Physiology of Lens and Vitreous

Question 1

Oxidation

Answer 1

can be caused by molecular oxygen or free radicals (reactive oxygen series)

• Generated by mitochondrial activity, metabolic processes, or by the absorption of light
• Easily take electrons from (oxidise) molecules they remain in contact with
• Results in chain reactions that lead to cell structure damage (proteins, DNA)

Cell cytoplasm is generally maintained as a reducing environment to prevent oxidation. The generation of reducing equivalents requires energy

Question 2

Endogenous

Answer 2

within cell

mitochondria 
peroxisomes 
lipoxygenases 
NADPH oxidase 
Cytochrome P450

Question 3

Exogenous

Answer 3

• Ultraviolet light
• Ionizing radiation
• Chemitherapeutics
• Inflammatory cytokines
• Environmental toxins

Question 4

Antioxidant defences

Answer 4

• vitamin A
• vitamin
• glutathione

Question 5

too many antioxidants

not enough free radicals

Answer 5

no normal function of cell. Decrease proliferation, decrease defence

Question 6

too many free radicals, not enough antioxidants

Answer 6

cell damage

Question 7

problem of lens

Answer 7

avascular - no route for waste removal, supply of nutrients

loss of organelles - central lens fibres loose organelles to aid with fibre packing and maintenance of lens transparency

Question 8

Lens Anatomy

Answer 8

Capsule –> basement membrane with elastic properties

Epithelium –> simple cuboidal cells with central non dividing zone and equatorial germinative zone

Cortex –> formed from the epithelium in the germinative zone as elongating lens fibres

Nucleus –> adult, fetal and embryonic

Question 9

Hyaloid System

Answer 9

During development the lens is supplied by the tunica vasculosa lentis:
- Served by the hyaloid artery with veins connected to the choroidal system

Development of retinal vasculature around 4-5 months of gestation triggers atrophy and regression of the hyaloid system

Need systems in place to perform the role of the tunica vasculosa lentis once it regresses

Need blood supply when developing because lens development is a highly metabolic process

Question 10

how is transparency achieved

Answer 10

• The absence of blood vessels
• Reduction in number of organelles along the optical axis
• Orderly lens fibre arrangement
  Close packing of lens components

Question 11

lens metabolism

Answer 11

main location in epithelium. Requires continuous ATP production.

Epithelial cell and fibre maintenance, equatorial mitosis

Active transport of ions and amino acids, lens dehydration, and production of protein and glutathione

All directed towards maintenance of transparency:
- Cell division, protein metabolism, cellular differentiation, and maintenance of cellular homeostasis all contribute to transparency
- Maintenance of lens hydration is important for maintaining transparency
Protection of the lens from oxidative damage is also critical for transparency

Question 12

Oxygen in lens

Answer 12

avascular - limited oxygen

helps protect lens proteins and lipids from oxidative damage

energy production must therefore occur through anerobic mechanisms

Question 13

Glucose of lens

Answer 13

Lens almost entirely dependent on the metabolism of glucose for the production of ATP

Aqueous humour glucose levels maintained by facilitated diffusion across the ciliary epithelium

Glucose enters the lens via facilitated diffusion: GLUT1 transporters in the epithelium, GLUT3 transporters in the lens fibres

Glucose is rapidly metabolised so that the concentration of glucose in the lens in 1/10 that in the aqueous

Glucose metabolism largely occurs in the epithelium and the cortex: nucleus relatively inert

Around 70-80% of glucose metabolised by anaerobic glycolysis

Remainder metabolised by sorbitol pathway, hexose monophosphate (pentose-phosphate) shunt, Krebs cycle:
- Also produce free radicals
Sorbitol pathway important in diabetic cataract formation

Question 14

Oxidants and Lens

Answer 14

aerobic glucose metabolism however produces free radicals that lead to oxidative stress

Hydrogen peroxide also thought to cause oxidative stress in the lens:

• Produced in mitochondria
• Also produced during oxidation of ascorbic acid, which is found in high levels in the aqueous and vitreous

UV light has the potential to induce oxidative damage in the lens:
Lens contains a series of UV filter compounds to help prevent this

Question 15

Glutathione

Answer 15

reducing agent (antioxidant) high conc in lens. Protection from oxidative damage in lens

tripeptide formed from glycine, leucine, glutamic

Oxidised glutathione (GSSG) converted to GSH by glutathione reductase and NADPH: 
NADPH produced via the hexose monophosphate shunt pathway of glucose metabolism

Question 16

Glutathione Diffusion

Answer 16

Glutathione concentrations highest in the superficial layers of the lens:
- GHS must diffuse towards the nucleus

GSSG must diffuse to more superficial layers of the lens to be reduce to regenerate GSH:
- Probably via connexin gap junctions

Rate of diffusion diminishes with age, leading to increased oxidative damage in the lens

Question 17

Mechanisms to get metabolites in/out cell

Answer 17

• Gap junction-based transport
• Membrane based transport
• Transcellular transport

Question 18

Ionic Current Lens

Answer 18

comes out of lens at equator and in at the poles

Na+K+/ATPase pumps locations coincide with this pattern:
- Pump density maximum in equatorial epithelium
Pump actively generated an electrochemical gradient with the interior of the lens more negative than the surrounding environment

Fibre cells low Na+ permeability but once inside cell flows quickly between cells via gap junctions to surface epithelium. Transported out by Na+K+/ATPase pump

Fibre cells have low K+ conductance compared to epithelial cells:
- K+ than enters via Na+K+/ATPase pumps thus leaks back out of epithelium rather than through fibre cells
Helps to maintain the ionic current and Na+ flow

Question 19

Gap Junctions type

Answer 19

Cx43, Cx46 and Cx50

Cx43, Cx50 in lens epithelial cells. Cx43 expression lost at transition between epithelium and developing lens fibres

Cx46, Cx50 expressed in developing lens fibres. Functional Cx50 channels lost in mature lens fibres although protein still present

Question 20

role of circulating current

Answer 20

circulate solutes to the deep lens fibres and transports waste out

Transport of Na+ thought to be linked to movement of fluid via local osmosis:
- As water moves into the lens through extracellular spaces it carries metabolites (glucose) , antioxidants, and amino acids to deeper lens fibres
- Enter at the poles
Exists the lens at the equator (intracellular pathway)

Question 21

Aquaporins

Answer 21

membrane proteins enhance membrane water permeability

AQP1 expressed in lens epithelium

AQP0 abruptly replaces it in lens fibres

Question 21

Aquaporins

Answer 21

membrane proteins enhance membrane water permeability

AQP1 expressed in lens epithelium

AQP0 abruptly replaces it in lens fibres

Question 22

Regulation of fluid volumes

Answer 22

tightly regulated lens hydration is the pump-leak system

Anterior:

• Passive Na+ diffusion into lens cortex
• Na+ ions actively pumped out in exchange for K+ ions by ATPase
  
  • Maintains an osmotic balance at the required level for transparency

Posterior:

• No epithelial barrier
• Permeable capsule
• Free diffusion of ions, solutes and water between lens and vitreous

Individual lens fibres have a negative resting voltage:
- Inhibits the influx of Cl-
- Maintains a steady state ion concentration
- Water movement into and out of the cells is thus in equilibrium (same volume in as out)
Maintains a constant cell volume

Question 23

gradient refractive index

Answer 23

proteins make up 40% of wet weight of lens fibre cells

Protein concentration 3x higher than in cytoplasm of typical cells

80-90% are water soluble crystallins, remainder membrane bound

made during lens development and need to be durable to last life time

Lower refractive index at poles compared to centre. Directly correlated with protein gradient. Inverse to water gradient in the lens

Question 24

Vitreous

Answer 24

Majority of the vitreous (>99%) is water:
- Acts as a gel with an interconnected meshwork of solids surrounding and stabilising the large amount of water

Structure providing by long, thick, non-branching collagen fibrils suspended in hyaluronic acid: mostly type II collagen

Inorganic and organic substances dissolved in the water of the vitreous:
- Blood-ocular barriers, retinal and ciliary body metabolism, and vitreous diffusion all lead to a gradient od solutes between the vitreous and blood plasma

Normal physiology of the vitreous divided into four main groups:
- Support for the retina and filling of vitreous cavity
- Diffusion barrier between the anterior and posterior segments
- Metabolic buffer
Ocular transparency

Question 25

Vitreous Diffusion Barrier

Answer 25

Diffusion through gels is slow, and bulk flow of solutes is very limited

Prevents topically applied substances reaching the retina in significant concentrations

Question 26

Vitreous metabolic buffer

Answer 26

Substances present in or produced by the retina are diluted through diffusion into the vitreous in cases where poor transport across the blood retina barrier

Glucose in the vitreous can supplement retinal metabolism, especially during anoxic conditions

Metabolite supply to the lens