Sclera

-The largest component of the fibrous tunic or outer coat. -Comprises of most of the outer coat of the eye ahout (5/6) -Main function is to protect the intraocular components and maintain an overall shape of the eyeball (the globe)

-Mesodermal origin -Initially the sclera is transparent early in gestation but becomes opaque prior to birth. In newborn infants, the sclera has a subtle bluish shade due to the greater delicacy, but becomes opaque white for adults. -The most anterior portion of the sclera is formed first during embryogenesis to allow for insertion of the EOM. -The limbus, which initially is found further back lying over the ciliary body will later move forward. -Embryologically Tenon's capsule, which encases the globe and is joined to the septum orbitale, develops in a similar way to the sclera, but later.

-Male sclera is thicker than the female sclera. -Sclera is thickes at the posterior globe (1 to 1.35mm) and decreases in thickness toward the equator to reach a minimum under the tendons of the rectus muscles (around 0.3mm). -The tendons are often thick as the sclera itself (around 0.3mm) and consequently, when the tendons merge, the thickness of the sclera increases to about 0.6mm. -From this point on, the sclera increaes in thickness toward the limbus. At the peripheral conreal (limbus) the sclera is about 0.83mm thick. -The external diameter of the scleral coate (the globe) varies from 23 to 25 mm (wider horizontally than vertically) but can vary according to gender, race and extreme refrative error.

1. Episclera 2. Stroma 3. Lamina Fusca

-No, the three layers are all variations of the same structure, but slightly different. -The three layers are composed of connective tissue, made up of collagen, and another type of connective tissue protein called elastin.

-Connective tissue of the episclera is anterior to the stroma of the sclera and generally less compact and more vascular than the scleral stroma. -The vascularity of the episclera differentiates it from the avascular Tenon's capsule (also composed of collagen). -The episclera is thick anteriorly and becomes thinner posterior to the rectus muscle insertions. -Anteriorly the episclera is connected to the conjunctiva.

-Composed of small bundles of cllagen fibrils, but some fibrocytes and melanocytes are also present. -The collagen fibrils are of medium diameter, approximately 50 to 60nm. -Overall, the diameter and spacing of the collagen fibrils appears to be fairly uniform but not as uniform as the stroma. -Some moderate sized elastin fibers may also be present.

-Derived from the anterior and posterior ciliary arteries -Anteriorly the anterior ciliary arteries provide the vascular supply, while posteriorly, this is accomplished by the posterior ciliary arteries.

-Referred to as the substantial propria, composed of dense bundles of collagen fibrils and some sclerocytes (which are similar to the keratocytes of the corneal stroma) -As can be seen, the basic components of the scleral stroma are similar to the corneal stroma, but there are fundamental differences in structure, which results in the cornea being transparent and the scleral opaque.

-The bundles of collagen in the scleral stroma are not organized into the uniform flat sheets (lamellae) that are seen within the corneal stroma. -Instead the bundles are less regular, often having an undulating pattern, espcially when the bundles are more flattened (as occurs in mid stroma). -The collagen fibrils in the scleral stroma do not have the rather uniform diameter that is found int he corneal stroma, neither is there uniform spacing between the fibrils.

-Diameter of the scleral collagen fibrils varies according to the depth within the sclera. -At the mid-sclera, they appear to be the most variable with diameters ranging from as small as 50nm to as large as 400nm. -The diameter of these fibrils are responsible for the opaque nature of the sclera. -The fibrils are sufficiently large, unlike those of the corneal stroma, to scatter light. -Scleraocytes are sufficietly infrequent such that their processes do not form junctions. -Deeper in the scleral stroma, and espcially more posteriorly and in proximity to the rectus muscles, a substantial network of elastin fibers can be found weaving between the bundles.

-This is a modified part of the most internal aspect of the stroma. -Where many pigmented cells (melanocytes) are found, which have migrated from the choroid. -Fibrous connective tissue of the laminca fusca is more loosely packed in the scleral strom due to its pigmentation, appears similar to the suprachoroid.

-The optic nerve is the most prominent penetration, which leaves the eye slightly nasal and just superior to the posterior pole of the eyeball. -At the exit of the optic nerve, the sclera becomes a thin sieve-like membrane called the lamina cribosa. -The axons of the ganglion cells of the retina pass through the holes of the sieve.

-As the axons leave the eyeball, they become myelinated, and thus increase in the overall diameter of the optic nerve. -Therefore, this opeining has a conical shape; the internal diameter is 1.5-2.0mm, and the external diameter is 3.0-3.5mm.

-The lamina cribosa forms the weakest part of the sclera and it is this part of the eye that tends to give away, pushed back, in glaucoma. -The increased intraocular pressure often present in glaucoma is an important factor leading to the lamina cribosa becoming cupped. -Ischemic changes of the nerve head are also important in the formation of the a pathological cupping. -Therefore, the anatomy of the optic disc is important clincical sign in diagnosing glaucoma.