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The cornea (Latin: cornea) is the part within the fibrous layer of the eyeball that makes up its frontal one-sixth. The cornea is a circular and transparent layer covering the pupil, iris, and anterior chamber. Compared to the sclera, the cornea is more convex to the outside. The scleral sulcus is seen within the corneoscleral junction due to the differences in curvature.
Structure of the cornea
The cornea is around 10.6 mm in the vertical and 11.7 mm in the horizontal direction. Being the thickest in the periphery, the cornea's diameter measures 0.7 mm, while in the center around 0.5 mm.
The cornea does not contain any blood vessels but has receptors and is made of proteins. The aqueous humor provides nutrition to the cornea, but from the front, it is moistened by tears.
Layers of the cornea
The cornea consists of five layers. Each layer differs in its thickness and cell shape.
Stratified corneal epithelium
The stratified corneal epithelium is the first or superficial layer of the cornea. Even being a part of the cornea, the corneal epithelium itself can consist of layers - five cell layers centrally, while in the periphery, it can have ten layers. The corneal epithelium is stratified and the total thickness of this layer is around 50 micrometers. The superficial cells are flattened, nucleated, nonkeratinized squamous cells, while the deepest cells are columnar.
The top layer is composed of two to three cells. The cells within the corneal epithelium are attached to one and another by desmosomes. Some of these cells have many microvilli, while some have fewer. It is thought that microvilli assist in retaining the tear film and keeping the cells moist.
Within the corneoscleral junction, the corneal epithelium can be formed by up to ten layers. In the junction, the corneal epithelium is continuous with the bulbar conjunctiva. The cells in the middle-zone have oval nuclei, convex anterior surfaces. Also, these cells are attached to one and another by desmosomes.
Bowman's membrane is the second layer from the top. It is also called the anterior limiting lamina of the cornea. Bowman's membrane has no cells within it while consisting only of irregularly arranged collagen fibers. Bowman's membrane is immediately below the basement membrane of the corneal epithelium. Its thickness is around 8 to 12 micrometers.
Substantia propria or corneal stroma is the thickest of all five layers. The corneal stroma makes up almost 90% of all the cornea. This corneal stroma also consists of collagen fibers, but this time they are arranged parallelly. The corneal stroma is transparent, fibrous, and compact.
Descemet's membrane is also called the posterior limiting lamina. Descemet's membrane is considered a basement membrane for the following layer (the corneal endothelium). Descemet's membrane also consists of collagen fibers. Descemet's membrane has extensions that start from the periphery of the cornea and move into the anterior chamber. These extensions are called the Hassal-Henle bodies. Descemet's membrane connects to a network of the canal of Schlemm's, and their connection site is known as the line of Schwalbe. Descemet's membrane is strong and homogenous. This layer is thicker than endothelium and has a sharp border from the corneal stroma.
The corneal endothelium is the last and deepest layer of the cornea. The corneal endothelium has only cell layer. The corneal endothelium is continuing with the endothelium of the iridocorneal angle and the anterior surface of the iris. The corneal endothelium works as a barrier separating the cornea from surrounding structures and controlling the flow of the aqueous humor into the cornea, and keeping proper corneal hydration and nutrition.
The function of the cornea
The cornea is the main structure that is responsible for the refraction of light entering the eye. The refractive power happens on the anterior surface of the cornea. Here, the refractive index of the cornea is higher than that of the air. Maintenance of corneal transparency is essential to keep the cornea functioning. As the eye's outermost layer, the cornea also serves as a protective barrier to the eye's components.
Vasculature and innervation of the cornea
The cornea is avascular and has no lymphatic drainage. The capillary blood vessels end at the circumference of the cornea. The cornea receives its nutrients from the aqueous humor and capillaries from the cornea's edge. The central part of the cornea receives oxygen indirectly from the air, while the peripheral part - by diffusion from the anterior ciliary arteries.
The cornea is innervated by the fibers coming from the ophthalmic division of the trigeminal nerve (CN V), mainly through the long ciliary nerves. Within the sclera, the long ciliary nerves divide and form the annular plexus. Branches from the annular plexus enter the substantia propria of the cornea. After this, the fibers have another division, and they form the subepithelial plexus. Branches from the subepithelial plexus enter Bowman's membrane, go between the epithelial cells, and form the intraepithelial plexus.