Lens (MT2)

Crystalline Lens

Focuses light to a point source on the retina

Zonules

Keep lens in place and help with accomodation

Lens thickness

Never stops growing, shape is elliptical and biconvex

Anterior is flatter than posterior (has larger radius of curvature)

Brunescence

Yellows with age

Transparency depends on

Fiber arrangment, few nuclei, metabolic balance, protein composition

Crystalline proteins

Pack very orderly, higher refractive index (highest protein content in body)

Refractive index is higher than surrounding medium

Capsule

Surrounds entire lens

Basement membrane, laminin and type IV collagen, secreted by lens epithelial cells, zonule fibers insert

Where does the pliability of the lens come from?

Laminin and type IV collagen

Capsule thickness

Changes with age

Thinnest - Posterior pole

Thicker- Anterior pole and equator

Thickest - Annular area around anterior pole

What area of the capsule has the highest number of zonule fibers inserting?

Annular area

Lens epithelium

Anterior- single layer of cuboidal cells

Posterior - None

Why do we not have a posterior epithelium?

In embryology, a complete sphere of epithelium creates the basement membrane → Posterior cells differentiate and pack themselves with protein and fill in inner space → Posterior lens epithelium is lost and are now primary lens fibers

Anterior epithelium

Hexagonal for surface area

Tight junction based junctions (probably leaky), gap junctions for communication

Most metabolically active part of the lens

Function of epithelial cells differ depending on location

Central - Low proliferation
Pregermative - Small amounts of mitosis
Germative - High amounts of mitosis
Transition/Equatorial zone - Terminal differentiation

Central lens epithelium

Low proliferation - Cells can die and adjacent cell fills space

Germinative zone

Where mitosis of lens epithelium occurs

Cells are pushed toward equatorial zone and stop proliferating

Equatorial zone

Daughter cells pushed from germinative zone and undergo differentiation process into lens fibers

Epithelial cell zones

Central → pre-germinative → germinative → equatorial

Lens polarity

Maintained during differentiation

Lens bow

Where terminal differentiation is occurring, lens fibers forming

Tucked at periphery so not a lot of light scatter

Lens growth

More lens epithelium cell added as lens develops and grows (like tree rings)

Lens fibers

Elongated progressively changing lens epithelium cell, inhibition of proliferation, lose organelles, alter protein expression

Ball and socket joints

Allow for movement of plasma membrane while keeping them close together, where gap junctions are usually located

Embryonic nucleus

At very center, highest protein packed

Primary lens fibers, formed from posterior epithelium

Fetal nucleus

Secondary lens fibers, formed from equatorial region

Y sutures (distinct configuration)

Adult nucleus

Secondary lens fibers

Stellate-shaped sutures

Cortex

Newest fibers

Least protein packed

Lens fiber shells

Embryonic nucleus → Fetal nucleus → Adult nucleus → Cortex

Mature lens fibers

Lose attachments with basement membrane

Apical and basal sides touch neighboring cells

Sutures

Mature lens fibers attaching apical and basal sides

Y or stellate sutures

Deep and internal in lens

Anterior sutures

Apical-apical interaction

Posterior suture

Basal-basal interaction

Y suture

Apical-apical (anterior)/basal-basal (posterior) interaction between two mature fibers

Embedded in center of lens within embryonic nucleus, for transparency

Upright on anterior, inverted on posterior

Opposite end curvature shape

Apical + basal point in opposite directions
S shape

Stellate suture pattern

Laid down in adult nucleus layer
Keeps opposite end curvature
Staggering sutures staggers light scatter

What happens to ball and socket joints as we age?

Get compressed and less pliable leading to a stiffer lens → Presbyopia

Lenticular/nuclear sclerosis

Increase in the central density of the lens
Part of presbyopia

Cataract

Opacity of the lens or its capsule which results in loss of transparency

Cataract etiologies

Age, drugs, genetics, trauma, UV exposure, etc

What does the lens capsule and Descemet’s membrane have in common

Both thicken with age
Have pliability yet have no true elastin