Sclera Flashcards
What are the 2 main functions of the sclera?
- protects intraocular contents
- maintains shape of globe when distended by intrinsic intraocular pressure
What happens to the globe shape during contractino of the extraocular muscles and why is this remarkable?
it maintains its shape; the EOM tendons insert on the scleral surface
What is different about the corneoscleral envelope (fibrous coat) in infants?
it is not mature and can increase in size (buphthalmos in congenital glaucoma)
What is the vascularisation like of the sclera?
relatively avascular
What does the viscoeleastic nature of the sclera (great tensile strength, extensibility and flexibility) permit in terms of changes in intraocular pressure?
it allos only limited distension and contraction to accommodate minor variations in IOP
At what point is the sclera thickest and what is its thickness here?
posteriorly - 1mm thick
At what point is the sclera thinnest and what is its thickness here?
behind the insertions of the aponeurotic tendons of the extraocular muscles: 0.3-0.3mm
What covers the sclera a) posteriorly and b) anteriorly?
- a) fascia bulbi
- b) conjunctiva
What is the struture of the sclera?
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)
How does the structure of the cornea differ significantly from the sclera? 2 key ways
- the scleral collagenous lamellae are irregularly arranged
- scleral lamellae are interspersed with elastic fibres
What is the principal type of collagen present in the matrix of the sclera?
collagen type I
What is the structure of the elastic fibres present interspersed in the scleral collagenous lamellae?
they consist of an elastin core surrounded by longitudinally arranged microfibrils composed of a number of glycoproteins including fibrillin
What are the microfibrils around the elastic core that form elastic fibres in the sclera formed of?
glycoproteins including fibrillin
What are 5 factors that contribute to the opacity of the sclera (compared with the transparent cornea)?
- irregular arrangement of the collagen fibres
- variable fibre diameter (25 -250 nm)
- variable and irregualar fibrillar spacing
- higher water content
- reduced coating of GAGs on collagen fibres
What proportion of proteroglycans and glycosaminoglycans (GAGs) are there in the sclera compared with the cornea that contribute to its opacity?
sclera contains 1/4 of the content of the cornea
What are the 2 most abundant proteoglycans in the sclera?
- dermatan sulphate
- chondroitin sulphate
What is the cause of buphthalmos?
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
What is shown in the image?
collagen bundles in the sclera ( x 7000)
What is the cause of yellowing of the eyeballs?
bilirubin deposition in the conjuctiva nad not the sclera
What can lead to a blue tinge of the sclera?
abnormal thinning of the sclera, such as occurs in some connective tissue disorders e.g. Ehlers Danlos syndrome
What is staphyloma?
bulging - can be caused by localised thinning of the stromal collagenous layers
Why do infants have blue sclera?
underlying choroidal pigment showing through the thin collagenous stroma
Why do elderly individuals sometimes have a yellowish hue to the sclera?
fat deposition in sclera
What is the function of the collagen fibrils in the sclera?
take up tensile force and are aligned with the direction of greatest tensile strength
How can the arrangement of scleral collagen be studied?
using the ‘split line’ technique
What formation are the collagen bundles in the outer sclera arranged in?
whorls, loops and arches, particularly around the muscle insertions and optic nerve
What formation are the collagen bundles in the internal aspect of the sclera arranged in?
rhombic pattern
Where does the sclera extend between?
from the limbus anteriorly to the lamina cribrosa posteriorly
How are the collagen fibrils in the sclera arranged at the lamina cribrosa?
in circles or figure-of-eight patterns
What structures traverse the sclera? 3 things
- anterior ciliary arteries
- vortex veins
- long and short posterior ciliary arteries
What are the 3 histological layers of the sclera?
- lamina fusca
- stroma
- episclera
