Limbus anatomy

The limbus of the eye marks the transition between the cornea and sclera. It is a ring-shaped area that starts off transparent by the cornea and becomes opaque near the sclera. The limbus is typically around 1-2mm wide and helps keep the cornea healthy and clear.

The cornea is the clear covering at the center of the eye. It helps protect the eye and focuses light. The sclera, sometimes called the “white of the eye,” covers the rest of the eye, from the edge of the cornea to the optic nerve. It also acts as protection and helps maintain the eye’s shape.

The limbus is not a distinct anatomical structure. Instead, it is a transition zone between the clear cornea and opaque sclera. On the surface of the eye, it is located where the cornea’s curvature protrudes out from the sclera. The word limbus means “border” in Latin.

There are several important ocular structures located at the limbus. The outermost layer consists of the conjunctiva, which lies on top of a unique tissue called the palisades of Vogt. Under that is a layer of episcleral tissue, followed by the corneoscleral stroma. This is the junction where the corneal and scleral stroma meet. The deepest part of the limbus contains the collector channels.

Conjunctiva 

The conjunctiva is a thin, clear layer that covers the inside eyelids and sclera. It provides mucus to help the eye make tears and is imperative for keeping the eyes moist.

Palisades of Vogt

The palisades of Vogt (POV) are folds of fibrovascular tissue in the limbus. Fibrovascular tissue is a combination of blood vessels and fibrous tissue. Fibrous tissue, or connective tissue, supports stability and movement throughout the body. It is made up of fibers that intertwine like ropes. Blood vessels — arteries, veins and capillaries — are tubes that carry blood between the heart and the rest of the body.

The POV folds fan out radially from the cornea. They contain limbal crypts or niches that are home to limbal epithelial stem cells (LSCs). The blood vessels in these crypts supply nutrients to the LSCs. The palisades of Vogt protect them from UV light.

Episclera

The episclera sits between the conjunctiva and the sclera. It is a thin layer made up of loose connective tissue. Connective tissue helps bind things together and provides structure and protection.

Junction of corneoscleral stroma

The stroma is a structural layer of connective tissue. The corneoscleral stroma is made up of collagen fibers. Collagen is the most common protein in the body. It helps form everything from skin and muscles to bones and connective tissue.

The collagen fibers in the corneal stroma are consistent in both thickness and density. The fibers in scleral stroma, on the other hand, are more randomized. This is partially what makes the sclera opaque. The junction where the stroma changes from one structure to the other is in the limbus.

Collector channels

Collector channels help with the outflow of aqueous humor. This important liquid is located in the anterior segment (front part) of the eye. It brings nutrients to the front of the eye and helps the eye retain its shape. The eye is constantly producing, and getting rid of, aqueous humor. 

However, if too much aqueous humor builds up, it can increase intraocular pressure. This can cause issues like glaucoma and vision damage. This is why collector channels are so important — they help ensure that intraocular pressure doesn’t get too high.

Function of the limbus

The limbus is a critical structure for the cornea. It provides nourishment, healing and a barrier between the cornea and sclera. 

The cornea is avascular, meaning there are no blood vessels that directly feed into it. This is what keeps the cornea clear. However, just like any other part of the body, it needs nourishment. The blood vessels of the limbus and the tear film on the surface of the eye provide this nourishment. The existence of the limbus allows the cornea to remain clear and avascular.

The limbus also provides a structural barrier between the cornea and sclera. This helps prevent conjunctival cells from entering the cornea. 

Limbal stem cells

Limbal stem cells (LSCs) play a major role in the cornea’s everyday health, as well as in cases of injury. These cube-shaped cells multiply rapidly. This allows them to immediately move into the cornea if it becomes injured or damaged. They also contain melanin to help prevent damage from UV rays. Melanin helps produce pigmentation, particularly in the eyes, skin and hair. 

LSCs reside in the palisades of Vogt in the limbal epithelial crypts. As people age, the size and efficiency of LSCs decreases.