GLOBES ANTERIOR STRUCTURES: LIMBUS

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GLOBES ANTERIOR STRUCTURES LIMBUS
SCLERA
LIMBUS is where the cornea meets the sclera
CILIARY BODY
CORNEA
PUPIL
ZONULE
LENS
IRIS
ANTERIOR CHAMBER
POSTERIOR CHAMBER
LIMBUS corneoscleral junction
LIMBAL epithelium is the sole source of cells, by
mitosis migration, for turnover repair of
the corneal epitheium
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OPTICAL ASPECTS I
dark UVEA prevents light reflecting around inside
is vascular
CILIARY BODY includes the muscle to relax the
lens shape for more focusing
Transparent CORNEAS extra curvature focuses light
IRIS controls the amount of light reaching the
retina
Vitreous body (a jelly) lets light through, and
keeps the layers attached
PUPIL size reflects iris muscles activity
LENS
Chambers are filled with the aqueous humor which
provides tranparency, and its pressure holds the
eye in shape
White of the eye is dense scleral connective
tissue for strength, and to keep light out, aided
by dark inner uvea
posterior/neural RETINA is sensitive to light
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OPTICAL ASPECTS I (Tabular)
White of the eye is dense scleral connective
tissue for strength, and to keep light out, aided
by dark inner uvea
Transparent CORNEAS extra curvature focuses light
Chambers are filled with the aqueous humor which
provides tranparency, and its pressure holds the
eye in shape
Dark UVEA prevents light reflecting around inside
is vascular
PUPIL size reflects iris muscles activity
IRIS controls the amount of light reaching the
retina
CILIARY BODY includes the muscle to relax the
lens shape for more focusing
LENS adds focusing
Vitreous body (a jelly) lets light through, and
keeps the layers attached
Anterior RETINA is a double-layered epithelium
unresponsive to light
Posterior/neural RETINA is sensitive to light
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OPTICAL ASPECTS II Focusing
On neural RETINA is a tiny
REAL INVERTED IMAGE of object
CILIARY BODY includes the muscle to relax the
lens shape for more focusing
Transparent CORNEAS extra curvature focuses light
OBJECT
Visual axis
LENS
Vitreous jelly lets light through, and keeps the
layers attached
Elongated lens focuses light further onto the
retina
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OPTICAL ASPECTS III Near-vision Focusing
CILIARY MUSCLE contracts tension in zonule
decreases the lens elasticity changes it into a
rounder shape
NEURAL RETINA
Close OBJECT
Visual axis
LENS
Rounder lens focuses light more onto the retina
The older lens loses this elastic ability to
change shape, so that one cannot clearly see
close objects - presbyopia
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OPTICAL ASPECTS IV Near-vision Focusing 2
CILIARY MUSCLE contracts tension in zonule
decreases the lens elasticity changes it into a
rounder shape
At first hearing, these sound to be contradictory
Under parasympathetic control the
CILIARY MUSCLE contracts tension in zonule
decreases the len capsules elasticity changes
lens into a rounder shape
CILIARY MUSCLE
ZONULE
LENS
LENS
CILIARY MUSCLE - within the eye - is an
intra-ocular muscle
A-P view
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OPTICAL ASPECTS V Central vision
NEURAL RETINA
The eyes are moved by the extra-ocular muscles
to bring the object of attention onto
the visual axis
OBJECT
Visual axis
LENS
On the visual axis, the
MACULA/FOVEA is the region of retina for precise
central vision
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OPTICAL ASPECTS VI Peripheral vision
OBJECT
Visual axis
LENS
This part of the anterior non-neural retina is
the PARS PLANA
MACULA for central vision
Outside the macula, the remainder of the neural
retina is responsible for peripheral vision - for
things off the visual axis
Can all of the neural retina see things? No
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OPTICAL ASPECTS VII NerveHead Blind spot
The neural retina contains not only the
photoreceptors, but nerve and glial cells for the
initial processing of the signals. The result
is patterns of firing in retinal ganglion
neurons whose axons leave the eye, at one
place, to become the optic nerve
LENS
The place is the Nerve head/ optic disc/ optic
papilla
Also where vessels enter
The optic nerve leaves on the nasa/ medial side
to the macula
OPTIC NERVE
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OPTICAL ASPECTS VIII Blind spot
UNSEENOBJEC T
The neural retina contains not only the
photoreceptors, but nerve and glial cells for the
initial processing of the signals. The result is
patterns of firing in retinal ganglion neurons
whose axons leave the eye, at one place, to
become the optic nerve
OPTIC NERVE
Nerve head/ optic disc/papilla has no
photoreceptors- hence is blind
The optic nerves leave on the nasa/ medial side
to the macula, so that they can connect at the
midline OPTIC CHIASMA the starting device for
binocular integration - seeing one object with
two eyes and two sides to the brain
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OPTICAL ASPECTS IX Experiencing the Blind spot
Place a small bright coin on a dark table, e.g.,
a quarter
Place another bright coin, e.g. a dime, to the
right of the first coin
R
L
Cover your left eye with your left hand
Fixate your right eye on the first coin
Move the second coin to right left with your
right-hand forefinger, but not obscuring your view
At about 17 cm separation, the second coin
should disappear from view as its image falls on
the right eyes blind spot/ nerve head
Reverse the side of second to first coin for
testing the left eye
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OPTICAL ASPECTS X Retinal layers
The neural retina contains not only the
photoreceptors, but nerve and glial cells for the
initial processing of the signals.
These cells are arranged in layers with the
photorecptors next to the uvea, and
the retinal ganglion output neurons innermost,
close to the vitreous
Thus, for peripheral vision, the light has to
pass through all the retinal layers to reach the
photoreceptors
For precise central vision, the nerve and glial
cells lie as an annular hump around a depression
- fovea - where the cone photoreceptors can
respond to the unimpeded light
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Retinal layers I Cells simplified
RETINAL GANGLION NEURONS
Between neurons and stretched across the layers
BIPOLAR NEURONS
PHOTORECEPTORS
PIGMENT CELLS
BRUCHS MEMBRANE
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Retinal layers II Connection layers
RETINAL GANGLION NEURONS
BIPOLAR NEURONS
Outer Zone of Synapsing processes
PHOTORECEPTORS
Reference point for inner outer is interior
of the eye
PIGMENT CELLS
BRUCHS MEMBRANE
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Retinal layers III Orientation
Reference point for inner outer is interior
of the eye
GANGLION NEURONS
Inner Zone of Synapsing processes
The presence of a major basement membrane outside
the pigment cells, here, is NOT the starting
point for orientation. The reference point
instead is opposite - inside the eye , where
there is an inconspicuous basal lamina around the
vitreous
BIPOLAR NEURONS
Outer Zone of Synapsing processes
PHOTORECEPTORS
BRUCHS MEMBRANE is a substantial basement
membrane
PIGMENT CELLS
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Retinal layers IV Terminology
INNER LIMITING MEMBRANE
NERVE FIBER LAYER
RETINAL GANGLION NEURONS
GANGLION CELL LAYER
INNER PLEXIFORM LAYER
BIPOLAR NEURONS
INNER NUCLEAR LAYER
Outer Zone of Synapsing processes
OUTER PLEXIFORM LAYER
PHOTORECEPTORS
OUTER NUCLEAR LAYER
OUTER LIMITING MEMBRANE
PHOTORECEPTOR LAYER
PIGMENT CELLS
PIGMENT CELL LAYER
BRUCHS MEMBRANE
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Glial cell layer
RETINAL GANGLION NEURONS
Inner Zone of Synapsing processes
BIPOLAR NEURONS BODIES
Outer Zone of Synapsing processes
PHOTORECEPTOR BODIES
PHOTORECEPTOR CONES RODS
RETINAL PIGMENT CELLS
Where Muller cells tightly attach - a discarded
term
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Retinal layers V HE stained
INNER LIMITING MEMBRANE
NERVE FIBER LAYER
GANGLION CELL LAYER
INNER PLEXIFORM LAYER
INNER NUCLEAR LAYER
OUTER PLEXIFORM LAYER
OUTER NUCLEAR LAYER
PHOTORECEPTOR LAYER
PIGMENT CELL LAYER
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Retinal layers V HE stained
NERVE FIBER LAYER
GANGLION CELL LAYER
INNER PLEXIFORM LAYER
INNER NUCLEAR LAYER
OUTER PLEXIFORM LAYER
OUTER NUCLEAR LAYER
PHOTORECEPTOR LAYER
PIGMENT CELL LAYER
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Retinal layers VI More cell types?
GANGLION NEURONS
Why not have the photoreceptors directly
stimulate action potentials in the ganglion cells?
BIPOLAR NEURONS
The light causes changes in photoreceptor
membrane potentials, but it takes STEPS to
achieve actual ganglion-cell firing
The synaptic arrangement shown transmits
signals just inwards
Additional synapses and cell types provide for
integrative influence and interactions across the
retina
PHOTORECEPTORS
Two types of photoreceptor - rods cones - have
somewhat different connection patterns
very different light sensitivities
PIGMENT CELLS
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Retinal layers VII More cells 2
GANGLION NEURONS
AMACRINE CELL
Both provide for cross-wise connections, and need
more investigation
BIPOLAR NEURONS
HORIZONTAL CELL
ROD
For low light black-grey perception
For daylight color perception Provides the
visual acuity of the fovea
CONE
PIGMENT CELLS
BRUCHS MEMBRANE
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PHOTORECEPTOR STRUCTURE I
CONE
INNER FIBER
PEDICLE
ROD

MULLER CELLS
Attachment of Muller cell
INNER FIBER

INNER SEGMENT
MITOCHONDRIA
OUTER SEGMENT
INNER SEGMENT
CILIUM
connecting segments for transport
Photopigment - iodopsin(s) - absorbs in red,
green or blue regions of light spectrum
Stacked BILAMINAR DISCS with photopigment
OUTER SEGMENT
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PHOTORECEPTOR STRUCTURE II
CONES are larger than rods are far fewer, e
xcept in the fovea have a differently shaped o
uter segment have different photopigments - NO
T rhodopsin - and responsiveness to light
their synaptic end - pedicle - is much larger t
han the rods spherule do not shed discs for ph
agocytosis by pigment cells
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Signal transduction Electrical activity I
GANGLION NEURONS
Light passes through the retina to be absorbed by
the photopigment stacked in the rod/cone outer
segments
LIGHT
The light has to alter electrical activity in
photoreceptors, the light stimulus counteracts
an existing depolarized state from cyclic
nucleotide-gated ion channels - so reduced, that
a hyperpolarization occurs, causing
AMACRINE CELL
BIPOLAR NEURONS
HORIZONTAL CELL
the receptor to stop releasing transmitter from
vesicles in its spherule, so changing
ROD
membrane potentials in the bipolar cells, which
signal to
CONE
the ganglion cell that it should produce an
action potential for its optic-nerve fiber
Outer segments
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Signal transduction Electrical activity II
In the DARK
SODIUM CHANNEL - held open by bound
allows to leave, DEPOLARIZING the cell
In the LIGHT
SODIUM CHANNEL - closes because dissociates
With rising intracellular a
hyperpolarization occurs
Na
Why the dissociation?
cGMPphosphodiesterase hydrolyzes cGMP, so
lowering its intracellular level
But what activates the enzyme?
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Signal transduction Electrical activity III
In the LIGHT
RHODOPSIN
2
11-cis-RETINAL
Changed shape of retinal forces OPSIN molecule to
alter its conformation
OPSIN
to
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Light is absorbed by the photopigment stacked in
the rod outer segment
all-trans-RETINAL
OPSIN
a1 subunit activates
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Altered OPSIN binds TRANSDUCIN, releasing a1
subunit
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cGMPphosphodiesterase, which hydrolyzes cGMP, so
lowering its intracellular level
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resulting in a cGMP dissociation from the Sodium
channel
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Signal transduction IV Recovery adaptation
In the DARK
SODIUM CHANNEL - held open by bound
also allows to enter
In the LIGHT
SODIUM CHANNEL - closes because
dissociates
Falling
entry is blocked
unbinds inactivating Ca 2 from RECOVERIN which
can then stimulate
Intracelllular falls
Recovery? With cGMP restored, it can quickly
associate again with the sodium channnels
Guanylyl cyclase
to make more
cGMP
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RHODOPSIN
TRANSDUCIN - a G protein
OPSIN
retinal
The G-protein cascade allows amplification of the
signal initially detected by the retinal
Lamb TD, EN Pugh Jr. Phototransduction, dark
adaptation, and rhodopsin regeneration. Invest
Ophthalmol Vis Sci 2006 475138
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Simplified sample sequence
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Signal transduction Electrical activity VI
COMPLICATING aspects include
As in the CNS, inhibition is used extensively
LIGHT
There are many subtypes of ganglion, amacrine ,
horizontal even bipolar cells
The GABA interplexiform is an additional type
Amacrine cells use electrical (nexus) synapses in
addition to chemical, e.g., dopaminergic, ones
ON cells respond to a stimulus brighter than
background, OFF to one darker than surround
Great convergence of connections characterizes
the rod system
Arrangements for color movement signal
processing are elaborate
More than this you dont really want to know
Kolb H. The architecture of functional neural
circuits in the vertebrate retina. Invest
Ophthalmol Vis Sci 1994352385-2404 is almost
comprehensible
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OPTIC NERVE
Nerve fibers acquire myelin as they leave the eye
NERVE-FIBER LAYER un-myelinated
LAMINA CRIBROSA
Holes in the sclera for the nerve fibers
A weak spot
RETINA
SCLERA
DURA
ARACHNOID PIA
DURA
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RETINA in OPHTHALMOSCOPY
All this transparency to let light in means that,
when the interior of the eye is illuminated, one
can look in, with magnification, at the inside
of the back of the eye - the fundus
NORMAL VIEW
FUNDUS
MACULA
VESSELS
OPTIC DISK
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SOME RETINA QUESTIONS in OPHTHALMOSCOPY
NORMAL VIEW
FUNDUS - Correct color for race? Any spots? No
unevenness?
MACULA - Any vessels over it? Too red?
OPTIC DISK - Not too pale? No bulge, or excessive
excavation?
VESSELS - Right size? Not bent? Correct course?
Engorged veins?
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SCLERA regional specializations
Dense irregular connective tissue
LENS
Some vessels by limbus ciliary body
Insertions of extraocular muscles
Vitreous
RETINA
Lamina fusca
Melanocytes
SCLERA proper
OPTIC NERVE exits
Loose episcleral CT
LAMINA CRIBROSA
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UVEA Components
sagittal view
outer
middle tunic
UVEA
SCLERA
RETINA
inner
LENS
1
IRIS
2
3
CILIARY BODY
CHOROID
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ZONULE or SUSPENSORY LIGAMENT OF LENS
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ORA SERRATA
Posterior-to-Ant. view
PARS PLANA
CILIARY MUSCLE
The junction between the neural retina and the
double cuboidal epithelium on the plars plana and
the ciliary body is very irregular - creating a
serrated mouth
LENS
NEURAL RETINA
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UVEA Choroid
The structure of the iris conveys much of the
structure of the choroid
ROD
CONE
IRIS
PIGMENT CELLS
BRUCHS MEMBRANE
CHORIOCAPILLARIS
Wide fenestrated capillaries to nourish the retina
Melanocytes
loose vascular connective tissue
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ANGLE OF ANTERIOR CHAMBER Aqueous Humor
Corner of ant chamber between cornea iris,
where sclera starts
Next Figure
LENS
ANTERIOR CHAMBER
PUPIL
POSTERIOR CHAMBER
Chambers filled with aqueous humor
SCLERAL ANGLE is another name
Epithelium of CILIARY PROCESSES makes AH
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ANGLE of ANTERIOR CHAMBER
Corner of ant chamber between cornea iris,
where sclera starts
CORNEA
Canal of Schlemm
Trabecular meshwork
ANTERIOR CHAMBER
Spaces of Fontana in the meshwork
SCLERA
IRIS
POSTERIOR CHAMBER
CILIARY PROCESSES make
aqueous humor
CILIARY MUSCLE
Uveoscleral outflow is another drainage route
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AQUEOUS HUMOR Production Flow II
Epithelium of CILIARY PROCESSES makes Aqueous
Humor
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POSTERIOR CHAMBER
5
2
4
PUPIL
3
ANTERIOR CHAMBER
PUPIL
LENS
SCLERAL ANGLE with
Uveoscleral outflow
Trabecular meshwork
Canal of Schlemm
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Weinreb RN, Khaw PT. Primary open-angle glaucoma.
Lancet 20043631711-1720
AQUEOUS HUMOR Glaucoma
Epithelium of CILIARY PROCESSES makes AH
POSTERIOR CHAMBER
PUPIL
ANTERIOR CHAMBER
SCLERAL ANGLE with
Trabecular meshwork
Blocked drainage/venous return of AH raises
intra-ocular pressure, damaging vessels the
retina
Canal of Schlemm
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CORNEA I Layers
CORNEAL EPITHELIUM
Bowmans membrane
No vessels
STROMA
Keratocyte
Descemets membrane
ENDOTHELIUM
Anterior chamber
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CORNEA II Layer constituents
CORNEAL EPITHELIUM is stratified squamous, with
nerve fibers
Thin basal lamina
Bowmans membrane
of dense fibrillar collagen
STROMA
of collagen fibers in very orderly lamellae,
with regular alternating fiber orientations
much special proteoglycan
Keratocytes are fibroblasts of the corneal stroma
No vessels are present
Bowmans membrane is modified stroma, not the
basal lamina
Descemets membrane
- a thick basal lamina
ENDOTHELIUM
Not a vascular endothelium, but pumps water out
of the stroma
Transparency factors, not present in the sclera
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CORNEA III Tear-film constituents
oily/lipid layer - eyelid glands
aqueous phase - Lacrimal
Mucin layer
From conjunctival tear-duct goblet cells
CORNEAL EPITHELIUM
TEARS
Protect the conjunctival corneal surfaces
Nourish the avascular cornea
Wash out discrepancies to corner of the eye
Kill restrain microorganisms
Smooth corneal-surface optics
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LACRIMAL/LACHRYMAL GLAND PASSAGES
LACHRYMAL GLAND
LACHRYMAL DUCTS
LACHRYMAL SAC
NASOLACRIMAL DUCT
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LACRIMAL GLAND II
Gland is superior and temporal to the eye
facilitating the spread of tears across the eye
to the collection points - the lacrimal puncta
medially at the eyelids medial/nasal margins
evaporation is slowed by surface film of lipid
from Meibomian glands
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LACRIMAL GLAND III
LACRIMAL GLAND
Compound tubulo-alveolar gland
Alveoli lined by pale columnar/cuboidal serous
cells with myoepithelial cells
Secretion - tears - comprises
water
antimicrobials - lysozyme, defensins, antibodies
electrolytes - plasma-like (tears taste salty)
Innervation - Parasympathetic in CN VII via
Pterygopalatine ganglion
Blinking - eyelid movement - is necessary to
spread tears
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UPPER EYELID II
Levator palpebrae superior. Muscle
Palpebral part of Orbicularis oculi Muscle
Inserts into Tarsus, etc
EYELID SKIN
BULBAR CONJUNCTIVA
Dense connective-tissue
TARSAL PLATE
with embedded
fornix
Meibomian glands
PALPEBRAL CONJUNCTIVA
Stratified cuboidal epithelium with some goblet
cells on loose CT
By the eyelash follicle are other small glands
LID MARGIN
Where secretion of Meibomian modified sebaceous
glands emerges
EYELASH
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LENS EQUATOR AXIS
EQUATOR
Anterior
AXIS
LENS
Posterior pole
Posterior--Anterior view
Lateral view
Lens shape is not quite as depicted the anterior
part is an ellipsoid the posterior bulges back
more as a parabyloid
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LENS PARTS
Subcapsular epithelium (cuboidal)
becomes elongated LENS FIBERS (cells)
at the LENS BOW
by filling themselves with crystallins -
the proteins that confer long-lasting transparency
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LENS CAPSULE
CATARACT - Lens becomes opaque
Common in old age
UV radiation is an accelerating factor
Naphthalene (in mothballs) is another agent, as is
Overheating with infrared radiation from
furnaces e.g., in glassblowers,
Traumatic damage to the lens capsule and
epithelium
Lentectomy, and replacement with an artificial
lens usually cure
ZONULE FIBERS
Posterior-capsule opacification is one risk
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EYE DEVELOPMENT I Some specifications
The eye comprises many tissues, structures, and
layers that require contributions from three main
sources
Using multiple sources needs tight coordination
of signals and controls
The bodys covering has to have a transparent
region
For optics, the lens needs to be roundish, the
eye almost spherical, with the retina precisely
hemispherical
Spaces - chambers and cavity - have to be created
inside
Blood vessels have to be introduced early into
the soon- to-be-enclosed round eye
Nerves (afferent efferent) to from the brain
are needed
External internal muscles other auxilliary
structueres are needed
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35 days pc
DEVELOPMENT of the EYE I from CNS
3 brain vesicles are subdividing
Mesencephalon
now four, then Rhombencephalon divides into Met-
Mel-encephalons
Rhombencephalon
BRAIN
Cephalic flexure/bend
Diencephalon
Cervical flexure
start the folding
Telencephalon
Already before 35d pc, on each side of the
head, interactions have started between surface
ECTODERM, a bulge of the FOREBRAIN the
MESENCHYME
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EYE PARTS EMBRYONIC SOURCES
Surface ECTODERM
Neural ECTODERM
MESENCHYME
RETINA
UVEA
SCLERA
RETINA
LENS
OPTIC NERVE
CORNEAL EPITHELIUM
CORNEAL STROMA
VITREOUS
Two ectoderms drive events and shaping
Connective tissue muscle ( vessels) come from
cranial mesenchyme
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Mesenchyme
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LENS OPTIC CUP DEVELOPMENT I
While still growing, both placode and end of the
optic vesicle invaginate
Mesenchyme
optic vesicle
Intraretinal space
lens placode
Double wall of optic cup is starting to form
Optic vesicle precedes the lens vesicle and is a
distinct structure
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OPTIC CUP DEVELOPMENT II Choroid fissure
Blood vessels have to be introduced early into
the soon to be enclosed round eye
Mesenchyme
Together with the invagination centrally at the
end of the optic cup,
an invagination along the cup stalks inferior
surface occurs, to create the choroid fissure
in which runs the hyaloid artery
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OPTIC CUP DEVELOPMENT II Coloboma
Blood vessels have to be introduced early into
the soon to be enclosed round eye
Together with the invagination centrally at the
end of the optic cup,
an invagination along the cup stalks inferior
surface occurs, to create the choroid fissure
in which runs the hyaloid artery
Also, an annular vessel runs around the
outside of the optic cup
Imagine a penis in which the urethra near into
the glans is still open on its underside - the
condition of hypospadias - (but now contains an
artery)
Defects in the eye from failure of the choroid
fissure to close are colobomas
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OPTIC DEVELOPMENT III Lens vesicle
LENS VESICLE
Mesenchyme
lens placode
Inner wall thickens
Deeper part of Placode sinks into mesenchyme
makes a vesicle
Optic cup becomes deeper
Attachment to surface ectoderm will be broken
so that surface ectoderm can become corneal
epithelium intervening mesenchyme can form the
corneal stroma
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OPTIC DEVELOPMENT IV Lens differentiation
Attachment to surface ectoderm lost
Mesenchyme
Mesenchyme
Anterior vesicle cells become subcapsular
epithelium
Basal lamina becomes lens capsule
Posterior vesicle cells become elongated lens
cells
Posterior vesicle cells form the nucleus of the
lens. Subsequent lens cells derive from the
subcapsular epithelium
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OPTIC DEVELOPMENT IV Lens differentiation
Anterior-vesicle cells become subcapsular
epithelium
Basal lamina becomes lens capsule
Lumen obliterated
LENS
Posterior-vesicle cells elongate to lens cells
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OPTIC DEVELOPMENT V Retina differentiation I
Outer layer of cup stays thin and beomes pigment
cell layer
Intra-retinal space occluded
Inner layer of cup thickens and becomes Neural
layer
Hyaloid artery reaches inside cup
After a while, the lens and vitreous no longer
need it, and it atrophies. Only the neural
retina continues to depend on it, but under
another name - central artery of the retina
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OPTIC DEVELOPMENT VI Retina differentiation II
Inner layer of cup thickens and becomes Neural
layer
Where cells multiply, form layers and
differentiate to the several cell types of the
neural retina
Outer layer of cup stays thin and beomes pigment
cell layer