

what part of the retina is the vitreous normaly attached to

ILM

where does a PVD happen

its a separation of the posterior vitreous from the ILM of the retina (posterior to the vitreous base)

whats the order a PVD detaches in

1. peripherally

2. macula

3. optic nerve

   1. total PVD w a Weiss ring

what are symptoms of a PVD

1. floaters

2. flashes of light

   1. as vitreous detaches it pulls on the retina

in a regular PVD, _________ and ________ occur simultaneously

synchysis and syneresis

what happens in an anomalous PVD

1. pathology or high RE causes the processes of synchysis and syneresis to not happen simultaneously

2. some areas stay attached while some separate

3. can tug on BV and cause bleeding

what factors contribute to a PVD

1. ocular inflammation

2. aphakia/psuedoaphakia

3. aging

4. RE - high myope >6

what happens in an asteroid hyalosis

phospholipid and calcium adherent to colagen strands

healthy eye

what happens in synchysis scintillans

cholesterol crystals

trauma - retinal detachment

what are anterior attachments of the vitreous???

1. anterior hyaloid membrane

2. bergers space

how does the intact vitreous provide protection against nuclear lens changes

1. anterior vitreous adherent to posterior lens in younger patients

2. much higher incidence of NS following vitrecotmy in patients over 50

what specific thing protects the lens from oxygen and how

1. ascorbate

   1. regulates intraocular molecular oxygen

   2. protects against oxidative stress

   3. maintained by a sodium dependent transporter in the pigmented layer of the CB ep

does the gel or liquid state of the vitreous have a higher conc of ascorbate

gel

• myope gets a cataract gaster bc they liquify faster

if the vitreous, how does this impact the lens

we get an accelerated cataract

PVD and peripheral structures

1. may also damage the cells of teh TM

   1. intact gel and crystalline lens protective against open angle glaucoma (POAG)

2. may induce apoptosis and neuronal cell death

how many blood supplies to retina

2

high oxygen demand due to metabolic activity

one of the best perfused organs

how does blood get to the retina?

1. aortic arch —> (right only: brachiocephalic artery) —> common carotid —>ECA and ICA —> ophthalmic artery —> Posterior ciliary arteries —> choroid

2. —>ophthlamic artery —> Central retina artery —> retina

what part of the retina does the CRA supply

inner 2/3

what part of the retina does the choriocapillaris supply

outer 1/3 - photoreceptors

what has he highest blood flow/ is most perfused

choroid

what does it mean that the eye is a closed sphere

blood vessels enter as arteries, merge w capillaries, and exit as viens

retinal vasculature (veins and arteries) travel together as they enter the gloe thru the center of the ONH and branch into _______________________________

4 quadrants throughout the posterior pole and peripheral retina

describer retinal arterioles

1. arteriole more narrow

2. brighter reflex

3. 2/3 A/V

4. no pulsation

describe arteries

1. high pressure

2. smaller lumen

3. thick smooth muscle

4. sensitive to sympathetic

5. tight jxns = BRB of vascular endo

6. autoregulation = maintain itself by vasoconstriction/dilation

wheres the outer BRB

RPE adn tight jxns

wheres the inner BRB

retinal vascular endo tight jxns

does the choriocapillaris have tight jxns

no, fenestrated - leaky

how does the retina drain

1. venous outflow is mediated by the CRV and vortex veins

2. these are branches of the superior and inferior ophthalmic veins

path of venous outflow / drainage

1. CRV

2. ophthalmic vein

3. CAV sinus

describe a venule

1. not holding high velocity blood

2. thinner wall

3. large lumen

4. tight jxns - BRB

5. autoregulation

6. more sensitive to parasymp

describe retinal veins

1. larger

2. no prominent central reflec

3. VOLD

4. pulsate

whatas the equatio nfor perfusion pressure

Perfusion pressure = P arterioles - P veins

P arterioles = 65- 70

P veins = 10-21 similiar to IOP

what do arteries and veins share at crossing

a common adventitia

autoregulation of retinal vasculature

1. ability of retinal vasc to constrict or dilate in response to blood pressure, allowoing maintenance/ consistency of blood flow

autoregulation is done in the

retina

inner 2/3 only - not choroid bc those are fenestrated

autonomic system impacts

choroid