Crystalline Lens

What is the anatomy of the crystalline lens?

Enclosed

What is the structure of the crystalline lens?

Enclosed

What is the structure of the capsule?

Modified basement membrane made of collagen, thickness varies to govern lens shape in accommodation

What is the function of the capsule?

Blocks bacteria and leucocytes, permeable to small molecules such as glucose and lactate

What is the structure of the epithelium?

Anterior lens surface located under capsule, contains junctions, central cells and equatorial cells

What’s the structure of junctions in the epithelium?

Contains desmosomes, gap junctions connect to cortex

What is the structure of central cells in the epithelium?

Cuboidal, hexagonal, interdigitated, transports nutrients and waste between aqueous and lens

What is the structure of equatorial cells within the epithelium?

Columnar, less interdigiated (less finger-like projections to hold everything together) and produce less lens fibres

What happens to cells in the epithelium as they move closer to the centre?

Cells become more columnar as they move towards centre- taller than they are wider

What is the structure of the stroma?

Contains lens fibre cells, with newest fibre cells in the cortex and older fibre cells in the nucleus, concentric lamellae structure (layers), produced throughout life

What is the structure of lens fibre cells within stroma?

fibre cells are hexagonal, thin and long, elongate anteriorly and posteriorly meeting at Y shape structures,containing specialised junctions

What’s the structure of specialised junctions in stroma?

Gap junctions with interlocking junctions influencing lens shape in accommodation

What is the function of interlocking junctions in stroma?

Maintains the shape to ensure flexibility to keep focus

What happens to lens fibre cells as they get older?

They lose their nuclei and organelles

What is the function of Zonules of Zinn?

Holds the lens in position and are lined around the equator

What is the structure of Zonules of Zinn?

Attaches crystalline lens to ciliary body and forms vitreous attachment

What happens accommodation occurs?

Ciliary muscles contract, Zonules slacken and lens surface steepens

What happens when there’s no accommodation occurring?

Ciliary muscles relax, Zonules are under tension and lens surface flattens

How does the structure of lens fibres minimise lens scatter?

Lens fibres have high concentration of soluble crystallins, narrow cell membranes, minimal extracellular space, no nuclei/organelles, highly regular arrangement of fibres in extracellular space

What does crystalline lens allow?

Allows light to pass through unobstructed

How is the low water content of the crystalline lens maintained and regulated?

Tightly regulated electrolyte balance by sodium potassium pump in epithelium which manages cell potential and regulates how much water is in the lens

How is oxygen obtained?

Some oxygen is obtained from aqueous which is used for high metabolic activity like the pump

What are other pathways used to obtain oxygen?

Mainly anaerobic glycolysis but alternatively pentose phosphate, sorbitol

Where does the highest metabolic activity occur?

Epithelium- te of lens fibre production, sodium potassium pump

How is oxygen harmful for tissue?

Hydrogen peroxide may form during aerobic respiration- causing damage to proteins and membrane- leading to cataract and macular degeneration

How does the crystalline lens prevent oxygen damage?

Has natural antioxidants to prevent tissue damage from ROS (free oxygen radicals) and H2O2 as well as vitamin A, C and E

What are the metabolic effects of aging?

Reduced bioactivity of anti-oxidants and hexokinase

What are the molecular effects of ageing?

ROS modify lens proteins, crystallins become insoluble, causing them to come out of solution and become opaque

What are the structural effects of ageing?

Lens fibres added throughout life, lens becomes less pilable (presbyopia), reduced sodium potassium pump activity leads to increased water intake, losing cell nuclei and organelles causes reduced metabolic activity as ROS changes structure of proteins

What are the optical effects of ageing?

Increased scatter due to insoluble crystallins and water uptake, increased brunescence (chromophores)/lens go brown

What is nuclear cataract?

Nuclear oppalescence (white scatter) due to crystallin aggregation (coming out of solution)

Nuclear brumenescnece due to chromphore aggregation

What is the effect of nuclear cataract?

May cause myopic shift due to increased refractive index of lens, making it more powerful (+ve)

What are symptoms of myopic shift?

Patients report problems with distant vision but find it easier to read (when mild)

What is cortical cataract?

Due to sunlight, breakdown and swelling of lens fibres, and crystallin aggregation

What are symptoms of cortical cataract?

May first appear as vacuoles and water clefts, excited fibres often appear and stokes.

Vision becomes worse under dim lighting as pupil diabetes so cataract becomes more obvious

What is posterior subcapsular cataract?

Due to breakdown and swelling of lens fibres, normal epithelial cells move to posterior poles, extracellular debris and fibrillar deposits causing central cataracts, drug induced (steroids)

When is vision more affected with posterior subcapsular cataract?

Bright light- pupil constricts so patient is just looking through cataract, and when reading

What are other causes of cataract?

Diabetes- high blood glucose encourages sorbitol pathway which is too big to move back out of lens capsule eating to water uptake, increase in refractive index and myopic shift, leading to cortical vacuoles and nuclear cataract

Down’s syndrome- cataracts in late childhood

Trauma/radiation/high myopia

What does cataract surgery entail?

Small limbal incision, portion of anterior surface of capsule removed, probe breaks up and removes cloudy lens, intraocular lens inserted into capsular bag

How is cataracts generally caused?

Damage to lens proteins and fibres by reactive oxygen species (ROS)