Sclera

The sclera, commonly referred to as the white of the eye, is the opaque, fibrous outer coat of the globe. Along with the transparent cornea, it makes up the eye's tunica fibrosa. The sclera surrounds most of the eyeball, provides structural support, maintains globe shape, and offers attachment points for the extraocular muscles. It also acts as a protective barrier against intraocular contents and contributes to the integrity of the intraocular pressure environment.

Anatomy and composition: The sclera consists primarily of dense irregular connective tissue rich in type I collagen arranged in layered lamellae. It contains relatively few cells and a low elastin content, contributing to its rigidity. The tissue is largely avascular; nourishment reaches the sclera from the adjacent vascularized episclera and the choroid. The sclera is thickest near the posterior pole and thins toward the equator; its boundary with the cornea is at the limbus. Posteriorly, the sclera forms part of the wall around the optic nerve canal.

Nerve supply and development: Sensory innervation arises mainly from branches of the nasociliary nerve, and pain is prominent in inflammatory disorders such as scleritis. Developmentally, the sclera is derived from mesenchymal tissue of neural crest origin.

Clinical significance: Inflammation of the sclera is called scleritis and can be associated with systemic autoimmune diseases; inflammation of the episclera is episcleritis and is usually milder. Thinning of the sclera (blue sclera) can occur in osteogenesis imperfecta and other conditions. The sclera also serves as the site for surgical procedures such as scleral buckling in retinal detachment.