Cornea - Transparency (M2)

Question 1

what are the only transparent tissues in the body

Answer 1

cornea
crystalline lens
vitreous

Question 2

contributions to corneal transparency

Answer 2

lack of blood vessels.
lack of melanocytes centrally and midperipherally.
lack of myelin sheath surrounding nerves in cornea.
smooth epithelial surface.
large amounts of water-soluble proteins (crystallins) in corneal cells.
highly organized stromal collagen fibrils.
maintenance of 78% water content in stroma.

Question 3

how does smooth epithelial surface contribute to corneal transparency

Answer 3

overlying tear film fills in slight irregularities between cells

Question 4

how does large amounts of water-soluble proteins in corneal cells contribute to corneal transparency

Answer 4

enable cytoplasm to appear homogeneous

Question 5

characteristics of highly organized stromal collagen fibrils that contribute to corneal transparency

Answer 5

uniformly straight.
uniform spacing between fibrils.
uniform diameter of 30 nm.

Question 6

Goldman and Bendek’s theory of corneal transparency

Answer 6

theory that uniform distance spacing between fibrils increases transparency

Question 7

Maurice’s lattice theory of corneal transparency

Answer 7

theory that the uniform diameter increases transparency

Question 8

what does the maintenance of 78% water content in the stroma allow

Answer 8

uniform spacing between collagen fibrils in stroma

Question 9

what 2 fluids is the cornea located between

Answer 9

tear film and aqueous humor - must be a balance between water into and out of the stroma

Question 10

factors that increase water content in stroma

Answer 10

1. Desmosomes between endothelial cells
2. Stromal swelling pressure (SP)
3. Intraocular pressure (IOP)

Question 11

what do desmosomes between endothelial cells allow

Answer 11

entrance of nutrients (ex. glucose, amino acids, lipids, minerals, vitamins, water) from aqueous humor

Question 12

stromal swelling pressure

Answer 12

pressure exerted when GAGs/proteoglycans pull water into corneal stroma.
swelling pressure = 55 mmHg

Question 13

intraocular pressure

Answer 13

pressure exerted from aqueous humor in anterior chamber.
pushed water into stroma.
intraocular pressure = 10 - 21 mmHg

Question 14

factors that decrease water content in the stroma

Answer 14

1. tight junctions between epithelial cells
2. metabolic pumps in the cell membranes of endothelial cells
3. metabolic pumps in the cell membranes of epithelial cells

Question 15

how do tight junctions between epithelial cells decrease water content in stroma

Answer 15

prevent excessive flow of water from tear film into stroma (complete barrier).
forces all molecules (including water) to pass through epithelial cells’ membranes

Question 16

metabolic pumps in cell membranes of endothelial cells

Answer 16

ion channels, cotransporters, energy-utilizing ion pumps.
- Na+/K+ ATPases in cell membrane of endothelial cells –> move Na+ out of stroma/endothelial cells into aqueous.
- Cl-/HCO3^- exchangers –> moves HCO3^- and Cl- out of stroma/endothelial cells into aqueous.
ion movement in and out of stroma/endothelial cells aids in fluid balance.
- water follows the movements of ions –> transported through aquaporins

Question 17

metabolic pumps in the cell membranes of epithelial cells

Answer 17

Na+ channels in apical membrane:
- move Na+ into epithelium from tear film.
Na+/K+/2Cl- cotransporters in the basolateral membrane:
- move Na+, K+, Cl- into epithelium from stroma.
Na+/K+ ATPases in basolateral membrane:
- move Na+ out of epithelium into stroma.
Cl- channels in apical membrane:
- move Cl- out of epithelium into tear film.
K+ channels in basal membrane:
- move K+ out of epithelium into stroma.
- opposed by Na+/K+ ATPases and Na+/K+/Cl- cotransporters

Question 18

tear film osmolarity

Answer 18

hypertonic tear film will pull water out of corneal epithelium