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.
- 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.
- 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.
- 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.