Corpus: Sclera

The sclera is the outer covering of the eyeball (bulbus oculi), which extends from the edge of the cornea to the dura mater of the optic nerve. Together with the cornea, it forms the fibrous layer of the eyeball and is responsible for the shape constancy of the eye.

The sclera is white, opaque and surrounds the entire eyeball – except for the corneal area. Its thickness varies between 0.3 and 1 mm, depending on the region. In the border area between the cornea and sclera, the so-called corneal limbus, the sclera protrudes over the cornea like a roof tile. This area is known as the scleral spur.

The sclera is a predominantly coarse connective tissue layer consisting of collagen and individual elastic fibers. The tendons of the outer eye muscles radiate into the sclera. At the posterior pole of the eyeball, the sclera is perforated (lamina cribrosa sclerae), as this is where the optic nerve emerges from the eyeball. At the anterior pole it is covered by the conjunctiva. Towards the orbit, it is otherwise completely enclosed by the Tenon's capsule.

Histologically, three layers of the sclera can be distinguished:

• Episclera
• Corneal stroma
• Lamina fusca

The episclera contains numerous small blood vessels that send their capillaries into a fine network of collagen fibrils and elastic fibers. Immune cells (lymphocytes, macrophages) are interspersed in this tissue layer.

The corneal stroma predominantly contains collagen fibrils. They are arranged in 0.5 to 6 µm thick fibril bundles and are strongly interwoven. They are predominantly type I collagen.

The main characteristic of the lamina fusca is the pigmentation caused by melanocytes. This layer of the sclera consists only of a thin layer of fibril bundles arranged like a scissor lattice, which merges into the choroid without a defined border. In addition to melanocytes, numerous fibroblasts can also be seen here.

The shape of the eyeball results from the intraocular pressure acting from the inside and the counterpressure of the sclera acting from the outside. The integrity of the sclera is therefore essential for the shape of the eyeball.

4.1. Degenerative changes

The biomechanical properties of the sclera change with age. The rigidity of the sclera increases due to increasing cross-linking of the collagens. In young people, the sieve plate of the sclera (lamina cribrosa sclerae) in particular is easily deformed so that fluctuations in intraocular pressure can be absorbed. The changes in the collagen structure in the sclera of older patients lead to its hardening. This is particularly important for glaucoma patients, as the sclera can no longer react as flexibly to fluctuations in intraocular pressure.

4.2. Discolouration

The sclera of older people can be yellowish in colour, as lipids are increasingly deposited in the tissue, especially in hyperlipoproteinaemia. This discolouration is occasionally confused with bilirubin deposits in the context of jaundice. Blue sclerae occur in osteogenesis imperfecta.