Histology Of Cornea

Histology Of Cornea


HISTOLOGY OF CORNEA 

  • The cornea has five distinct layers (from superficial to deep)?:-

1. Epithelium :

  • Outermost part of cornea.
  • Composed of stratified squamous non-keratinized epithelial cells.

2. Bowmann’s membrane:

  • Not a true membrane but simply a condensed superficial part of stroma.
  • Once destroyed, it does not regenerate.

3. Stroma (Substantia propria)

  • This layer constitutes most of the cornea (90% of thickness).
  • It consists of collagen fibrils (lamellae) embedded in hydrated matrix of proteoglycans.

4. Descemet’s membrane:

  • This layer bounds the stroma posteriorly. 
  • In the periphery it appears to end at the anterior limit of trabecular meshwork of Schwable’s ring.

5. Endothelium: 

  • Single layer of flat polygonal cells.
  • The endothelial cells contain ‘active pump’ mechanism & is the most important layer in maintaining the transparency of cornea.
  • The cell density of endothelium is around 3000 cells/ mm2 in young adults, which decreases with advancing age.
  • There is functional reserve for the endothelium.
  • Therefore owing to the high functional reserve & good metabolic activity of the endothelial layer, regeneration occurs rapidly after injury. 

CORNEAL TRANSPARENCY

  • Factors which maintaining corneal transparency are:

a) Anatomical factors

  1. Corneal epithelium & tear film:

  • Homogenity of the refractive index throughout the epithelium.

2. Absence of blood vessels (avascularity) & absence of myelinated nerves (Presence of unmyelinated nerves)

3. Peculiar uniform arrangement of the stromal lamellar collagen fibrils in a regular lattice.

b) Physiological factors

  • Physiological factors are those which keep the cornea in a relative state of corneal dehydration by maintaining corneal hydration.
  • These factors are:

i) Stromal swelling pressure & imbibition pressure.

ii) Barrier function of limiting layers (epithelium & endothelium)

iii) Endothelium pumps:

  • Corneal endothelium plays a predominan role in controlling fluid transport due to several enzymes systems: Na/K+ ATPase Bicarbonate dependent ATPase, Na+/H+ pump.

iv) Evaporation from corneal surface

v) Normal intraocular pressure 

  • These five factors help to maintain corneal hydration in a state of relative dehydration.
  • The most important of these is the endothelial pump mechanism.
Exam Question
 
  • Metabolically active layer of cornea is Endothelium & Epithelium.
  • Corneal epithelium composed of stratified squamous non-keratinized epithelial cells.
  • Schwable’s ring is seen in Descemet’s membrane layer of cornea.
  • Bowmann’s membrane has no regeneration capacity.
  • Corneal transparency is maintained by Endothelium.
  • Corneal deturgescence is maintained by an active sodium-potassium pump situated in endothelium.
  • The cell density of endothelium is around 3000 cells/ mm2 in young adults.
  • Corneal endothelium regenerates rapidly after injury.
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