Limbus

• What is the limbus?

  border/junction between cornea and sclera
• Limbus features

  - Band that encircles the periphery of the cornea

  - Approx 1.5 to 2.0 mm wide
• Limbal profile

  The radius of curvatureabruptly changes at the junction of cornea and scleracreating a shallow furrow or sulcus externally
• Where is the external scleral sulcus? What does it result from?

  External scleral sulcus: transition between flatter sclera and steeper cornea. Results from product of radius of curvature change
• Describe the internal scleral sulcus

  -where cornea and sclera meet

  -bounded anteriorly by cornea and posteriorly by scleral spur

  -allows for aqueous drainage structures (trabecular mesh work and canal of Schemm)
• Changes that occur at the limbus

  1. The corneal epithelium becomes conjunctival epithelium

  2. Regular corneal stroma becomes irregular scleral stroma

  3. Corneal endothelium become discontinuous

  4. Bowman's and Descemet's membrane terminate at the anterior border

  5. The conjunctival stroma, episclera and Tenon's capsule begin within the limbal area
• Why does the corneal endothelium become discontinuous at the limbus?

  In order to wrap around the strand of the trabecular meshwork
• Histologic (cells and tissues study) features of the limbus

  1. Limbal epithelium can increase to a layer 10-15 cells thick

  2. Melanocytes may be present within the basal layer

  3. Pigmentation may be evident within the limbus and conjunctiva
• The limbus contains the transition from...

  The very regular corneal lamellae to the irregular and random organization of collagen bundles in the sclera
• the transition between corneal lamellae and collagen bundles are...

  GRADUAL (no line of demarcation may be identified)
• Where do scleral (collagen) fibrils extend

  They extend further anteriorly on the external than on the internal side of the limbus
• Describe the scleral spur

  1. Within the stroma

  2. Group of collagen fibrils lying circumferentially

  3. Formed by collagen that is continuous with the collagen of the trabeculae

  4. Forms an annulus (ring shape)

  5. exists to help maintain correct corneal curvature
• (Corneal layers) features of descemet's membrane

  1. Tapers at the anterior limbal boundary

  2. Posterior portion becomes interlaced with the connective tissue for the anterior sheets of the trabecular meshwork
• (Corneal layers) Endothelium

  Continues into the anterior chamber angle as the endothelial covering
• Where does the tenon's capsule lie?

  inner to the conjunctival submucosa
• Where does the episclera lie?

  inner to Tenon's capsule
• Where does the tenon's capsule, the episclera and the conjunctival stroma fuse?

  Within the anterior limbal area
• What is common to both the tenon's capsule and the episclera?

  They both begin in the limbus but do not continue into the cornea
• What does the bulbar conjunctival epithelium contain?

  Goblet cells, melanocytes, underlying blood vessels
• What does the limbal epithelium contain?

  Melanocytes and underlying blood vessels (does not contain goblet cells)
• What does the corneal epithelium contain?

  No goblet cells, melanocytes or underlying blood vessels.
• What are palisades of vogt?

  Small lines within limbal area, may need to be assessed in clinics as an optometrist
• Describe Palisades of Vogt

  1. Radial projections of limbal epithelium and stroma

  2. Extend into the corna in spoke like fashion

  3. 0.5mm wide, 2-4mm long

  4. Surface of the limbal area where the palisade of Vogt arise remains flat

  5. Epithelium within this area is suspected to be the source of corneal epithelial basal cell replication (stem cell origin)
• How does corneal epithelium regeneration occur?

  1. Epithelial wound with basement membrane intact

  2. Sliding of adjacent epithelial cells (after 1 hour)

  3. Formation of pseudopods (After 15 hours)

  4. Cells become more cuboidal increasing cell coverage (After 24-48 hours)

  
  

  (DNA synthesis and hemidesmosomal attachment begins)
• What is the limbal blood vessel network formed by?

  1. Capillary loops that surround the cornea

  2. Conjunctival and episcleral vessels
• Where do limbal veins collect blood from?

  The anterior conjunctival veins (25-30 collector channels)
• Where does the collected blood from limbal veins drain into?

  Into the radial episcleral veins (which empty into the anterior ciliary veins)
• What are the main functions of the limbus?

  1. To provide nutrients to adjacent tissue

  2. To provide a pathway for the drainage of aqueous humour from the eye

  3. To provide stem cells for the regeneration of corneal cells
• How does the limbus provide nutrients to adjacent tissue?

  The limbus contains vessels that loop in the conjunctival and episcleral tissue and provide metabolites for the peripheral cornea
• How does the limbus provide a pathway for the drainage of aqueous humour from the eye?

  The limbus has structures in the internal scleral sulcus, the trabecular meshwork and the canal of schlemm that provide the major route for aqueous humour drainage