Download Developmental biology 2008 Lecture 3

Developmental biology 2008
Lecture 3:
Development of the eye
Chap. 12 p. 397-401
Chap. 13 p. 436-441
Stine Falsig Pedersen
[email protected]/room 527
Dept. of Biology
University of Copenhagen
Fates of the ectoderm
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Fig. 12.1
2
The vertebrate eye
Aqueous
humor
Cornea
Iris + ciliary body
Lens
Optic
nerve
Neural retina
Pigmented retina
3
The sensory organs of the head develop from interactions between the neural tube and a series
of epidermal thickenings formed at the anterior border between the epidermal and neural
ectoderm, the cranial sensory placodes.
The cranial sensory placodes give rise to the sensory apparatus of nose, ears, and taste
receptors, to the lens, and to the glossopharyngeal, facial, and vagal sensory neurons.
Fig. 13.11
4
Development of the vertebrate eye
Fig. 6.5A-B
Mouse day 9-10
The neural tube (presumtive forebrain region) is induced by the chordamesoderm (mostly
presumptive notochord) to bulge out and form the optic vesicle. Subsequently:
(A) The optic vesicle induces the overlying ectoderm to become the lens placode
(B) The lens now induces the optic vesicle to become the optic cup (reciprocal induction)
(C) The optic cup differentiates into neural retina (inner) and pigmented retina (outer) 5
Development of the vertebrate eye
Fig. 6.5D-E
Mouse day 11-13
(D) The lens placode separates from the overlying ectoderm to become the lens vesicle,
and the lens cells start to differentiate
(E) The lens induces the overlying ectoderm to become the outer layer of the cornea
(helped by enzymes from neural crest cells). Retinal ganglion cell axons begin to grow
along the inner surface of the retina toward the optic disc, the head of the optic nerve 6
Lens induction is a multistep process with more than one
inducer and reciprocal inductive processes
Fig. 6.5F
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Development of the vertebrate eye
Fig. 12.26
Mouse ca. day 9-10
Mouse ca. day 11-13
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Role of the cytoskeleton in lens formation
Contraction of the apical F-actin cytoskeleton in
the lens placode contributes to lens closure
Eye development in stage 15 chick embryo
Zolessi & Arruti (2001) BMC Developmental Biology 1:7
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Transcription factors in lens induction in amphibians - I
Fig. 6.4
Otx2 expression is induced in the presumptive lens ectoderm in the late gastrula stage,
possibly as a result of signals from the pharyngeal endoderm and heart-forming mesoderm
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Transcription factors in lens induction in amphibians - II
Fig. 6.4
Pax6 makes the presumptive lens ectoderm
Signals from the anterior neural plate
(including presumptive retina) induce Pax6 competent to be induced (likely by BMP
expression in the presumptive lens ectoderm family proteins) by the optic vesicle to express
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Sox3 and initiate lens formation
Induction of the optic vesicle in vertebrates requires at least three
transcription factors expressed in the anterior neural plate:
Rx1 (of the retinal homeobox genes)
Six3 (SIX homeodomain family)
Pax6 (paired box and paired-like homeobox gene)
• Otx2 upregulates Rx1
• Rx1 is required for continued expression of Six3 and Pax6
in the prospective retina
• Six3 is a direct activator of Pax6
These are important, but many more factors
contribute to the complete development of the eye!
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Role of Rx genes in vertebrate retina development
Xr1 is the Xenopus Rx1 gene
Early Xenopus
neurula
Newly hatched
tadpole
Mouse and human blindness
due to Rx mutations
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Pax 6 is widely expressed in the eye in early development
mRNA in situ hybridization showing Pax6 expression in the prospective
lens ectoderm and the optic cup, and later in lens, retina, and cornea
mouse embryo day 10
mouse embryo day 15
Grindley et al (1995) Development 121: 133-142
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Pax6 acts as a competence factor for lens induction
The inductive signal from the optic vesicle is necessary, but not sufficient: Pax6 plays an
important role in making the prospective lens ectoderm in which it is expressed competent
to be induced by the optic vesicle to form the lens.
The lens-inducing factors from the optic vesicle appear to be BMP4 (which leads to Sox2
and Sox3 production) and FGF8
Fig. 6.1
Fig. 6.2
Pax 6 knockout mice lack eyes
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The essential role of Pax6 in eye development is highly conserved
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Generation of two eyes
Sonic hedgehog from the prechordal plate inhibits Pax6 expression in the center of the
embryo, causing the eye field to be split in two.
Inactivating mutations of sonic hedgehog cause cyclopism:
development of only one eye.
Wt and shh-/- mouse embryos
Pax6 in Xenopus in ctrl.
(C) and with prechordal
mesoderm removed (D)
Red: Otx2
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Conversely, loss of eyes in the cavefish Astynanx mexicanus reflects
reduced Pax6 expression
(caused by upregulation
of sonic hedgehog, not shown).
5-somite
18-somite
Jeffery (2001) Dev. Biol. 231: 1–12
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Development of the neural retina - I
2 week mouse ~ 7 week human embryo
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Development of the neural retina - II
Cell division, migration, and differentiation of pluripotent precursor cells of
the neural retina germinal layer gives rise to all the cell types of the neural
retina. Here shown: photoreceptors, neurons, and glial cells
B-galactosidase labeling of retinal precursor cell
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Development of the neural retina - III
The germinal neuroepithelium of the
retina is located closest to the lens, in
the ganglion cell layer
To become retinal neurons, precursor
cells must exit from mitosis and migrate
out to the correct location
Specification of cell fate is thought to
occur just after the last mitosis
Retinal neurons migrate along the
elongated cell bodies and processes of
retinal neuroepithelial cells
Migration and localization is modulated
by specific adhesion molecules (e.g.
Cadherins, N-CAM, integrins), and
secreted and cell surface guidance
cues and extracellular matrix
Malicki (2004) Curr. Op. Neurobiol. 14:15–21
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Development of the lens
The inner cells of the lens vesicle elongate and become the lens fibers, which fill
up with crystallins and loose their nuclei and organelles, becoming transparent
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From neural crest
Development
of the lens,
cornea, and
iris
Cornea develops
from neural crest
mesenchymal cells
Iris (which controls
pupil size) and the
ciliary body (which
forms the aqueous
humor) develop from
the optic cup
Fig. 12.32
Day 13
Day 15
Day 14
Day 15½
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Roles of Sox2 and Pax6 in development of the lens
Induced in the
lens placode by
BMP4 secreted
from the optic
vesicle
Fig. 12.32
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Development of the lens
Lens cell division and elongation in the anterior/equatorial zone
Crystallin-containing lens fibres
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8 week old human embryo
Development of the cornea and iris
The lens induces the overlying ectoderm to form the cornea, the shaping of which requires
the aqueous humor. The inner layer of the cornea derives from cranial neural crest cells.
The iris (a pigmented muscular tissue responsible for controlling pupil size) develops from
the outer rim of the optic cup (i.e., the iris is an ectodermally derived muscle!).
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Migration of the retinal ganglion neuron axons
The axons of retinal ganglion neurons (RGNs) travel through the optic nerve to the optic
tectum: the dorsal portion of the midbrain/mesencephalon, mediates reflective responses to
visual stimuli (in humans, the corresponding region is the lateral geniculate nucleus).
Fig. 13.32
Lateral
geniculate
nucleus
Fig. 22.21
Visual cortex
In mammals, most RGNs cross over
at the optic chiasm to project
contralaterally to the lateral geniculate
nucleus, but the ventrotemporal RGNs
project ipsilaterally.
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Forces working on the axon growth cone
Laminin/integrin
Netrins/DCCs + UNCs
EphA7/ephrinA5
semaphorin 1/plexin
Modified from
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
Eph/ephrin
Neurotrophins:
BDNF, NGF,
NT-3, NT-4
semaphorin 3,5/plexin
Slit/Robo
NB: this is a simplified summary of some important players – whether a
given signal is in fact repulsive or attractive is often cell-type specific!
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Guidance cues direct the growth cone by modulation
of actin polymerization and organization
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
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Guidance cues direct the growth cone by modulation
of actin polymerization and organization
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
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Roles of Netrin and semaphorin 5 in directing retinal
ganglion neurons through the optic nerve head
Netrin (red) from the neuroepithelial cells of the optic nerve head is a chemoattractant for
the RGNs and guides them through the optic nerve head (although prior to the entry into
the optic nerve head, netrin is a chemorepellant, due to the additional presence of laminin).
Semaphorin 5 (green) which is expressed in the perifery of the optic nerve head, is a
chemorepellant for the RGNs, and prevents the RGN axons from diverging from their path.
Oster et al (2004) Sem. Cell & Dev. Biol. 15:125–136
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Migration of the retinal ganglion neuron axons
Fig. 13.32
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Slit proteins inhibit axon outgrowth from retinal neurons
Slits are secreted proteins which
act via the Roundabout (Robo)
receptors, and which are generally
chemorepellants for axon growth
cones
Slit proteins in the pre-optic area inhibit outgrowth of the
retinal neurons before they reach the optic chiasm
R: Retinal explant
POA: pre-optic area
WT: wildtype
MOCK: control
Plump et al (2002) Neuron 33:219–232
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Role of Slit proteins in preventing retinal ganglion neurons
from prematurely crossing over before the optic chiasm
Plump et al (2002) Neuron 33:219–232
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Migration of the retinal ganglion neuron axons
Fig. 13.32
35
Ventrotemporal RGNs are very sensitive to repulsion by ephrin,
and are therefore prevented from crossing at the optic chiasm
Ephrin
Ephrin B2 is expressed at
the midline of the optic
chiasm
Axon outgrowth from the ventrotemporal (VT)
RGNs, which cannot cross the midline (project
ipsilaterally), is more strongly prevented by
Ephrin B2 than that from dorsotemporal (DT)
RGNs, which do cross and project
contralaterally, because the VT neurons
express high levels of EphB1 receptors
Williams et al (2003) Neuron 39:919-35
Migration of the retinal ganglion neuron axons
Fig. 13.32
37
Role of Eph/ephrin interactions in the targeting of retinal
ganglion neurons the correct position in the optic tectum
Once the RGN axons reach the optic
tectum (in humans aka superior
colliculus), they must target to a specific
region:
Those coming from the temporal part of
the retina go to the rostral/anterior part
of the tectum, and those from the nasal
part of the retina go to the
caudal/posterior part of the tectum.
This is achieved in part by a caudal-torostral gradient of ephrins in the tectum,
and a temporal-to-nasal gradient of Eph
receptors in the RGNs
A Wnt3 gradient in the tectum
contributes in a similar manner
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Neurotrophins and neuronal survival in the retina
A large fraction – up to 80% - of the retinal neurons must undergo apoptosis during eye development
Chick retina TUNEL stained for apoptotic cells
Pinon-Duarte et al (2004) Eur J Neurosci 19:1475-84
Postnatal day 7 (P7) rat retinas grown in vitro for 7
days with or without BDNF: retinal neuron and glial
survival during development is regulated by BDNF.
Mayordomo et al (2003) Eur J Neurosci 18:1744-50
A comparable competition for neurotrophins
takes place in the optic tectum (p. 710-12)
Essential take-home points I
Fig. 6.5F
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Essential take-home points II
Optic vesicle (from neural tube)
induces lens from ectoderm,
becomes optic cup, then retina,
and differentiates into neural and
pigmented retina
RGN axons grow through optic
nerve head, controlled by e.g.
netrins and semaphorins
Correct crossing/noncrossing at
chiasm involves slits and ephrin
Targeting involves ephrin
gradient at the optic tectum
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So much for the fates of the ectoderm……..
Fig. 12.1
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Slit-Robo signaling
Fernandis & Ganju, Sci STKE 2001
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Overview of major guidance cues in retinal ganglion guidance
Semaphorin 5
Wnt3
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