4. Anatomy of Sclera & Cornea. Clinical Evaluation.

Question 1

what is cornea?

Answer 1

avascular tissue
it is transparent
important optical element of +43D
very sensitive

Question 2

what is the main function of the cornea ?

Answer 2

70 percent of the total refractive power of the eye

protective barrier to infection and trauma

Question 3

describe the morphology of the cornea ?

Answer 3

the anterior surface is elliptical
its posterior surface is circular
it is thinnest centrally and thicker peripherally

Question 4

what is the corneal composition ?

Answer 4

78 percent water and 15 percent collagen and 5 percent other parts and 1 percent GLYcosaminoglycans

Question 5

what is the corneal layers ?

Answer 5

ABCDE

anterioir epithelium - stratified squamous non keratinising
cells - overlapping polygonal cells
easily regenerated
most signficant component of the refractiveness of the eye
continuous with conjunctival epithelium
6 layers of cells

umbrella cells - polyhedral convex anteriorly with elongated processes
basal cells - arranged in palisade manner - germinative layer - columnar with oval nucleus

bowmans layer - a cellular
layer of collagen fibrils
does NOT REGENERATE- replaced by opaque scar

central stroma - 80 percent of corneal thickness
its main component is water
also collages and proteoglycans
keratocytes - modified fibroblasts involved in remodelling after injury
mesodermal origin - stroma

descendants membrane - highly elastic
aceelular
produced by endothelium
can regenerate if destroyed

corneal endothelium - MONOlayer of hexagonal cells
forming continous mosaic best seen in spectral microscopy
MESODERMAL ORIGIN -MOST IMPORTANT FOR CORNEAL TRANSPARENCY

Question 6

what accounts for the corneal epithelium transparency ?

Answer 6

the tight junctions in the basal cells

Question 7

the corneal epithelium sheds regularly how long does it take for the complete replacement of the corneal epithelium ?

Answer 7

7 days

Question 8

where does the corneal endothelium receive its nutrients from ?

Answer 8

the aqueous humour

Question 9

when cells are lost in the corneal endothelium what occurs?

Answer 9

they corneal endothelium cannot regret

instead with old age it is compensated by enlargement and migration of neighbouring cells

Question 10

what is the blood supply of the cornea ?

Answer 10

conjuctival and episcleral capillary network from limbal region
the healthy cornea does not have or need blood supply in it instead oxygen dissolves in tears and then diffuses through the cornea

Question 11

what is the nerve supply to the cornea

Answer 11

very sensitive

trigeminal nerve 
ophthalmic nerve 
nasocillary nerve 
long cillarynerve 
pericorneal plexus 
stromal plexus 
subepithelila plexus 
supplies all ver epithelia cells

neutroptrophis from the nerves supplying the cornea becomes the nutrient

Question 12

cornea form how much of the outer coat and what is the rest formed by ?

Answer 12

1/6 of the outer coat

sclera is the 5/6 of the outer coat

Question 13

the cornea and the sclera borders each other and this border is called ?

Answer 13

the limbus

Question 14

where does the sclera finish ?

Answer 14

near the optic nerve

Question 15

the whole outer surface of the sclera is covereed by what ?

Answer 15

tenons capsule and bulbar conjuctiva in the anterior part

inner surface in contact with the choroid

Question 16

function of the sclera

Answer 16

protect the eye and maintain the shape of the eye

provides an attachment for the extraocular muscle insertions

Question 17

why does the human sclera look white ?

Answer 17

because of the scattering of all wavelengths of light by dense irregular bundles of collagen in sclera

Question 18

why in children is the sclera blue ?

Answer 18

because of the extremely thin sclera, allows the visibility of underlying choroid

Question 19

lder age the sclera
may appear slightly
yellowish why ?

Answer 19

the deposition of fat.

Question 20

Sclera is thicker in? and where is it thickest ? and thinnest in ?

Answer 20

males than in females
thickest near the optic nerve
thinnest at the insertion of extra ocular muscles

Question 21

Scleral rupture following trauma primarily occurs where ?

Answer 21

behind the insertion of the

rectior parallel to the limbus

Question 22

Histologically, sclera consist of?

Answer 22

Epicleral tissue - composed of loose fibrous elastic tissue
attaching to tenons capsule
thin vascularised connective tissue

Sclera proper- avascular dense bundles of collagen fibres

Lamina fusca - pigmented cells present giving brownish in colour

Question 23

the sclera has numerous apertures for various arteries which are?

Answer 23

posterior scleral aperture formed by optic nerve - near the optic nerve
long and short posterior ciliary arteries

middle scleral aperture - vortex veins

anterior scleral aperture - anterior to the insertion of the rectus muscle (supplying that swell)
episclera receives its blood supply from the anterior cilliary

Question 24

what are the nerves supplying the sclera ?

Answer 24

anterior - branches from the long ciliary nerves

posteriorly - short colliery nerves

Question 25

where in the sclera does the optic nerve puncture it ?

Answer 25

at lamina cribrosa

Question 26

why unlike the corna is the sclera opaque

Answer 26

opaque due to the irregularity of the Type I collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen