Physiology of Lens and Vitreous Flashcards
Oxidation
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
Endogenous
within cell
mitochondria peroxisomes lipoxygenases NADPH oxidase Cytochrome P450
Exogenous
- Ultraviolet light
- Ionizing radiation
- Chemitherapeutics
- Inflammatory cytokines
- Environmental toxins
Antioxidant defences
- vitamin A
- vitamin
- glutathione
too many antioxidants
not enough free radicals
no normal function of cell. Decrease proliferation, decrease defence
too many free radicals, not enough antioxidants
cell damage
problem of lens
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
Lens Anatomy
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
Hyaloid System
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
how is transparency achieved
- The absence of blood vessels
- Reduction in number of organelles along the optical axis
- Orderly lens fibre arrangement
Close packing of lens components
lens metabolism
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
Oxygen in lens
avascular - limited oxygen
helps protect lens proteins and lipids from oxidative damage
energy production must therefore occur through anerobic mechanisms
Glucose of lens
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
Oxidants and Lens
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
Glutathione
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
Glutathione Diffusion
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
Mechanisms to get metabolites in/out cell
- Gap junction-based transport
- Membrane based transport
- Transcellular transport
Ionic Current Lens
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
Gap Junctions type
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
role of circulating current
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)
Aquaporins
membrane proteins enhance membrane water permeability
AQP1 expressed in lens epithelium
AQP0 abruptly replaces it in lens fibres
Aquaporins
membrane proteins enhance membrane water permeability
AQP1 expressed in lens epithelium
AQP0 abruptly replaces it in lens fibres
Regulation of fluid volumes
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
gradient refractive index
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
Vitreous
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
Vitreous Diffusion Barrier
Diffusion through gels is slow, and bulk flow of solutes is very limited
Prevents topically applied substances reaching the retina in significant concentrations
Vitreous metabolic buffer
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
