Ocular Anatomy 8 Lens

Question 1

What is the lens formed from

Answer 1

Lens placode - Disc shaped thickening of surface ECTODERM overlying optic vesicle on day 27

Question 2

HOW DOES THE LENS DEVELOP

Answer 2

Placode cells elongate to form lens vesicle

Cells from posterior part of vesicle lengthen to form the primary lens fibres that fill vesicle lumen

Secondary lens fibres from anterior epithelial cells migrate and taper to from Y shaped (ant) and inverted Y (post) sutures

Anteroposterior diameter greater initially than equatorial diameter

Question 3

Describe the lens and size in mm

Answer 3

Biconvex
10mm indiameter and 4mm thick

Apices fo each convex surface are the poles

Growth throughout life increases anterior curvature, increaseing hte power

Question 4

Describe the lens capsule
What is ti made of?

Answer 4

Elastic capsule which is thickened in periequatorial regions and thinned at anterior and posterior poles

Type IV collagen

Anterior capsule synthesis continues throughout life
Post capsule remains constant

Question 5

What epithelium is on the lens and where?

Answer 5

Lenticular epithelium lies below the capsule found only on anterior surface of lens
Cuboidal, becoming columnar as cells approach the equator

Centrally cuboidal, stable cells, slow rate of decline
Pre-equatorial epithelial cells form germinative zone - lens epithelial mitotic activity

Question 6

What are the nuclei of the lens?

Answer 6

Embryonic nucleus is innermost
Surrounded by fetal nucleus and adult nucleus

Question 7

How does size of the lens increase?

Answer 7

Cells elongate becoming lens fibre running from posterior to anteiror lens surface. U shaped lens fibres bend around then ucleus towards anteiror pole
Laterally opposing secondary fibres within the same layer connect to form the classical Y suture

Posteriorly elongating fibres curve around the nucleus and joint wihtin posteiror pole to form an inverted Y sutures

Question 8

what connects the lens equator to the ciliary processes?

Answer 8

Zonules are fibrous structures that join to the external peri-equatorial areas of the lens in radial pattern.
Derived from basal laminae of non-pigmented ciliary epithelium

Each zonule has multiple tight packed fibrillin filaments which integrate with lens capsule

Weakening of zonues may result in lens sublaxation

Question 9

What is the shape and refractive power of the lens

Answer 9

Biconvex
+15D

Question 10

Which surface is more convex?

Answer 10

Posterior surface

Question 11

Describe the function of zonues

Answer 11

When viewing objects at distance, zonules under max tension to flatten the lens

When viewing near objects, ciliary muscle contraction - forward and inward movement of ciliary zonule attachment - zonule relax tension, lens resumes natural more convex shape