Sclera and Limbus

Question 1

Describe the sclera

Answer 1

Outer coat of eyeball
Relatively avascular
Inactive metabolism
Collagenous and spherical

Question 2

What is the embryological origin of sclera?

Answer 2

Mesoderm around optic vesicle
Scleral condensation at 6 weeks gestation
5th month sclera has formed complete layer around optic cup

Question 3

Where does the sclera sit?

Answer 3

Extends from corneal limbus to optic nerve

Question 4

What are the dimensions of the sclera?

Answer 4

22mm diameter

1mm thick

Question 5

What covers the sclera internally?

Answer 5

Choroid posteriorly

Ciliary body and muscle anteriorly

Question 6

What covers the sclera externally?

Answer 6

Tenons capsule
Episclera, then conjunctiva
Rectus muscle tendons

Question 7

Describe the histology of the sclera

Answer 7

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

Question 8

What is the blood supply of the sclera?

Answer 8

Relatively avascular
Anterior and posterior ciliary arteries (ophthalmic artery)
Internal sclera receives a small supply from choroid

Question 9

What is the nerve supply of sclera?

Answer 9

Trigeminal nerve/V1
Short posterior ciliary nerves- parasympathetic and sympathetic
Long posterior ciliary nerves- sensory mainly around limbus

Question 10

What are the functions of the sclera?

Answer 10

Supports structure of eye and intraocular pressure

Tectonic patch graft for ocular surgery

Question 11

What is the limbus?

Answer 11

Annular zone border of cornea and sclera

Location of corneal stem cells

Question 12

What is the embryological origin of the limbus?

Answer 12

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

Question 13

Where does the limbus begin and end?

Answer 13

Begins at termination of Bowman’s and Descemet’s membrane

Ends at perpendicular plane through scleral spur

Question 14

How thick is the limbus?

Answer 14

1.5mm wide

10-15 epithelial layers thick

Question 15

What are the palisades of Vogt?

Answer 15

Specialised radial fibrovascular ridges between thickened conjunctival epithelium
1mm in size
Location of limbal epithelial stem cells

Question 16

What are the layers of the limbus?

Answer 16

Conjunctiva: non keratonised sqaumous epithelium +goblet cells
Vascularised conjunctival stroma: collagen fibres less organised than corneal collaged
Tenon’s capsule
Episclera
Limbal stroma

Question 17

What are the limbal crypts?

Answer 17

Epithelial invaginates from peripheral cornea

Limbal stem cells line their edges and bases

Question 18

What are focal stromal projections?

Answer 18

Projections through epithelium encircled by a ridge of stroma posteriorly
Site of limbal stem cells

Question 19

What are the structures of the aqueous outflow apparatus?

Answer 19

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

Question 20

What is the blood and nerve supply to limbus?

Answer 20

Trigeminal nerve V1 via long posterior ciliary nerve
Mostly sensory
Anterior ciliary artery and episcleral venous vessels
Lymphatics in conjunctiva

Question 21

What is the key function of the limbus?

Answer 21

Renews and maintains corneal epithelium

Stem cells lie in basal epithelial layer of limbus

Question 22

What are the impacts of limbal stem cell deficiency?

Answer 22

Poor corneal epitheliasation
Chronic corneal inflammation +scarring
Corneal vascularisation
Conjunctivalisation (conjunctival epithelial ingrowth onto corneal surface)

Question 23

Where is the sclera thickest and thinnest?

Answer 23

Thinnest: 0.3 mm posterior to rectus muscle insertion
Thickest: posterior pole at optic nerve head

Question 24

How much of the eye does the sclera cover?

Answer 24

5/6th of the eye