Week 9: Crystalline Lens & Accommodation

Question 1

What are the functions of the lens?

Answer 1

• Focus light on the retina (1/4 of total refractive power of the eye)
• Lens provides about 15-20D
• Filters ultraviolet light
• Separates between more anterior & posterior structures of the eye

Question 2

Structure/characteristics and location of the lens

Answer 2

• Biconvex, transparent & avascular
• Location: behind the iris & in front of vitreous chamber
• Suspended by zonule fibres which arise from ciliary processes
• Anterior surface follows an elliptical curve
• Posterior surface follows a parabolic curve

Question 3

What are the dimensions/characteristics of the lens?

Answer 3

• 6.5 mm diameter at birth
• 10 mm diameter in adult
• Provides 15-20 D
• No cranial nerve innervation
• Adult lens has NO blood supply

Question 4

What are the layers of the lens?

Answer 4

From anterior to posterior:

1. Lens capsule
2. Lens Epithelium
3. Lens Fibres
   a. Cortex
   b. Adult nucleus
   c. Foetal nucleus
   d. Embryonic nucleus

Question 5

What is the lens capsule structure?

Answer 5

• Thickened elastic basement membrane
• Composed layers of lamellar matrix of type IV collagen fibrils and GAGs
• Produced by lens epithelium cells
• Thickest at pre and post equatorial locations
• Thinnest at posterior pole

Question 6

What is the function of the lens capsule?

Answer 6

• Serves as a diffusion barrier
• Mould the shape of the lens during accommodation
• When not under tension of zonules, elastic capsule and cortex it becomes rounded

Question 7

What is the relationship of lens capsule and age?

Answer 7

• Both anterior & posterior capsules get thicker with age

Question 8

Describe lens epithelium?

Answer 8

• Single layer of cuboidal cells lining the inside of anterior lens capsule
• Proliferation occurs in the germinative zone
• Cells in the transitional zone migrate to elongate and become lens fibres
• Migrating below the equator to elongate and become secondary lens fibre cells

Question 9

Describe lens fibres

Answer 9

• As fibres migrates internally, nucleus migrates anteriorly, creating a lens bow
• Fibres are held together by interdigitations
• Extensive intercellular communication occurs via lens gap junction like MIP26
• In the foetus, ends of opposing lens fibres in the nucleus meet anteriorly & posteriorly to form Y sutures
• Anterior suture is erect Y
• Posterior suture is inverted (downward Y)
• Y sutures form during gestation (2 to 8 months)
• Gradient refractive index in periphery = 1.38, nucleus = 1.41

Question 10

Lens suspension

Answer 10

1. Lens is held in place by zonules
2. Arise from non-pigmented epithelium of the ciliary processes and run towards the equator of the lens
3. Fuse together to form bundles
4. Larger bundles: attach anterior to the equator (anterior zonular sheet)
5. Smaller bundles: attach posterior to the equator (posterior zonular sheet)

Question 11

What is the physiology of the lens?

Answer 11

• Fibres behave like a large syncytium both electrically & chemically
• Na+ and K+ ions
• Water follows ions to ensure substances move in and out of the lens
• Movement of ions & water facilitated by Na+/K+ ATPase protein pumps and gap junctions
• Between epithelium and fibres, communication is by MIP26

Question 12

What are the lens metabolites?

Answer 12

1. Glucose
   - From the aqueous humour, main source of energy for lens metabolism
2. Protein
   - Crystallins and MIP26
3. Lipid
   - Lens fibre cell membranes have high concentration of fatty acids giving rigidity to cell membrane
4. Redox
   - Oxidative agents can result in oxidative damage
   - Lens contains enzyme systems to buffer effects

Question 13

What are the ciliary body muscle fibres?

Answer 13

1. Longitudinal (meridional) fibres
   - Most external and closest to sclera
2. Oblique (radial) fibres
   - Radiate outward from scleral spur
3. Circular fibres
   - Anterior & inner portion of ciliary body
   - Nearest to lens
   - Acts as a sphincter

Question 14

What is the process when the lens is at rest (not accommodating)?

Answer 14

• Ciliary muscle is relaxed
• Radial fibres pull on zonules
• Lens is held under tension by zonules
• Lens is flatter, allows for clear vision in distance

Question 15

What is the process when the lens is accommodating?

Answer 15

• The ciliary muscle contracts
• Circular fibres contract
• Moving ciliary body forward and inward
• Releases tension on zonules
• Lens becomes more spherical, increasing in dioptric power
• Allows for clear near vision

Question 16

What is the relationship with accommodation and age?

Answer 16

• Ability to accommodate decreases with age, due to thickening of the lens with increase in number of lens fibres

Question 17

What are the the changes in lens during accommodation (Von Helmholtz Theory)?

Answer 17

1. Lens thickness increases from anterior to posterior
2. Lens thins along the equator
3. Anterior lens surface moves forward, anterior chamber becomes shallower
4. No change in the position of the posterior pole

Question 18

What are some environment injury to the lens?

Answer 18

1. Ultraviolet radiation (UVR)
   - Lens absorbs wavelength between 300 – 400 nm
   - UVR absorbed by lens causes oxidative damage and is associated with risk of lens opacification
2. Infrared (IR)
   - Cataracts formed by years of exposure to IR
3. Smoking
   - Increased risk of nuclear and posterior sub-capsular cataracts, due to oxidative stress
4. Steroids
   - Can induce posterior sub-capsular cataracts

URV, IR and smoking are modifiable

Question 19

Explain the loss of lens transparency in nuclear sclerosis, cortical & sub-capsular

Answer 19

• Nuclear sclerosis: hardening, cloudy, yellowing/browning (brunescence)
• Cortical: wedge shaped opacities
• Sub-capsular: posterior