The Limbus

• What is the limbus also known as?

  Corneo-scleral junction

  
• How wide is the limbus?

  1. 1.5 – 2.0 mm
• What occurs at the limbus regarding curvature?

  Abrupt change of radius of curvature
• What are the two sulci associated with the limbus?

  External and internal scleral sulcus
• What defines the anterior boundary of the limbus histologically?

  Plane connecting Bowman’s and Descemet’s layers
• What defines the posterior boundary of the limbus histologically?

  Plane perpendicular to the globe surface
• What happens to the corneal epithelium at the limbus?

  It becomes conjunctival epithelium
• How does the corneal stroma change at the limbus?

  Regular corneal stroma becomes irregular scleral stroma
• What happens to the corneal endothelium at the limbus?

  It becomes discontinuous around trabecular meshwork
• Where do Bowman’s and Descemet’s membranes terminate?

  At the anterior border of the limbus
• What structures begin within the limbal area?

  Conjunctival stroma, episclera, and Tenon’s capsule

  
• How thick is the limbal epithelium?

  10-15 cells thick
• What may be present in the basal layer of the limbal epithelium?

  Melanocytes
• What type of collagen organization is found at the limbus?

  Transition from regular to irregular collagen bundles
• Is there a clear line of demarcation at the limbus?

  No line of demarcation may be identified
• How do scleral fibrils extend at the limbus?

  Further anteriorly on the external than internal side
• What are the components of the limbus and their functions?

  • Epithelium
  • Loose connective tissue stroma
  • Limbal conjunctiva
  • Tenon’s capsule
  • Episclera
  • Limbal stroma
  • Provides nutrients, drainage, and stem cells
• What is the function of the scleral spur?

  Helps maintain correct corneal curvature
• How does Descemet’s membrane change at the limbus?

  Tapers at the anterior limbal boundary
• What does the endothelium do at the limbus?

  Continues into the anterior chamber angle
• Where does Tenon’s capsule lie in relation to the conjunctival submucosa?

  Just inner to the conjunctival submucosa
• What happens to Tenon’s capsule and episclera at the limbus?

  Both begin in the limbus but not in the cornea
• What are the palisades of Vogt?

  Radial projections of limbal epithelium and stroma
• How wide and long are the palisades of Vogt?

  0.5 mm wide and 2-4 mm long
• What is the suspected site of stem cell origin in the limbus?

  Surface of the limbal area
• What does the X, Y, Z hypothesis explain?

  Corneal epithelial maintenance
• What does 'X' represent in the X, Y, Z hypothesis?

  Proliferation of basal epithelial cells
• What does 'Y' represent in the X, Y, Z hypothesis?

  Centripetal movement of peripheral cells
• What does 'Z' represent in the X, Y, Z hypothesis?

  Epithelial cell loss from the surface
• What forms the limbal blood vessel network?

  Capillary loops surrounding the cornea
• How many collector channels drain into the radial episcleral veins?

  25-30 collector channels
• What are the functions of the limbus?

  Provide nutrients, drainage, and stem cells
• What do the vessels in the limbus provide for the peripheral cornea?

  Some metabolites
• What major route does the limbus provide?

  Drainage of aqueous humour from the eye
• What structures provide the major route for drainage of aqueous humour?

  Trabecular meshwork and canal of Schlemm
• What are the three P's related to the limbus?

  Provide nutrients, drainage, and stem cells
• What are the histological features of the limbus?

  • Increased limbal epithelium thickness
  • Presence of melanocytes
  • Pigmentation in limbus and conjunctiva
  • Transition from regular to irregular collagen organization
• What are the changes that occur at the limbus?

  • Corneal epithelium to conjunctival epithelium
  • Regular corneal stroma to irregular scleral stroma
  • Corneal endothelium becomes discontinuous
  • Termination of Bowman’s and Descemet’s membranes
  • Beginning of conjunctival stroma, episclera, and Tenon’s capsule