Cornea, Limbus, canal of Schlemm

Question 1

What are the vertical and horizontal dimensions of the cornea?

Answer 1

Vertical: 10.6mm
Horizontal 11.7mm

Question 2

What is the posterior diameter of the cornea?

Answer 2

11.7mm

Question 3

What is the central thickness of the cornea?

Answer 3

500-600microns (0.5-0.6mm)

Question 4

What is the thickness of the cornea peripherally?

Answer 4

1.2mm

Question 5

What is the radius of curvature of the:
a) anterior surface of the cornea?
b) posterior surface of the cornea?

Answer 5

Anterior: 7.8mm
Posterior: 6.5mm

Question 6

What is regular astigmatism?

Answer 6

When cornea is more curved in vertical than horizontal plane

Question 7

What is the refractive index of air?

Answer 7

1.00

Question 8

What is the refractive index of aqueous?

Answer 8

1.33

Question 9

Name the layers of the cornea

Answer 9

1) corneal epithelium
2) Bowman’s membrane
3) Substantia propria/stroma
4) Descemet’s membrane
5) Corneal endothelium

Question 10

Corneal epithelium:
1) How many layers centrally and at limbus?
2) total thickness
3) types of cells and arrangement (broadly)

Answer 10

1) 5 centrally, 10 or more at limbus (becomes continuous with bulbar conjunctiva)
2) 50-60microns
3) superficial - flattened, non-keritanized squamous cells. Deep = columnar. Cells are stratified.

E= 5th letter of alphabet; epithelium = 10 letters

Question 11

Corneal epithelium:
Describe the structure of superficial vs middle vs deep zones. (shape, number, attachments, microstructure)

Answer 11

• Superficial:
  2-3 cells thick, flat cells, horizontal nuclei, attached by desmosomes.
  Microvilli and micropliae on superficial cell surface that extend into superficial tear film and retain tear film. (as cells age they loose attachments to one another and are lost in the tear film)
  light cells have more microvilli than dark cells. Dark cells located more centrally.
• Middle:
  Polyhedral cell shape (aka wing cells) Convex anteriorly + concave posteriorly
  Ovoid/round nuclei
  Desmosomes join cells, lateral borders interdigitate
• Deep:
  Single layer columnar cells that rest on basement membrane
  Lateral borders interdigitate, attached by desmosomes
  Hemidesmosomes attach basal plasma membrane to basement membrane
  Basement membrane strongly attached to Bowman’s membrane + stains positive with periodic acid-Schiff (PAS)

Naked nerve endings of sensory nerve fibres (ie non-myelinateed nerves) run between cells, sensitive to pain

Question 12

Are any other types of cells found in corneal epithelium?

Answer 12

Yes!
lymphocytes, macrophages, pigmented melanocytes, wandering histocytes.
Antigen-presenting Langerhan’s cells (derivatives of macrophages) founding limbus and peripheral cornea. Migrate centripetally in response to trauma + infection, but also as part of ageing)

Question 13

What is found between the corneal epithelium and Bowman’s layer?
What is it subdivided into, how thick are these layers?
How is the corneal epithelium anchored to Bowman’s layer?

Answer 13

Basal lamina
- Lamina lucida (25nm) and lamina densa (50nm)
- anchored with complex mesh of connecting tissue via basal lamina. Known as anchoring fibrils (TVII collagen) and anchoring plaques (TVI collagen)

Note: F looks simulate to 7 in your handwriting, P upside down is 6

Question 14

Bowman’s layer:
a) where found
b) thickness
c) what is it made of?
d) what happens to it at its peripheral and deep borders?

Answer 14

a) immediately beneath basement membrane of cornea
b) 8-10microns thick
c) acellular. = interwoven collagen fibres embedded in intercellular substances. Type 1, 3, 5 and 6 collagen.
d) peripheral borders - ends abruptly at limbus
deep border - merges with substancia propria (aka stroma) of cornea

Question 15

Corneal stroma/’substancia propria’
a) what proportion of the corneal thickness does the storm comprise?
b) what is it made of?
c) how many lamellae are there estimated to be? How thick are they?…So how thick does this make stroma?
d) what is the orientation of collagen fibres within each lamella relative to each other?
e) what is the orientation of collagen fibres in adjacent lamellae relative to each other?
f) how are lamellae bound together?
g) how long are the collagen fibres? where are they generally largest?
h) what are the collagen fibres embedded in?
i) which other cells are found in the stroma?
j) which cell type occupies 3-5% volume of the stroma? Where are they found, how are they bound, which organelles do they contain?

Answer 15

a) 90%
b) lamellae of collagen fibres that run parallel to corneal surface
c) 200-250, 2microns thick…400-500microns
d) same direction
e) perpendicular/right angles
f) fibres that pass from one lamella to another
g) 21-65mm
h) glycosaminoglycans
i) fibroblasts, macrophages, lymphocytes, polymorphonuclear leukocytes
j) keratocytes - anterior cornea. Bound by gap junctions. Mitochondria, rough endoplasmic reticulum, Golgi apparatus.

Question 16

Descemet’s Membrane:

a) What is the thickness?
b) which is thicker, Descemet’s membrane or the endothelium?
c) What happens when Descemet’s is incised?
d) describe it’s microstructure
e) It stains positive with…(which stain lol)
f) What are Hassal-Henle bodies?
g) what happens to Descemet’s membrane at the peripheral corneal margin?
h) What is the anterior border ring of the trabecular meshwork known as?

Answer 16

Descemet’s membrane

a) 10microns
b) Descemet’s
c) curls into anterior chamber
d) hexagonally arranged fine type 4 collagen fibres that are embedded in matrix
e) Periodic Acid Schiff (PAS)
f) protrusions of Desmet’s membrane into anterior chamber that are covered with endothelium. Number increases with age
g) terminates abruptly and becomes continuous with trabecular tissue on inner wall of sinus venous sclerae
h) Schwalbe’s line

NB: D=4th letter of alphabet => T4 collagen.

Question 17

Corneal Endothelium:

a) Describe structure
b) what happens to it at the peripheral corneal border?
c) How are cells connected to one another?
d) what is seen on the cell’s free edge?
e) What is the main role of the cells of the corneal endothelium + how do they do this?
f) how thick is the endothelium?

Answer 17

Corneal Endothelium

a) single layer of polygonal flattened cells who’s plasma membranes interdigitate with one another
b) becomes continuous with endothelial cells that line the spaces of the iridocorneal angle and anterior surface of the iris
c) to control hydration of cornea. Acts as barrier (to limit water from aqueous –> stroma) + has active transport mechanism
d) a few microvilli
e) maintaining normal hydration of cornea: barrier function (tight cell-cell junctions stop water from aqueous) + active transport ( => lots of mitochondria, rough endoplasmic reticulum and Golgi apparatus)
f) 15microns

Question 18

Blood supply of Cornea

a) Does the cornea have it’s own blood supply?
b) Where does it receive nourishment from centrally and peripherally?

Answer 18

Blood supply of Cornea

a) No

b)
- centrally: indirectly from air via oxygen dissolved in tear film + diffusion from lid vasculature when eyes closed

• aqueous humour + capillaries at its edge (derived from anterior ciliary arteries of conj + sclera)

Question 19

Does the corneal have lymphatic drainage?

Answer 19

No

Question 20

What is the nerve supply of the cornea?

Answer 20

Ophthalmic division of trigeminal nerve (through long ciliary nerve)

Question 21

Describe route of nerves supplying cornea

Answer 21

Long ciliary nerves enter sclera from perichoroidal space short distance from limbus

Divide to form annular plexus

Branches pass forward in radial pattern to enter substancia propria of cornea

Further divisions occur and fibres lose myelin sheath

Fibres unite to form sub-epithelial plexus

Fine terminals pierce Bowman’s membrane and pass between epithelial cells to form intraepithelial plexus

Question 22

Do the nerves of the intraepithelial plexus have specialised endings?

Do nerves have a sheath, if so what is it formed by?

Answer 22

No specialised endings

No myelin sheath/Swann cell sheath

Question 23

What is the refractive index of the cornea?

Answer 23

1.38

Question 24

List 6 changes that occur in cornea with age

Answer 24

1) Less translucent
2) dust-like opacities in deeper parts of stroma
3) Descemet’s increases in thickness
4) Bowman’s increases in thickness
5) Arcus senalis
6) Increase in Hassall-Henle bodies

Question 25

What is Arcus senalis?

Answer 25

white arc separated from limbus by clear margin roughly 1mm
Formed by extracellular lipid

Question 26

What is the normal cell count of the cornea (in adults)?

Answer 26

1500 to 3500 cells/mm2

Question 27

What happens to the following structures when damaged (eg in abrasion):
a) corneal epithelium
b) Bowman’s membrane
c) stroma
d) Descemet’s membrane
e) endothelium

Answer 27

a) corneal epithelium - can regenerate. Centripetal, amoeboid, movement of cells from periphery to centre. (there is increased mitotic activity at palisades of Vogt). Displace existing cells superficially and centripetally to centre of cornea

b) Bowman’s membrane - replaced by fibrous tissue

c) stroma - replaced by fibrous tissues

d) Descemet’s membrane - resistant to trauma. CAN be regenerated by endothelial cells. When torn, curls into AC

e) endothelium - cannot regenerate. Flatten themselves to cover larger surface area. Can be stripped out as a whole because tightly bound.

Question 28

Trauma to which layers results in corneal oedema?

Answer 28

Trauma to epithelium/enothelium can resort in corneal oedema (Note that Bowman’s membrane is generally dense enough to prevent spread of wager to deeper stroma)

Question 29

How does angle closure glaucoma –> cornal oedema?

Answer 29

Acute glaucoma –> oedema by opening gaps between endothelial cells. High pressure also causes Aq to be pushed into stroma where is accumulates below and amongst epithelial cells

Question 30

Where do vessels arise from in cases of infection/inflammation? What can this result in?

Answer 30

Neovascularisation arises from deep and superficial conjunctival plexi (from anterior ciliary arteries). Normally capillaries end at limbus. In infection/inflammation, neovasularisation occurs to bring cellular and humoral defence closer to site of infection

New vessels can enter cornea around entire circumference, or just in one segment

Normally blood vessels atrophy –> ghost vessels. If not –> corneal opacity

Question 31

How wide is the limbus?

Answer 31

1.5-2mm wide

Question 32

What is the external scleral sulcus?

Answer 32

shallow groove on outer surface of limbus

Question 33

what is the inner scleral sulcus and what does it contain

Answer 33

shallow groove on inner surface of limbus, contains TM and coal of Schlemm (aka sinus venosus sclerae)

Question 34

a) What is the scleral spur?
b) What shape is it?
c) What does it attach to at anterior and posterior surfaces?

Answer 34

a) Projection of scleral tissue formed by posterior lip of inner scleral sulcus

b) triangular, apex pointing anteriorly and inwardly

b)
- anteriorly: TM
- posteriorly: ciliary muscle

Question 35

What happens to corneal epithelium at the limbus?

Answer 35

Thrown into folds by sub epithelial connective tissue & becomes continuous with conjunctival epithelium.

Note: crests run radially into cornea and folds are known as Palisades of Vogt

Question 36

What do palisades of Vogt contain?

Answer 36

connective tissue, blood vessels, lymphatics
NB: palisades of Vogt = important for corneal epithelial regeneration

Question 37

What happens to Bowman’s layer at the limbus?

Answer 37

Becomes continuous with the lamina propria of the conjunctiva and Tenon’s capsule

Question 38

What happens to the lamina propria/stroma of the cornea at the limbus?

Answer 38

Loses uniform structure and becomes the sclera

Question 39

What happens to Descemet’s membrane at the limbus?

Answer 39

Ends abruptly at Schwalbe’s line
TM begins just posterior to this

Question 40

Describe the structure of the trabecular meshwork

Answer 40

Branching sheets of fenestrated connective tissue that form sponge-like system of passages that communicate with anterior chamber

Question 41

What happens to the corneal endothelium at the limbus?

Answer 41

Continuous laterally with a layer of flattened cells that line the passage of the TM and continue to anterior surface of iris
(note actually unclear if these are two distinct different cell types or if they are continuous lolz)

Question 42

What is the canal of Schlemm also known as?

Answer 42

sinus venosus sclerae

Question 43

Describe the anatomical structure of the canal of Schlemm + what shape is it in cross-section

Answer 43

Canal lined with endothelium that runs circularly at cornea-scleral junction + lies in internal scleral sulcus
Breaks up into branches at places that coalesce again
Oval/triangular in cross-section

Question 44

What are the canal of Scheme’s inner and posterior relations?

Answer 44

inner = AC + TM
posterior = scleral spur

Question 45

Why do corneal transplants work so well?

Answer 45

The cornea is an ommunologically privileged site for grafting, this is because of the avascularity of the cornea and the absence of dendritic, antigen presenting Langerhan’s cells in the corneae epithelium (these cells are however present, in the peripheral cornea)

Question 46

Why do we get a sulcus sclerae?

Answer 46

The transparent cornea forms the anterior 1/6th of the eyeball, however because its curvature is greater than the rest of the eyeball, a slight sulcus forms which marks the junction of the cornea with the sclera