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FRCOphth part 1 Anatomy Eyeball > Cornea, Limbus, canal of Schlemm > Flashcards

Cornea, Limbus, canal of Schlemm Flashcards

(pg 128 - 140)

1
Q

What are the vertical and horizontal dimensions of the cornea?

A

Vertical: 10.6mm
Horizontal 11.7mm

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2
Q

What is the posterior diameter of the cornea?

A

11.7mm

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3
Q

What is the central thickness of the cornea?

A

500-600microns (0.5-0.6mm)

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4
Q

What is the thickness of the cornea peripherally?

A

1.2mm

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5
Q

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

A

Anterior: 7.8mm
Posterior: 6.5mm

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6
Q

What is regular astigmatism?

A

When cornea is more curved in vertical than horizontal plane

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7
Q

What is the refractive index of air?

A

1.00

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8
Q

What is the refractive index of aqueous?

A

1.33

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9
Q

Name the layers of the cornea

A

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

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10
Q

Corneal epithelium:
1) How many layers centrally and at limbus?

A

5 centrally, 10 or more at limbus (becomes continuous with bulbar conjunctiva)

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

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11
Q

Total thickness corneal epithelium

A

50-60microns

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12
Q

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

A
  • 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.
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13
Q

Corneal epithelium:
Describe the middlezone
(shape, number, attachments, microstructure)

A
  • Middle:
    Polyhedral cell shape (aka wing cells) Convex anteriorly + concave posteriorly
    Ovoid/round nuclei
    Desmosomes join cells, lateral borders interdigitate
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14
Q

Corneal epithelium:
Types of cells and broad arrangement

A

superficial = flattened, non-keritanized squamous cells.
Deep = columnar. Cells are stratified.

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15
Q

Corneal epithelium:
Describe the structure deep zone.

(shape, number, attachments, microstructure)

A
  • 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

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16
Q

Are any other types of cells found in corneal epithelium?

A

Yes!
1. lymphocytes
2. macrophages
3. pigmented melanocytes
4. wandering histocytes.
5. 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)

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17
Q

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?

A

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

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18
Q

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?

A

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

B looks like 8

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19
Q

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?

A

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.

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20
Q

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?

A

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.

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21
Q

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?

A

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) 5microns

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22
Q

Blood supply of Cornea

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

A

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)
23
Q

Does the corneal have lymphatic drainage?

A

No

24
Q

What is the nerve supply of the cornea?

A

Ophthalmic division of trigeminal nerve (through long ciliary nerve)

25
Q

Describe route of nerves supplying cornea

A

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 between Bownma’s and Stroma

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

26
Q

Do the nerves of the intraepithelial plexus have specialised endings?

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

A

No specialised endings

No myelin sheath/Swann cell sheath

27
Q

What is the refractive index of the cornea?

A

1.38

28
Q

What happens to the corneal epithelium when damaged

A

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

29
Q

What happens to Bowman’s layer when damaged

A

replaced by fibrous tissue

30
Q

What happens to the stroma when damaged

A

replaced by fibrous tissues

31
Q

What happens to Descemet’s membrane when damaged

A

resistant to trauma. CAN be regenerated by endothelial cells. When torn, curls into AC
.

32
Q

List 6 changes that occur in cornea with age

A

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

33
Q

What is Arcus senalis?

A

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

34
Q

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

A

1500 to 3500 cells/mm2

35
Q

What happens to the endothelium when damaged

A

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

36
Q

Trauma to which layers results in corneal oedema?

A

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

37
Q

How does angle closure glaucoma –> cornal oedema?

A

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

38
Q

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

A

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

39
Q

How wide is the limbus?

A

1.5-2mm wide

40
Q

What is the external scleral sulcus?

A

shallow groove on outer surface of limbus

41
Q

what is the inner scleral sulcus and what does it contain

A

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

42
Q

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

A

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

43
Q

What happens to corneal epithelium at the limbus?

A

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

44
Q

What do palisades of Vogt contain?

A

limbal stem cells + connective tissue contaning blood vessels, lymphatics
NB: palisades of Vogt = important for corneal epithelial regeneration

45
Q

What happens to Bowman’s layer at the limbus?

A

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

46
Q

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

A

Loses uniform structure and becomes the sclera

47
Q

What happens to Descemet’s membrane at the limbus?

A

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

48
Q

Describe the structure of the trabecular meshwork

A

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

49
Q

What happens to the corneal endothelium at the limbus?

A

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)

50
Q

What is the canal of Schlemm also known as?

A

sinus venosus sclerae

51
Q

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

A

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

52
Q

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

A

inner = AC + TM
posterior = scleral spur

53
Q

Why do corneal transplants work so well?

A

The cornea is an Immunologically 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 corneal epithelium (these cells are however present, in the peripheral cornea)

54
Q

Why do we get a sulcus sclerae?

A

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

FRCOphth part 1 Anatomy Eyeball (10 decks)

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