Cornea - Layers (M2)

Question 1

how many layers make up conjunctiva

Answer 1

5 layers (outer to inner):
1. epithelium
2. Bowman’s layer
3. stroma
4. Descemet’s membrane
5. endothelial
(6. dua’s layer)

Question 2

function of the epithelium

Answer 2

barrier to the outside environment - prevents microbes from entering globe

Question 3

thickness of epithelium

Answer 3

thicker in the periphery, thinner centrally

Question 4

what epithelial cells make up the epithelium

Answer 4

basal cells
wing cells
surface cells (apical cells)

Question 5

characteristics of basal cells found in the epithelium

Answer 5

Single layer of columnar cells.
Undergo mitosis and migrate into outer layers.
Replaced by limbal stem cells as they move outer.

Question 6

characteristics of wing cells found in the epithelium

Answer 6

2-3 layers of cells.
Transition zone: basal –> wing –> surface

Question 7

characteristics of surface cells (apical cells) found in the epithelium

Answer 7

2 layers of non-keratinized, squamous cells.
Microvilli are present at the apex of surface cells.
Secrete glycocalyx from apex of surface cells.
tight junctions join lateral sides of adjacent cells.
undergo desquamation

Question 8

function of microvilli in surface cells

Answer 8

helps bind mucin to corneal surface

Question 9

what does the glycocalyx in surface cells consist of and what is the function

Answer 9

consists of transmembrane mucins (glycoproteins) - MUC1, MUC4, MUC16.
Barrier to pathogens and injury.
Helps bind mucin to corneal surface.
Helps spread tear film across corneal surface

Question 10

function of tight junctions in surface cells

Answer 10

forms complete barrier - preventing fluid and molecules from passing between cells.
fluid and molecules must pass through cells

Question 11

what is desquamation

Answer 11

surface cells degenerate and are shed.
replaced by wing cells.
turnover time for entire corneal epithelium: 7 - 10 days

Question 12

what attached basal cells to basement membrane

Answer 12

hemidesmosomes

Question 13

what are hemidesmosomes composed of and secreted by

Answer 13

composed of collagen type IV.
secreted by basal epithelial cells.
anchoring fibrils (collagen type VII) anchor hemidesmosomes to underlying connective tissue.

Question 14

is bowman’s layer acellular or cellular?

Answer 14

acellular

Question 15

what is bowman’s layer composed of

Answer 15

interwoven collagen fibrils in ground substance.
collagen types : I, III, V, VI

Question 16

what is bowman’s layer secreted by and when

Answer 16

secreted prenatally by corneal epithelial cells = does NOT regenerate

Question 17

how much of the cornea does the stroma make up

Answer 17

90% of total corneal thickness

Question 18

what is the stroma composed of

Answer 18

connective tissue with WBCs dispersed throughout.
- keratocytes
- collagen
- ground substance

Question 19

what are keratocytes, where are they found, and what is their function in the stroma

Answer 19

Flattened cells with extensive branching.
Higher density in anterior stroma.
Lie between (sometimes within) layers of collagen.
Synthesize collagen and ground substance.

Question 20

what type of collagen found in stroma

Answer 20

predominately type I, but does contain type III, V, VI

Question 21

what are fibrils like in collagen type I

Answer 21

uniformly straight, spaced, and have a diameter of 30 nm.
fibrils form 200-300 lamellae that lie parallel to the corneal surface.

Question 22

characteristics of the lamella in the stroma

Answer 22

Extend from limbus to limbus.
Course in the same direction.
Adjacent lamellae lie at angles to one another.
Anteriorly (1/3): thin and branch and interweave –> allows for more crosslinking –> increases rigidity.
Posteriorly (2/3): thicker with less branching and interweaving.

Question 23

function of collagen type V

Answer 23

maintains uniform diameter of type I

Question 24

function of collagen type VI

Answer 24

stabilizes keratocytes found between collagen layers

Question 25

what is ground substance composed of in the stroma

Answer 25

transparent gel composed of water, glycosaminogylcans, and proteoglycans.
- Chondroitin sulfate and dermatan sulfate are abundant in anterior stroma; if attached to protein its called decorin.
- Keratin sulfate is abundant in posterior stroma; if attached to protein its called keratocan, lumican, or mimican

Question 26

function of ground substance in the stroma

Answer 26

maintain precise spacing between fibrils.
contribute to relatively high stromal hydration - GAGs are very hydrophilic.

Question 27

what is descemet’s membrane

Answer 27

basement membrane of endothelium - connects endothelial cells to stroma.
weak attachment

Question 28

what is descemet’s membrane composed of

Answer 28

type VII collagen and barely any type IV collagen.

Question 29

what does the collagen arrangement give descemet’s membrane

Answer 29

elastic property without elastin being present

Question 30

what are the 2 layers of descemet’s membrane

Answer 30

anterior layer: secreted during embryonic development.
posterior layer: secreted by endothelium throughout life

Question 31

where does descemet’s membrane terminate

Answer 31

Schwalbe’s line.
thickens.
at the end of cornea towards limbus

Question 32

what is the endothelium

Answer 32

innermost layer of the cornea

Question 33

what is the endothelium composed of

Answer 33

single layer of flattened endothelial cells.
basal surface adheres to descemet’s membrane.
apical surface lines anterior chamber - microvilli.

Question 34

shape of cells in endothelium and why

Answer 34

70-80% of cells are hexagonal - most efficient shape to provide area coverage without gaps.
endothelial mosaic

Question 35

function of desmosomes in endothelial layer

Answer 35

form incomplete barrier, which allows nutrients to pass between endothelial cells and into stroma.
excess water accompanies these nutrients into the cell and need to be pumped out.

Question 36

function of metabolic pumps

Answer 36

about 3 million Na+/K+ -ATPases present in single corneal endothelial cell.
pump ions out of cornea and into aqueous humor –> water follows through aquaporins into aqueous humor.
maintain water constant of corea at 78%

Question 37

does the endothelium have a small or large number of mitochondria

Answer 37

large.
reflects amount of ATP needed for metabolic pumps

Question 38

what is dua’s layer

Answer 38

controversial 6th layer of cornea.
if it does exist it is a transition layer