The Six Layers of Cornea

Cornea is the transparent, dome-shaped membrane that covers the front part of the eye. It covers the pupil, iris and anterior chamber of the eye. It is an avascular tissue except at the margins that serve to protect the inner structures of the eye and prevent any infections.

Composition and Structure

The cornea is horizontally oval and generally measures 11-12 mm horizontally and 9-11 mm vertically. It is composed of both cellular and acellular components. The cellular components of the cornea include epithelial cells, endothelial cells and keratocytes. The acellular components include glycosaminoglycans and collagen fibres.

The layers of the cornea include – Epithelium, Bowman’s layer, stroma, Descemet’s membrane and endothelium. Up to very recently these 5 layers were considered to complete the structure of cornea until 2013 when a new discovery was made. In 2013 Harmindar Singh Dua, an Indian-British doctor and researcher from University of Nottingham detected another layer between the stroma and Descemet’s membrane. Because this layer was discovered by Harminder Singh Dua and his colleagues, it was named Dua’s layer.

Let us dive into each of the layers in detail.

Layers of the Cornea

1. Corneal Epithelium
• The corneal epithelium is a stratified structure, i.e., made up of 5-7 layers of squamous non-keratinised epithelium, that gives the cornea a regular smooth surface.
• It is usually 50-60 µm in thickness.
• The three types of cells found in the 5-7 layered epithelium are wing cells, basal cells and superficial cells.
• The superficial cells are flat and polygonal and exist up to 2-3 layers of the epithelium. It has microvilli on its surface that increases the surface area.
• The wing cells are 2-3 layered and are named so because they give a wing-like appearance. They express keratins on their surface.
• The basal cells are single layered cells of epithelium that are either cuboidal or columnar. They are heavily populated with organelles and take active part in mitotic division.
• The epithelial surface cells are made of large dark cells and small light cells.
• The adjacent cells are held to each other by desmosomes and to the basal membrane by hemidesmosomes. These strong attachments hold the epithelium tightly to the other layers. Any damage to these can lead to corneal erosions.
• The microvilli found on the anterior surface of the epithelium helps in stabilising the precorneal tear film by interacting with glycocalyx.
• The basal cells divide actively and replace the existing cells of the epithelium both centripetally and superficially.
• The functions of the corneal epithelium include protection from bacterial infection to the stroma and preventing the loss of fluids from the tears.
• The corneal epithelium is attached to the next membrane, the Bowman’s layer, by a mesh of anchoring fibrils and plaques.
2. Bowman’s Layer
• The Bowman’s layer, also known as anterior limiting membrane, is made up of type I and type IV collagen fibrils and proteoglycans.
• It is named after the English ophthalmologist Sir William Bowman who discovered the layer.
• It is not a true layer as it is a cellular condensate of the anterior part of the stroma.
• The layer is 8–12 µm thick.
• This smooth layer helps in maintaining the shape of the cornea.
• The Bowman’s layer has no regenerative capacity. Damage to this layer can result in a scar.
• The Bowman’s layer is separated from the corneal epithelium by basal lamina anteriorly. Posteriorly, it merges with the stromal boundary.
3. Stroma of Cornea
• The stroma of the cornea is the thickest layer (comprising 80-85% of cornea) made up of dense connective tissues.
• The stroma appears characteristically transparent because of the arrangement of stromal fibres and extracellular matrix.
• It is composed of thick and flattened collagenous fibres that are arranged in parallel bundles called fibrils. These fibrils are arranged in layers of lamellae (up to 200-250 layers) that are continuous with the sclera of the eye.
• Each lamellar layer is up to 2µm in thickness.
• Keratocytes which are modified flattened fibroblasts are found between the lamellae.
• The transparency of the cornea is maintained by the spacing between the collagen fibres. This spacing is regulated by glycosaminoglycans and proteoglycans that form bridges between the collagen fibrils.
• This layer does not contain any vessels but a few sensory nerve fibres are found that extend towards the epithelium.
4. Dua’s Layer
• The Dua’s layer was discovered by Harminder Singh Dua in 2013. It is found between the stroma and Descemet’s membrane.
• It is made up of five to eight thin and compact layers of lamellae of type I and type VI collagen.
• The layer may be sparsely or not at all populated with keratocytes.
• This layer is impervious to air.
• There is still an air among the scientists about the credibility of this discovery. More research needs to be done.
5. Descemet’s Membrane
• The Descemet’s membrane, also known as posterior limiting lamina, is a thin and homogenous membrane found between the stroma and endothelium.
• It is 8–12 µm in thickness and is a modified basement membrane of the corneal endothelium.
• The Descemet’s membrane can be studied in two parts – the banded one third anterior region and a posterior two thirds homogenous or non-banded region.
• The layer is rich in laminin, type IV collagen and basement membrane glycoproteins.
• The Descemet’s membrane is secreted by the endothelial cells.
• The membrane is elastic in nature. Upon damage, it tends to curl towards the anterior chamber.
6. Corneal Endothelium
• The corneal endothelium is a single layer of flattened hexagonal cells that form a boundary between the anterior chamber and corneal stroma.
• It is made up of simple squamous epithelium, is almost 5μm thick and found on the posterior side of the cornea.
• The primary function of this layer is to maintain stromal hydration.
• The endothelial cells are mostly non-regenerative because they get arrested in the G1 phase and do not duplicate. This leads to a decrease in about 0.3 – 0.5% of cell density every year. However, some mitotic division events have been reported in human corneal endothelium.
• The hydration of the stroma is controlled by Na+ K+ ATPase pumps present in the endothelial layers.

This is all about the six layers of cornea. Visit BYJU’S for more important information related to NEET.

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