The Vitreous

(1)
University of Sydney, Sydney, Australia
 

Overview

The vitreous is an almost spherical transparent gel that makes up 80 % of globe volume [1] (Fig. 7.1).
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Fig. 7.1
Vitreous structure

Development (See Table 7.1)

Table 7.1
Vitreous development [1–4]
Structure
Fetal age of development
Formation process
Adult derivative
Primary vitreous
Weeks 4–6
Fibrillary material secreted by the embryonic retina and hyaline artery in the optic cup
Cloquet’s canal
Secondary vitreous
Week 6 onward
The secondary vitreous is secreted by developing retinal cells
The secondary vitreous forms as the vitreous cavity enlarges and hyaloid artery regresses
Most of the adult vitreous (except Cloquet’s canal)
Tertiary vitreous
Week 12 onward
There is condensation of fibrillary material anterior to the vitreous base
Zonular fibers

Functions

The vitreous:
1.
Provides structural support to the retina posteriorly and lens anteriorly
  • The outermost portion, the cortex, consists of densely packed collagen fibrils [5].
  • The cortex is attached to the retinal internal limiting membrane (ILM) via anchoring fibrils.
  • The adhesion is strong in younger individuals and decreases with age [6]. It is most strong at the vitreous base, optic disk, retinal vessels, foveola, and at sites of retinal degeneration [7, 8].
 
2.
Acts as a viscoelastic shock absorber for the eye [9]
  • The vitreous exists as a gel with >99 % water [1, 10].
  • The gel is maintained by long, thick, non-branching collagen fibrils suspended in hyaluronic acid.
  • The fibrils predominantly consist of type II collagen [11].
  • Type IX collagen is also present which may act as a bridge linking the type II fibers [12].
  • The combination of type II collagen and hyaluronic acid provides viscoelastic gel properties [5, 13, 14].
 
3.
Maintains image clarity
  • Regulated collagen fibrillary structure within a hyaluronic acid matrix minimizes light scattering [15].
  • Hyaluronic acid is highly hydrated; the molecules form large open coils with widely separated anionic sites [16].
  • This stabilizes the structure and conformation of the collagen fibrils and minimizes light scatter [12].
 
4.
Acts as a gel barrier for diffusion of solutes
  • The vitreous gel slows but does not impede diffusion of solutes [17].
  • Plasma solutes slowly diffuse from retinal vessels into the posterior vitreous and then the center [18].
  • The gel properties prevent solutes from the anterior segment reaching high retinal concentrations; however, a small amount of solute will cross the anterior vitreous face from the aqueous fluid [19].
  • The vitreous prevents high oxygen levels reaching the posterior lens surface [20].
  • Oxygen is supplied by diffusion from the retinal arterioles; vitreous oxygen tension is low and reduces toward the center [21].
  • The vitreous can act as a reservoir extending the half life of intravitreal medications.
 
5.
Slows bulk flow of large molecules moving from the anterior chamber toward the retina [22]
 
6.
Acts as a metabolic buffer
  • The vitreous acts as a reservoir of glucose for ciliary body and retinal metabolism [1, 10].
  • It is a reservoir of antioxidants and ascorbate important for lens metabolism [21].
  • It provides a metabolic buffer that is particularly useful for retinal metabolism and K+ homeostasis (See Chap. 8. The Retina) [23].
 

Aging Changes

With age disintegration of the gel structure results in formation of vacuoles and opacities [24].
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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Vitreous

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