Cornea and Sclera Flashcards
if the cornea is steeper in the vertical meridian, it is:
WTR
if the cornea is steeper in the horizontal meridian, it is:
ATR
what is the average corneal thickness in the center and the periphery?
center: 550 microns
periphery: 670 microns up to 710 microns
what is the refractive index of the cornea
1.376
what is the central radius of curvature of the cornea, anterior and posterior surfaces?
anterior surface: 7.8mm curvature
posterior surface: 6.5mm curavture
prolate
steeper in center and flatter in periphery, normal cornea
oblate
flatter in center and steeper in periphery, ex. post-LASIK, ortho-K corneas
average anterior horizontal and vertical diameters of the cornea
anterior horizontal diameter: 11.7mm
anterior vertical diameter: 10.6mm
average posterior horizontal and vertical diameter of the cornea
posterior both horizontal and vertical are 11.7mm (spherical from behind)
what are the corneal layers?
epithelium (surface, wing, basal cells), basement membrane, Bowman’s layer, stroma, Descemet’s membrane, endothelium
what is the corneal epithelium thickness?
52 microns, 5 to 6 cell layers
what is Bowman’s layer thickness?
8-14 microns, acellular, random type 1 and type 5 collagen fibrils
what is the thickness of corneal stroma?
450 microns
Descemet’s membrane
5-15 microns, the basement membrane of the endothelium, composed of type 4 collagen, very resistant to trauma and damage, becomes Schwalbe’s line at the limbus
what is the thickness of the corneal endothelium?
5 microns
the 3 factors behind RCEs
- poor hemidesmosome attachments
- epithelial basement membrane dystrophy
- age-related thickening of the basement membrane
Reis-Buckler’s dystrophy
rare corneal epithelial dystrophy that appears early in life and is secondary to damage to Bowmans
corneal stroma is made up of
dense, regular connective tissue composed of keratocytes (fibroblasts), collagen fibrils, ground substance, and water (75-80% of the stroma)
keratocytes
fibroblasts of the cornea that produce collagen fibrils and the extracellular matrix
hydrops
occurs in keratoconus when Descemet’s membrane ruptures
Haab’s striae
folds in Descemet’s membrane that occur in congenital glaucoma
Hassall-Henle bodies
small areas of thickened Descemet’s membrane in the corneal periphery that protrude towards the AC, increase with age and have no visual significance
pleomorphism
corneal endothelial cells no longer being hexagonal, change shape
(HEX marker, 70-80% is normal)
polymegathism
when endothelial cells die, neighbouring cells enlarge to cover the empty space, to compensate for the decrease in cell density
stromal edema
- loss of endothelial cells results in a decrease of Na+/K+ ATP pumps and leads to edema in the stroma
- hypoxia of cornea leads to increase of lactate and acidity
deturgescence
the physiological process in which the stroma is kept relatively dehydrated to maintain normal corneal clarity and transparency, depends on GAGs, cell barriers, and ion transport
78%
how does the avascular cornea obtain nutrients?
diffusion from the aqueous humor, limbal conjunctival and episcleral capillary networks, palpebral conjunctival networks
O2 under open-eye conditions - comes from tear film, O2 under closed-eye conditions - comes from palpebral conjunctival blood vessels
corneal neovascularization
defense response to oxygen deprivation, the new vessels arise from endothelial cells of the limbal capillary network in response to cytokines and growth factors (including VEGF)
how does the cornea get sensory innervation
sensory innervation comes from V1, the LPCNs and SPCNs enter the mid-stroma of the cornea. inside the stroma the nerves lose their myelin sheaths and become highly sensitive with nocireceptors (pain)
neurotrophic keratitis
characterized by poor corneal sensitivity and poor wound healing, secondary to damage of V1 (ex. HSV, HZO, CVA, DM)
guttata
protrusions of Descemet’s membrane in central cornea (Hassall-Henle bodies are in periphery in young pts)
bullous keratopathy
low cell density causes swollen cornea because the endothelial cells aren’t pumping fluid as efficiently due to low count
corneal plana
flat cornea due to mutated corneal keratocan, also can cause sclerocornea
which layers of the cornea have nerves running through?
nerves anter mid-stroma and go anteriorly, through to the epithelium but terminate in the wing cell layer
how does the anterior third of the corneal stroma differ from the posterior third?
in the anterior third of the corneal stroma, the lamellae are thin, have more crosslinking, more rigid/harder, more homogenous, more abundant fibroblasts (keratocytes), and have nerves running through. in the posterior third collagen lamellae are larger
lumican
a protein that is essential for maintaining collagen sizing, especially in the posterior stroma
UVR absorption and the cornea
epithelium and Bowmans layer are the primary sites for UV-C absorption, some UV-B
corneal hypoxia
cornea switches to anaerobic glycolysis for energy production in the absence of O2, produces lactate which is hard to remove and changes concentration gradients -> leads to edema (stromal) also increasing H+ means loss of K+ causing cells to shrink and function poorly
what is the average eccentricity of the normal cornea?
e-value is usually around 0.50 for a normal cornea
sclera
forms the posterior 5/6th of the protective connective tissue coat of the eye and helps to maintain the shape of the globe and to protect intraocular structures
What is the mean radius of curvature for the sclera?
11.5mm
Where is the thickest area of the sclera?
1.0mm at the posterior pole
Where is the thinnest area of the sclera?
0.30mm under the recti tendon insertions
What is the weakest area of the sclera?
the lamina cribrosa (within the optic nerve)
How does the sclera receive blood flow?
Sclera is considered avascular, it receives minimal blood supply from episcleral vessels, choroidal vessels, and branches of the LPCAs
What innervates the sclera?
minimally innervated by the LPCNs and SPCNs
episclera
loose connective tissue layer that contains a capillary network (from ACAs) that surrounds the cornea
What causes characteristic ciliary flush/circumlimbal injection?
inflammation of the ciliary body or iris will cause dilation of ACAs causing the circumlimbal flush
sclera proper
thick, dense, avascular connective tissue that is continuous with the cornea stroma
What is the sclera proper composed of?
composed of irregular collagen bundles that provide strength but no transparency. contrains less fibroblasts and GAGs but similar ground substance to the corneal stroma
lamina fusca
the innermost layer of the sclera adjacent to the choroid that contains elastin fibers and numerous melanocytes
What can make the sclera look blue?
blue sclera can be seen in osteogensis imperfecta or Ehlers-Danlos syndrome
also in infants (seeing underlying uvea) as the sclera is sitll thin
What can make the sclera look yellow?
lipids become trapped in the sclera proper connective tissue in elderly patients, or can also be a sign of liver disease
Name the layers of the eye from anterior to posterior (sclera part not corneal)
conjunctival epithelium, conjunctival stroma, Tenon’s capsule, episclera, sclera proper, and lamina fusca
Which layers of the cornea are hydrophilic and which are hydrophobic?
epithelium and endothelium are hydrophobic
stroma is highly hydrophilic
Which corneal layers absorb shorter wavelengths of UV light (UV-C and UV-B) to protect the inner layers of the eye?
Corneal epithelium and Bowmans layer
What light wavelengths does the cornea transmit?
300nm (UV) to 2500nm (infrared)
corneal crystallins
located in the cytoplasm of epithelial and endothelial cells and help maintain corneal transparency by limiting light scattering
Vitamin C/ ascorbate and glutathione
located within the epithelial cells and help to protect the cornea from UV rays and free radical scavengers
What features help the cornea to transmit light?
- proteoglycans, corneal crystallins, ascorbate
- presicely spaced lamellae (destructive interference)
- being avascular
- high water content helps transparency
the major proteoglycan in the corneal stroma is
keratin sulfate
partial pressure of O2 in tear film during open eye conditions
155 mmHg
partial pressure of O2 in closed eye conditions
55 mmHg
critical partial pressure of O2 for the cornea is
10-20 mmHg
the entire corneal epithelium replaces itself every…
7-14 days
Which layers of the cornea regenerate and which do not?
Epithelium and Descemets membrane can regenerate
Bowmans and the endothelium CANNOT regenerate
Aging of lid tension leads to what change in corneal cyl in older patients?
increase in ATR
What is the only mitotic layer in the corneal epithelium?
the basal layer, (secretes its own BM that attaches with hemidesmosomes)
Which layers of the cornea thickens with age?
basement membrane of epithelium doubles with age,
descemets triples in size
Bowmans function?
resistance to damage/injury, also may maintain the correct curvature of the cornea
Clinical conditions related to Bowmans
band keratopathy, pterygia, crocodile shagreen, Reis-Bucklers, keratoconus starts here, refractive sx
Clinical conditions related to Descemets
hydrops, Haab’s striae (horizontal), Hassall-Henle bodies (normal, insignificant, peripheral guttata)
min amount of endo cells before cornea swells
400-700 cell/mm2
average amount of corneal endo cells
1,000-2,000
What germ cell layers do the corneal layers derive from?
Surface ectoderm - corneal and conjunctiva epithelium
Neural crest cells - cornea endothelium, stroma, Bowmans, Descemets
Which layers of sclera/conj are vascular and which are not?
vascular: conj and episclera
avascular: Tenons, sclera proper
