Sklera

Sklera, also spelled sklera in some languages and known in English as the sclera, is the tough, opaque fibrous outer coat of the eyeball. Along with the cornea, it forms the protective external envelope of the globe. The sclera provides structural support, helps maintain the eye’s shape, and offers anchor points for the extraocular muscles.

Anatomically, the sclera is a thick, dense irregular connective tissue rich in type I collagen with relatively little elastin. It is thickest at the posterior pole and becomes thinner toward the anterior segment, where it meets the cornea at the limbus. The anterior sclera is covered by conjunctiva; the inner surface interfaces with the choroid and retina. The sclera also contains a scleral canal for the optic nerve. The lamina fusca lies adjacent to the choroid on the inner scleral surface.

Vascularization of the sclera is limited; nourishment mainly occurs by diffusion from the overlying episclera and adjacent choroidal and conjunctival vessels. Blood supply is primarily via the posterior ciliary arteries. Sensory innervation is mainly through the ciliary nerves, making scleral inflammation painful.

Developmentally, the sclera arises largely from neural crest-derived periocular mesenchyme. Clinically relevant conditions include episcleritis and scleritis (inflammation of the sclera), scleral thinning or scleromalacia (seen in systemic diseases such as rheumatoid arthritis or osteogenesis imperfecta), and visible changes like blue sclera in certain connective tissue disorders.