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Ocular Physiology #2 > Vitreous Humor > Flashcards

Vitreous Humor Flashcards

2
Q

how much of the eye is vitreous humor

A

80% (largest structure)

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3
Q

how long is the vitreous chamber in newborns

A

10.5mm

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4
Q

how long is the vitreous chamber in adults

A

16.5mm

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5
Q

how much of the vitreous is water

A

about 98%

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6
Q

what are the non-aqueous biochemical components of the vitreous

A

collagen and glycosaminoglycans (GAGs) that form the vitreous into a viscoelastic gel

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7
Q

when does the vitreous gel become liquified

A

it decreases with age-starting the day we are born, 20% (volume) by age 18 and progresses to 50% by 80th decade

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8
Q

what are the 2 types of collagen in the vitreous

A

type 2 (75%) and combined type 5/11 (10%)

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9
Q

what are 2 difficulties that arise in attempting to investigate the vitreous

A

it is “invisible” tissue and previous techniques are combined with artifacts, it is hard to make interpretations on the true in vivo situation

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10
Q

how is the primary vitreous formed

A

the optic cup is maintained by the lens vesicle, as the cup grows it is filled by fibrillar material (secreted by cells of embryonic retina), then the hyaloid artery penetrates and more fibrillar material from blood vessel cells fills the space

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11
Q

when does the secondary vitreous form

A

at the end of the 6th week

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12
Q

what happens as the secondary vitreous is formed

A

the size of the vitreous cavity increases and the hyaloid vascular system regresses

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13
Q

what is present after the hyaloid artery regresses

A

a tube in the primary vitreous surrounded by the secondary vitreous- Cloquet’s canal

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14
Q

where does cloquet’s canal extend to and from

A

from the retrolental space to the optic nerve

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15
Q

what type of vitreous are the zonules

A

tertiary vitreous

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16
Q

what shape does the mature vitreous have

A

spherical except anteriorly which is concave shape for the lens

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17
Q

What does the vitreous base extend from

A

a 3-D zone extending from 2mm anterior to 3mm posterior to the ora serrata and it is several mm thick

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18
Q

what is the cortex

A

the outermost part of the vitreous; divided into anterior cortex and posterior cortex

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19
Q

where in the vitreous is collagen densely packed

A

in the base area

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20
Q

what is the vitreoretinal interface

A

the outer part of the vitreous cortex, including anchoring fibrils of the vitreous body and the internal limiting membrane of the retina

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21
Q

how thick is the internal limiting membrane on the retina

A

between 1 and 3 microns thick

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22
Q

where are the 4 places the vitreous cortex is firmly attached to the internal limiting membrane at

A

the vitreous base, around the optic disc (weiss ring), at the vessels and in the area surrounding the foveola

23
Q

the INM (internal limiting membrane) is the basal lamina of which cells

A

the muller cells

24
Q

what is the internal limiting membrane made of

A

mainly type 4 collagen and proteoglycans

25
Q

what does the gel structure of the vitreous act as a barrier against

A

the movement of solutes

26
Q

what are the two processes substances move by in the vitreous

A

diffusion or bulk flow

27
Q

why does bulk flow occur in the vitreous

A

it is a result of a pressure gradient from the anterior to posterior pole of the eye

28
Q

is bulk flow a significant way to distribute low molecular weight substances in the vitreous

A

no

29
Q

what type of molecules move with bulk flow

A

large, high molecular weight substances

30
Q

what disolves and replaces the gel structure

A

aqueous lacunae: small pockets of liquid (melt together over time and creates a detachment)

31
Q

why does the apparent molecular weight of vitreous collagen increase with age

A

because of the formation of new covalent cross-links between the peptide chains

32
Q

what are the 4 main aspects of vitreous physiology

A
  1. support function for the retina and filling-up function of the vitreous cavity
  2. diffusion barrier between the anterior and posterior segments
  3. metabolic buffer function
  4. establishment of an unhindered path of light
33
Q

what is the Maillard reaction

A

insoluble proteins produced by a covalent bond between an amino group and glucose (why proteins are cross-linked)

34
Q

how does the vitreous support the retina

A

helps prevent a large retinal detachment and absorb external forces to protect globe deformation

35
Q

how much is the concentration of glucose increased in the vitreous of diabetic patients

A

it is doubled in concentration

36
Q

what is a PVD

A

posterior vitreous detachment: central degeneration is large and causes a collapse –> the cortex sinks to the center of the vitreous body

37
Q

when can a PVD result in a retinal tear

A

if there is a strong attachment between the posterior cortex and the internal limiting membrane and the vitreous pulls the retina away from that spot

38
Q

who is most likely to get a PVD

A

it is considered a common pathophysiologic condition (normal aging phenomena)

39
Q

what is the first step in a rhegamatogenous retinal detachment

A

a PVD (RRD is the worst type of retinal detachment- fluid accumulates between the neurosensary retina and the RPE)

40
Q

in untreatable edema, will performing a vitrectomy improve VA’s

A

yes and it will decrease the edema

41
Q

what causes macular edema

A

increased passive permeability and decreased outward active transport (normally the blood-retinal barrier is tight- low passive permeability)

42
Q

how is the vitreous a diffusion barrier between anterior and posterior segments

A

diffusion is slow and movement by bulk flow is limited in a gel (also preventing topical substances from reaching the retina and optic nerve)

43
Q

what happens to the barrier after removal of lens and anterior vitrectomy

A

the exchange between the anterior and posterior is fast and easy

44
Q

what happens in diabetic patients (who don’t have bleeding) after a vitrectomy

A

the pre-retinal oxygen tension is improved- oxygen transport increases and retinal neovascularization and macular edema regress

45
Q

how can the vitreous act as a metabolic buffer

A

it can be a reservoir for metabolism of the ciliary body and retina

46
Q

what do the glucose and glycogen reserves in the vitreous supplement

A

the metabolism of the retina especially during anoxic conditions

47
Q

can a vitrectomy stop or cure diabetic retinopathy if there is active blood leaking

A

no it can lead to rapid movement of vasoproliferative factors from the posterior pole to the anterior pole- leading to neovascularization of the anterior segment (neovascular glaucoma)

48
Q

what does the vitamin C reserve in the vitreous supplement

A

it acts as a reservoir of antioxidants for stress situations protecting the retina from metabolic and light induced free radicals

49
Q

why is light unhindered in the vitreous

A

it produces a low concentration of structural macromolecules

50
Q

what are 6 pathological conditions that interfere with transparency of the vitreous

A

synchysis scintillations, asteroid degeneration, hemorrhages, inflammatory material, fibrous tissue, and lack of regression of the hyaloid artery

51
Q

what is a vitreo-macular traction

A

where the vitreous is lifting away from the retina and pulling on the fovea (seen with an OCT)- patients are unaware/no symptoms

Ocular Physiology #2 (7 decks)

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