Cornea

Cornea

Also known as: corneal epithelium, Bowman layer, Descemet membrane, corneal endothelium, corneal stroma

The transparent, avascular, densely innervated front window of the eye; with the tear film it provides ≈2/3 of the eye's refractive power (≈43 D).

Show on body map Eye — Horizontal Section

Overview

Horizontal diameter≈11.5–12 mm

Central thickness≈0.52–0.55 mm

Refractive power≈43 D (≈2/3 of total)

Refractive index≈1.376

Adult endothelial cell density≈2 500 cells/mm²

The cornea forms the anterior one-sixth of the fibrous tunic. It is more steeply curved than the sclera (anterior radius of curvature ≈7.8 mm), ≈11.5–12 mm across horizontally and ≈10.5–11 mm vertically, and ≈0.52–0.55 mm thick centrally, thickening to ≈0.7 mm peripherally. Because the largest change in refractive index occurs at the air–tear interface, the cornea is the eye's most powerful refracting surface (≈43 D), but unlike the lens its power is fixed.

Its transparency depends on (1) the absence of blood vessels and keratinisation, (2) the uniform small diameter and regular lattice spacing of stromal collagen fibrils (≈200 orthogonally layered lamellae), which causes destructive interference of scattered light, and (3) a relative state of dehydration (deturgescence) maintained by the endothelial Na⁺/K⁺-ATPase and bicarbonate "pump-leak" mechanism together with the barrier functions of epithelium and endothelium. Oxygen comes mainly from the air via the tear film, nutrients from aqueous humour and limbal capillaries.

The cornea is among the most densely innervated tissues in the body (≈300–600 times the density of skin): free nerve endings from the long ciliary nerves (V1) mediate pain and the corneal (blink) reflex — afferent limb V1 (nasociliary), efferent limb VII (orbicularis oculi), bilateral (direct and consensual) response.

Location: Anterior one-sixth of the outer coat of the eye, continuous with the sclera at the limbus.

Parts & subdivisions

1. Epithelium
Non-keratinised stratified squamous, 5–7 cell layers (≈50 µm). Regenerates within days from limbal stem cells; richly innervated by free nerve endings.
2. Bowman layer
Acellular condensed layer of collagen beneath the epithelium (≈12 µm); does not regenerate — injuries reaching it leave scars.
3. Stroma
≈90% of thickness; regularly arranged lamellae of type I (with V) collagen fibrils in a proteoglycan matrix (keratan sulphate, dermatan sulphate) with flattened keratocytes.
(Dua layer)
A thin, tough pre-Descemet layer described in 2013; its status as a distinct layer remains debated.
4. Descemet membrane
Thick basement membrane of the endothelium (type VIII collagen), thickening with age. Copper deposition here produces the Kayser–Fleischer ring of Wilson disease.
5. Endothelium
Single layer of hexagonal cells pumping fluid out of the stroma. Cells do not divide in humans; density falls from ≈3 000–3 500 cells/mm² in youth to ≈2 500 in adults; below ≈500 cells/mm² the cornea swells and becomes cloudy.

Anatomy

Innervation: Long ciliary nerves, branches of the nasociliary nerve (ophthalmic division of the trigeminal nerve, V1), forming a subepithelial plexus.
Arterial supply: Avascular. Nutrients from aqueous humour and the limbal capillary arcades (anterior ciliary arteries); oxygen mainly from atmosphere via the tear film (and from palpebral conjunctival vessels when the eyes are closed).
Histology: Five classic layers: epithelium, Bowman layer, stroma, Descemet membrane, endothelium.
Development: Epithelium from surface ectoderm; stroma and endothelium from neural crest mesenchyme migrating between lens and surface ectoderm.

Function

Refract light (≈43 D — about two-thirds of the eye's power)
Transmit light (transparent, avascular)
Protect the intraocular contents and form a barrier to infection
Trigger the protective blink and tear reflexes (dense sensory innervation)

Physiology: Refractive index ≈1.376. Corneal hydration is held at ≈78% water by the endothelial pump; endothelial failure causes stromal oedema and loss of transparency. Avascularity and the lack of lymphatics confer relative "immune privilege", explaining the high success of corneal transplantation.

Clinical significance

Corneal abrasion and ulcer (keratitis)

Abrasions are very painful, with photophobia and tearing, and stain green with fluorescein under blue light. Contact-lens wearers are at risk of Pseudomonas and Acanthamoeba keratitis; herpes simplex keratitis gives a branching dendritic ulcer (never treat with topical steroids alone).

Keratoconus

Progressive thinning and conical ectasia of the cornea from adolescence, causing irregular astigmatism; associated with atopy, eye rubbing and Down syndrome. Treated with rigid lenses, collagen cross-linking or keratoplasty.

Corneal reflex

Touching the cornea causes bilateral blinking (afferent V1, efferent VII). Loss suggests a lesion of V1 (e.g. cerebellopontine angle tumour compressing V) or VII, and is a component of brainstem death testing.

Refractive surgery

LASIK, PRK and SMILE reshape the corneal stroma with lasers to correct myopia, hyperopia and astigmatism, exploiting the fact that the cornea is the eye's main refracting element.

Test yourself

Which structure contributes the greatest share of the eye's total refractive power?

Lens
Aqueous humour
Cornea (air–tear interface)
Vitreous body

Answer: Cornea (air–tear interface)

The largest change in refractive index occurs at the air–tear/cornea interface, so the cornea provides ≈43 of the eye's ≈60 D. The lens adds ≈15–20 D but is the only component whose power can be varied (accommodation).

A 65-year-old hyperopic woman has a sudden painful red eye, haloes, a hazy cornea, vomiting and a fixed mid-dilated pupil after watching a film in a dark cinema. Which drug would be contraindicated?

Pilocarpine
Acetazolamide
Atropine
Timolol

Answer: Atropine

This is acute angle-closure glaucoma. Mydriasis (darkness, anticholinergics) crowds the peripheral iris into the angle. Atropine dilates the pupil and would worsen closure; pilocarpine, acetazolamide and timolol lower IOP, followed by laser peripheral iridotomy.