Week 5: The Cornea and Limbus Flashcards
What are the functions of the cornea?
- Optical function
- Transparency = allows light energy to be transmitted to the retina
- Refraction - Protection
- Corneal nerves detect foreign bodies = results in blinking and reflex tearing
- Physical barrier = protect the eye from injury
What are the dimensions of the cornea: general shapes, measurement horizontally, vertically, anterior and posterior surface dimensions, corneal thickness, and periphery thickness, refractive power, refractive index and chemical compositions?
- General shape: convex & elliptical
- Measurement: 11.7 mm horizontally, 10.6 mm vertically
- Anterior surface: 7.8 mm, posterior surface: 6.5 mm
- Corneal thickness: centre 544 +/- 34 um, periphery 700 um
- Refractive power = 40 diopters
- Refractive index = 1.376
- Chemical composition = water (80%), collagen (15%), proteins (5%)
What is the corneal shape, curvature constant in optical zone and what does the corneal map show?
- Shape: toroidal - radius of curvature increases from centre to peripheral
- Curvature constant in the optical zone (4 mm central)
- Corneal map = contours of the cornea identifying the principle meridians (power and axis)
Define astigmatism and the types of astigmatism
- Definition: uneven curvature of the cornea
- Types: with the rule (vertical) & against the rule (horizontal)
What are the layers of the cornea from anterior to posterior and their thickness
- Epithelium (50 um thick)
- Bowman’s Membrane (10 um thick)
- Stroma (500 um thick)
- Descemet’s Membrane (10 um thick)
- Endothelium (5 um thick)
What is epithelium and list the layers only
- Contains 5 to 6 layers of tightly packed cells
- Continuous with conjunctival epithelium
- Non-keratinised, hence fragile
Layers:
1st layer: superficial cells/squamous cells
2nd layer: wing cells
3rd layer: basal cells
Describe the superficial cells/squamous cells
- Outer layer
- Flattened, non-keratinised cells that contain microvilli & microplicae
- Trap mucous layer of tear film
Describe the wing cells
- Intermediate layer, 1 to 2 layers of epithelium
- Wing shaped characteristics
Describe the basal cells
- Inner most layer
- Tall columns attached to the Bowman’s membrane. If attachment is broken by injury, reattachment is difficult
Explain the days and process of the epithelial healing phase and the xyz hypothesis
- 4% of epithelial cells are undergoing mitosis at any given time
- After trauma, initial epithelial response is to inhibit mitosis & promote cell migration
- Mitosis @ day 3 is 50% of epithelial cells
- @ day 6 is 14% of epithelial cells
- @ day 10 is 4% of epithelial cells
xyz hypothesis = X (proliferation of basal cells) + Y (centripetal movement of cells) = Z (loss of cells from epithelial surface)
What are the functions of epithelium?
- Physical barrier
- Optical
- Tear stabilisation
- Barrier to fluid movement
- Microorganism shield
What is the Bowman’s Membrane and its function?
- Exists posterior to epithelium
- Acellular (lacks fibroblasts/cells)
- Composed of interwoven collagen fibers
- Allows nerves to perforate
- Ends abruptly at limbus
- Not replaced following injury
- Function: adhere the epithelium to stroma
What is the Stroma?
- Thickest part of the cornea (500 um)
- Composed of lamellae & keratocytes
- Stromal cells do not regenerate
- Posterior stroma is more easily separated than anterior stroma (one of the first signs of odema)
What is the Descemet’s Membrane and its function?
- 10 um thick
- Thickens with age
- Function: protective barrier against injury & infection
- Attachment to the stroma is weak
- Ends at the limbus = Schwalbe’s line
What is the endothelium?
- Single layer of flattened cells and leaky compared to epithelium
- Pleomorphism = change in cell shape
- Polymegathism = change in cell size
- Cell density decreases & causes shape changes with age, uv damage & genetic factors
- Hexagonal in shape
- Do not divide/replicate but metabolically active
Describe corneal transparency
- Epithelium + collagen matrix + glycosaminoglycans (GAGs) + endothelial pump
- Endothelial pump: 90% of fluid/waste is pumped out of endothelial pump, 10% of fluid is pumped out of epithelial pump
Describe the process of corneal hydration
- Epithelium
- Sodium potassium pumps, ion channels, aquaporins - Stroma
- Pulls water in via osmotic gradient - Endothelium
- Pumps, ion channels, aquaporins through stroma and anterior chamber
Describe the corneal metabolism and nutrition
- Corneal depends on a constant supply of oxygen & glucose
- Primary = supply of oxygen is derived from tear film (80%)
- Secondary = sources from aqueous & limbal capillaries (20%)
- Most nutrients are supplied by aqueous through the endothelium
Describe the effect of contact lens wear on the cornea
- Epithelial and stromal thinning
- Decreased number of keratocytes
- Changes endothelial mosaic pattern
- Pleomorphism & polymegathism have been found in long-term extended wear contact
Describe the degenerative process leading to keratoconus
- Focus disruption to Bowman’s layer
- Adhesion between collaged fibrils is lost causing slippage and displacement of lamellae
- Stroma degenerates & thins
Describe the corneal innervation
- Mostly dense innervated tissue of the body
- Afferent pathway via the long ciliary nerve to brainstem
- Stimulation of corneal nerves causes:
1. blinking reflex: efferent facial nerve
2. lacrimation: parasympathetic innervation of lacrimal gland
3. miosis (pupil constriction)
What is the corneal healing and repair process?
- Corneal epithelium
- Replaces itself every 10 days
- Epithelial damage –> chemical messengers –> neighbouring epithelium to detach from the basement membrane –> migrate to close the defect - Corneal stroma
- Slow & collagen must be replaced causing irregularity - Corneal endothelium
- Changes in shape - Corneal nerves
- After injury, a rapid nerve degeneration occurs and normal innervation is restored by 4 weeks
Explain Corneal Vascularisation
- Oxygen deprivation
- Leads to blood vessels forming
- Ghost vessels form
Describe the limbus
- Transition zone between cornea & conjunctiva, as well as the cornea & sclera
- Limbus forms physical barrier to blood vessels
- Palisades of Vogt located in the limbus
