Vitreous Humor Flashcards
how much of the eye is vitreous humor
80% (largest structure)
how long is the vitreous chamber in newborns
10.5mm
how long is the vitreous chamber in adults
16.5mm
how much of the vitreous is water
about 98%
what are the non-aqueous biochemical components of the vitreous
collagen and glycosaminoglycans (GAGs) that form the vitreous into a viscoelastic gel
when does the vitreous gel become liquified
it decreases with age-starting the day we are born, 20% (volume) by age 18 and progresses to 50% by 80th decade
what are the 2 types of collagen in the vitreous
type 2 (75%) and combined type 5/11 (10%)
what are 2 difficulties that arise in attempting to investigate the vitreous
it is “invisible” tissue and previous techniques are combined with artifacts, it is hard to make interpretations on the true in vivo situation
how is the primary vitreous formed
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
when does the secondary vitreous form
at the end of the 6th week
what happens as the secondary vitreous is formed
the size of the vitreous cavity increases and the hyaloid vascular system regresses
what is present after the hyaloid artery regresses
a tube in the primary vitreous surrounded by the secondary vitreous- Cloquet’s canal
where does cloquet’s canal extend to and from
from the retrolental space to the optic nerve
what type of vitreous are the zonules
tertiary vitreous
what shape does the mature vitreous have
spherical except anteriorly which is concave shape for the lens
What does the vitreous base extend from
a 3-D zone extending from 2mm anterior to 3mm posterior to the ora serrata and it is several mm thick
what is the cortex
the outermost part of the vitreous; divided into anterior cortex and posterior cortex
where in the vitreous is collagen densely packed
in the base area
what is the vitreoretinal interface
the outer part of the vitreous cortex, including anchoring fibrils of the vitreous body and the internal limiting membrane of the retina
how thick is the internal limiting membrane on the retina
between 1 and 3 microns thick
where are the 4 places the vitreous cortex is firmly attached to the internal limiting membrane at
the vitreous base, around the optic disc (weiss ring), at the vessels and in the area surrounding the foveola
the INM (internal limiting membrane) is the basal lamina of which cells
the muller cells
what is the internal limiting membrane made of
mainly type 4 collagen and proteoglycans
what does the gel structure of the vitreous act as a barrier against
the movement of solutes
what are the two processes substances move by in the vitreous
diffusion or bulk flow
why does bulk flow occur in the vitreous
it is a result of a pressure gradient from the anterior to posterior pole of the eye
is bulk flow a significant way to distribute low molecular weight substances in the vitreous
no
what type of molecules move with bulk flow
large, high molecular weight substances
what disolves and replaces the gel structure
aqueous lacunae: small pockets of liquid (melt together over time and creates a detachment)
why does the apparent molecular weight of vitreous collagen increase with age
because of the formation of new covalent cross-links between the peptide chains
what are the 4 main aspects of vitreous physiology
- support function for the retina and filling-up function of the vitreous cavity
- diffusion barrier between the anterior and posterior segments
- metabolic buffer function
- establishment of an unhindered path of light
what is the Maillard reaction
insoluble proteins produced by a covalent bond between an amino group and glucose (why proteins are cross-linked)
how does the vitreous support the retina
helps prevent a large retinal detachment and absorb external forces to protect globe deformation
how much is the concentration of glucose increased in the vitreous of diabetic patients
it is doubled in concentration
what is a PVD
posterior vitreous detachment: central degeneration is large and causes a collapse –> the cortex sinks to the center of the vitreous body
when can a PVD result in a retinal tear
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
who is most likely to get a PVD
it is considered a common pathophysiologic condition (normal aging phenomena)
what is the first step in a rhegamatogenous retinal detachment
a PVD (RRD is the worst type of retinal detachment- fluid accumulates between the neurosensary retina and the RPE)
in untreatable edema, will performing a vitrectomy improve VA’s
yes and it will decrease the edema
what causes macular edema
increased passive permeability and decreased outward active transport (normally the blood-retinal barrier is tight- low passive permeability)
how is the vitreous a diffusion barrier between anterior and posterior segments
diffusion is slow and movement by bulk flow is limited in a gel (also preventing topical substances from reaching the retina and optic nerve)
what happens to the barrier after removal of lens and anterior vitrectomy
the exchange between the anterior and posterior is fast and easy
what happens in diabetic patients (who don’t have bleeding) after a vitrectomy
the pre-retinal oxygen tension is improved- oxygen transport increases and retinal neovascularization and macular edema regress
how can the vitreous act as a metabolic buffer
it can be a reservoir for metabolism of the ciliary body and retina
what do the glucose and glycogen reserves in the vitreous supplement
the metabolism of the retina especially during anoxic conditions
can a vitrectomy stop or cure diabetic retinopathy if there is active blood leaking
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)
what does the vitamin C reserve in the vitreous supplement
it acts as a reservoir of antioxidants for stress situations protecting the retina from metabolic and light induced free radicals
why is light unhindered in the vitreous
it produces a low concentration of structural macromolecules
what are 6 pathological conditions that interfere with transparency of the vitreous
synchysis scintillations, asteroid degeneration, hemorrhages, inflammatory material, fibrous tissue, and lack of regression of the hyaloid artery
what is a vitreo-macular traction
where the vitreous is lifting away from the retina and pulling on the fovea (seen with an OCT)- patients are unaware/no symptoms
