cornea

Question 1

what are the six layers of cornea?

Answer 1

1. Epithelium
2. Bowman’s layer
3. Stroma
4. Dua’s layer
5. Descemet’s Membrane
6. Endothelium

Question 2

what are the three(3) cells present in the corneal epithelium?

Answer 2

• superficial squamous cells
• wing cells
• basal columnal cells

Question 3

what is superficial squamous cells?

Answer 3

• Forms the outermost layer of corneal epithelium
• Has flat, horizontal nuclei, non-keratinized cells
• Attached to one another by desmosomes
• 2 to 3 cell layer thick
• Has microvilli on the surface
• Microvilli of the superficial squamous cells
  > Finger-like projections that increase surface area for mucin of the tear film to adhere
  > Important for the uniform spreading of the tear film

Question 4

what is corneal epithelial turnover? + process

Answer 4

• Occurs every 7 days
• Maintained by a balance of basal cell division (source of epithelial cells) and shedding of the oldest cells from the corneal surface
• Process of Epithelial Turnover
  > 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 wine cells , and then squamous cells
  > Shedding of oldest cells from the corneal surface into the tear film

Question 5

what is corneal bowman’s layer?

Answer 5

• Also know as anterior limiting lamina, lies beneath basement membrane
• Scars upon damage
• Strong barrier against injury and infection
• Acellular and 8 - 12 um thick
• Has interwoven collagen fibrils
• Ends abruptly at the limbus

Question 6

what is corneal stroma?

Answer 6

• Approx. 500 um thick (90% of the corneal thickness)
• Consists of regularly arranged lamellae of collagen fibrils and keratocytes
• Occasionally seen cells
  > macrophages, lymphocytes and polymorphoneuclear leukocytes

Question 7

what is corneal dua’s layer?

Answer 7

• acellular

- has 5 to 8 lamellae collagen fibrils

Question 8

what is corneal Descemet’s membrane?

Answer 8

• Basement membrane of the endothelium
• Also known as the posterior limiting lamina
• Strong and homogenous, 10um(micrometer) thickness
• Made up of collagen fibrils that is secreted by the endothelium

Question 9

what is corneal endothelium?

Answer 9

• Single layer of hexagonal cells posterior to the Descemet’s membrane and adjacent to the anterior chamber
• Cells are rich in mitochondria; reflects high metabolic activity (active fluid transport system) required to maintain corneal deturgescence (relative dehydration state of the cornea required for transparency)
• Cells do not regenerate
• Endothelial cell density
  > About 6000 cells/mm? at birth
  > Cell density reduces with age, and stabilizes in middle age to approx 2500 cells/mm^2
  > If cell density falls below 800 cells/mm^2, this leads to corneal oedema with resultant loss of transparency

Question 10

what are the factors that contribute to corneal transparency?

Answer 10

1. arrangement of collagen fibrils
2. absence of blood vessels
3. absence of pigments
4. corneal deturgescence

Question 11

barrier function of epithelium & endothelium (corneal deturgescence)

Answer 11

• prevent excessive flow of water into the stroma
• epithelial barrier is a high resistance structure that serves to protect the cornea from influx of unwanted substances & micro-organisms
• endothelium has a combined leaky barrier & fluid pump mechanism
  > leaky barrier that allows the entrance of nutrients, such as glucose & amino acids from aqueous humour (cornea is avascular so nutrients are provided by diffusion from aqueous humour)
  > simultaneously, active metabolic pump mechanism are present to draw excess water out of the stroma so as to maintain proper hydration(deturgescence)

Question 12

active metabolic pump mechanism of endothelium (corneal deturgescence)

Answer 12

• Metabolic pump mechanisms are active throughout the endothelial cells and function continually to move ions across the cell membranes.
• With changes in solute concentration, water flows down the concentration gradient, thus maintaining a balance of fluid movement across the endothelium.

Question 13

evaporation at the ocular surface/changes in osmolarity of tears (corneal deturgescence)

Answer 13

• factors leading to corneal swelling/oedema
  > swimming in a fresh water with eyes wide open
  > during lacrimation(tearing): osmolarity is lowered, water enters the cornea
  > during hypoxia: built-up of lactase ions thus upsetting the osmotic balance leading to oedema(susceptible to contact lens wearers)

Question 14

epithelial repair (cornea wound healing)

Answer 14

1. Epithelial injury (abrasion)
2. Prompts healing response to cover the exposed basement membrane with cells
3. Cells at wound edge retract, thicken, and loosen their hemidesmosomal attachment to the basement membrane
4. Cells travel in an amoeboid movement to cover the defect
5. After wound closure, mitosis resume to re-established the normal epithelium configuration
6. Surface tight junctions re-established
7. Adhesion with Bowman’s layer within 7 days

Question 15

stromal repair (cornea wound healing)

Answer 15

• repairs of a stromal injury involves resynthesis and cross-linking of collagen
• keratocytes are activated for tissue remodelling to restore tensile strength
• this repair process eventually causes scarring

Question 16

endothelial repair (cornea wound healing)

Answer 16

• endothelial cell loss may occur due to ageing, surgical trauma, or disease
• wounded area is covered primary by spreading cells from areas adjacent to the wound

Question 17

corneal nutrition

Answer 17

oxygen - mainly from the atmosphere through tear film, w small amounts supplied by the aqueous and limbal blood vessels(vasculature)
glucose and solutes - from the aqueous

Question 18

purpose of corneal metabolism

Answer 18

• metabolism refers to all chemical reactions involves in maintaining the living state of cells
• energy is generated by the breakdown of glucose in the form of ATP
• cornea metabolism is required for:
  > cells renewal (for normal functioning of epithelium)
  > wound healing (new cells production)
  > metabolic pump mechanism of endothelium
  > maintenance of temperature (for normal functioning of the cells)

Question 19

process of corneal metabolism

Answer 19

• What are the implication of metabolism that occurs under anaerobic condition (insufficient oxygen = hypoxia)?
  > It does not yield ATP energy efficiently, i.e. 2 molecules compared to 36 molecules under aerobic condition.
  > More critically, the build up of lactate in the corneal stroma changes the osmotic gradient, giving rise to oedema.
  > Over-wearing of contact lens is an example that may give rise to this.

Question 20

drug permeability across the cornea

Answer 20

• Depending on the corneal layer, drug permeability across the cornea is affected by:
  > Lipid and water solubility of the drug
  > Ionic form of the drug
  > Molecular size of the drug
• Epithelium
  > Favors lipid soluble, non-ionized compounds
  > Tight barriers, limits absorption of hydrophilic, ionized substances
• Stroma
  > Hydrophilic nature, favors ionized, more water soluble drugs
  > Barrier for lipid-based drugs
• Endothelium
  > Determined by molecular size
• Other factors influencing drug permeability
  > Surface active agents that reduce surface tension, increase permeability
  > Increased viscosity, which increases contact results increased corneal absorption
  >Increased drug concentration

Question 21

corneal blood supply

Answer 21

ophthalmic artery

Question 22

corneal blood drainage

Answer 22

superior ophthalmic vein

Question 23

corneal nerve supply

Answer 23

trigeminal nerve (CN V)