L7 crystalline lens

Question 1

Compare the anterior and posterior curvature crystalline lens

Answer 1

Posterior curvature is steeper

Anterior curvature is flatter

Question 2

What is the lens refractive power

Answer 2

+10D

+10 to 20D

Question 3

What are the layers of the lens?

Answer 3

1. Capsule
   - envelops the lens
2. Epithelium
   - single layer of cuboidal epithelium
   - cuboidal epithelium
   - columnar cells elongate to form the lens fibres
3. Lens fibres
   - newer fibres seen in cortex (lens periphery)
   - older fibres are pushed to the centre of the lens, Hence seen in the nucleus

Question 4

Name the 3 lens proteins

Answer 4

1. Crystallins
2. Albuminoids
3. Glutathione

Question 5

Describe crystallins

Answer 5

90% of the lens proteins

Present in the lens cortex

Water soluble

Formed by the displaced lens organelles of the epithelial cells

Question 6

Describe albuminoids

Answer 6

10% of the lens proteins

Present in the lens nucleus

Water insoluble

Question 7

Describe Glutathione

Answer 7

Is a polypeptide made of 3 amino acids

Is synthesised in the lens

Prevents oxidation damage of the lens
- oxidation damage results in loss of transparency of the lens known as cataract

The Amount of glutathione reduces with age, hence cataract develops (usually aft age of 60)

Question 8

What are the sources of nutrition for the lens?

Answer 8

Both Aqueous humour and vitreous humour as the lens is avascular

Question 9

What are the 4 metabolic pathways

Answer 9

1. Anaerobic glycolytic pathway
2. Aerobic Kreb’s cycle.
3. Hexose Monophosphate
4. Sorbital pathway

Question 10

What are the Aging changes in the lens?

Answer 10

1. Increase in lens size
2. Increase in lens diameter
3. Increase in lens thickness
4. Hardening of older lens fibres in the nucleus
5. Yellowing of the crystalline proteins
6. Loss of lens transparency
7. Loss of accommodation

Question 11

Explain the process of accommodation

Answer 11

1. When The Eye changes fixation from far to near
2. Contraction of ciliary muscle occurs
3. This decreases the tension of the suspensory ligaments
4. This allows the lens to acquire a rounder shape, anterior curvature steepens
5. Hence, increasing the power of the lens and the near object focuses on the retina

Question 12

Explain presbyopia

Answer 12

1. As the lens ages and its cell population increases, the cells becomes more tightly packed within the lens capsule
2. The lens become less flexible which decreases the contraction and relaxation of ciliary muscles
3. Hence, it is more difficult to accommodate and restore its unaccomodatied state
4. This loss of accommodation is called presbyopia

Question 13

Describe cataract

Answer 13

Happens after 60-65 years old

Possible causes:

1. Protein aggregation
2. Oxidation damage
3. Disruption of membrane lipid which contributes to membrane permeability
4. Reduction in glutathione synthesis

Question 14

Name and describe the types of cataract

Answer 14

1. Nuclear cataract
   symptoms:
   - blurred vision
   - myopic shift
   - glare
   - desaturation of colours

2. Cortical cataract 
Symptoms: 
- blurred vision 
- hyperopic shift 
- glare in dim light/ night time 

3. Posterior subcapsular cataract
   Symptoms:
   - reduced near vision
   - glare in bright light