Sclera and Limbus Flashcards
Describe the sclera
Outer coat of eyeball
Relatively avascular
Inactive metabolism
Collagenous and spherical
What is the embryological origin of sclera?
Mesoderm around optic vesicle
Scleral condensation at 6 weeks gestation
5th month sclera has formed complete layer around optic cup
Where does the sclera sit?
Extends from corneal limbus to optic nerve
What are the dimensions of the sclera?
22mm diameter
1mm thick
What covers the sclera internally?
Choroid posteriorly
Ciliary body and muscle anteriorly
What covers the sclera externally?
Tenons capsule
Episclera, then conjunctiva
Rectus muscle tendons
Describe the histology of the sclera
Episclera: superficial stroma, dense vascular connective tissue, collagen irregular
Stroma: bundles of collagen and fibroblasts, circular
Laminal fusca: pigmented cells amoungst thinner bundles of collagen
What is the blood supply of the sclera?
Relatively avascular
Anterior and posterior ciliary arteries (ophthalmic artery)
Internal sclera receives a small supply from choroid
What is the nerve supply of sclera?
Trigeminal nerve/V1
Short posterior ciliary nerves- parasympathetic and sympathetic
Long posterior ciliary nerves- sensory mainly around limbus
What are the functions of the sclera?
Supports structure of eye and intraocular pressure
Tectonic patch graft for ocular surgery
What is the limbus?
Annular zone border of cornea and sclera
Location of corneal stem cells
What is the embryological origin of the limbus?
Corneo-scleral junction appears week 8 gestation + well demarcated by week 11
Aqueous outflow develops 3rd-5th month
Trabecular meshwork 7th -8th month
Palisades of Vogt (site of stem cells) develop in 6 months after birth
Where does the limbus begin and end?
Begins at termination of Bowman’s and Descemet’s membrane
Ends at perpendicular plane through scleral spur
How thick is the limbus?
1.5mm wide
10-15 epithelial layers thick
What are the palisades of Vogt?
Specialised radial fibrovascular ridges between thickened conjunctival epithelium
1mm in size
Location of limbal epithelial stem cells
What are the layers of the limbus?
Conjunctiva: non keratonised sqaumous epithelium +goblet cells
Vascularised conjunctival stroma: collagen fibres less organised than corneal collaged
Tenon’s capsule
Episclera
Limbal stroma
What are the limbal crypts?
Epithelial invaginates from peripheral cornea
Limbal stem cells line their edges and bases
What are focal stromal projections?
Projections through epithelium encircled by a ridge of stroma posteriorly
Site of limbal stem cells
What are the structures of the aqueous outflow apparatus?
Schlemm’s canal: circular venous channel for aqueous
Trabecular meshwork: sheets of perforated collagen
Scleral spur: collagen posterior site of insertion of trabecular meshwork and anterior site for insertion of ciliary muscle
Deep corneolimbus: transition zone where corneal collagen becomes trabecular meshwork collagen
What is the blood and nerve supply to limbus?
Trigeminal nerve V1 via long posterior ciliary nerve
Mostly sensory
Anterior ciliary artery and episcleral venous vessels
Lymphatics in conjunctiva
What is the key function of the limbus?
Renews and maintains corneal epithelium
Stem cells lie in basal epithelial layer of limbus
What are the impacts of limbal stem cell deficiency?
Poor corneal epitheliasation
Chronic corneal inflammation +scarring
Corneal vascularisation
Conjunctivalisation (conjunctival epithelial ingrowth onto corneal surface)
Where is the sclera thickest and thinnest?
Thinnest: 0.3 mm posterior to rectus muscle insertion
Thickest: posterior pole at optic nerve head
How much of the eye does the sclera cover?
5/6th of the eye
