L24 - Cyrstalline Lens

Question 1

What are the three chambers of the globe?

Answer 1

Anterior chamber, posterior chamber, vitreous body.

Question 2

What is the hyaloid canal?

Answer 2

A channel running through the vitreous body.

Question 3

What are the main functions of the crystalline lens?

Answer 3

Allows passage of light to the retina (transparency), enables focusing of images from near to infinity (accommodation), helps in refraction (about 1/4 of total dioptric power).

Question 4

Where is the lens located?

Answer 4

Between the iris and vitreous, in the patellar fossa.

Question 5

What is Wieger’s ligament?

Answer 5

The ligamentum hyaloideocapsular, attaching the lens to the vitreous.

Question 6

What is Berger’s space?

Answer 6

A small potential space between the hyaloid face and lens capsule.

Question 7

What is the shape and size of the lens?

Answer 7

Transparent, biconvex.

Question 8

What is the refractive power of the lens?

Answer 8

16–17 diopters (D).

Question 9

How does the lens change with age?

Answer 9

Equatorial diameter increases (birth: ~6.5mm, adult: 9–10mm), axial width increases (birth: 3.5–4mm, adult: 4.75–5mm), weight increases (birth: 65mg, old age: 258mg), accommodative power decreases (birth: 14–16D, 25yrs: 7–8D, 50yrs: 1–2D), color changes from colorless to yellowish to amber.

Question 10

What is the composition of the lens?

Answer 10

64% water, 35% proteins, 1% lipids/carbohydrates/trace elements.

Question 11

What are the three main structural components of the lens?

Answer 11

Lens capsule, lens epithelium, lens fibers.

Question 12

What is the lens capsule?

Answer 12

A thin, transparent, hyaline collagenous membrane surrounding the lens, mainly type IV collagen, thickest basement membrane in the body.

Question 13

Where is the lens capsule thickest and thinnest?

Answer 13

Thickest anteriorly and at the equator; thinnest at the posterior pole.

Question 14

What is the ciliary zonule (zonule of Zinn)?

Answer 14

Suspensory ligament of the lens; series of fine, transparent, stiff glycoprotein fibers (diameter 0.35–1.0μm) connecting ciliary body to lens.

Question 15

What are the functions of the ciliary zonule?

Answer 15

Holds the lens in position, enables ciliary muscle to act on lens during accommodation.

Question 16

What are the types of zonular fibers?

Answer 16

Thick/wavy (near vitreous), thin/flat, very fine/circular.

Question 17

What are the main arrangements of zonular fibers?

Answer 17

Orbiculo-posterior capsular, orbiculo-anterior capsular, cilio-posterior capsular, cilio-equatorial capsular.

Question 18

What is the lens epithelium?

Answer 18

Single layer of cuboidal nucleated cells on the anterior surface; main site for metabolic, synthetic, and transport processes.

Question 19

Is there posterior lens epithelium?

Answer 19

No.

Question 20

What are the zones of the lens epithelium?

Answer 20

Central zone, intermediate (pre-germinative) zone, pre-equatorial (germinative) zone.

Question 21

What are the functions of the lens epithelium?

Answer 21

Centrally: transport between aqueous humor and lens interior; equatorially: mitotic division and differentiation into lens fibers.

Question 22

How do lens fibers differentiate?

Answer 22

Elongation of lens epithelial cells, osmotically driven cell volume increase, cytoskeletal rearrangement, sustained protein synthesis, accumulation of crystallin protein, regulated by cyclins and proteins (p57, p27).

Question 23

What is the role of apoptosis and autophagy in lens fiber differentiation?

Answer 23

Key features of apoptosis not met; autophagy mediators present but not required.

Question 24

How does the lens grow?

Answer 24

Throughout life, without removal of cells; new growth occurs at the lens bow; most fiber elongation occurs apically and basally.

Question 25

What are the dimensions of lens fibers?

Answer 25

Length: 5–15μm; width: 1–5μm; length/radius up to 20mm.

Question 26

What are lens sutures?

Answer 26

Meeting points of fibers at the poles; anterior suture (Y) = apical, posterior suture (Y) = basal; star-shaped in primates.

Question 27

What is the main type of cell junction in lens fibers?

Answer 27

Gap junctions (30–65% of lens fiber membrane).

Question 28

What is the composition of lens fibers?

Answer 28

Crystallin proteins (40%), high cholesterol content (increases with age/cataract), phospholipids.

Question 29

How does the lens get nutrients?

Answer 29

From aqueous humor; epithelium is main site of energy production; nutrients reach fibers by diffusion.

Question 30

What is the main metabolic pathway in the lens?

Answer 30

Anaerobic glycolysis (due to low oxygen tension).

Question 31

What is the ionic gradient in the lens?

Answer 31

Within lens: Na+ = 25 mmol, K+ = 140 mmol; aqueous humor: Na+ = 163 mmol, K+ = 4 mmol.

Question 32

How is the lens divided anatomically?

Answer 32

Embryonal nucleus, fetal nucleus, adult (infantile) nucleus, cortex, anterior epithelium, anterior/posterior capsule.

Question 33

What is the gradient refractive index (GRIN) of the lens?

Answer 33

Increases from anterior to center (equator: 1.375, pole: 1.385, nucleus: 1.406), decreases toward posterior; due to protein concentration.

Question 34

What percentage of light does the lens transmit?

Answer 34

90% (while absorbing UVA and UVB).

Question 35

What causes loss of lens transparency?

Answer 35

Formation of opaque fibers, fibrous metaplasia, epithelial opacification, pigment accumulation, deposits of extracellular materials.

Question 36

What is accommodation?

Answer 36

Mechanism by which the eye changes refractive power by altering lens shape to focus objects at various distances.

Question 37

Define far point and near point.

Answer 37

Far point: object position where image falls on retina with no accommodation; near point: nearest point seen clearly with maximum accommodation.

Question 38

What is amplitude of accommodation?

Answer 38

Dioptric power difference between rest and fully accommodated eye.

Question 39

What is the AC/A ratio?

Answer 39

Accommodative Convergence/Accommodation Ratio; normal range is 3:1 to 5:1.

Question 40

What are the types of accommodation?

Answer 40

Tonic, proximal, reflex, convergence-accommodation.

Question 41

What is the relaxation theory of Helmholtz?

Answer 41

Accommodation occurs by ciliary muscle contraction, releasing tension on zonular fibers, allowing lens to become more spherical.

Question 42

What changes occur in the lens during accommodation (Fincham 1937)?

Answer 42

Central radius of curvature changes from 12mm to 5.5mm, lens thickness increases axially by 0.5mm, 3rd Purkinje image moves forward and decreases in size, anterior pole moves forward, posterior pole moves backward, lens diameter reduces, nucleus changes shape.