11 Crystalline Lens

What is the shape of the crystalline lens?

transparent biconvex structure

What does the crystalline lens look like when it is cut in half?

an onion cut in half - layers

Where is the lens located?

Behind the iris & in front of the vitreous humor

How much accommodative power does the lens contribute when it’s UNaccommodated?

18 to 20 D

How much accommodative power does the lens contribute when it’s accommodated?

58 to 60 D

What is the diameter of the lens? How does this change with age?

• grows throughout life increases 0.023 mm per year

• 6.5 mm diameter at birth

• 10 mm diameter as adult

What’re the 4 functions of the lens?

• transmit visible light

• absorb UV light (3-hydroxykynurenin)

• refract

• focus

What is the transmittivity (what light get thru) of a young vs. old lens?

• young lenses act as a UV blocker (transmittance under 400 nm is zero aka the lens absorbs all the light & none of it reaches the retina)

• older lens act as a UV & blue blocker (we can only see crystalline lens in people who are 80 years old because before it ages it is so opaque & there will be a right shift in the transmittance graph

What is the center point of the anterior & posterior sides of the lens?

anterior pole & posterior pole

What is the “axis” of the lens? What is the axial thickness?

The “axis” is the line that connects the anterior & posterior pole. The axial thickness is around 4mm.

How does the anterior & posterior surface of then lens very in curvature?

• anterior surface radius of curvature is 10 mm (it’s less convex than the posterior surface aka flatter)

• posterior surface radius of curvature is 6 mm

Where is the equator located in the lens?

At the outer edges

What is the concentration of water & crystalline proteins in the lens composition?

60% water & 40% protein

The lens composition has a high concentration of what compound?

Glutathione (GSH, an organic compound)

What does glutathione maintain in the lens?

maintains protein sulfhydryl group in reduced form

What does glutathione protect against in the lens?

protects from oxidative damage by detoxification of peroxide

What does glutathione remove from the lens?

removes xenobiotics by conjugating with hydrophobic compounds having an electrophilic center

Which protein in the lens are soluble?

alpha, beta & gamma crytallins

What do lens crystallins help maintain in the lens?

maintains spacing of the lens fiber to promote destructive interface, minimize light scattering & maximize transparency

What do alpha crystallin do in the lens?

alpha crystallins act as a molecular chaperone to help beta & gamma crystallins recover from injury

What happen to the crystallin proteins in the lens as we age?

• become insoluble

• form aggregates

• contribute to loss of transparency & cataract formation

What causes a cataract development in the lens?

lens proteins becoming insoluble with age

What’re the layers of the lens?

1. lens capsule (outermost elastic layer)

2. epithelium (only on anterior surface)

3. lens fibers: cortex & nucleus (innermost)

What is the lens capsule?

an acellular basement membrane that completely envelopes the lens

Where is the lens capsule the thickest & thinnest?

• thinnest at the poles & equator

• thickest in the mid region

What is the lens capsule formed by?

• anteriorly by lens epithelium

• posteriorly by lens fibers

What does the lens capsule consist of?

consist of 40 lamella made up of reticular fibers (net-like or entangled) embedded in sulfate glycoaminoglycan

What does the lens capsule composed of?

collagen IV

What does the lens capsule do?

• forms a barrier to bacteria & inflammatory cells

• allow diffusion of smaller molecules

What is a capsulorhexis?

its an anterior capsulotomy & its a surgical technique to create a window in the anterior capsule of the lens during cataract surgery

What is the histology of the lens epithelium?

• single layer of simple cuboidal epithelium on anterior surface UNDER the capsule & is NOT found posteriorly

• becomes columnar towards the equator & convert into lens fiber

How does the capsule effect the lens epithelium?

The lens epithelium can’t shed because the lens capsule does not allow it to do so

What cell junction is found in the lens epithelium?

• sodium potassium pumps (low in Na+ & high in K+)

• strands of zonule fuse with the outer surface of the capsule; anchoring junction on the inner surface bind it to the epithelium

What’re the lens epithelium zones?

1. central zone (ion transfer)

2. germative or proliferative zone (replication)

3. transition zone & equator (growth)

What’re the characteristics of the central zone of the lens epithelium?

cells are flattened & hexagonal

What’re the characteristics of the germative/proliferative zone of the lens epithelium?

• cells are columnar & smaller in area but higher in density

• new cells are generated here, nuclei divide & migrate posteriorly to become new lens fibers in the cortex

• layers are laid down in close contact& the lens fibers are interlocked through “ball & socket” like joints

What’re the characteristics of the transitional zone & equator of the lens epithelium?

cells elongate & rotate so long axis is parallel to cortical surface

Which lens fibers/cells are nucleated vs. anucleate?

• new superficial fibers are nucleated

• deeper fibers are anucleate (true fibers)

How is the nuclear bow formed?

in order for a epithelium cells to become a lens fiber it must change chape (elongated) & lose it’s nucleus. there is a region around the nucleus where those early lens fibers still have there nucleus & thats called the lens bow!

forms as nuclei move anteriorly & fiber pass deeper into the lens to form the cortex

What happens to the lens cells preceding new generations?

they’re pushed deeper into center of lens

Which direction do lens fibers grow?

from the equator towards the pole

How are lens fibers interlocked with each other?

ball & socket joint (forms loops like a puzzel)

How are lens sutures formed?

lens fiber grow from the equator towards the poles & when the fiber meet, they form a suture

What is the difference between an anterior & posterior lens suture?

• anterior lens suture form an upright Y

• posterior lens suture form an inverted Y

How does the lens accommodate? What theory does this involve?

Helmholtz theory

1. ciliary muscle contracts moving ciliary body inward & forward

2. zonular tension is released which releases the tension from the lens (it is no longer being pulled flat & will become more curved - especially the anterior surface)

3. the capsule molds to the lens

What’re the 2 categories of lens metabolism?

• 50% oxidative metabolism (lens epithelium)

• 50% anaerobic via glycolytic pathway (lens fiber)

How much energy does the lens use? When?

uses only SMALL amount of energy

• growth

• synthesis

• pumping nutrient in & waste out

• maintaining water balance

What is the ion movement thru the lens?

Active transport:

1. Na⁺,K⁺ ATPase

2. Ca²⁺ ATPase

3. AA (entering ant. lens)

Passive transport:

1. simple diffusion (water & chlorine)

2. facilitated diffusion (glucose)

What’re the varying concentrations in the lens?

• High potassium & calcium

• Low sodium & chlorine

What is the varying concentrations outside the lens?

• Low potassium & calcium

• High sodium & chlorine

What is the largest component of the eye? %?

vitreous humor; 80% of internal volume

What’re color is the vitreous humor?

colorless (transparent gel)!

What is the vascularization of the vitreous humor?

Avascular

What does the vitreous humor consist of? (4 bullets)

• 99% water

• salt (some)

• soluble proteins

• hyaluronic acid contained within meshwork of insoluble protein collagen

What structure helps the eye to hold its shape?

vitreous humor

How does the VH components aid the eye?

help eye to transmit light, there is minimal light scattering due to low concentration of particles & wide interfibrillar spacing

How does the VH properties aid the eye?

acts as a shock absorber due to viscoelastic properties, protecting the retina in rapid movements & physical activity

How does the VH specifically provide to the retina & lens?

storage area for metabolites for retina & lens, providing avenue for movement of these substances in the eye

What is the organization of the VH?

1. hyaloid membrane - outermost layer of vitreous

2. cortex - around the periphery, closer to the retina

3. center - towards the middle of the globe

4. face - anterior surface (i.e. anterior hyaloid membrane)

How long in the vitreous chamber?

15 to 16 mm

How is the vitreous chamber measured? Why is this important?

• can be measured with a-scan (like immersion) ultrasound, b-scan or optical biometer

• important when following myopia progression

What is lenstar biometer useful for?

• low-coherence optical biometer

• often used for myopia research (viewing vitreous chamber)

• uses partial coherence interferometry

How much vitreous is in an adult eye?

4 to 5 mL

What is the refractive index of the VH?

1.3349

What precent of light gets transmitted by the VH? What is the wavelength of this light?

transmits 90% of light between 300 & 1400 nm

How does the VH consistency change with age? Pathology?

• 100% gel at birth, 40% gel at age 65

• posterior vitreous detachment is common with age

What component of the VH are you looking at when you look directly at your patient?

vitreous face (anterior surface of vitreous aka anterior hyaloid)

What is patellar fossa?

the indentation of the vitreous face where the lens sits

draw what makes up the vitreoretinal interface?

What is the composition of the vitreous?

• water (99%)

• collagen (mainly type II)

• hyaluronic acid (a type of GAG)

• glycoproteins

• A few types of cells:

  • hyalocytes

  • fibrocytes

  • glial cells

What is the function of hyalocytes in the vitreous cells?

• produces hyaluronic acid

• synthesis of extracellular matric (ECM)

• modulator of immune reaction

• modulator of inflammation

What is the function of fibrocytes in the vitreous cells?

• primarily located near optic disc & ciliary processes

• involved in collagen synthesis, especially in pathology

What is the function of glial cells in the vitreous cells?

• unknown purpose, possibly misplaced cell

• no neural element in the vitreous

Is the vitreous metabolically active?

yes - for an avascular tissue w few cells

In terms of vitreous metabolism, where is the different oxygen tensions?

• lowest oxygen tension in the center

• greatest in the posterior cortex (which is where most cells are & closest to the blood supply)

How are vitreous metabolism & retinal metabolic waste related?

Vitreous metabolism serves as a temporary repository for retinal metabolic waste

Can vitreous regenerate?

no, but it can fill the space with fluid

What drops relieve dry eye & discomfort?

0.2 to 0.4% HA eye drops and as slow release contact lenses

What is a vitrectomy?

the removal of VH (if there’s an oxygenation problem to the eyes, VH buffers the amount of oxygen reaching the skin)

Vitreous aging (birth, 2, 25, 65)?

• birth: 0% liquid & 100% gel

• age 2: 5% liquid & 95% gel

• age 25: 20% liquid & 80% gel

• age 65: 60% liquid & 40% gel

What is the pathology associated with age & VH?

liquefaction causes traction & 65% of individuals have posterior vitreous detachment

What the vitreous attachments?

1. vitreous base (spanning the ora serrata)

2. hyaloid capsular aka retrolental ligament of Weigert (posterior lens in the midperiphery)

3. area of martigiani (around optic nerve)

4. at the fovea

What is the strongest attachment of vitreous to the eye wall?

Vitreous base - very adherent

Where does the vitreous base span? What is the consequence of this?

• spans from 2mm anterior to ora serrata & 2-4mm behind ora serrata

• peripheral retina is susceptible to detachment from continued traction from the vitreous

Where is the area of martegiani?

end of hyaloid canal in front of optic disc (very adherent)

What is a symptom of posterior vitreous detachment?

• occurs when vit pulls away & detaches from retina

• collagenous condensation leave Weiss’s ring & often floaters