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Ocular anatomy > The cornea > Flashcards

The cornea Flashcards

1
Q

What is the function of the cornea

A
  • A transparent, avascular tissue with smooth optical surface
  • A major refractive structure to refractm transmit and focus light onto the retina
  • Its refractive index is 1.376
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2
Q

Which part of the cornea is the thickest and the thinnest

A

Thinnest at centre (500-600 microns)

Thicker at the periphery (700 microns)

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3
Q

What are the six corneal layers

A

Epithelium

Bowman’s layer

Stroma

Dua’s layer

Descemet’s membrane

Endothelium

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4
Q

What are the layers of cells in the epithelium

A

Superfical squamous cells

Wing cells

Basal columnal cells

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5
Q

Why is there microvilli on the surface of the superfical sqaumous

A

Finger- like projections that increase surface area for mucin of the tear film to adhere

Important for the uniform spreading of the tear film

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6
Q

Give a description of superfical squamous cells

A
  • Outermost layer
  • has flat, horizontal nuclei, non- keratinized cells
  • attached to one another by desmosomes
  • has microvilli on the surface
  • 2 to 3 cell layer thick
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7
Q

Give a description of wing cells

A
  • Transitional phase between squamous and basal cells
  • Convex anterior n concave posterior surface
  • Attached to one another by desmosomes
  • 2 to 3 cells layer thick
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8
Q

Give a description of basal columnal cells

A
  • Forms innermost layer of corneal epithelium
  • Single layer resting attached to the underlying basement membrane by hemidesmosomes
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9
Q

Name the basement membrane of the epithelium

A

Basal lamina

40 to 60nmm thick, strongly attached to Bowman’s layer

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10
Q

When does epithelial turnover take place

A

Occurs every 7 days

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11
Q

Describe the process of epithelial turnover

A

Migration of new basal cells from the limbal stem cells to the central cornea

Migration of basal cells upwards to the epithelium

Transformation of basal cells to wing cells, and the squamous cells

Shedding of oldest cells from the corneal surface into the tear film

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12
Q

Does the Bowman’s layer scar upon damage?

A

Yes

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13
Q

What is a Bowman’s layer the barrier against

A

Strong barrier against injury and infection

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14
Q

What does the Bowman’s layer containt

A

Interwoven collagen fibrils

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15
Q

Which layer ends arubtly at the limbus

A

Bowman’s layer

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16
Q

What does the stroma layer of the cornea consist of

A

Regularly arranged lamellae of collagen fibrils and keratocytes

Occasionally seen cells:
- macrophages, lymphocytes and polymorphoneuclear leukocytes

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17
Q

Which layer is the thickest layer in the cornea

A

Stroma (90% of corneal thicknest)

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18
Q

How are the lamellae of collagen fibrils arranged

A

Arranged in layerse running parallel with each other and with corneal surface

Direction of collagen fibrils in a given lamellae are the same, but are perpendicular to the adjacent lamellae

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19
Q

Where are keratocytes located

A

posisition between the lamellae

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20
Q

What are keratocytes responsible for

A

The production of collagen fibrils (and structural integrity of the lamellae of collagen fibrils)

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21
Q

What are the characteristics of Dua’s layer

A

Well defined - acellular and strong

22
Q

What is located in the Dua’s layer

A

5-8 lamella of collagen fibrils

has NO keratocytes

23
Q

Which layer is the basment membrane of the endothelium

A

The descemet’s membrane aka posterior limting lamina

24
Q

What is the descement’s membrane made of

A

collagen fibrils (secreted by the endothelium)

25
Q

name three characteristics of the endothelium

A

Single layer of hexagonal cells

Cells are rich in mitochondria
- requied to maintain corneal deturgescence

Cells do not regenerate

26
Q

What is the average Endothelial cell density and what happens if it falls to low

A

6000 cells/mm^2 at birth

Cells density reduces with age, stablize to approx 2500 cells/mm^2

If it falls between 800 cells/mm^2, can result in corneal oedema with resultant loss of transparency

27
Q

What are the factors that contribute to corneal transparency

A

Arrangement of collagen fibrils

Absence of blood vessels

Absence of pigments

Corneal deturgesence

28
Q

What are the two theories of the arrangment of collagen firbirls

A

Maurice: The transparency of the stroma is due to the lattice arragnment of the collagen fibrils. As the fibrils have equal diametr and seperated by less than half the wavelength of visible light, light scattered by individual fibrils is canceled by destructive interfernce. Hence, it is transmitted only in a forward direction.

Goldman et al: Conreal transparency existed due to optical homogeneity rather than lattice arragnment of collagen fibrils

29
Q

What are the four ways that corneal deturgesence is maintained

A

Barrier function of epithelium and endothelium

Active metabolic pump mechanism of endothelium and epithelium

Evaporation at the ocular surface/ changes in osmolarity of tears

Intraocular pressure (IOP)

30
Q

What does the barrier function of epithelium and endthelium do

A

Prevent excessive flow of water into the stroma

31
Q

How does the epithelial barrier prevent excessive flow of water

A

High resistance structure taht serves to protects the cornea from influx of unwatned substances and micro-organisms

32
Q

How does the endothelium prevent excessive flow of water (barriere function of eptithelium and endothelium)

A

Leaky barrier allows entrance of nutreinets from the aqueous humour

Simultaneously, active metabolic (fluid) pump mechanisms are present to draw excess water out of the stroma to maintain deturgesecene

Rate of leakage of nutrient n solutes (from aqueous to stroma) is counter balanced by the active fluid pump acitivty of the endothelium

33
Q

What happens if the barrier integrity of the endothelium breaks down

A

The pump mechainsm cannot cope with the leak and stromal edema would be induced

34
Q

How does the active metabolic pump mechanism of endothelium (also present in epithelium) mainatin corneal transparency

A

Moves ions across the cell membranes, with changes in solute concentration, water flows down the concentration gradient thuys maintaing a balance of fluid movement across the endothelium

35
Q

What are the factors that lead to oedema/ corneal swelling (evaporation at ocular surface)

A

Swimming in fresh water pool with eyes wide open

During lacrimation, osmolarity is lowered and water enters the cornea

During hypoxia: Built-up of lactate ions thus upsets the osmotic balance leading to oedmea

36
Q

What would eleveated IOP result in and what is the vaule of elevated IOP

A

Elevated IOP of 50mmHg or more results in corneal oedema

One of the signs of glaucoma

37
Q

Explain in detail how corneal wound healding takes place

A

Epithelial injury promts healing response to cover the exposed basement membrane with cells

Cells at wound edge retract, thicken and loosen hemidesmosomal attachemnt to basement membrane

Cells travel in amoeboid movement to cover the defect

After wound closure, mitosis resumes to re-establish the normal epithelium configuration

Surface tight junctions re-established

Adhesion with Bowman’s layer in 7 days

38
Q

State how stromal repair is carried out

A

Resynthesis and cross-linking of collagen, keratocytes are activated for tissue remodeling to restore tensil strength

This repair process causes scaring

39
Q

State how endothelial repiar is carried out and waht it could result in

A

Wounded area is covered by spreading of cells from areas adjacent to the wound

Results in reduction of cell density and in turn affects the endothelium fluid pump

Also affects the hexagonal shape of the endothelium

40
Q

What is the source of nutrition of the cornea

A

Oxygen is mainly from the atmosphere thr tear film, small amt supplied by aquesous and limbal blood vessels

Glucose and solutes from aqueous

41
Q

What is the purpose of corneal metabolism

A

Cell renewal (for normal functioning of epithelium)

Wound healing (new cells production)

Metabolic pump mechanisms of endothelium

Maintainece of temp (for normal functioning of cells)

42
Q

What is the process of corneal metabolism

A

Glucose undergoes glycosis and become pyruvate. Pyruvate is oxdised in Krebs cycle under aerobic conditions

43
Q

Difference of aerobic and anaerboic conditions

A

Anaerobic
- glucose undergone glycosis to become pyruvate
- Lactate and 2 molecules of ATP energy

Aerobic
- Glycosis, pyruvate oxidised in kreb cycle
- Co2, H2O, 36 molecules of AP energy

44
Q

What are the implications of metabolism that occurs under anaerobic condtion (hypoxia)?

A

Does not yield ATOp energy effficiently (2 vs 36)

Build up of lactate in corneal stroma changes the osmotic gradient, giving rise to oedema

EG: overwearing contact lens

45
Q

What is the drug permeability across the Cornea

A

Epithelium:
- Favours lipid soluble, non ionised compounds

Stroma
- Hydrophillic nature, favors ionsied more water soluble drugs

Endothelium
- Determined by molecular size

46
Q

How are drugs made to be able to penetrate through the cornea

A

Lipid soluble to dissolve into epithelium

Drug saturated epithelium acts as a reservior to release drugs into hydrophillic stroma
- drugs in epithelium dissociates into water soluble ionzied form, facilitating penetration into stroma

Drg penetratets thr endothelium if the molecular size is small enough

47
Q

What are the other factors that affect drug permeabillity across the cornea

A

Surface active agents that reduce surface tension, increase permeablity

Increased visocity, increased contact, increased corneal absorption

Increased drug conc.

48
Q

What is the corneal blood supply

A

Vascular arached in limbus formed by anterior ciliary artery anstamoses with posterior conjunctival artery

49
Q

What is the corneal blood drainage

A

Superior opthalmic vein to anterior ciliary vein to episclera vein

50
Q

What is the corneal nerve supply

A

Innervated by trigeminal nerve (CN V) => long ciliary nerves

Long ciliary nerves pierces through the back of the sclera, pass forward between scerla and choroid to cornea, enters stroma in radial pattern, lose their myelin sheath and further divides,

Fine terminal branches pierce Bowman’s layer and pass between epithelial cells as sensory nerves

Ocular anatomy (15 decks)

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