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.