Lecture 9: Crystalline Lens

What are some characteristics of the lens?

• Transparent

• Elliptic

• Malleable

• High protein content

Where is the crystalline lens located?

It is located within the posterior chamber.

Anterior → vitreous

Posterior → iris and anterior chamber

What are some functions of the lens?

1. Refract and transmit light.

2. UV protection

3. Accommodation

What shape is the lens?

Biconvex

What are the dimensions of the lens? (e.g. anterior and posterior radius, diameter, thickness)

Anterior radius: ~10mm

Posterior radius: ~5-6mm

Diameter: ~9mm

Thickness (relaxed): ~4mm

The thickness of the lens _________ ~0.2 mm annually.

increases

What is the refractive power of the relaxed lens?

20 D

Power ______ with accommodation.

increases

What is the gradient refractive index?

It is the index of refraction that increases from periphery to the center of the lens.

What are the divisions of the lens?

• Lens capsule

• Lens epithelium

• Lens cortex

• Lens nuclei

What is the lens capsule?

• Transparent envelope (surrounds the lens)

• Basement membrane

Where do zonular fibers insert?

A lens capsule

What is the function of the lens capsule?

• Encloses all lens components

• Semipermeable barrier

What produces the lens capsule?

Anterior lens epithelial cells

Which part of the lens capsule increases in thickness over time and which part does not?

Anterior lens capsule: increases in thickness over time

Posterior lens capsule: thickness doesn’t increase over time

What part of the lens capsule is the thinnest region?

Posterior lens capsule

What is the lens capsule composition?

Mainly type IV collagen

What gives lens capsule the elastic quality desite lacking elastic fibers?

Arrangement of collagen provides flexibility (e.g. cross linking and scaffolding).

What is the lens epithelium?

It is a single layer of epithelial cells adjacent to anterior lens capsule.

Does the lens have a posterior lens epithelium?

No

Describe the orientation of the basal and apical aspect of the lens epithelium.

Basal aspect: attaches to capsule

Apical aspect: oriented toward center of lens

What type of junction is found in the lens epithelium?

Desmosomes and gap junctions

Name the lens epithelium zones.

• Central

• Intermediate (Pre-germinative)

• Germinative

• Equatorial (Transitional)

What is the central zone?

The central and anterior portion of the lens epithelium where low mitotic activity occurs.

What is the function of the central zone?

Maintenance and metabolic support

What is the intermediate zone?

The zone that directly surrounds the central zone where there is increased mitotic activity (compared to central).

Where is the germinative zone?

Zone anterior to lens equator.

What zone is known as the primary site of epithelial cell division? And why?

Germinative zone because it has high mitotic activity and produce new epithelial cells.

Where is the equatorial zone located?

Lens equator

What occurs at the equatorial zone?

Newly formed epithelial cells migrate here.

• To elongate and differentiate into lens fibers

• For lens growth and development

What is the lens cortex composed of?

Secondary lens fibers which are formed after puberty / in adulthood.

Where is the lens cortex located?

Peripheral portion of the lens (internal to lens epithelium + lens capsule)

What forms the lens fibers?

Differentiation of lens epithelial cells

What are some characteristics of the lens fibers?

• Hexagonal (cross section)

• Contain network of gap junctions (e.g. circulation + nutrition)

• Extend from anterior pole to posterior pole

What is the composition of the lens fiber?

• Protein (30-35%)

• Water (65-70%)

• Antioxidants

• Cholesterol

• Other solutes

The lens fiber contains a ______ concentration of crystallin proteins.

High

What is the role of crystallin proteins in the lens fiber?

Contributes to structure and transparency / gradient refractive index.

Why are the lens fibers described like an onion?

• Deposited into layers

• Newer fibers are external to older fibers (contain organelles)

• Older fibers are deeper (organelles degrade)

Where is the embryonic nucleus found and when are they formed?

• Innermost

• Fibers formed during gestation / embryonic development (primary lens fibers)

Where is the fetal nucleus found and when are they formed?

• Surrounds embryonic nucleus

• Fibers formed before birth

Where is the adult nucleus found and when are they formed?

Fibers formed b/w birth and puberty.

What are lens sutures?

Junction where lens fibers meet as they elongate towards poles during differentiation and growth.

Where are lens sutures found?

Fetal nucleus

How are the anterior suture and posterior suture formed? What shapes are they?

Anterior: Joining apical aspects of fibers (Upright Y shape)

Posterior: Joining basal aspects of fibers (Inverted Y shape)

What happens to the lens and lens sutures when more lens fibers form?

The lens will grow and sutures will grow too.

Lens sutures → more asymmetric and dissimilar.

What are the zonules (of Zinn)?

• Suspensory ligaments of the lens

• Microfibrils → extracellular matrix w/ fibrillin and elastin

What is the function of zonules?

Connect ciliary body to crystalline lens

What are two places that zonules extrend form?

• Basement membrane of nonpigmented ciliary epithelium (pars plana)

• Valleys of Kuhnt b/w ciliary processes (pars plicata)

Describe the lens circulation and innervation.

• Avascular

• No innervation

Since the lens is avascular, how does it obtain nutrients?

• Gap junctions (allow nutrients and ions to pass)

• Aqueous humor (brings in nutrients)

What wavelenths does the lens absorb?

• 300-400 nm

• >400 nm are transmitted to retina