Expansion
Epithelium, Bowman layer, stroma, Descemet membrane, endothelium
Expansion
From outside in:
- Epithelium: stratified squamous non-keratinised, regenerating from limbal stem cells
- Bowman layer: acellular collagen; does not regenerate, so injury here scars
- Stroma: about 90 per cent of thickness. Regularly arranged collagen lamellae with keratocytes
- Descemet membrane: the basement membrane of the endothelium; regenerates
- Endothelium: single layer of cells that pump water out of the stroma. Does not regenerate, with cell density falling steadily through life
Transparency depends on three things: the regular lattice arrangement of stromal collagen, relative dehydration maintained by the endothelial pump, and the absence of blood vessels.
Failure of any of these causes opacity:
- Endothelial failure (Fuchs dystrophy, surgical trauma) causes stromal oedema and clouding, and is a leading indication for corneal transplantation
- Neovascularisation after chronic hypoxia, as with contact lens overwear, causes scarring
- Oedema from acute glaucoma disrupts the lattice
The cornea is avascular, obtaining oxygen from the atmosphere and nutrition from aqueous humour and limbal vessels, which is why it is the most successfully transplanted tissue: it is immunologically privileged.