The Cornea
Corneal dimensions: the cornea isn't precisely circular, instead it's elliptical in shape with a horizontal diameter longer than the vertical one. The horizontal diameter measures 11-12 mm While the vertical measures 10-11 mm.This difference is due to conjunctival and scleral overlap which is more vetically than from the sides.
Corneal thickness: The thickness of the cornea ranges from 500 𝞵m at the central area and it increases toward the periphery of the cornea and may reach 1000 𝝻m at the limbus (the corneo-scleral junction)
Microscopically the cornea is formed of the following layers:
1- Epithelium: formed of 5-6 layers of stratified squamous non-keratinized epithelium.It's about 50 𝝻m in thickness.It's highly regenerative (capable of regeneration). So abrasions in this layer heals within 24 hours.
2-Bowman's membrane : It's a structureless,non-elastic, acellular membrane that's present just below the epithelium and it's made of condensed superficial layer of the stroma. It isn't capable of regeneration so injury in this layer will heal by fibrosis (scar) leading to permanent corneal opacity that interferes with vision.
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| Layers of the cornea |
3-Stroma: It forms about 90% of the corneal thickness. It's formed of 200-250 lamellae of collagenous bundles that are regular and packed (opposite to those of the sclera which are irregular and not packed thus the cornea is transparent while the sclera is opaque)
4-Descemet's membrane: It's an elastic, highly resistant and regenerable membrane. It's secreted by the endothelium and considered as it's basement membrane.
5-Endothelium: It's a single layer of flat cells (in cut section), hexagonal cells (in front view). They count 2000-3000/mm2
their number decrease with age and after any intra-ocular opeation.
They are responsible for corneal dehydration (endothelial pump) which is very important to keep the cornea clear. Endothelial cells don't regenerate, instead they increase in size to cover the gap between the cells.
Why cornea is transparent ?
*Avascularity of the cornea (Normally no blood vessels in the cornea).
*Corneal dehydration by the endothelial pump which keeps the cornea dehydrated by pumping water from the cornea back to the aqueous in the anterior chamber.
* The corneal epithelium is made of non keratinized stratified squamous epithelium.
*The stroma is regular and packed in lamellae.
*The corneal nerves are unmyelinated nerves.
*No lymph vessels.
*The tear film which abolishes any surface irregularities.
From where the cornea receives it's nutrition?
Despite avascularity of the cornea,It's metabolism is aerobic (it uses oxygen) opposite to the lens which is anaerobic (not using oxygen). The cornea receives it's nutritional supply from:
*The limal capillaries (the main source).
*The aqueous humor (by diffusion).
*The tear film (oxygen from the atmosphere dissolves in the tear film enabling the cells to make benefit of it).
4-Descemet's membrane: It's an elastic, highly resistant and regenerable membrane. It's secreted by the endothelium and considered as it's basement membrane.
5-Endothelium: It's a single layer of flat cells (in cut section), hexagonal cells (in front view). They count 2000-3000/mm2
their number decrease with age and after any intra-ocular opeation.
They are responsible for corneal dehydration (endothelial pump) which is very important to keep the cornea clear. Endothelial cells don't regenerate, instead they increase in size to cover the gap between the cells.
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| Endothelial cells as seen by specular micoscopy showing their hexagonal appearance. |
Why cornea is transparent ?
*Avascularity of the cornea (Normally no blood vessels in the cornea).
*Corneal dehydration by the endothelial pump which keeps the cornea dehydrated by pumping water from the cornea back to the aqueous in the anterior chamber.
* The corneal epithelium is made of non keratinized stratified squamous epithelium.
*The stroma is regular and packed in lamellae.
*The corneal nerves are unmyelinated nerves.
*No lymph vessels.
*The tear film which abolishes any surface irregularities.
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| The cornea (Note how it's transparent looking like the watch glass) |
Despite avascularity of the cornea,It's metabolism is aerobic (it uses oxygen) opposite to the lens which is anaerobic (not using oxygen). The cornea receives it's nutritional supply from:
*The limal capillaries (the main source).
*The aqueous humor (by diffusion).
*The tear film (oxygen from the atmosphere dissolves in the tear film enabling the cells to make benefit of it).
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