Ocular Biochemistry Flashcards
What are the cells in the corneal epithelium
Basal cells - responsible for mitosis, adhere to basal membrane
Wing cells - three layers as cells move superficially
differentiated squamous cells (3-4) layers superficial
What is the role of basal cells of the cornea
Mitosis
store of glycogen
More mitochondira, ER and Golgi
What do sruface corneal cells have?
Microvilli - adherent glycocalyx
Superficial cells contain MUC1 and MUC4 genes
What is the basal lamina of corneal epithelium
40-60um thick
Composed of type 4 and 6 collagen, laminin, fibronectin, fibrin and proteoglycan perlecan
Basement membrane of corneal epithelium basal cells
What is Bowman’s layer
Lies deep to basement membrane
12um thick
Randomly arranged collagen fibres
Acellular layer
Modified superficial stromal layer
What are hemidesmosomes
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
What are hemidesmosomes bound to in basement membrane?
Type 6 collagen - anchoring fibrils
These penetrate the stroma and banch to end in anchoring plaques - composed of laminin
What link basal, wing and superficial cells?
Desmosomes
More numerous between superficial cells
What are zonula occludens
Tight junctions between superficial cells of epithelium
Anastamosis of lipid bilayers of two cells
Superficial cells form semipermeable membrane at anterior surface of eye
What is the cornea formed from?
Collagen fibres running parallel to the corneal surface
Separated by proteoglycan matrix
What collagen makes up corneal stroma
55% type 1 colalgen
Fibril diameter < 30nm and interfirbillary distance 55nm
10% type5
1% type 3
33% type 4
How many lamellae of collagen fibres in stroma
200-250
eAch lamella is bundle of collagen fibres
What are the proteoglycans that make up extrafibrillar matrix?
Keratin sulphate
Dermatan sulphate
Chondroitin sulphate
Decorin (CD/DS)
Lumican (KS)
Keratocan (KS)
Mimecan (KS)
What does sulphation of proteoglycans do?
Affects water retention
Important in corneal transparency
Function of proteoglycans
Important in sapcing of collagen fibrills
What % hydration is cornea? Sclera?
Cornea 80%
Sclera 70%
Cornea can take up more water with welling due to sulphate proteogluycans
How is water removed form cornea?
Metabolic pump in endothelium - removes water from cornea
ATP driven ion pumps
What is the corneal endothelium
Monolayer of hexagonal cells
Large nucleus, numerous mirochondria, ER golgi
What doe the endothelial cells lie on?
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
What connects endothelial cells
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
Where does corneal epithelium get glucose
Stroma
Glycolysis to pyreuvate
Most to lactic acid but some citric acid cycle
Rest is with pentose phosphate for free radicals
Where does corneal epitehlium get oxygen?
Tear film
During sleep from the palpebral conjunctiva
How does corneal epithelium metabolise glucose?
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
What does the corneal endothelium use? Where does it get oxygen
Anaerobic glucosis, citric acid cycle, pentose phosphate pathway
High activity compared to epithelium
Obtains oxygen from aqueous humour
Where does corneal mitosis occur
Mostly at limbus in stem cells which can differentiate into basal cells
What happens in epithelial damage?
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
What happens to corneal stroma when injured
BEcomes opague due to oedema and resulting disorganisation of regular fibrillar arrangement
Water attracted by GAGs
What happens during corneal wound healing
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
What is the sclera made up of
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
