cornea Flashcards
1
Q
what are the six layers of cornea?
A
- Epithelium
- Bowman’s layer
- Stroma
- Dua’s layer
- Descemet’s Membrane
- Endothelium
2
Q
what are the three(3) cells present in the corneal epithelium?
A
- superficial squamous cells
- wing cells
- basal columnal cells
3
Q
what is superficial squamous cells?
A
- 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
4
Q
what is corneal epithelial turnover? + process
A
- 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
5
Q
what is corneal bowman’s layer?
A
- 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
6
Q
what is corneal stroma?
A
- 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
7
Q
what is corneal dua’s layer?
A
- acellular
- has 5 to 8 lamellae collagen fibrils
8
Q
what is corneal Descemet’s membrane?
A
- 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
9
Q
what is corneal endothelium?
A
- 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
10
Q
what are the factors that contribute to corneal transparency?
A
- arrangement of collagen fibrils
- absence of blood vessels
- absence of pigments
- corneal deturgescence
11
Q
barrier function of epithelium & endothelium (corneal deturgescence)
A
- 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)
12
Q
active metabolic pump mechanism of endothelium (corneal deturgescence)
A
- 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.
13
Q
evaporation at the ocular surface/changes in osmolarity of tears (corneal deturgescence)
A
- 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)
14
Q
epithelial repair (cornea wound healing)
A
- Epithelial injury (abrasion)
- Prompts healing response to cover the exposed basement membrane with cells
- Cells at wound edge retract, thicken, and loosen their hemidesmosomal attachment to the basement membrane
- Cells travel in an amoeboid movement to cover the defect
- After wound closure, mitosis resume to re-established the normal epithelium configuration
- Surface tight junctions re-established
- Adhesion with Bowman’s layer within 7 days
15
Q
stromal repair (cornea wound healing)
A
- 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
16
Q
endothelial repair (cornea wound healing)
A
- 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
17
Q
corneal nutrition
A
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
18
Q
purpose of corneal metabolism
A
- 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)
19
Q
process of corneal metabolism
A
- 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.
20
Q
drug permeability across the cornea
A
- 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
21
Q
corneal blood supply
A
ophthalmic artery
22
Q
corneal blood drainage
A
superior ophthalmic vein
23
Q
corneal nerve supply
A
trigeminal nerve (CN V)
