Sclera

Question 1

What are the 2 main functions of the sclera?

Answer 1

1. protects intraocular contents
2. maintains shape of globe when distended by intrinsic intraocular pressure

Question 2

What happens to the globe shape during contractino of the extraocular muscles and why is this remarkable?

Answer 2

it maintains its shape; the EOM tendons insert on the scleral surface

Question 3

What is different about the corneoscleral envelope (fibrous coat) in infants?

Answer 3

it is not mature and can increase in size (buphthalmos in congenital glaucoma)

Question 4

What is the vascularisation like of the sclera?

Answer 4

relatively avascular

Question 5

What does the viscoeleastic nature of the sclera (great tensile strength, extensibility and flexibility) permit in terms of changes in intraocular pressure?

Answer 5

it allos only limited distension and contraction to accommodate minor variations in IOP

Question 6

At what point is the sclera thickest and what is its thickness here?

Answer 6

posteriorly - 1mm thick

Question 7

At what point is the sclera thinnest and what is its thickness here?

Answer 7

behind the insertions of the aponeurotic tendons of the extraocular muscles: 0.3-0.3mm

Question 8

What covers the sclera a) posteriorly and b) anteriorly?

Answer 8

• a) fascia bulbi
• b) conjunctiva

Question 9

What is the struture of the sclera?

Answer 9

consists of dense, irregular connective tissue comprising extracellular matrix and marix-secreting fibroblasts

matrix contains collagen (type I mainly, also III, IV, V, VI, VIII, XII, XIII)

Question 10

How does the structure of the cornea differ significantly from the sclera? 2 key ways

Answer 10

1. the scleral collagenous lamellae are irregularly arranged
2. scleral lamellae are interspersed with elastic fibres

Question 11

What is the principal type of collagen present in the matrix of the sclera?

Answer 11

collagen type I

Question 12

What is the structure of the elastic fibres present interspersed in the scleral collagenous lamellae?

Answer 12

they consist of an elastin core surrounded by longitudinally arranged microfibrils composed of a number of glycoproteins including fibrillin

Question 13

What are the microfibrils around the elastic core that form elastic fibres in the sclera formed of?

Answer 13

glycoproteins including fibrillin

Question 14

What are 5 factors that contribute to the opacity of the sclera (compared with the transparent cornea)?

Answer 14

1. irregular arrangement of the collagen fibres
2. variable fibre diameter (25 -250 nm)
3. variable and irregualar fibrillar spacing
4. higher water content
5. reduced coating of GAGs on collagen fibres

Question 15

What proportion of proteroglycans and glycosaminoglycans (GAGs) are there in the sclera compared with the cornea that contribute to its opacity?

Answer 15

sclera contains 1/4 of the content of the cornea

Question 16

What are the 2 most abundant proteoglycans in the sclera?

Answer 16

1. dermatan sulphate
2. chondroitin sulphate

Question 17

What is the cause of buphthalmos?

Answer 17

the corneoscleral envelope of children with congenital glaucoma responds to raised IOP by irreversibly stretching, owing to the immaturity of the collagen fibres –> produced characteistically enlarged buphthalmos (ox-eye) of this condition

Question 18

What is shown in the image?

Answer 18

collagen bundles in the sclera ( x 7000)

Question 19

What is the cause of yellowing of the eyeballs?

Answer 19

bilirubin deposition in the conjuctiva nad not the sclera

Question 20

What can lead to a blue tinge of the sclera?

Answer 20

abnormal thinning of the sclera, such as occurs in some connective tissue disorders e.g. Ehlers Danlos syndrome

Question 21

What is staphyloma?

Answer 21

bulging - can be caused by localised thinning of the stromal collagenous layers

Question 22

Why do infants have blue sclera?

Answer 22

underlying choroidal pigment showing through the thin collagenous stroma

Question 23

Why do elderly individuals sometimes have a yellowish hue to the sclera?

Answer 23

fat deposition in sclera

Question 24

What is the function of the collagen fibrils in the sclera?

Answer 24

take up tensile force and are aligned with the direction of greatest tensile strength

Question 25

How can the arrangement of scleral collagen be studied?

Answer 25

using the ‘split line’ technique

Question 26

What formation are the collagen bundles in the outer sclera arranged in?

Answer 26

whorls, loops and arches, particularly around the muscle insertions and optic nerve

Question 27

What formation are the collagen bundles in the internal aspect of the sclera arranged in?

Answer 27

rhombic pattern

Question 28

Where does the sclera extend between?

Answer 28

from the limbus anteriorly to the lamina cribrosa posteriorly

Question 29

How are the collagen fibrils in the sclera arranged at the lamina cribrosa?

Answer 29

in circles or figure-of-eight patterns

Question 30

What structures traverse the sclera? 3 things

Answer 30

1. anterior ciliary arteries
2. vortex veins
3. long and short posterior ciliary arteries

Question 31

What are the 3 histological layers of the sclera?

Answer 31

1. lamina fusca
2. stroma
3. episclera