outer coat Flashcards
What is the outer coat of the eye formed by?
- sclera
- as the eye is very delicate need a tough outer coat
What does the outer coat consist of ?
- very thin
- made of rigid, collagenous matrix
- Outer coat constitutes the cornea and the sclera
Why does the outer coat of the eye need be tough ?
as the eye contains IOP (pressure which is pushing out)- so needs to resist the pressure within.
-Eye is constantly moving and is always under action of eye muscles - so needs to be tough outer coat
Protects the delicate internal ocular tissues
How is the cornea shape?
slightly oval
-longer in the horizontal dimension than the vertical
How much of the cornea is in the outer coat ?
(7%- of the total surface area of the globe)
How much of the sclera is in the outer coat?
(93%)
What can we measure of the structure ?
- thickness
- curvature
What is the instrument to measure the , curvature of the cornea?
Keratometer
What would the radius of curvature be for the front part of the eye?
radius of curvature of the front part of the eye - 7.8mm- in the centre and flatten towards periphery- more elliptical in shape- more spherical in middle.
What would the radius of curvature be of the posterior part of the eye?
11.5mm
What happens prior to cataract surgery ?
Has 2 measurements :
- length of eyeball from front back
- shape of front of the eye ball
- which tells them the power of intraocular lens to go in.
What is the sclera?
- Largest part of the fibrous coat of the globe
- Thickest posteriorly (1mm), thinnest under tendons of rectus muscles (0.3mm)
- comprises collagen and proteoglycans
What is foramina?
holes - which in the sclera allows for blood vessels and nerves to pass through
What are the number of foramina needed for in the sclera?
- Optic nerve
- Short posterior ciliary arteries and nerves
- Long ciliary nerves
- Vortex veins
- Anterior ciliary arteries
What are the 3 layers of the sclera?
Episclera
Scleral stroma
Lamina fusca
- comprises collagen and proteoglycans
What is episclera ?
- The uppermost layer
- blood vessels are located here
- made up of loose connective tissue outside the sclera
- Connects the sclera to the conjunctiva
- Lies under Tenon’s capsule
- Highly vascular
- Vessels larger and darker than vessels of the conjunctiva
What is the Scleral stroma?
- bulk of sclera
- regular scleral collagen- which makes up the main substance of the slcera
What is the Lamina fusca?
-looser connective tissue which is pigmented
What is episcleritis?
inflammation of the superficial layer of the sclera
- distinct area of inflammation within the episclera- blood vessels dilated
- blood vessels in the conjunctiva
What happens during eye muscle surgery ?
in children- they have a squint
-want to straighten eye- make sure visual axis is parallel again
can do this by strengthen one muscle or weaken one eye muscle- this involves detaching one muscle and stiching it back in a different location
What is the tenons capsule ?
- sheet of connective tissue
- thin fibrous sheath that envelops the globe from the limbus to the margins of the optic nerve.
- runs from optic nerve to sclera
- Inferiorly Tenons capsule is thickened to form the suspensory ligament of Lockwood
What is the tenons capsule known as?
Fascia Bulbi,
What is the cornea?
- it is transparent
- it bends light
- big dioptiric power
What is the corneas 2 important functions ?
- Protection
- Major refractive surface (2/3 of the refractive power of the eye (refracts light)- principle refractive surface
What is the measurement of the central corneal thickness ?
0.53mm
520um
What is the measurement of the peripheral corneal thickness ?
0.67mm
What is the average corneal thickness in the centre in population ?
0.54mm
What is the radius of anterior central curvature of the cornea ?
7.8mm
What is the radius of the posterior central curvature of the cornea ?
6.5mm
What is the 5 different layers the cornea can be resolved to ?
- Epithelium- made up to 5-7 layers of cells
- stroma- bulk of cornea
- endothelium
- bowmans layer
- descements membrane
What is the anterior limiting membrane of the cornea?
bowmans membrane
What is the posterior limiting membrane ?
descements membrane
What is the corneal epithelium ?
- 10% of the cornea
- Stratified squamous non-keratinized- which makes it vulnerable - as keratin is waterproof - so cornea needs to compensate.
- 50 microns thick
- 5-6 cell layers
What are the 3 cell types in the corneal epithelium ?
-Squamous cells - 2-3layers
-Wing cells- 2-3 layers
Basal cells- 1 layer
What are the Non-epithelial cells in the corneal epithelium?
include lymphocytes and Langerhans cells
What are the squamous cells ?
- 2-3 layers of cells
- Extensive surface projections (microvilli and microplicae)
- irregular appearance- due to microvilli -
What is in the microvilli (layer) of the squamous cells?
- Glycocalyx provides a hydrophilic surface and anchors the tear film- they emanate from the microvilli
- Most superficial cells linked by tight junctions forming an effective permeability barrier
What are the wing cells ?
2 layers of cells
Polygonal with inferior indentations
What are the wing cells like inside?
- they have adjacent cells which are linked by extensive interdigitations (interlocking) and abundant desmosomes (anchoring component of cell) (adaptation to resist damaging shear forces)
- Sparse organelles
- Numerous intermediate filaments in the cytoplasm
- No presence of organelles as would scatter light- dont need mitochondria- dont need lots of ATP for energy.
What are basal cells ?
- Single layer of cells
- Mitotic cells
- Structurally similar to wing cells
What can you use with a Electron microscope ?
scan surface of structures with high mag
What are the basal cells like inside ?
- they have adjacent cells linked by desmosomes
- Attached to the underlying stroma via a complex anchoring system
- Nerve terminals seen between basal cells
What is the epithelial anchoring system?
- series of filaments which are anchoring the epithelium to the underlying stroma-
- need an equally strong anchoring system between the epithelia and underlying stroma- in order to keep the epithelia on the surface.
What is the bowmans membrane?
anterior limting membrane
-homogenous thin membrane
What would we see in the electron microscope of the bowmans layer?
see that it is made up of randomly orientated collagen fibre
What is the bowmans layer composed of?
-It is composed of a randomly orientated array of fine collagen fibrils
How thick is the bowmans layer?
-8-14 microns thick
What is the function of the bowmans layer?
Function is unclear
Thought that it is not critical to corneal integrity- can take it away and still have a healthy cornea
What is the stroma made up of?
- 90% corneal thickness
- 78% water 1% salts -21%biological macromolecules
- VERY HYDRATED and collagenouns structure
How thick is the stroma?
500 microns thick
What is the stroma composed of?
Composed of collagen fibrils embedded in a matrix of proteoglycans- attract water (very hydrophilic molecules)
Synthesized by keratocytes
How is the collagen fibrils arranged in the stroma?
-arranged in lamellae
What is the role of fibroblast?
synthesise collagen and proteoglycans
What is the keratocytes ?
found in tendons
- same function as fibroblasts- but are in the eye
- synthesise collagen and proteoglycans matrix which fulls the space between the collagen fibrils
What do you see in the electron microscope of the stroma collagen fibres?
- organised in distinct layers
- looks like a series of stripes
- regular collagen, fine fibrils /thin
- Electron microscopy shows that the collagen fibrils are small (25-35nm) and regularly arranged (a pre-requisite for transparency)- the reason cornea is like that is the reason it is transparent.
What are the collagen types?
Collagen types I and V predomominantly (fibril type) with lesser amounts of type VI and type XII (non-fibril)
What is the collagen orientation ?
-Vertically
-Horizontally
-Collagen fibrils have a preferred orientation corresponding to the medial lateral and superior-inferior axes of the cornea
This preferrental arrangement tends to occur more posteriorly than anteriorly and increases towards the limbus
-Limbal collagen takes the form of an annulus- when get to limbus. (periphery) they then start to form an annulus (a circle)
What is the difference in the cornea to the sclera?
- cornea is clear
- sclera is opaque
- collagen fibrils is much less regular organised in the sclera and very variable in size and no distinct pattern
- collagen fibrils are very small and spaced and regularly arranged in the cornea- giving its transparent appearance.
What is decements membrane?
-Posterior limiting membrane
What is the descements membrane that the bowmans layer isn’t?
- it is a basement membrane of the corneal endothelium .
How thick is the basement membrane/descements membrane?
3-4 microns thick at birth, increasing to 10-12 microns in the adult
What does the anterior and posterior part of decements membrane represent?
The anterior 1/3 of Descemets membrane represents that part produced in foetal life
The posterior 2/3 is formed postnatally
How can you tell the age of the cornea?
looking at the thickness of the decements membrane
What is the structure of the decement membrane?
- bottom endothelium
- decements membrane middle
- stroma at top
How do collagen fibre position themsleebs?
sometimes 90degrees to one another layers
What is the corneal endothelium ?
- -Monolayer of squamous cells-single layer of cells- doesnt need to be more than 1 layer as it is not there for protection.
- it has metabolic support for the cornea
- needs a lot of energy for its function- as it is involved in maintaining corneal hydration
- Adjacent cells are linked by tight junctions (barrier function)- needs a barrier
- Large numbers of mitochondria reflects their high metabolic activity compared to epithelium with none.
- These cells can not regenerate- existing. cells need to spread to fill gaps when we are losing the corneal endothelial cells.
What does the corneal endothelium appear as under the electron microscope?
Under the electron microscope, the lateral borders of endothelial cells are markedly convoluted
What can we see in a specular microscope in a corneal endothelium ?
- In tangential section the endothelium appears as a mosaic of polygonal (typically hexagonal) cells
- Limited capacity for mitosis to replace damaged cells
- There is a progressive reduction in endothelial cell number with age
- Variation in cell size (polymegathism) and shape (pleomorphism) occurs with age, trauma, and contact lens wear
What does the corneal endothelial cell appear as when you are young compared to old in a specular microscope ?
comparing a endothelium in a 13 yr old and 76 yr old -much larger in a older person than younger
- very regular hexagonal shape in a young person
- but a change in older person
What is the properties of the cornea?
- The cornea is the most richly innervated surface tissue in the body
- Predominantly sensory (trigeminal innervation)
- Reflex and trophic function- in terms of supporting the epithelium - reflex- intiates protection and trophic is the function of the corneal nerves to protect and support the corneal epithelium.
What enters the cornea radially?
- Nerve fibre bundles
- They branch repeatedly forming a rich sub-epithelial plexus
- Individual axons penetrate Bowmans membrane and terminate in the epithelium
