Anterior segment - ocular surface
Sclera
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Get access- Fibrous coat of eyeball
- Relatively avascular
- Almost acellular apart from a few fibroblasts
- 70% water
- Thickest posteriorly around optic nerve (lamina cribrosa)
- Thinner towards the equator and thinnest around EOM insertions
- The four vortex veins penetrate the sclera behind the equator
Clinical Correlate
The most common site of scleral rupture following blunt trauma is in the area parallel with the limbus on the opposite side of the impact (a contre-coup effect) and at the insertion of the rectus muscles or equator. The superonasal limbus is most common.
Embryology
- Derived from mesenchyme around optic cup
- Two layers: inner vascular layer forms choroid and outer fibrous layer forms sclera
Collagen fibres and elastin are laid down by fibroblasts and reach the optic nerve by 3 months
- Scleral spur formed by 5 months
Episclera
- Outermost layer
- Attached to the outer fascia bulbi (Tenon’s capsule)
- Episclera is vascular: supplied by anterior ciliary artery
- Progressively thins towards the posterior globe
Clinical Correlate
Episcleral vessels do not constrict with topical adrenaline
Stroma
- Irregular collagen lattice (mainly types I and III)
- Fewer GAGs than the cornea
Lamina fusca
- Pigmented layer containing melanocytes
- Some collagen connections to underlying choroid
Lamina cribrosa
- Perforated part
Contains ganglion cells (which become myelinated after they leave), retinal artery and retinal vein
- Weak point of the sclera and may stretch
- Pushed posteriorly by raised IOP
- Pushed anteriorly by raised ICP
- No photoreceptors (blind spot)
- High concentration of glial cells