Sclera

Sclera

Also known as: white of the eye, fibrous tunic, lamina cribrosa

The tough, opaque white outer coat forming the posterior five-sixths of the fibrous tunic; it maintains the shape of the globe and anchors the extraocular muscles.

Overview

Thickness near optic nerve≈1 mm

Thickness behind rectus insertions≈0.3 mm (thinnest)

Share of fibrous tunic≈5/6

The sclera is a dense, relatively avascular connective tissue of irregularly interwoven type I collagen bundles with some elastic fibres. Unlike the cornea its fibrils vary in diameter and are randomly arranged, and it is relatively dehydrated — which is why it is opaque and white. It is thickest posteriorly near the optic nerve (≈1 mm), thins to ≈0.5–0.6 mm at the equator and is thinnest (≈0.3 mm) just behind the insertions of the rectus tendons.

Posteriorly, the optic nerve axons pierce the sclera through the sieve-like lamina cribrosa, a mechanically weak point that is deformed posteriorly ("cupping") in glaucoma. Here the outer two-thirds of the sclera become continuous with the dural sheath of the optic nerve. The sclera is pierced also by the posterior ciliary arteries and nerves around the optic nerve, the vortex veins behind the equator, and the anterior ciliary vessels near the limbus.

Three layers are described: the vascular episclera (loose connective tissue beneath Tenon capsule), the scleral stroma (scleral proper) and the pigmented lamina fusca, which is loosely attached to the choroid through the suprachoroidal space. Anteriorly the sclera meets the cornea at the limbus, where its inner surface is grooved by the scleral sulcus containing the canal of Schlemm and trabecular meshwork.

Location

Posterior five-sixths of the outer coat of the eyeball, deep to Tenon capsule and the bulbar conjunctiva.

Parts & subdivisions

Episclera

Thin vascular connective tissue on the outer surface; inflamed in episcleritis.

Scleral stroma

Dense irregular type I collagen with fibroblasts; opaque.

Lamina fusca

Inner pigmented layer containing melanocytes, adjacent to the suprachoroidal space.

Lamina cribrosa

Perforated region where ≈1.2 million retinal ganglion cell axons exit as the optic nerve.

Scleral spur

Circular ridge at the limbus giving attachment to the longitudinal ciliary muscle and the trabecular meshwork.

Anatomy

Innervation

Short and long ciliary nerves (V1 via the nasociliary nerve); deep pain from scleritis is often severe.

Arterial supply

Largely avascular stroma; episcleral vessels from the anterior ciliary arteries and short posterior ciliary arteries.

Histology

Dense irregular collagenous connective tissue (type I collagen, variable fibril diameter), few fibroblasts, some elastic fibres; relatively dehydrated.

Development

Neural crest-derived mesenchyme condensing around the optic cup, beginning anteriorly near the limbus.

Clinical significance

Scleral colour changes

Scleral icterus (yellow) — earliest clinically visible sign of jaundice (bilirubin binds elastin), detectable at serum bilirubin >≈2.5–3 mg/dL (≈40–50 µmol/L). Blue sclerae — thin sclera revealing the dark uvea, classically in osteogenesis imperfecta (defective type I collagen), also Ehlers–Danlos syndrome.

Scleritis vs episcleritis

Episcleritis: mild discomfort, sectoral redness, blanches with phenylephrine, benign. Scleritis: severe boring pain that wakes the patient, violaceous hue, often associated with rheumatoid arthritis or vasculitis (e.g. granulomatosis with polyangiitis); necrotising forms can perforate (scleromalacia perforans).

Test yourself

Through which route does most aqueous humour leave the eye?

Uveoscleral outflow through the ciliary muscle
Posterior diffusion through the vitreous to the retinal veins
Across the corneal endothelium into the tear film
Trabecular meshwork → canal of Schlemm → episcleral veins

Answer: Trabecular meshwork → canal of Schlemm → episcleral veins

About 80–90% of aqueous drains by the conventional (trabecular) pathway into Schlemm canal and then to aqueous and episcleral veins; ≈10–20% leaves by the uveoscleral route, which prostaglandin analogues enhance.