The lens

Question 1

why does the cornea have very little refractive power under water?

Answer 1

because the water has a high refractive index and so fish lenses have a very high protein content and are too rigid for accommodation

Question 2

what are the three parts that make up the lens?

Answer 2

-capsule: an elastic basal membrane which envelopes the entire lens
-lens epithelium: lies under the capsule and is only found on the anterior surface of the lens
-lens fibres formed by the multiplication and differentiation of the lens epithelial cells at the equator

Question 3

what are the functions of lens epithelium?

Answer 3

-homeostasis and osmolarity
-synthesis of crystallin
-production of lens fibres

Question 4

give the pathway of lens fibre production

Answer 4

1. Cuboidal Epithelial cells elongate, forming columnar cells
2. Apical surface of the cell grows and pushes anteriorly
3. Basal surface grows and pushes posteriorly
4. Both processes continue to grow, meeting opposite fibres at irregular sutures

Question 5

what is the lens bow?

Answer 5

the alignment of the cell nuclei of the newly produced lens fibres in the lens cortex

Question 6

what are sutures

Answer 6

the irregular lines the lens fibres meet at (they dont meet at the poles)

Question 7

what forms the lens nucleus?

Answer 7

older lens fibres whose nuclei have fragmented - its much more compact than the cortex due to higher protein density

Question 8

what forms the lens cortex

Answer 8

newer lens fibres which have nuclei

Question 9

what does the change in protein density between the nucleus and the cortex of the lens create?

Answer 9

the Refractive Index Gradient

Question 10

why does presbyopia occur?

Answer 10

As the lens is constantly growing as cells constantly get added to the central portion of the lens as lens fibres
-central part of the lens becomes less pliable and compact as lens capsule increases in thickness with age and so reduces accomodation

Question 11

what percentage of the lens is protein?

Answer 11

90% Highest percentage of any tissue in the body

Question 12

what are the three types of crystallin in the lens?

Answer 12

alpha, beta and gamma crystallin

they are all globular and have different isoforms

Question 13

what are the two types of proteins in the lens?

Answer 13

-insoluble so membrane and cytoskeletal proteins ‘
-soluble: crystallin

Question 14

how is crystallin transparent chemically?

Answer 14

-insoluble: has a long range order of atoms or molecules so forms a crystallin lattice
-soluble: short range order for liquid

Question 15

how do lens proteins change with age?

Answer 15

the percentage of soluble protein decreases and insoluble protein increases

Question 16

give the denaturing processes of crystallins

Answer 16

-glycation: the addition of sugar to protein making it become less flexible
-oxidation: production of free radicals
-

Question 17

how does alpha crystallin act as a molecular chaperone?

Answer 17

prevents the aggregation of other lens crystallins and proteins that have become unfolded by “trapping” the protein in a high molecular weight complex.

Question 18

how can UV radiation & oxidation affect the lens?

Answer 18

by impairing the chaperoning functions of alpha crystallin causing a loss of lens transparency

Question 19

what is a cataract?

Answer 19

light scattering due to insoluble proteins acting as opacities in the lens hence reducing visual acuity

Question 20

which part of the lens has the greatest refractive index?

Answer 20

The lens nucleus