4. Anatomy of Sclera & Cornea. Clinical Evaluation. Flashcards
what is cornea?
avascular tissue
it is transparent
important optical element of +43D
very sensitive
what is the main function of the cornea ?
70 percent of the total refractive power of the eye
protective barrier to infection and trauma
describe the morphology of the cornea ?
the anterior surface is elliptical
its posterior surface is circular
it is thinnest centrally and thicker peripherally
what is the corneal composition ?
78 percent water and 15 percent collagen and 5 percent other parts and 1 percent GLYcosaminoglycans
what is the corneal layers ?
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
what accounts for the corneal epithelium transparency ?
the tight junctions in the basal cells
the corneal epithelium sheds regularly how long does it take for the complete replacement of the corneal epithelium ?
7 days
where does the corneal endothelium receive its nutrients from ?
the aqueous humour
when cells are lost in the corneal endothelium what occurs?
they corneal endothelium cannot regret
instead with old age it is compensated by enlargement and migration of neighbouring cells
what is the blood supply of the cornea ?
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
what is the nerve supply to the cornea
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
cornea form how much of the outer coat and what is the rest formed by ?
1/6 of the outer coat
sclera is the 5/6 of the outer coat
the cornea and the sclera borders each other and this border is called ?
the limbus
where does the sclera finish ?
near the optic nerve
the whole outer surface of the sclera is covereed by what ?
tenons capsule and bulbar conjuctiva in the anterior part
inner surface in contact with the choroid
function of the sclera
protect the eye and maintain the shape of the eye
provides an attachment for the extraocular muscle insertions
why does the human sclera look white ?
because of the scattering of all wavelengths of light by dense irregular bundles of collagen in sclera
why in children is the sclera blue ?
because of the extremely thin sclera, allows the visibility of underlying choroid
lder age the sclera
may appear slightly
yellowish why ?
the deposition of fat.
Sclera is thicker in? and where is it thickest ? and thinnest in ?
males than in females
thickest near the optic nerve
thinnest at the insertion of extra ocular muscles
Scleral rupture following trauma primarily occurs where ?
behind the insertion of the
rectior parallel to the limbus
Histologically, sclera consist of?
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
the sclera has numerous apertures for various arteries which are?
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
what are the nerves supplying the sclera ?
anterior - branches from the long ciliary nerves
posteriorly - short colliery nerves
where in the sclera does the optic nerve puncture it ?
at lamina cribrosa
why unlike the corna is the sclera opaque
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
