What Is Limbus?
The limbus is the border between the clear part of the eye (cornea) and the white part of the eye (sclera). If you look into the mirror at the colored part of your eye, the limbus is the transparent ring that forms a border around it. The limbus performs many functions including healing wounds that occur to the surface of the eye, protecting the eye from invading bacteria, and controlling the eye pressure. Corneal stem cells are located at the limbus, and they are important in renewing the outer layer of the cornea after it’s damaged. If the limbus becomes damaged, such as by a chemical burn, the cornea cannot heal itself properly any more. Signs that the limbus has been damaged includes blurry vision and light sensitivity.
Key Takeaways
- The limbus is the border zone between the clear cornea and the white sclera.
- The limbus contains stem cells that are responsible for renewing the outer layer (epithelium) of the cornea, especially after the eye is wounded, such as from a fingernail scrape.
- Damage to the limbus may be caused by contact lenses, chemical burns, or Stevens-Johnson syndrome.
Understanding Limbus
The eye has many layers. The cornea is the clear tissue that covers the front of the eye. The cornea protects the eye from damage and focuses light into your eye. The sclera is the white part of the eye. The sclera is the supporting wall of the eyeball and helps the eye to keep its proper shape. The limbus is the transition zone between the clear cornea and the white sclera. It forms a circular border around the clear cornea. The width of the limbus is approximately 1.5 millimeters.
The limbus contains stem cells, which are important in keeping the front of the eye healthy. Limbus stem cells are constantly regenerating the outer layer of the cornea. The stem cells ensure that the cornea’s outer layer (epithelium) has a new layer of cells every week. By keeping the cells new and replacing them often, the cornea stays clear and prevents bacterial infection. The limbus is able to defend itself from invading bacteria and other pathogens because it has immune cells called MHC cells. The limbus also helps regulate eye pressure, which is important to prevent eye diseases like glaucoma.
Gross Anatomy
The Limbus is defined as a circular shaped zone that is the border between the clear cornea and white sclera. The limbus contains stem cells which regenerate the outer layer of the cornea (epithelium). Limbal stem cells are immature cells that eventually become epithelial cells, which form the top layer of the cornea. The stem cells are released from the limbus and migrate towards the center of the cornea in a whorl-like pattern. The stem cells replace the outer layer of the cornea with a new layer every seven to ten days. Without limbal stem cells, the corneal epithelium cannot regenerate and the cornea will become cloudy, or more prone to bacterial infections.
If the limbus is damaged, it can lead to a condition called limbal stem cell deficiency. In limbal stem cell deficiency, the stem cells are destroyed so the outer layer of the cornea cannot regenerate. The cornea becomes irregular and white colored. The vision becomes blurry because the cornea is less clear. The cornea can also grow new blood vessels and get inflamed. Symptoms of limbal stem cell deficiency include blurry vision, pain, watery eyes, and light sensitivity.
Limbal stem cell deficiency is caused by:
- Aniridia (absence of iris)
- Contact lens overwear
- An improperly fitting contact lens
- Chemical burn to the eye
- Stevens-Johnson syndrome
Histologic Anatomy
The front part of the limbus goes from Bowman’s membrane to Schwalbe’s line. The back part of the limbus goes from Schwalbe’s line to the scleral spur. The limbus is defined by a V-shaped line that follows the transition of the cornea to the sclera. The limbus contains collagen connective tissue.
Eye conditions that occur at the limbus include:
- Limbal stem cell deficiency: The stem cells become damaged and the cornea loses its ability to repair itself after injury.
- Vogt limbal girdle: Normal age-related degeneration of the limbus.
- Dellen: A thinning of the cornea that occurs at the limbus.
- Corneal neovascularization: The growth of new blood vessels onto the cornea caused by lack of oxygen, often due to sleeping in contact lenses.
- Marginal keratitis: The bacteria from the eyelids deposits onto the limbus, and the eye gets inflamed.
Surgical Limbus
The limbus is the location of the eye where the eye is cut into for eye surgery. The surgical limbus is an external landmark that helps an ophthalmologist decide where to cut the eye for procedures like glaucoma surgery. Also, injections that treat diabetic eye disease are injected 4 millimeters behind the limbus.
Aqueous Outflow Pathways
The fluid inside of the eye helps maintain the shape of the eye and provides nutrients to the eye’s structures. The fluid is drained out of the eye by a structure close to the limbus, which is called the Schlemm’s canal. If the limbus is damaged it is possible that the eye pressure can spike. Elevated eye pressure can cause glaucoma, which is an eye disease that damages the optic nerve and causes vision loss.
Limbal Relaxing Incisions (LRIs)
Limbal Relaxing Incisions are incisions made in the peripheral areas of the cornea in order to treat astigmatism. Limbal relaxing incisions are used to correct a low to moderate amount of astigmatism (under 3.00 diopters of cylinder). The surgeon will make a small partial-thickness incision in the limbus region with a blade during cataract surgery. The surgical procedure takes around five minutes per eye and is relatively safe and well tolerated.
