Cornea

cornea dimensions

11.7mm horizontal and 10.6mm vertical

Optical zone

area responsible for image formation

central zone is mostly spherical, flattens near periphery

Corneal epithelium

50 - 60 um thick

5 layers

non keratinised stratified squamous → wing cells → columnar basal cells

Superficial cell layer

attached by desmosomes

outer surface protrudes microvilli and microplicae - binding of tear film

Wing cells

2-3 layers thick

tight, lateral, intercellular junctions and multiple gap junctions - free communication between cells

XYZ hypothesis

refers to maintenance of epithelium

corneal stem cells at limbus replenish basal cells

proliferation of basal cells → central movement → cell loss from surface

basal cells

single layer of tall columnar cells on basement membrane

desmosomes and gap junctions

adherent via anchoring complex

basement membrane

secreted by epithelial cells

made of type 4 collagen, laminin, HSPGs

aids with binding of epithelium to bowmans layer

Bowmans layer

Acellular collagen fibril matrix

mainly type 1 collagen

corneal stroma

Dense regular CT

90% of thickness

water, collagen in lamellae, keratocytes, proteoglycans and GAGs

mainly type 1 collagen

keratocytes

corneal stroma cells

lie between lamellae

maintain stromal collagen and ECM

Decorin

Proteoglycan most abundant in anterior stroma

some control of collagen fibril diameter

Lumican

Proteoglycan most abundant in posterior stroma

major control of collagen fibril diameter

Keratan sulfate

GAG that makes up 60% of corneal GAGs

45nm long

most in posterior corneal stroma

attracts and binds water to help maintain spacing between collagen fibrils

Dermatan sulfate

GAG that makes up 40% of corneal GAGs

70 nm long

most in anterior corneal stroma - prevents water evaporation

attracts and binds water to help maintain spacing between collagen fibrils

Descemet’s membrane

basement membrane of the endothelium

type 4 collagen

has ant and post laminae

terminates at limbus - Schwalbe’s line

Corneal endothelium

innermost layer adjacent to aqueous humour and anterior chamber

single layer of flat hexagonal cells (most efficient)

role in fluid production and transportation for stromal hydration

non replenishing cells

Nutrient supply

Diffusion from aqueous humour and limbal vessels

oxygen from atmosphere that is dissolved in tear film

Palpebral cappilaries

structure that provides nutrients to cornea when eye closed

transparency of cornea

Regulation by

• collagen sizing, spacing and lattice pattern

• lattice fibril arrangement

• keratocyte spacing

• endothelial fluid pump