Vitreous Flashcards
3 functions for vitreous
- screen out UV and IR light
- provide a clear media for optical transmission
- protection of the retina
gross anatomy of the vitreous (simple) is:
largely an acellular, connective tissue structure
mass of vitreous
3.9 grams
vitreous is approximately ___% water
99%
vitreous occupies ___ % of the globe
60%
index of the aqueous is
n= 1.334
in the vitreous, there are some cells and they are located:
in the anterior portion of the vitreous near the ciliary epithelium
when does the primary vitreous develop?
at the end of the third embryologic week
the primary vitreous is behind the lens vesicle and is formed by what tissue and what migration pattern?
mesoderm that migrates between the optic cup and the lens vesicle
the primary vitreous is primarily the ___ ____
hyaloid vasculature
what is the hyaloid vasculature
artery that supplies nutrients to the tissue behind the lens and the lens
when does the hyaloid vasculature dissolve and how?
dissolves before birth, autolytic process- dissolves itself
the canal that is left after the primary vitreous dissolves is called the:
canal of Cloquet of hyaloid canal
the secondary vitreous starts to develop by the:
9th week
which vitreous becomes the mature vitreous?
secondary vitreous
what tissue origin and synthesis does the secondary vitreous come from?
- neuroectoderm in origin
- synthesized by the primary virtual cells and retinal glial cells
how does the secondary vitreous interact with the primary
- secondary eventually fills the globe and compacts the primary vitreous
- there is a condensed area of vitreous that separates the primary and secondary vitreous
where does the secondary vitreous have a condensed area at?
at its periphery (acts like an outer skin)
the tertiary vitreous (zonular fibers) begin to develop at:
6 months embryonically
the fibrous structure of the secondary vitreous condenses and forms the:
zonules
where do the zonules merge with
with the lens capsule and the basement membrane of the ciliary body
2 vitreous attachments anteriorly
- wieger’s hyaloideo-capsular ligament
2. ora serrata (anterior vitreous base)
2 vitreous attachments posteriorly
- anterior nerve head (posterior vitreous base)
2. blood vessels of the retina and macula
what are the cells of the vitreous called
hyalocytes
where do hyalocytes possibly originate from?
from monocytes thus not intrinsic to the vitreous
where are hyalocytes located?
in the anterior cortical vitreous near the ciliary body
half life of hyalocytes
about a week
only known function of hyalocytes is:
to produce hyaluronic acid
what gives the vitreous its substance?
collagen
what gives the vitreous its viscoelasticity?
sodium hyaluronate
vitreous body is composed of:
collagen + hyalonic acid
vitreous humor is composed of:
fluid, composition similar to aqueous
the human vitreous if typically gel, composed of;
randomly arranged collagen fibers and soluble hyaluronic acid (sodium hyaluronate)
the collagen protein of the vitreous is called
vitrosin
3 ways vitrosin was shown to be similar to collagen:
- has the ammoniated hydroxyproline (specific to collagen)
- has a shrinkage temperature of 60 to 65 degrees centigrade, same as collagen
- has the same X-ray diffraction properties as collagen (64 repeated chains in length)
3 ways vitrosin was shown to be different from collagen:
- 4% to 9% of the weight of vitrosin is a complex polysaccharide that cannot be separated from it
- polyacrylamide gel electrophoretic pattern of vitrosin does not match collagens type I, II, III, or IV
- only 60-93% of the vitreous framework can be dissolved with collegenase
where is the collagen content of the vitreous highest?
where it is a gel, at the vitreous cortex
the higher the collagen content, the ___ the viscosity of the vitreous
higher
recent studies show ___ % of collagen in the vitreous is type ___
80 % is type II
___ of all the protein in the body is collagen
1/3
how many types of collagen
> 20 types
what is sodium hyaluronate made up of?
a repeating chain of disaccharide units made up of N-acetyl-D-glucosamine and D-glucuronic acid
molecular weight of sodium hyaluronate
between 10^4 and 10^6
is sodium hyaluronate hydrophilic or hydrophobic
hydrophilic
where is sodium hyaluronate the most concentrated?
in the cortical vitreous
turnover rate for sodium hyaluronate is
0.45 micrograms/ day
sodium hyaluronate controls the ____ of the vitreous
viscosity
which portion of the vitreous increases with age
the liquid portion
where does the oxygen in the vitreous come from?
from the arteries of the retina (lower concentration in the center vitreous)
the water in the vitreous is exchanged about every
30 minutes
water movement rate in the vitreous is:
85 mm^3/ min
how does sodium enter the vitreous?
enters anteriorly from the ciliary body mostly and posterior chamber
what % of the sodium in the vitreous is exchanged in 24 hours (mainly flowing to aqueous)?
90%
how does potassium enter into the vitreous?
enters by active transport through the ciliary epithelium into the posterior chamber, then diffuses into the vitreous from lens and posterior chamber
there is a gradient of chloride from the ____ to _____
vitreous to aqueous
how is chloride removed from the vitreous?
via the retina and posterior chamber
phosphate enters the vitreous via the
ciliary body
where is where a low concentration of phosphate and why?
low concentration in vitreous base because it is used by the retina
if the retina is damaged, how would the phosphate concentration change in the vitreous?
concentration of phosphate goes up in vitreous
how does glucose enter the vitreous?
glucose diffuses into the vitreous from all tissues, principally the retina
how is the rate of glucose diffusion different in the vitreous versus the aqueous?
due to the viscosity of the vitreous, diffusion into vitreous slower than into aqueous
what does the blood-vitreal barrier block?
the movement of most proteins into the vitreous
intra-vitreal injections demonstrate what difference in the permeability in the outward direction versus in for fluorescein?
it is 27-38 time more permeable in the outward direction than inward direction
do antibiotics get into the vitreous
no, most antibiotics do not have good penetration info the vitreous
how are antibiotic injections into the vitreous removed?
readily removed by a carrier mechanism
how can the antibiotic carrier mechanism be inhibited?
simultaneous injection of probenecid- which poisons pumping mechanism to allow antibiotic to stay longer
penetration of antibiotics into the vitreous appears to be related to ____ of the compound
liposolubility
what is an example of an antibiotic that can penetrate from the blood to the vitreous
chloramphenicol- highly lipid soluble- therapeutic doses are achieved
what is unique about the metabolism of the hyalocytes?
if the hyalocytes are all removed with an intravitreal injection of hyaluronidase it will totally be reformed in 6 weeks
what is syneresis
breakdown of the vitreous gel, fluid filled cavities form (they can enlarge and cause PVD)
list 3 age related changes of vitreous
- syneresis
- asteroid hyalosis
- synchysis scintillans
% of people over 60 with syneresis
65%
higher incidence of syneresis with what refractive error
myopes
b/c they have bigger eye?
what is seen in asteroid hyalosis?
hundreds of small spheres of calcium soaps are seen in the vitreous
is there any general significance of asteroid hyalosis?
- not associated with any systemic condition
- no clinical significance
- does not affect vision
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?
more common in elderly
3x more likely to be unilateral than bilateral
is synchysis scintillans bilateral or unilateral?
usually bilateral
what is synchysis scintillans?
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)
any associated disease with synchysis scintillans? what age is it seen in?
not associated with any systemic conditions
- no loss of vision
- usually occurs before 40
primary type vitreous inflammations are called
endophthalmitis
describe the symptoms of endophthalmitis
painful, associated with photophobia, redness and edema of the conjunctiva and lids
how is endophthalmitis usually caused?
usually caused by a penetrating injury that introduces bacillus subtilis (found in soil) into the vitreous
what is the treatment/ results of endophthalmitis?
infection usually destroys the eye, even with administration of antibiotics
what are the majority of vitreal infections?
secondary to inflammations of the choroid or retina
secondary inflammations to the vitreous usually cause that effects?
result in white blood cells in the vitreous that cause blurring of the retina and decrease in vision
how can vitreal hemorrhages be caused?
trauma, diabetes, HBP, or blood dycrasias (such as leukemia)
how is a vitrectomy performed?
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
