Ocular Biochemistry

Question 1

What are the cells in the corneal epithelium

Answer 1

Basal cells - responsible for mitosis, adhere to basal membrane

Wing cells - three layers as cells move superficially

differentiated squamous cells (3-4) layers superficial

Question 2

What is the role of basal cells of the cornea

Answer 2

Mitosis
store of glycogen
More mitochondira, ER and Golgi

Question 3

What do sruface corneal cells have?

Answer 3

Microvilli - adherent glycocalyx

Superficial cells contain MUC1 and MUC4 genes

Question 4

What is the basal lamina of corneal epithelium

Answer 4

40-60um thick
Composed of type 4 and 6 collagen, laminin, fibronectin, fibrin and proteoglycan perlecan

Basement membrane of corneal epithelium basal cells

Question 5

What is Bowman’s layer

Answer 5

Lies deep to basement membrane
12um thick
Randomly arranged collagen fibres

Acellular layer
Modified superficial stromal layer

Question 6

What are hemidesmosomes

Answer 6

Link epithelium and basement membrane

Membrane protein complexes in epithelial basal cell plasma membrane

Linked to keratin filaments intracelularly

Bound to type 6 collagen - anchoring fibrils in basement membrane

Question 7

What are hemidesmosomes bound to in basement membrane?

Answer 7

Type 6 collagen - anchoring fibrils
These penetrate the stroma and banch to end in anchoring plaques - composed of laminin

Question 8

What link basal, wing and superficial cells?

Answer 8

Desmosomes
More numerous between superficial cells

Question 9

What are zonula occludens

Answer 9

Tight junctions between superficial cells of epithelium

Anastamosis of lipid bilayers of two cells

Superficial cells form semipermeable membrane at anterior surface of eye

Question 10

What is the cornea formed from?

Answer 10

Collagen fibres running parallel to the corneal surface
Separated by proteoglycan matrix

Question 11

What collagen makes up corneal stroma

Answer 11

55% type 1 colalgen

Fibril diameter < 30nm and interfirbillary distance 55nm

10% type5
1% type 3
33% type 4

Question 12

How many lamellae of collagen fibres in stroma

Answer 12

200-250

eAch lamella is bundle of collagen fibres

Question 13

What are the proteoglycans that make up extrafibrillar matrix?

Answer 13

Keratin sulphate
Dermatan sulphate
Chondroitin sulphate

Decorin (CD/DS)
Lumican (KS)
Keratocan (KS)
Mimecan (KS)

Question 14

What does sulphation of proteoglycans do?

Answer 14

Affects water retention
Important in corneal transparency

Question 15

Function of proteoglycans

Answer 15

Important in sapcing of collagen fibrills

Question 16

What % hydration is cornea? Sclera?

Answer 16

Cornea 80%
Sclera 70%

Cornea can take up more water with welling due to sulphate proteogluycans

Question 17

How is water removed form cornea?

Answer 17

Metabolic pump in endothelium - removes water from cornea

ATP driven ion pumps

Question 18

What is the corneal endothelium

Answer 18

Monolayer of hexagonal cells

Large nucleus, numerous mirochondria, ER golgi

Question 19

What doe the endothelial cells lie on?

Answer 19

Descemet’s membrane (absement membrane 1-15um thick

Secreted by endothelial cells
Main components are type 4 collagen, laminin, fibronectin

Highly resistant to proteolytic enzymes and may remain intact in severe corneal ulceration

Question 20

What connects endothelial cells

Answer 20

Tight junctions

Apical tight junction = macula occludens - do not completely encircle cell - more leaky than zonula occludens of epithelium

Cap junctions on lateral membranes - roel in intercellular communications

Question 21

Where does corneal epithelium get glucose

Answer 21

Stroma

Glycolysis to pyreuvate
Most to lactic acid but some citric acid cycle

Rest is with pentose phosphate for free radicals

Question 22

Where does corneal epitehlium get oxygen?

Answer 22

Tear film

During sleep from the palpebral conjunctiva

Question 23

How does corneal epithelium metabolise glucose?

Answer 23

In hypoxia and normal conditions
Some glucose divereted to hexose monophosphate shunt - regulates elvels of NADP

Glucose from aqueous or epitelium converted to pyruvate by the Embden Meyerhof (glycolysis) pathway 2 ATP for 1 glucose

Pyruvate oxidised via citric acid cycle to produce 36 aTP

Question 24

What does the corneal endothelium use? Where does it get oxygen

Answer 24

Anaerobic glucosis, citric acid cycle, pentose phosphate pathway

High activity compared to epithelium

Obtains oxygen from aqueous humour

Question 25

Where does corneal mitosis occur

Answer 25

Mostly at limbus in stem cells which can differentiate into basal cells

Question 26

What happens in epithelial damage?

Answer 26

Cells slide horizontally to fill the defect
Hemidesmosomes and intercellular contacts reform and 6 layer structure is reformed

Migrate due to action and redistribution of actin-myosin fibrils

Question 27

What happens to corneal stroma when injured

Answer 27

BEcomes opague due to oedema and resulting disorganisation of regular fibrillar arrangement

Water attracted by GAGs

Question 28

What happens during corneal wound healing

Answer 28

Wound closed by deposition of fibrin and activation of fibroblasts to generate collagen and GAGs

Irregular size/arrangement results in corneal opacity

Epithelialisation of the wound

Endothelial cells spread out and change size and morphology as they do not undergo mitosis
Remodelling to normal hexagonal shape and pump and barrier function restored when confluent monolayer

Question 29

What is the sclera made up of

Answer 29

Type I and type III collagen of varying diameter, distributed irregularly

Fewer proteoglycans than in the cornea - proteodermatan and proteochondroitin sulphate are the amin ones - less hydrated than cornea