The cornea

• Summary
• Corneal basics
• Corneal structure
• Layers of the cornea
• Epithelium
• Bowman’s layer
• Stroma
• Descemet’s membrane
• Endothelium
• Nutrient and nerve supply
• Tear film
• References
• Author(s)

Summary

The cornea, alongside the aqueous humour, lens, and vitreous body, makes up the refractive media of the eye. In this section, we will describe the detailed anatomy of the cornea.

Corneal basics

The cornea and the sclera make up the outer covering of the eyeball, meeting at at the limbus. Unlike the sclera however, the cornea is a transparent and avascular tissue.

This coating is a structural barrier which works to protect the eye against infections, in addition to providing the anterior refractive surface of the eye together with the tear film.

Corneal structure

The cornea has an oval shape, with a horizontal diameter measuring 11-12mm and vertical diameter measuring 9-11mm. It is convex, with the anterior curvature measuring 7.8mm and posterior curvature measuring 6.5mm. Its thickness increases from the central cornea to the periphery, attributed to increased collagen in the peripheral stroma (the stroma is one of the corneal layers - we talk about it later on in the article!).

The cornea is composed of both cellular and acellular components. The cellular components include epithelial cells, keratocytes and endothelial cells. The acellular part consists of collagen and glycosaminoglycans. From an embryological point of view, the epithelial cells are derived from epidermal ectoderm, whilst the keratocytes and endothelial cells are derived from the neural crest.

Layers of the cornea

The layers of the cornea are as follows:

1. Epithelium
2. Bowman’s layer
3. Stroma
4. Descemet’s membrane
5. Endothelium

We’ll now go into each layer in turn.

Epithelium

The corneal epithelium is around 5-7 layers of cells thick, around 50 microns. It is made up of non-keratinised stratified squamous epithelium. The epithelium is closely associated with the overlying tear film - particularly the mucin layer, which interacts with epithelial cells and allows the tear film to spread across the eye with each blink.

The epithelium consists of three types of cells - superficial cells, wing cells, and basal cells. The basal cell layer, which is the deepest layer of the epithelium, is the only layer capable of mitosis, and is the source of the superficial and wing cells.

The corneal is thinner centrally, made up of 5-7 layers, and has columnar basal cells. In the peripheral cornea, the epithelium is made up of around 7-10 layers, and the basal cells are cuboidal.

The epithelium is derived from surface ectoderm at around 5-6 weeks of gestation.

Bowman’s layer

The Bowman’s layer is essentially made up of Type 1 and Type 5 collagen and proteoglycans. It is around 12 microns thick (much thinner than the epithelium!). It is unable to regenerate, and when injured may result in a scar. It helps the cornea maintain its shape.

Stroma

The corneal stroma forms the bulk of the structural framework of the cornea, accounting for 80-85% of its thickness. It is a transparent structure, primarily made of Type 1 collagen.

The corneal stroma is made up of keratocytes and the extracellular matrix (collagen + glycosaminoglycans). The glycosaminoglycans here include keratan sulfate, chondroitin sulfate and dermatan sulfate. The major cell type however is the keratocytes - which are actually involved in maintaining the extracellular matrix environment, synthesising collagen molecules and glycosaminoglycans and creating matrix metalloproteinases which are important in maintaining stromal homeostasis. The majority are within the anterior stroma.

Collagen fibres making up the stroma are arranged in parallel bundles known as fibrils. These fibrils are packed in layers known as lamellae - the stroma of the human eye has over 200 distinct lamellae! The lamellae at the anterior surface of the stroma are densely packed and are highly interwoven, inserting into the Bowman’s layer. The posterior lamellae in the central cornea are more loosely packed and consequently have the capacity to swell more easily. This means the anterior part of the stroma resists morphological changes even following swelling (which can occur after injury, inflammation, or surgery), maintaining a smooth anterior surface curvature which is key for minimal astigmatism. The proteoglycan content also varies from anterior to posterior - keratan sulfate is a much more hydrophilic proteoglycan which exists in the posterior stroma, whilst dermatan sulfate is a much less hydrophilic proteoglycan and is present in the anterior stroma.

Keratoconus is thought to be a disease predominantly of the corneal stroma.

From an embryological point of view, the stroma is formed from the second wave of neural crest migration that occurs in the 7th week of gestation, after the primitive endothelium is established.

Descemet’s membrane

This membrane is around 7 microns, made up of predominantly Type 4 collagen and laminin. Descemet’s membrane is formed at around 8 weeks gestation. The endothelial cells continuously secrete Descemet’s membrane.

Endothelium

The endothelium is a single layer of hexagonal and metabolically active cells, around 5 microns thick. Two important ion transport systems exist here: 1) membrane bound Na+K+ATPase pumps and 2) the intracellular carbonic anhydrase pathway. These transport systems produce a net flux of ions from the stroma to the aqueous humour (link the glaucoma page here). The basal surface of the endothelium contains numerous hemidesmosomes that promote adhesion to Descemet’s membrane.

Endothelial cell density changes continuously throughout life, declining from 3000-4000 cells/mm2 to around 2600 cells/mm2. In fact, central endothelial cell density decreases at an average of about 0.6% per year in normal corneas throughout adult life. Endothelial cells do not regenerate in adults.

Nutrient and nerve supply

The cornea is one of the most heavily innervated and sensitive tissues in the body. Its sensation mainly comes from the ophthalmic division of the trigeminal nerve, via the anterior ciliary nerves and to a lesser degree, from the maxillary nerve. The limbus and peripheral cornea also receive sympathetic innervation from the superior cervical ganglion.

Nerves enter the middle third of the stroma and run forward in a radial fashion towards the centre, losing their myelin sheath around 1 mm from the corneal limbus, and giving rise to branches that innervate the anterior and mid-stromal layers. Between the Bowman's layer and the anterior stroma, the nerves form the subepithelial nerve plexus which perforates Bowman’s layer and then forms the sub-basal epithelial nerve plexus. This plexus innervates the basal epithelial layer and finally terminates within the superficial epithelial layers.

To maintain transparency, the cornea is avascular, with its main source of nutrients coming from diffusion from the tear fluid on the outside, and aqueous humour from the inside.

Tear film

The minute irregularities of the corneal surface epithelium are masked by the smooth and regular overlying tear film, which is actually the first layer of the cornea light comes into contact with.

The tear film not only provides lubrication and hydration to the ocular surface but is a source of oxygen, immunoglobulins, lysozymes, lactoferrin, and alpha and beta defensins. The tear film is traditionally thought to be composed of three distinct layers. The most superficial layer is the lipid layer, secreted by the meibomian glands. As oil is less dense than water, this layer floats to the top and provides an important barrier against evaporation of the tear film. The aqueous middle layer lies immediately beneath, produced by the lacrimal gland. The innermost mucin layer is produced mainly by the conjunctival goblet cells.

References

1. Sridhar, Mittanamalli S. ‘Anatomy of Cornea and Ocular Surface’. Indian Journal of Ophthalmology, vol. 66, no. 2, Feb. 2018, pp. 190–94. PubMed Central, https://doi.org/10.4103/ijo.IJO_646_17.
2. Srinivasan, Sathish, and Douglas A. M. Lyall. ‘27 - Neurotrophic Keratopathy’. Ocular Surface Disease: Cornea, Conjunctiva and Tear Film, edited by Edward J. Holland et al., W.B. Saunders, 2013, pp. 205–11. ScienceDirect, https://doi.org/10.1016/B978-1-4557-2876-3.00027-4.
3. Dua, Harminder S., et al. ‘Human Corneal Anatomy Redefined’. Ophthalmology, vol. 120, no. 9, Sept. 2013, pp. 1778–85. DOI.org (Crossref), https://doi.org/10.1016/j.ophtha.2013.01.018.

Author(s)

Dr Sara Memon

Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.

Published: 21/5/22

Last updated: 23/1/25