The Lens
Hana Abouzeid
Hôpital Ophtalmique Jules-Gonin
Basic Science Course
Introduction
• 2 principal refractive elements of the eye:
– Cornea 43 D
– Lens
23D
• Function:
refract light to be focused on the retina
through accomodation
with variable focals
Introduction
• Transparency is mandatory !
– Cornea: continuous pumping of interstitial fluid,
acellular collagen stroma
– Lens: intricate arrangement of highly specialized
cells to produce a gradient of refractive index
Introduction
• « Underlying lens function is lens structure. »
Kuszak JR
Plan
•
•
•
•
Embryology
Anatomy: gross, microscopic
Physiology
Cataract Genetics
Embryology
• Surface Ectodermal cells
• Lens Placode
• Cuboidal epithelial cells
fiber-like cells
Embryo
Optic groove
Optic vesicle
Neural
ectoderm
25 days: Surface Ectoderm
28 days: Lens Placode
www.med.unc.edu/embryo_images
Embryology
Kuszak in Albert and Jakobiec, 2000
29 days: Invagination of the lens
placode and optic vesicle
36 days: lens vesicle
www.med.unc.edu/embryo_images
6 weeks: lens vesicle and
hyaloid artery
7 weeks: Hyaloid vasculature,
Cornea
www.med.unc.edu/embryo_images
7 weeks: anterior chamber forms, cornea
www.med.unc.edu/embryo_images
8 weeks: final lens
Avascular, no innervation
www.med.unc.edu/embryo_images
Lens developement
Lens epithelium:
germ cell cuboidal
monolayer with stem cells
sequestrated at the
germinative zone
Kuszak in Albert and Jakobiec, 2000
Lens development
• Central zone: cells held in G0 stage of the cell
cycle, no contribution to fiber formation
• Pregerminative zone (between central and
germinative): small number undergo mitosis,
some contribution to fiber formation
Lens development
• Germinative zone (ring around the anterior
lens): highest mitotic activity
• Transitional zone (beyond germinative zone):
cells from the germinative zone that will
migrate posteriorly to form secondary fibers
Mitoses at germinative zone
Adler’s Physiology of the Eye 1987
Kuszak in Albert and Jakobiec, 2000
• Lens epithelium:
germ cell cuboidal monolayer with stem cells
sequestrated at the germinative zone
• Germinative zone (ring around the anterior lens):
highest mitotic activity
• Lens equator: terminal differentiation, secondary
fibers
Embryology
• Secondary fibers are formed throughout life,
they are overlain, in register, as age-related
concentric growth shells.
• Fibers of every shell lie atop fibers of the
previously formed shell and beneath the fibers
of the subsequently formed shell.
Concentric
layers
1400 newborns
2000 adults
Kuszak in Albert and Jakobiec, 2000
Microscopic Anatomy
• 3 types of fibers:
the elongating fibers,
the cortical fibers (after sexual maturation)
the nuclear fibers
• Terminal differentiation elimination of
most intracellular organelles: nucleus,
mitochondria, ER, Golgi bodies, and
lysosomes
The highly ordered arrangement of lens fibers
contributes to lens transparency by
transforming the individual fibers into a series
of coaxial refractive surfaces.
Trokel, 1962
www.med.unc.edu/embryo_images
Lens sutures
• Both ends of new hexagonal flat lens fibers
grow toward the poles where they join new
fibers from other areas at the sutures.
• The Y pattern results from the fact that the
anterior and posterior arms of the fiber have
different lengths.
Anterior Y
Posterior inverted Y
Kusyak JR et al, Int J Dev Biol, 2004
Cotlier and Weinreb, Survey of Ophthalmology, MayJune2004
Anterior
sutures
Hervouet F, Ertus M, Scanning electron microscopic studies of the eye structure, Masson 1973
Simple star and complex star
sutures with age
Kusyak JR et al, Int J Dev Biol, 2004
Lens Star Sutures
Kuszak in Albert and Jakobiec, 2000
Embryology
• The older lens cells are progressively more
internalized throughout life.
• The fibers are arranged in order of ascending
age from its periphery to its interior.
Kuszak in Albert and Jakobiec, 2000
Equatorial Cross-section
Kuszak et al, IOVS, 1996
Light Microscopy
Equatorial Section
Adult nucleus
Fetal Nucleus
Embryonic nucleus
Fibers: hexagonal radial cell columns (RCCs)
Kuszak in Albert and Jakobiec, 2000
Lens Growth
• Over time, cumulative fiber total increases.
Lens grows throughout life.
Birth
• Diameter equator 6.5 mm
ant-post 3 mm
Adulthood 80y
9-10 mm stop
5 mm
6 mm
Lens growth: Significant zonal variations
as a function of age
Kuszak et al, IOVS, 1996
Child
Adult
nucleus
nucleus
Destiny…
• lens epithelium eliminates some cells
throughout life
• 2 mechanisms: necrosis and apoptosis
• Dysfunction of normal apoptosis may lead to
cataract formation
Plan
•
•
•
•
Embryology
Anatomy: gross, microscopic
Physiology
Lens and Cataract Genetics
Gross Anatomy
Gross Anatomy
• Lens stability relies on
• 1. Zonules
• 2. Vitreous attachment : Wieger’s ligament
Zonules
• Zonular fibers run between the ciliary
processes.
• The longer zonules are at the ora serrata and
insert on the surface of the lens, just anterior to
the equator, the shorter insert just posterior to
the equator.
• Zonules also prevents the lens its inherent
tendency to become spherical.
Lens zonules
Canal of Petit: delimited by the 2 zonular fibers systems, longer and shorter
Kristic RV. Human Microscopic Anatomy, 1997
Lens zonules
Glycoprotein
fibrillin
Canal of Petit
Hervouet F, Ertus M, Scanning electron microscopic studies of the eye structure, Masson 1973
Superotemporal subluxation
Courtesy of F. Majo
Marfan. Autosomal Dominant
OD: VA 0.4 s-14
OS: VA 0.7 s-11
Alfred Vogt, 1979
Pseudoexfoliation
zonular weakness
Wieger’s ligament
Berger’s space
Kuszak in Albert and Jakobiec, 2000
Mittendorf’s dot
Site of the hyaloid artery
Mittendorf’s dot
Lens capsule
• Basal membrane of lens epithelial cells and
elongating fibers = lens capsule
• Lens capsule = thickest basement membrane in
the body, but elastic
• 13 microns anteriorly, 4 microns posteriorly
• Synthesis of anterior capsule takes place
throughout life, posterior remains stable
Anterior capsule
Lamellar fibers
striae parallel to
the surface
Hervouet F, Ertus M, Scanning electron microscopic studies of the eye structure, Masson 1973
Anterior capsule
• Primary components:
• type IV collagen
• the glycoprotein adhesion protein laminin
• the proteoglycan heparan sulfate
Lens transparency
Cortical lens
fibers
Nuclear lens
fibers
« ball-andsocket »
interdigitations
microvilli
Kristic RV. Human Microscopic Anatomy, 1997
Balls and sockets
Microvilli
Kuszak et al, IOVS, 1996
Vacuoles:
oedema of the lens fibers
Plan
• Embryology
• Anatomy: gross, microscopic
• Physiology: proteins, metabolism,
accomodation
• Lens and Cataract Genetics
Lens transparency
• Lens is the only transparent cellular tissue in
the body.
• Cornea, aqueous fluid and vitreous are
acellular.
• Lens: cells have almost no turn over,
embryonnic cells are the oldest cells in the
body !
Lens transparency
1. Loss of intracellular organelles through fiber
formation
2. Arrangement of fibers transforming the lens
into a series of coaxial refractive surfaces,
thus contributing to lens transparency by
reducing large particle scatter.
3. Presence of crystallins: medium of high
refractive index
Cataract
Cataract: Surgical definition
• Surgical nucleus :
– nucleus and the majority of cortex
– lacks the majority of the elongating region
• Secondary cataract (PCO):
from residual germinative zone and nascent
elongating fibers
Lens proteins
• Lens crystallins 95%: water soluble
– Role: maintain transparency
• Cytoskeletal proteins 4%: actin, vimentin,
tubulin, tropomyosin, spectrin
– Role: maintain fiber shape, organize the lens
crystallins
• Membrane proteins 1%: MIP (major intrinsic
protein)
– Role: aquaporin (water channel)
Crystallins
Highly water-soluble to avoid light scatter
« crystalline » lens gave the name
Refractive elements in the lens
Other functions in other organs, expressed
mostly outside the lens
• Must be extremely long lived cf fibers do not
synthesize novel proteins
•
•
•
•
Chromatographic separation
2 major families:
alpha and beta
in all
vertebrates
lenses, called
ubiquitous
3 types
Albert and Jakobiec, 2000
Alpha-crystallins
• The a-crystallins are highly conserved
• alphaA crystallin DNA sequence has changed
only 3% in 100 million years, one of the
lowest evolutionary rates described so far.
• Due to their role in maintaining lens
transparency
Alpha-Crystallins
• The largest, molecular mass 600 to 800kD
• 2 subunits: A, B
• Share sequence homology with «heat shock»
potein family
• Chaperone-like activity: ability to suppress
nonspecific aggregation of various proteins
denatured by heat or chemicals in response to
different stresses, to avoid light scattering
•
Lens transparency !
Beta- and gamma-crystallins
•
Beta: The most abundant ones.
–
–
–
•
Gamma
–
–
–
•
Complex group of oligomers
23-32kD
Dimers or units of up to 8 subunits
monomeric
20kD
Expressed early, most concentrated in the nucleus
No specific biological function identified for both
beta and gamma crystallins
Metabolism
Because there are no organelles, fibers relie on
1. Anaeorobic glycolysis : No need for oxygen !
(hexose monophosphates shunt, aldose reductase
pathway)
2. Extracellular diffusion (aqueous humor, vitreous)
3. Intercellular junctions (gap junctions)
Fibers maintain themselves over a lifetime
Pump
Maintenace of
steady-state volume
and resting voltage
Active transport, pump
predominates
Ionic movements accross
the lens: Na from post to
ant, K from ant to post
Sodium is low
Potassium is high
Adler’s Physiology of the Eye, 1987
Transport
Accomodation
• Mechanism by which the eye changes focus
from distant to near images
• Change in lens shape
• Resulting from action of the ciliary muscle on
zonular fibers
Accomodation
1853 Hermann von Helmholtz
Changes with accomodation
Ciliary muscle
Ciliary ring diameter
Zonular tension
Lens shape
Lens equ. Diameter
Lens thickness
Lens dioptric power
With Acc
Without
contraction
relaxation
decreases
decreases
increases
increases
more spherical
flatter
decreases
increases
increases
decreases
increases
decreases
Accommodation
• Amplitude of Accommodation:
amount of dioptric power change produced
Adolescents: 12-16 D
Adults 4-8D
Older 2D
• Diminishes with age: presbyopia
Plan
•
•
•
•
Embryology
Anatomy: gross, microscopic
Physiology
Lens and Cataract Genetics
Heritability of Cataract
as a function of age
Congenital
100%
H
Childhood
Adult
50%
Age
Heiba et al (1993) Genetic etiology of nuclear cataract: evidence for a major gene
Heiba et al. (1995) Evidence for a major gene for cortical cataracts
Hammond et al. (2000) Genetic and environmental factors in age-related nuclear cataracts in monozygotic and dizygotic twins
Hammond et al (2001) The heritability of age-related cortical cataract: the twin eye study
Congenital & Paediatric
Cataract
• 34 genetic loci, 20 specific genes mutations
• Autosomal dominant inheritance is the most
frequent
20 known genes
50% involve crystallins
25 % involve connexins (gap junctions)
25% aquaporin 0 (MIP), BFSP2
(cytoskeleton protein), HSF4 (heat
shock)
γ- crystallin gene mutations
Aculeiform
dense coraliform core with needle-like
extentions of different sizes