Limbus

-The limbus is the area where the cornea meets the sclera and conjunctiva. -The limbal area is about 1mm wide and see clinically as a transition zone, where the clear cornea becomes opaque sclera. -Maybe more define histologically. -Structurally it can be seen that in the limbal region, certain corneal layers end, whereas others are modified and continous with the sclera and conj.

1. To nourish the peripheral cornea 2. To provide an outflow for the aqueous humor. 3. To assist in corneal epithelial regeneration.

-Very important due to the presence of stem cells, which are vital to corneal epithelial regeneration and transparency. -The modifications of the endothelium to form the trabecular meses in the limbus is an imoprtant factor in the complex equation that determines the IOP. -When the resistane to the outflow of aqueous increases in this region, the IOP goes up and may result in glaucoma.

Epithelium ALL Stroma PLL Endothelium

-Continues into the limbus where it becomes modified. -Also continuous with the conj epithelium and epidermis of skin of the eyelid. -At the limbus, the epithelium becomes thickers and less uniform. -It contains 10 or more layers of cells and this increase is mainly caused by a greater number of wing cells. -The basal cells, a single row, are smaller and more densely packed.

-The BM lining their posterior side in the epi. continues in to the conj. -In the limbal region, the BM becomes somewhat undulating and not as flat as in the cornea. -On the conj side the epithelial thickness tapers down to 3-4 cell, except for the Palisades of Vogt, which are epithelial ridges with huge epithelial cell stock piles.

-Horn-line extensions from the internal aspect of the thickend limbal epithelium. -Found at intervals. -These epithelial features are believed to be the location of stem cells.

-Some pigmentation of basal cells is often seen, as melanin granules are found in these cells. -The melanin is derived from the conj. and its melanocytes through a process known as melanosis. -Blue eyes tend to contain less pigment here than heavily pigmented eyes.

-Goblet cells are not found in the limbal epithelium, but will be present later in the bulbar and palperbral conj epithelium -Ultrastructurally, the cells in the corneal and limbal epi show little difference.

-Stops at the extreme peripheral cornea. -The peripheral termination of the ALL demarcates anteriorly the histological limit of the cornea.

-The corneal stroma is continuous with the scleral stroma, but at the limus the stroma loses the regularity of its lamellae as found in the cornea. -Consequently, the cornea loses some of its transparency at the limbal region, but it does not become opaque as it is in the sclera. -Contributing to this gradual loss of transparency is the increasing variation in diameter of the collagen fibers and less uniform spacing of the fibers. -Anteriorly, among the bundles of collagen fibers are numerous blood vessels - Limbal Loops

-Literature reports, that there is an increase or subset of lamellae that have an annular orientation and that these lamellae provide physical strength or an anchoring point for lamellae that cross the cornea.

-The PLL as we know it in the cornea, is not found in the limbus or in the sclera. -The extreme periphery of the PLL forms a circumcorneal ring to dermarcate the peripheral limit of the cornea posteriorly. -It is known as Schwalbe's ring or line.

-The BM is continous into the trabeculum as these cells form the trabecular meshes, but now the membrane thickness is similar to that for a typical BM.

-The endothelium is continous from the cornea but shows some drastic modifications at the limbus. -The important drainage of the aqueous humor comes here and necessitates these endothelial modifications. -If this are area becomes blocked the IOP wil inrease and may result in glaucoma.