Prefix: kerat-
Definition: The cornea is a transparent, avascular, watch-glass like structure which forms anterior one-sixth of the outer fibrous coat of the eyeball and covers iris, pupil and anterior chamber.
Histology:
It consists of 5 distinct layers which can be remembered using the mnemonic “ABCDE“:
| Layers | Thickness (µm) | Composition | Pathophysiology |
| Anterior epithelium | 50 | a. Top: 3-4 layers of squamous cells (uppermost are apical cells)b. Middle: 1-3 layers of wing cells (flattened polygonal shape) c. Deep: 1 layer of basal cells d. Basal lamina: scaffold for epithelim; collagen type IV; secreted by basal cells | Epithlial basement membrane dystrophy (AD) Meesmann’s (AD) |
| Bowman’s membrane | 8-14 | Unorganized type I collagen fibers in GAG matrix Acellular | Reis-Buckler’s (AD) |
| Corneal stroma (Substantia propria) | 500 (~90%) | ~ 80% water by weightParallely organized lamellae of collagen I, IV and V in mucopolysaccharide matrix Cells: Keratocytes, Langerhans’ cells, pigmented melanocytes, macrophages, histiocytes Only MMP-2 is found in healthy cornea | Granular (AD) Macular (AR) Lattice (AD) |
| Descemet’s membrane | 10-12 | Anterior organized fetal banded layer (no change with age 3 µm)Posterior unorganized non-banded layer (thickens with age 2-10 µm) PAS + true basement membrane | |
| Endothelium | 5 | Single layer of interdigitating hexagonal cells (It is a misnomer as the cells are not endothelial cells)Schwalbe’s line: termination of corneal endothelium (junction between endothelium and trabecular meshwork) Posterior embryotoxon: thickening and anterior displacement of Schwalbe’s line | Posterior polymorphous corneal dystrophy (AD) Fuch’s dystrophy (AD) |
Corneal Wound Healing in Short:
Epithelium: by migration and mitosis; rapid (starts 24-48 hours and completed by 6-8 days)
Stroma: non-complicated wound (avascular healing); complicated wound (vascular healing)
Descemet’s membrane and endothelium: slow regeneration
- Endothelium: mitosis and migration
- Descemet’s membrane: replacement by hyaline material derived from endothelium