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
