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Posterior segment

Vitreous

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  • Largest cavity in the eye 
    • 2/3rds of total volume (approx 3.9ml in emmetropic eyes)
    • 3.9g weight
  • Transparent viscoelastic gel
  • 98% water and 0.15% macromolecules
  • Structure: collagen network interspersed with hyaluronic acid
    • Hyaluronan provides viscosity, viscoelasticity and stability. 
      • Polysaccharide GAG: nontoxic, noninflammatory and nonimmunogenic

Hot Topic

There is no regeneration of collagen in the vitreous or rebuilding of the structure after syneresis

  • Also contains: 

    • Albumin, transferrin, tPA (for fibrinolysis after VH?) fibronectin, fibulin, opticin, VIT1, hyaluronidase, fibrillin (component of zonules) and MMP-2

    • Opticin and VIT1 are novel glycoproteins that maintain the structure of the collagen fibrils

  • Higher glycoprotein content than aqueous: glycoproteins are produced by the ciliary epithelium

Hot Topic

Na/K concentrations are the same as in the aqueous

  • High ascorbic acid concentration due to active transport by the ciliary epithelium
  • Acellular, although occasional cells may be found in the cortex

    • Single layer of hyalocytes line the vitreous cortex

    • Produce hyaluronan
    • There are fibroblast-type and macrophage-type hyalocytes
  • Saturated and unsaturated fats are present
  • RI of 1.33 (similar to aqueous)
  • Does not transmit light of wavelengths below 300nm
  • Anterior depression called the hyaloid fossa holds the lens

  • The outer “cortical” zone comprises densely arranged collagen fibres compared to the central vitreous which is more liquid and has less collagen

  • There is a “previtreal” space anteriorly between the pre-lenticular part and the vitreous base at the ora

Hot Topic

The central vitreous contains a fluid filled canal (hyaloid or Cloquet’s canal) which is the remnant of the hyaloid artery

  • Viscosity decreases with age

Vitreous functions:

  • Maintain transparency
  • Conduit for nutrient/solutes to the lens
  • Inhibitor of angiogenesis (this quality is reduced in diabetic vitreoretinopathy)
  • Non-compressible shock absorber
  • Retards the bulk flow of fluid

Embryology

  • 5 weeks: Primary vitreous comprising hyaloid artery and its branches
  • 5-12 weeks: Secondary vitreous (avascular) deposited behind the primary vitreous 
    • Secreted by the retina
    • Attachments to ILM
    • Early hyalocytes of the mononuclear phagocytes system
    • Deposit fibrillar material
    • Generate the microglia of the retina
  • 3 months: Tertiary vitreous
    • Secreted by neural ectoderm of ciliary body
    • Condenses around the optic cup margin and lens
    • Matures to form the vitreous base
    • Includes the zonules

  • Tunica vasculosa lentis (hyaloid vessel and branches) regresses and atrophies while the retinal vessels form

    • 30 weeks: blood flow in the hyaloid artery stops
    • 34-35 weeks: loses connection to the optic nerve and disappears
    • Remnant becomes Cloquet’s canal

Clinical Correlate

Persistent hyperplastic primary vitreous can cause leucocoria (other causes being retinoblastoma, Coat’s disease and toxocara etc.)

  • Much of the hyaluronan and type II collagen is deposited after birth

Vitreous collagen

  • 90% is Type II collagen and forms major component of the fibrils

  • 10% is Type IX collagen and there are cross-links between some type II and type IX molecules

    • Forms the surface of the fibrils
  • Types V/XI (thought to be unique to the vitreous) are found in small amounts

Hot Topic

The posterior hyaloid membrane seen following PVD stains for type IV collagen 

Clinical Correlate

Stickler’s syndrome is an autosomal disorder of type II collagen causing vitreous abnormalities with premature vitreous collapse and a high risk of GRT and RD.

Vitreous attachments

  • Cortical vitreous attached by condensations of fine collagen fibres

Hot Topic

Areas of attachment:

  • Peripheral retina and pars plana (3-4mm wide band of vitreous base)
  • Posterior lens capsule (ligamentum hyaloide capsulare or Wieger’s ligament)

    • The canal of Petit lies between the anterior vitreous cortex and the postero-capsular bundle

  • Retina at the optic disc margins (base of the hyaloid canal)
  • Inner limiting membrane especially along the retinal vessels (variable)
  • Edges of lattice degeneration

Liquefaction

  • The vitreous gel undergoes liquefaction with age
    • Age-related loss of type IX collagen leading to fibril breakdown
    • Hyaluronan is degraded
    • Collagen fragments clump into fibrillar opacities
    • ILM adhesions loosen
  • Myopia is associated with earlier liquefaction
    • Associated with changes in the concentrations of collagen and hyaluronan
  • May lead to posterior vitreous detachment
    • Occurs in 50% of the population

    • Often sudden: central liquefied vitreous passes through a hole in the cortical vitreous and strips the remaining cortex off the ILM

    • Retinal tear can occur

    • PVD may protect against proliferative diabetic retinopathy by removing a scaffold for fibrovascular proliferation

Anterior persistent fetal vasculature**

  • A differential in leucocoria
  • Most common cause of unilateral congenital cataract
  • Persistent hyaloid artery results in a white retrolental mass
  • Associated with
    • Microphthalmos (cp with Rb which is very rare in microphthalmic eyes)
    • Shallow AC
    • Long ciliary processes
    • Guarded visual prognosis due to glaucoma, cataract, deprivational amblyopia
    • Tractional retinal detachment in severe cases
    • M=F
    • Present at birth

  • Management: difficult. Early cataract removal, membrane excision, CL wear, amblyopia treatment

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