Flashcards in Chapter 8: Cornea Deck (26):
What is the innervation of the cornea?
Long Posterior Ciliary Nerves (branch of CN V1) penetrate the cornea to in 3 planes: the scleral, episcleral, and conjunctival
Peripherally, 70-80 branches enter the cornea and lose their myelin sheaths 1-2 mm from the limbus
a plexus posterior to bowmans layer sends branches anteriorly into the epithelium
Describe the 02 supply of the cornea?
02 is supplied via the pre corneal tear film, eyelid vasculature, and aqueous humor
What is the source of glucose for the cornea?
Glucose is received primarily from the aqueous via carrier-mediated transport through the endothelium
Epithelium receives it via passive diffusion from the stroma
10% of glucose is supplied by the preocular tear film and limbal vessels
How is glucose metabolized by the cornea?
TCA cycle (much more active in the endothelium than the epithelium), anaerobic glycolysis, and HMP shunt
In epithelium, 35-65% go glucose metabolized via HMP pathway
How do keratocytes metabolize glucose? Why?
Keratocytes metabolize glucose via the HMP shunt, because they lack the enzyme 6-phosphogluconate dehydrogenase
What is the thickness of the corneal epithelium?
The corneal epithelium is 50 micrometers thick, and constitutes 5-10% of total corneal thickness
What is the composition of the corneal epithelium?
composed of 4-6 layers.
1-2 layers of superficial squamous cells
2-3 layers of broad wing cells
inner layer of columnar basal cells
What is the composition of bowman's layer of the cornea? What is the thickness?
8-12 micrometers thick
composed of randomly packed type I and V collagen
What cells produce the bowman's layer? When does the Bowmans layer develop?
Anterior stromal keratocytes and epithelium secrete bowmans layer during embryogenesis
What happens to Bowmans layer when it is inured?
As it is acellular, the cells do not regenerate. Therefore, a scar will form.
What is the function of Bowmans Layer of the cornea?
Bowmans layer seves to prevent expire of the stromal corneal keratocytes to GFs secreted by the epithelial cells
What is the composition of the corneal stroma? What percentage of total corneal volume does it occupy?
The stroma is composed of 200 layers of lamellae, which are collagen fibrils enmeshed in a matrix consisting of proteoglycans, proteins, and glycoproteins.
Makes up 90% of total corneal volume
What percentage of corneal volume is made up by keratocytes?
30-45 %, depending on age
What is the orientation of collagen fibrils in the corneal stroma?
The collagen fibrils run parallel to each other from limbus to limbus, with regular spacing of 55-60 nm from center to center
Adjacent lamellae are positioned at 90 degrees to each other (< 90 in anterior stroma, >90 in posterior stroma)
What type of collagen is the major component of the corneal stroma? What other types of collagen are also found in the stroma?
Type I collagen is the major component of the corneal stroma--constitutes 70% of total stromal dry weight
Types V, VI, VII, XII, XIV
Type III is also produced and is associated with wound healing
What is the second most abundant biological constituent of the corneal stroma?
Proteoglycans, which constitute 10% of dry weight of corneal stroma and confer hydrophilic properties to the stronga
What glycosaminoglycans are found in the corneal stroma?
What is the only MMP found in a healthy cornea? What types are pro ducted in response to injury?
S/P injury, MMP types 1, 3, and 9 are synthesized
What is the function of proteinase inhibitors in the cornea? Where do they come from?
-They restrict damage during corneal inflammation, ulceration, and wound healing.
- Some are synthesized in the cornea, others come from the tears, aqueous humor, and limbal blood cells
What is Descemets Membrane?
a specialized basement membrane composed predominately of type IV collagen that is 10-12 micrometers thick.
Has an anterior banded portion and a posterior non-banded portion
Where does Descemets membrane come from?
It is secreted by the corneal endothelium
What is the corneal endothelium? What is the cell density in young people? When do you begin to see guttata?
A single layer of polygonal cells that are 20 micrometers in diameter.
guttata form at < 2000 cells/mm^2
What is the function of the corneal endothelium?
it is a permeability barrier b/w the stroma and aqueous humor, and a pump to maintain the corneal stroke in a dehydrated state (generates a negative hydrostatic pressure)
Where does the endothelium receive 02 from?
The endothelium receives enough 02 from the aqueous to maintain its pumping function
Does healing/regeneration occur in the corneal endothelium? If so, how?
Healing occurs via cell migration, rearrangement, and enlargement of the residual cells.
Regenerative capabilities are limited