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Ocular Physiology > Vitreous > Flashcards

Vitreous Flashcards

1
Q

3 functions for vitreous

A
  1. screen out UV and IR light
  2. provide a clear media for optical transmission
  3. protection of the retina
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2
Q

gross anatomy of the vitreous (simple) is:

A

largely an acellular, connective tissue structure

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3
Q

mass of vitreous

A

3.9 grams

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4
Q

vitreous is approximately ___% water

A

99%

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5
Q

vitreous occupies ___ % of the globe

A

60%

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6
Q

index of the aqueous is

A

n= 1.334

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7
Q

in the vitreous, there are some cells and they are located:

A

in the anterior portion of the vitreous near the ciliary epithelium

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8
Q

when does the primary vitreous develop?

A

at the end of the third embryologic week

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9
Q

the primary vitreous is behind the lens vesicle and is formed by what tissue and what migration pattern?

A

mesoderm that migrates between the optic cup and the lens vesicle

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10
Q

the primary vitreous is primarily the ___ ____

A

hyaloid vasculature

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11
Q

what is the hyaloid vasculature

A

artery that supplies nutrients to the tissue behind the lens and the lens

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12
Q

when does the hyaloid vasculature dissolve and how?

A

dissolves before birth, autolytic process- dissolves itself

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13
Q

the canal that is left after the primary vitreous dissolves is called the:

A

canal of Cloquet of hyaloid canal

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14
Q

the secondary vitreous starts to develop by the:

A

9th week

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15
Q

which vitreous becomes the mature vitreous?

A

secondary vitreous

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16
Q

what tissue origin and synthesis does the secondary vitreous come from?

A
  • neuroectoderm in origin

- synthesized by the primary virtual cells and retinal glial cells

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17
Q

how does the secondary vitreous interact with the primary

A
  • secondary eventually fills the globe and compacts the primary vitreous
  • there is a condensed area of vitreous that separates the primary and secondary vitreous
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18
Q

where does the secondary vitreous have a condensed area at?

A

at its periphery (acts like an outer skin)

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19
Q

the tertiary vitreous (zonular fibers) begin to develop at:

A

6 months embryonically

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20
Q

the fibrous structure of the secondary vitreous condenses and forms the:

A

zonules

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21
Q

where do the zonules merge with

A

with the lens capsule and the basement membrane of the ciliary body

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22
Q

2 vitreous attachments anteriorly

A
  1. wieger’s hyaloideo-capsular ligament

2. ora serrata (anterior vitreous base)

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23
Q

2 vitreous attachments posteriorly

A
  1. anterior nerve head (posterior vitreous base)

2. blood vessels of the retina and macula

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24
Q

what are the cells of the vitreous called

A

hyalocytes

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25
Q

where do hyalocytes possibly originate from?

A

from monocytes thus not intrinsic to the vitreous

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26
Q

where are hyalocytes located?

A

in the anterior cortical vitreous near the ciliary body

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27
Q

half life of hyalocytes

A

about a week

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28
Q

only known function of hyalocytes is:

A

to produce hyaluronic acid

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29
Q

what gives the vitreous its substance?

A

collagen

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30
Q

what gives the vitreous its viscoelasticity?

A

sodium hyaluronate

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31
Q

vitreous body is composed of:

A

collagen + hyalonic acid

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32
Q

vitreous humor is composed of:

A

fluid, composition similar to aqueous

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33
Q

the human vitreous if typically gel, composed of;

A

randomly arranged collagen fibers and soluble hyaluronic acid (sodium hyaluronate)

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34
Q

the collagen protein of the vitreous is called

A

vitrosin

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35
Q

3 ways vitrosin was shown to be similar to collagen:

A
  1. has the ammoniated hydroxyproline (specific to collagen)
  2. has a shrinkage temperature of 60 to 65 degrees centigrade, same as collagen
  3. has the same X-ray diffraction properties as collagen (64 repeated chains in length)
36
Q

3 ways vitrosin was shown to be different from collagen:

A
  1. 4% to 9% of the weight of vitrosin is a complex polysaccharide that cannot be separated from it
  2. polyacrylamide gel electrophoretic pattern of vitrosin does not match collagens type I, II, III, or IV
  3. only 60-93% of the vitreous framework can be dissolved with collegenase
37
Q

where is the collagen content of the vitreous highest?

A

where it is a gel, at the vitreous cortex

38
Q

the higher the collagen content, the ___ the viscosity of the vitreous

A

higher

39
Q

recent studies show ___ % of collagen in the vitreous is type ___

A

80 % is type II

40
Q

___ of all the protein in the body is collagen

A

1/3

41
Q

how many types of collagen

A

> 20 types

42
Q

what is sodium hyaluronate made up of?

A

a repeating chain of disaccharide units made up of N-acetyl-D-glucosamine and D-glucuronic acid

43
Q

molecular weight of sodium hyaluronate

A

between 10^4 and 10^6

44
Q

is sodium hyaluronate hydrophilic or hydrophobic

A

hydrophilic

45
Q

where is sodium hyaluronate the most concentrated?

A

in the cortical vitreous

46
Q

turnover rate for sodium hyaluronate is

A

0.45 micrograms/ day

47
Q

sodium hyaluronate controls the ____ of the vitreous

A

viscosity

48
Q

which portion of the vitreous increases with age

A

the liquid portion

49
Q

where does the oxygen in the vitreous come from?

A

from the arteries of the retina (lower concentration in the center vitreous)

50
Q

the water in the vitreous is exchanged about every

A

30 minutes

51
Q

water movement rate in the vitreous is:

A

85 mm^3/ min

52
Q

how does sodium enter the vitreous?

A

enters anteriorly from the ciliary body mostly and posterior chamber

53
Q

what % of the sodium in the vitreous is exchanged in 24 hours (mainly flowing to aqueous)?

A

90%

54
Q

how does potassium enter into the vitreous?

A

enters by active transport through the ciliary epithelium into the posterior chamber, then diffuses into the vitreous from lens and posterior chamber

55
Q

there is a gradient of chloride from the ____ to _____

A

vitreous to aqueous

56
Q

how is chloride removed from the vitreous?

A

via the retina and posterior chamber

57
Q

phosphate enters the vitreous via the

A

ciliary body

58
Q

where is where a low concentration of phosphate and why?

A

low concentration in vitreous base because it is used by the retina

59
Q

if the retina is damaged, how would the phosphate concentration change in the vitreous?

A

concentration of phosphate goes up in vitreous

60
Q

how does glucose enter the vitreous?

A

glucose diffuses into the vitreous from all tissues, principally the retina

61
Q

how is the rate of glucose diffusion different in the vitreous versus the aqueous?

A

due to the viscosity of the vitreous, diffusion into vitreous slower than into aqueous

62
Q

what does the blood-vitreal barrier block?

A

the movement of most proteins into the vitreous

63
Q

intra-vitreal injections demonstrate what difference in the permeability in the outward direction versus in for fluorescein?

A

it is 27-38 time more permeable in the outward direction than inward direction

64
Q

do antibiotics get into the vitreous

A

no, most antibiotics do not have good penetration info the vitreous

65
Q

how are antibiotic injections into the vitreous removed?

A

readily removed by a carrier mechanism

66
Q

how can the antibiotic carrier mechanism be inhibited?

A

simultaneous injection of probenecid- which poisons pumping mechanism to allow antibiotic to stay longer

67
Q

penetration of antibiotics into the vitreous appears to be related to ____ of the compound

A

liposolubility

68
Q

what is an example of an antibiotic that can penetrate from the blood to the vitreous

A

chloramphenicol- highly lipid soluble- therapeutic doses are achieved

69
Q

what is unique about the metabolism of the hyalocytes?

A

if the hyalocytes are all removed with an intravitreal injection of hyaluronidase it will totally be reformed in 6 weeks

70
Q

what is syneresis

A

breakdown of the vitreous gel, fluid filled cavities form (they can enlarge and cause PVD)

71
Q

list 3 age related changes of vitreous

A
  1. syneresis
  2. asteroid hyalosis
  3. synchysis scintillans
72
Q

% of people over 60 with syneresis

A

65%

73
Q

higher incidence of syneresis with what refractive error

A

myopes

b/c they have bigger eye?

74
Q

what is seen in asteroid hyalosis?

A

hundreds of small spheres of calcium soaps are seen in the vitreous

75
Q

is there any general significance of asteroid hyalosis?

A
  • not associated with any systemic condition
  • no clinical significance
  • does not affect vision
76
Q

even though asteroid hyalosis only affects about 1% of the population, who is it more common in and is it in 1 eye or both?

A

more common in elderly

3x more likely to be unilateral than bilateral

77
Q

is synchysis scintillans bilateral or unilateral?

A

usually bilateral

78
Q

what is synchysis scintillans?

A

cholesterol crystals in the vitreous that are not attached to collagen strands (So they fall to the bottom of the globe if eye stays stationary)

79
Q

any associated disease with synchysis scintillans? what age is it seen in?

A

not associated with any systemic conditions

  • no loss of vision
  • usually occurs before 40
80
Q

primary type vitreous inflammations are called

A

endophthalmitis

81
Q

describe the symptoms of endophthalmitis

A

painful, associated with photophobia, redness and edema of the conjunctiva and lids

82
Q

how is endophthalmitis usually caused?

A

usually caused by a penetrating injury that introduces bacillus subtilis (found in soil) into the vitreous

83
Q

what is the treatment/ results of endophthalmitis?

A

infection usually destroys the eye, even with administration of antibiotics

84
Q

what are the majority of vitreal infections?

A

secondary to inflammations of the choroid or retina

85
Q

secondary inflammations to the vitreous usually cause that effects?

A

result in white blood cells in the vitreous that cause blurring of the retina and decrease in vision

86
Q

how can vitreal hemorrhages be caused?

A

trauma, diabetes, HBP, or blood dycrasias (such as leukemia)

87
Q

how is a vitrectomy performed?

A

incision made at pars plana of ciliary body, small micro-surgical instrument inserted which takes small piece of vitreous, cut sit, remove it, and then replaces it with an equal volume of saline or other fluid

Ocular Physiology (8 decks)

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