Lens Flashcards
Because the refractive power of the lens is variable, it permits
Focus on objects that’s are near or far
Lens vascularization and innervation
Lens is avascular and no innervation
What is vossius ring?
Blunt trauma causes iris to stick to the eye and you will see pigment cells on the anterior lens
The lens is a ______ structure without any nerves or blood vessels
Purely epithelial structure
The oldest cells of the lens are located
In the center of the lens
Youngest are always on the surface
Small cuts and tears in the lens capsule will result in
Lens opacities
The lens capsule is more fragile in
Diabetics
Having too much sugar makes the capsule weaker
Lens capsule is secreted by
The basal cell area of the lens epithelium
Lens is like an onion because ___ which affects ____.
New layers are added over a pts life span
Which produces an age related increase in thickness (harder to accommodate)
Active layer of the lens is
Lens epithelium: a single layer of cuboidal cells which lies deep to the anterior capsule
The bulk of the lens is composed of
Lens fiber
Mitotic cells ___ with age
Decrease
What layer is the most susceptible to radiation damage ?
Epithelial cells
Mature lens fibers are cells which
Have lost their nuclei
Eliminates light scatter
Extensive system of gap junctions that allow the lens to behave as one large cell
Syncytium
The reason the cells lose their nuclei is to allow light to pass through
The lens nuclei would create light scatter and distort retinal images
Epithelial transitions to
Fiber cell
Lens fibers are highly specialized terminally differentiated cells (cant do anything else)
Lens sutures
Where lens fibers terminates upon each other (where they meet)
Can have a y suture cataract
Zonules are primary made of
Fibrillin, a connective tissue protein
Contain cysteine
In Marfans syndrome
Mutation in the fibrillin gene, will cause lens to be dislocated
What is ectopic lentis
Lens dislocation/ subluxation
Zonular fibers are broken and the lens is no longer held securely in place but remains in pupil aperture
Hydration of the lens
Relatively dehydrated organ
Low water is a neutral consequence of need for maintaining refractive index
The majority of the lens protein is
Structural protein/ crystallin
Water soluble
Most insoluble proteins are found in
The lens nucleus / oldest part of the eye
Solubility decreases with age
Why we see cataracts on older people
Water soluble structural proteins
Crystallins
Crystallins are chaperones
Provides lens with oxidative stress resistant properties (chaperone like activity)
They also prevent aggregation
When crystallin become insoluble
Light scatter occurs
Oxidation releases
Free radicals that tear up membranes
Glutathione (GSH synthesis)
Important in lens, tries to minimize damage form oxidation
A way to keep lens transparent is by
Maintainance of low intracellular Ca+
High levels of calcium activate proteases
Glutathionine uses up ____ of the ATP generated by glycolysis
11%
The concentration in glutathione in cataracts
Is reduced
Glutathione is a
Tripeptide (glycine-leucine-glutamic acid)
GSH function
Protects sulhydryl groups (preventing aggregation via disulfide bonds)
Protective against free radicals
WHAT TRIGGERS OXIDATION?
Light
Lens metabolism:
No mitochondria
Low energy demands, consumes very little oxygen
Mostly anaerobic-> poor amount of ATP
Low oxygen tension around and within the lens helps
To protect lens proteins and lipids from oxidative damage
Hydrogen peroxide can also be produced during
The oxidation of ascorbic acid, which is present in the aqueous and vitreous humor
The lens is avascular so nutrients come from
Aqueous humor
Lens behaves as a single cells
Syncytium
In the lens Na+ ____ with age
Increases
Calcium concentration on the lens is ____ than in the aqueous
50x less
Healthy lens cell have
Low intracellular Ca+
In cortical cataracts calcium is
16x higher
The lens has a special amino acid called
Taurine who’s function is unknown
Essential scavenger of free radicals
Ascorbic acid
Protein synthesis in the lens
Does not exist, so you cant repair stuff
Aerobic glycolysis is limited mostly to
Epithelium
Pentose produces in HMP shunt can be used for DNA/RNA
Produces NADPH
Used in sorbitol path & glutathione reductase, fatty acid synthesis
Phosphofructokinase
Determines how the glucose gets shunted
Sorbitol increases
Osmotic concentration causing fluid influx into lens
Decreased GSH means
Increased oxidation
HMP shunt recreates glutathione
NADPH
Reduced glutathione
What are free radicals
Molecules with odd number of electrons
Enzymes involved in redox systems
Catalase
Glutathione peroxidase
Superoxidase dismutase
Ascorbic acid
When proteins in the lens precipitates
Lens color changes
Ciliary muscles contact and lens bulges. This creates a greater curvature of the lens and more refraction of the light
Accommodation
