Histology of Cornea and Corneal Dystrophies

Epomedicine

10 years ago

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)

The pre-Descemet’s layer/Dua’s layer, also termed the Dua-Fine layer and the pre-posterior limiting lamina layer, lies anterior to the Descemet’s membrane in the cornea, is 10 μm (range 6-16) thick, made predominantly of type I and some type VI collagen with abundant elastin, more than any other layer of the cornea.

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