Lens

Location of the Lens?

• In the posterior chamber

• Posterior to the iris

• Anterior to the vitreous

• Medial to the ciliary body

• Suspended by zonules (suspensory ligaments)

what is the shape of the lens?

biconvex elipse

what is in anterior and posterior of the lens

anterior - faces iris

posterior - faces the vitreous

what is the anterior radius of curvature of the lens?

8-14mm

• flatter

what is the posterior radius of curvatures of the lens?

• 5-8mm

• steeper

what does the equator do?

separate anterior and posterior segments

what occurs to size and weight of lens over time

• with age is becomes bigger and heavier

how does size change over a lifetime?

• anterior to posterior

  • 3.5mm in newborns

  • to 4.75-5.0 mm for >50yrs

• Equatorial diameter

  • 6.5mm for newborns

  • 9.0mm for >15 yrs

what are lens size trends over a lifetime generally?

• posterior face is steeper than anterior

• lens grows more in equatorial diameter than anterior to posterior

how does weight of lens change over a lifetime?

• Newborns

  • 65mg

• 1 yr

  • 130mg

• ~90 yrs old

  • gradually increases to 250mg

what is the trend in weight changes of a lens?

• lens doubles in first year of life and then gradually increases from there

what is the color of the lens over a lifetime?

• young

  • transparent to transmit light

    • clear bc part of visual axis

• Old

  • yellow/brown; opacified = cataract, impedes light

what is the blood and nerve supply to the lens?

Avascular = none

what is the function of the lens?

• refractive surface

  • metabolical maintains transparency and clarity

• filter UV

• facilitates accommodation through attachments of zones of the ciliary body

how much does the lens refract?

+15 to +20 diopters

list structures of lens from outside to inside

1. capsule

   1. anterior

   2. posterior

2. cortex

3. nucleus

what is lens capsule and what is it made from?

• thickened basement membrane

• made by anterior lens epithelia cells in anterior

• made by most superficial posterior lens fibers in posterior

what type of fibers make up the lens capsule?

• ~40 lamella of type IV (4) collagen filaments embedded in a mucopolysaccharide matrix

what kind of passage does the lens capsule allow?

• selective passage of nutrients into the lens and waste out of the lens

  • pulls nutrients from the aeros running through the posterior chamber

what inserts onto the lens capsule/annular region?

lens zones

where is the thickest and thinnest region of the lens?

• Annular region around anterior pole

  • Found on the anterior and posterior aspects of the lens

  • Serve as the insertion site for the lens zonules

  • 13.5 um -> 16 um with age

• Anterior pole

  • 11um → 15 um with age

• Equator

  • 7um

• Posterior pole

  • 3.5 um → does NOT include w age

what is the lens epithelium?

• only cells in lens

• have metabolic activity and cellular respiration

what is the shape and position of lens epithelium cells?

• monolayer of cuboidal epithelial cells ONLY on anterior of lens

  • apex of cells face interior lens

  • base of cells face interior lens

what does the base of anterior cells secrete?

• the basement membrane or the capsule → faces outwards

what are the 3 zones of the lens?

1. central

2. intermediate/germinative/proliferative

3. transition/equatorial

where is the central zone of lens and whats happening there?

• located at anterior pole

• cuboidal shape in saggital section, polygonal in coronal

• NO mitosis (G0) = true collection of aging cells

• cells laterally joined by desmosomes and gap jxns to allow nutrient transport

  • waste and nutrients in

what connects cells in the central zone?

desmosomes and gap jxns

what is the mitotic activity in the central zone?

no mitosis = true collection of aging cells

what is another name for the intermediate layer?

• germinative

• proliferative

where is the intermediate layer located?

• anterior to the equator

describe cell shape in the proliferative layer?

• columnar

describe mitotic activity in germinative layer?

• location of ALL cell mitosis

  • daughter cells displaced towards transitional/equaotiral region

what are cells in the proliferative layer joined by?

desmosomes and gap jxns

what are the names for the 3rd zone of the lens?

• transition

• equatorial

where is the equatorial zone located?

near the equator

what is the cell shape of the transitional zone cells?

• cells are pyramidal in shape

  • basal side is elongated compared to apical side

what is happening in the equatorial layer to the cells?

• cells differentiate from epithelia cells to lens fibers

  • cells elongate and internalize → basal portion of cell migrates posteriorly and apical portion of cell migrates anteriorly

  • inc ribosomes→ crystalline

how do lens cells become fibers?

• increase of ribosomes

• ribosomes produce a protein called crystalline

what is formed when cells are moving anterior to become lens fiber?

• nuclear bow

  • the nuclei of fiber cells move anterior and from the nuclear bow

where are new lens fibers found?

in the cortex

• the more anterior the newer the fiber

  • nuclei are still found in newer lens fibers

what is the arrangement of lens fibers and how it contributes to light?

• internalized (mature) lens fibers lose their nuclei

  • minimize light scatter to help lens transmit light to retina and maintain transparency

where are the older lens fibers found?

• found at the lens center

  • these cells have lost all their nuclei

what is the shape of lens fibers and how it helps?

• they are U shaped or C shaped; hexagonal in cross section

• this allows tight compacting → minimize extracellular matrics and extracellular space

  • minimize light scatter → transmit light and maintain transparency

what are characteristics of lens fiber cells?

• lack organelles

• cytoplasm has highest concentration of crystalline proteins

  • higher concentration in nuclear region(compared to cortex)

what is the lens fiber cytoskeleton made of?

microtubules and filaments for structure and stability

how do fibers have the ability to slide?

• fibers have interdigitations

  • allow for accommodation

how are lens fibers connected?

desmosomes and gap jxns

what happens as we age to the lens fibers?

• as new fibers are formed→ older ones displaced deeper into lens

  • oldest fibers in nucleus of lens

  • newest fibers directly under capsule

when does the lens have most of its layers?

• at 5 years old we have 75% of its fibers

  • 31% at birth

what happens to lens growth with age?

lens growth slows

what is happening in the outer 1/3 of the cortex?

• composed of young lens fibers

  • stabilizes lens fibers during accommodation.

  • maintains regular arrangement to support transparency

  • facilitates metabolic communication between fibers (low resistant gap jxns)

• fibers tightly packed with little extracellular space

  • minimize light scatter→ max refractive properties

• extensive system of low resistance gap jxns between fibers

  • facilitates metabolic communication

• cortical nuclear interface

  • no anatomical landmark between nucleus and cortex

where is the nucleus?

inner 2/3s of lens

• made of lens fibers

what are the parts of the nucleus?

1. Embryonic Nucleus

2. Fetal Nucleus

3. Adult Nucleus

4. Cortex

when is embryonic nucleus made? and other characteristics of it

• made in the frist 1 to 3 months of gestation

• primary lens fibers

• oldest part of lens

when is fetal nucleus made? and other characteristics of it

• made from 3 months gestation to birth

• Has 2 Sutures - Y and lambda

where is the Y and Lambda sutures found?

fetal nucleus

What does the upright Y represent and where is it found?

• anterior suture

• represents the interdigitations of the apical processes of lens fibers

what does the upside down Y represent and where is it found?

• posterior suture

• represents the interdigitations of basal processes of lens fibers

what happens to the lens sutures with age?

the sutures becomes more asymmetric and complex → less Y shaped

when is the Adult nucleus made?

• birth to puberty

when is the cortex made?

• post puberty

what is another name for zonules and what do they do?

• Zones of Zinn

• suspensatory ligaments

where do zonules come from?

• they come from the non pigmented ciliary epithelium in the pars plant and the valleys between the colliery processes in pars plicate

where do zonules insert?

• 360 degrees pre and post equatorial regions of the lens capsule with few attachments at the equator (2.5mm zone)

  • most attach at anterior and posterior annular regions of the lens capsule

  • some attach at equator

what are primary lens zonules?

• attach directly to the lens

what are secondary lens zonules?

• act as structural support

  • join primary lens zonules

  • connect the ciliary body origin to the middle of another lens zone (does not connect to the lens itself)

what is the function of lens zonules?

• suspend the lens to help accommodate

what is the compositor of the zonules from the smallest unit to he biggest unit?

• microfibrils

  • has extracellular matrix of fibrillar, elastin, and glycoproteins

• fiber

  • made of 8-12nm microfibrils

• Dense bundles (5-30um)

  • made up of a series of fine fibers

what happens to zonules when accommodating at near?

• ciliary muscles → contracts

• zonules → relax

• lens → thicker, rounder, and convex

• refractive power - increases

what happens to zonules when looking farther

• ciliary muscles → relax

• zonules → tighten

• lens → becomes thicker and flatter

• refractive power → decreases