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Neural development comprises the processes that generate, shape, and
MBNS 604 Developmental Neurobiology:
Neuronal Determination and Differentiation
Naiphinich Kotchabhakdi Ph.D.
Ph.D.
Research Center for Neuroscience,
Institute of Molecular Biosciences, Mahidol University, Salaya,
Email: scnkc@mahidol.ac.th
Web: http://www.neuroscience.mahidol.ac.th
Growth and Developmental Processes of the
Nervous System:
1.Induction of neural plate
2.Determination and Pattern formation
3.Proliferation of cells in different regions
4.Cellular communication and adhesion
g
5.Cell migration
6.Aggregation of cells to form identifiable parts of the
brain and spinal cord
7.Cell differentiation
8.Formation of specific connections
9.Selective death of certain cells
10.The elimination of some connections that were
initially formed and the stabilization of the others
11. Integrated of neural function
reshape the nervous system, from the earliest stages of embryogenesis to the final
years of life. The study of neural development aims to describe the cellular basis of
brain development and to address the underlying mechanisms. The field draws on both
neuroscience and developmental biology to provide insight into the cellular and
molecular mechanisms by which complex nervous systems develop. Defects in neural
development can lead to cognitive, motor, and intellectual disability, as well as
neurological disorders such as autism, Rett syndrome, and mental retardation.
Some landmarks of neural development include the birth and differentiation of neurons
from stem cell precursors, the migration of immature neurons from their birthplaces in
the embryo to their final positions, outgrowth of axons and dendrites from neurons,
guidance of the motile growth cone through the embryo towards postsynaptic partners
partners,
the generation of synapses between these axons and their postsynaptic partners, and
finally the lifelong changes in synapses, which are thought to underlie learning and
memory.
Typically, these neurodevelopmental processes can be broadly divided into two
classes: activity-independent mechanisms and activity-dependent mechanisms.
Activity-independent mechanisms are generally believed to occur as hardwired
processes determined by genetic programs played out within individual neurons.
These include differentiation, migration and axon guidance to their initial target areas.
These processes are thought of as being independent of neural activity and sensory
experience. Once axons reach their target areas, activity-dependent mechanisms
come into play. Although synapse formation is an activity-independent event,
modification of synapses and synapse elimination requires neural activity.
Embryology and
Developmental
Biology
Development of the
Nervous System
Developmental
Neuroscience
The future of
Neuroscience and
the future of Science,
Medicine and Mankind
ขัน้ ตอนการเจริญเติบโตและพัฒนาการของระบบประสาท
พัฒนาการของระบบประสาทกลางของสัตว์ที่มีกระดูกสันหลังประกอบด้ วยขันตอนดั
้
งต่อไปนี ้คือ
1.Induction of neural plate เป็ นขันตอนการเหนี
้
่ยวนําให้ เนื ้อเยื่อชันนอก
้
(Ectoderm)
ของเซลล์ตวั อ่อนในบริเวณตอนกลางทางด้ านหลังของลําตัวให้ กลายเป็ น “neural plate” ซึง่ เป็ น
ส่วนของเนื ้อเยื่อที่จะเจริญไปเป็ นระบบประสาทกลางในอนาคต รวมถึงการเหนี่ยวนําให้ สว่ นต่างๆ
ของ neural tube กลายเป็ นสมองบริเวณต่างๆ หรือกลายเป็ นไขสันหลัง เป็ นต้ น
2.Determination and Pattern formation เป็ นขันตอนในการกํ
้
าหนดขอบเขตการ
เจริญเติบโตของเนื ้อเยื
้ ่อสมอง โดยจะมีการกําหนดแบ่งขอบเขตว่า neural tube ส่วนต่างๆ จะ
เจริญไปเป็ นสมองส่วนใด เซลล์ประสาทในแต่ละส่วนจะไม่เจริญเติบโตข้ ามขอบเขตกัน ขันตอนนี
้
้
เชื่อว่าเกี่ยวข้ องกับ Homeobox gene และยีนส์อื่นๆ ที่เกี่ยวข้ อง
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3.Proliferation of cells in different regions เป็ นขันตอนการแบ่
้
งเซลล์เพื่อเพิ่มจํานวน
เซลล์ประสาท ทําให้ สมองส่วนต่างๆ เจริญเติบโตและเพิ่มขนาด ในขันตอนนี
้
้จะมีการแบ่งเซลล์ให้ ได้
เซลล์ประสาทจํานวนมากมายเกินปกติ หลังจากนันจะมี
้ ขบวนการกําจัดเซลล์ประสาทส่วนเกินที่มิได้
ทําหน้ าที่ใดๆ ออกไปในภายหลัง ดังจะได้ กล่าวในข้ อต่อไป
4.Cellular communication and adhesion เป็ นขันตอนการสื
้
่อสารระหว่างเซลประสาท
เพื่อที่จะเลือกเซลประสาทที่มีหน้ าที่คล้ ายคลึงหรือสัมพันธ์กนั ให้ มาเกาะกลุม่ อยูใ่ นบริเวณเดียวกัน
5.Cell migration เป็ นขันตอนการอพยพเคลื
้
่อนย้ ายของเซลล์ประสาท เพื่อให้ เกิดเป็ นโครงสร้ าง
ส่ว่ นต่า่ งๆ ของระบบประสาท
ป
6.Aggregation of cells to form identifiable parts of the brain and
spinal cord การรวมกลุ่มของเซลล์เป็ นสมองและไขสันหลังส่วนต่างๆ
7.Cell differentiation เป็ นขันตอนการเปลี
้
่ยนแปลงพัฒนารูปร่างและคุณสมบัติของเซลล์ตงั ้
ต้ นของเซลล์ประสาทให้ กลายเป็ นเซลล์ประสาท (neuron) หรือ เซลล์คํ ้าจุนระบบประสาท (glial
cell) ที่โตเต็มวัยพร้ อมที่จะทําหน้ าที่ตอ่ ไป
8.Formation of specific connections เป็ นขันตอนการสร้
้
างเครือข่ายเพื่อติดต่อสื่อสาร
ระหว่างกลุม่ เซลล์ประสาท โดยใยประสาทที่งอกออกจากตัวเซลล์ประสาทแต่ละเซลล์จะมีการ
เชื่อมโยงเข้ าด้ วยกัน และจุดเชื่อมต่อระหว่างใยประสาทดังกล่าวต่อไปจะถูกเรียกว่า synapse ซึง่ จะ
เป็ นบริเวณที่มีการส่งผ่านข้ อมูลู ในรููปของกระแสประสาทจากเซลล์ประสาทเซลล์หนึ่งไปยังอีกเซลล์
หนึง่
9.Selective death of certain cells การกําจัดเซลล์จําเพาะบางตัวหรือกลุม่ โดยการมี
โปรแกรมการตาย (Programmed cell death or apoptosis)
Neural induction
10.The elimination of some connections that were initially
formed and the stabilization of the others. เป็ นขันตอนการคั
้
ดเลือกวงจร
ประสาทให้ คงอยูห่ รือกําจัดทิ ้งไป โดยใยประสาทที่เชื่อมโยงถูกต้ องและสามารถทําหน้ าที่ได้
อย่างมีประสิทธิภาพก็จะคงอยูต่ อ่ ไป ส่วนใยประสาทที่เชื่อมโยงผิดพลาดหรือเซลล์ประสาทที่
ไม่ได้ รับการเชื่อมโยงติดต่อก็จะถูกกําจัดและตายไปในที่สดุ
11. Integrated of neural function เป็ นขันตอนสุ
้
ดท้ ายคือเมื่อการเชื่อมโยงวงจร
ของเซลล์ประสาท เกิดขึ ้นอย่
้ างถูกต้ องเรียบร้ อยก็จะทําให้ เกิดการทํางานที่สลับซับซ้ อนและมี
ประสิทธิภาพของระบบประสาท
Identification of neural inducers
A transplanted blastopore lip can convert ectoderm into neural tissue and is
said to have an inductive effect. Neural Inducers are molecules that can induce
the expression of neural genes in ectoderm explants without inducing
mesodermal genes as well. Neural induction is often studied in Xenopus
embryos since they have a simple body pattern and there are good markers to
distinguish between neural and non-neural tissue. Examples of Neural Inducers
are the molecules Noggin and Chordin
Chordin..
When embryonic ectodermal cells are cultured at low density in the absence of
mesodermal cells they undergo neural differentiation (express neural genes),
suggesting that neural differentiation is the default fate of ectodermal cells.
cells In
explant cultures (which allow direct cell-cell interactions) the same cells
differentiate into epidermis. This is due to the action of BMP4 (a TGF-β family
protein) that induces ectodermal cultures to differentiate into epidermis. During
neural induction, Noggin and Chordin are produced by the dorsal mesoderm
(notochord) and diffuse into the overlying ectoderm to inhibit the activity of
BMP4. This inhibition of BMP4 causes the cells to differentiate into neural cells.
During early embryonic development the ectoderm becomes specified to give rise to
the epidermis (skin) and the neural plate. The conversion of undifferentiated
ectoderm to neuro-ectoderm requires signals from the mesoderm. At the onset of
gastrulation presumptive mesodermal cells move through the dorsal blastopore lip
and form a layer in between the endoderm and the ectoderm. These mesodermal
cells that migrate along the dorsal midline give rise to a structure called the
notochord. Ectodermal cells overlying the notochord develop into the neural plate in
response to a diffusible signal produced by the notochord. The remainder of the
ectoderm gives rise to the epidermis (skin). The ability of the mesoderm to convert
the overlying ectoderm into neural tissue is called Neural Induction.
The neural plate folds outwards during the third week of gestation to form the neural
groove. Beginning in the future neck region, the neural folds of this groove close to
create the neural tube. The formation of the neural tube from the ectoderm is called
Neurulation. The ventral part of the neural tube is called the basal plate; the dorsal
part is called the alar plate. The hollow interior is called the neural canal. By the end
of the fourth week of gestation, the open ends of the neural tube (the neuropores)
close off.
Genetic
Nutrition
Environment
The Brain
SStructural
uc u Development
eve op e
Functional Development
Chemical Development
Behavioural Development
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Hierarchy of
Problems
and
Consideration
Sites and
M h i
Mechanisms
Utilization
and
Wisdom
Recall that capability to produce neural tissue is
conferred by neural plate induction genes that
have anti-TGFβ family activity (e.g. anti-BMP genes
noggin, chordin, follistatin, activin in vertebrates,
anti-dpp (sog) in insects) and anti-Notch/Delta
proneural genes (bHLH
bHLH family e.g. achaete/scute
)
in insects and Mash,, Xash etc in vertebrates).
BUT this activity actually leads to progenitor cells,
not neurons themselves. So how does a progenitor
decide to produce a neuron or glial or other type of
cell? How does it know how many to make?
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Patterning of the nervous system
In chordates, dorsal ectoderm forms all neural tissue and the nervous system.
Patterning occurs due to specific environmental conditions - different concentrations
of signaling molecules
Dorsoventral axis
The ventral half of the neural plate is controlled by the notochord, which acts as the
'organiser'. The dorsal half is controlled by the ectoderm plate which flanks the
neural plate on either side.
Ectoderm follows a default pathway to become neural tissue. Evidence for this
comes from single, cultured cells of ectoderm which go on to form neural tissue. This
is postulated to be because of a lack of BMPs, which are blocked by the organiser.
The organiser may produce molecules such as follistatin,
follistatin noggin and chordin which
inhibit BMPs.
The ventral neural tube is patterned by Sonic Hedgehog (Shh) from the notochord,
which acts as the inducing tissue. The Shh inducer causes differentiation of the floor
plate. Shh-null tissue fails to generate all cell types in the ventral tube, suggesting
Shh is necessary for its induction. The hypothesised mechanism suggests that Shh
binds patched, relieving patched inhibition of smoothened, leading to activation of
glia transcription factors.
In this context Shh acts as a morphogen - it induces cell differentiation dependent on
its concentration. At low concentrations it forms ventral interneurones, at higher
concentrations it induces motor neurone development, and at highest concentrations
it induces floor plate differentiation. Failure of Shh-modulated differentiation causes
holoprosencephaly.
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The dorsal neural tube is patterned by BMPs from the epidermal ectoderm
flanking the neural plate. These induce sensory interneurones by activating Sr/Thr
kinases and altering SMAD transcription factor levels.
Rostrocaudal (Anteroposterior) axis
Signals that control anteroposterior neural development include FGF and retinoic
acid which act in the hindbrain and spinal cord. The hindbrain, for example, is
patterned by Hox genes, which are expressed in overlapping domains along the
anteroposterior axis under the control of retinoic acid. The 3' genes in the Hox
cluster are induced by retinoic acid in the hindbrain, whereas the 5' Hox genes are
not induced by retinoic acid and are expressed more posteriorly in the spinal cord.
Hoxb-1 is expressed in rhombomere 4 and gives rise to the facial nerve. Without
this Hoxb-1 expression, a nerve which is similar to the trigeminal nerve arises.
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Specificity vs. plasticity
Determination of neuronal origin
and their fates through cell lineage
Homeobox genes
control the plan
for segmentation
of the body and
the brain
3 Nobel Laureates who won the 1995 prize in Physiology
or medicine worked on Homeobox genes which control
the segmentation of the body and the brain.
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Neuronal migration
Neuronal migration is the method by which neurons travel from
their origin or birth place to their final position in the brain.
There are several ways they can do this, e.g. by radial
migration or tangential migration.
Radial migration Neuronal precursor cells proliferate in the
ventricular zone of the developing neocortex. The first
postmitotic cells to migrate form the preplate which are
destined to become Cajal-Retzius cells and subplate neurons.
These cells do so by somal translocation. Neurons migrating
with this mode of locomotion are bipolar and attach the leading
edge of the process to the pia.
pia The soma is then transported to
the pial surface by nucleokinesis, a process by which a
microtubule "cage" around the nucleus elongates and contracts
in association with the centrosome to guide the nucleus to its
final destination.] Radial glia, whose fibers serve as a
scaffolding for migrating cells, can itself divide or translocate to
the cortical plate and differentiate either into astrocytes or
neurons. Somal translocation can occur at any time during
development. Subsequent waves of neurons split the preplate
by migrating along radial glial fibres to form the cortical plate.
Corticogenesis: younger
neurons migrate past older
ones using radial glia as a
scaffolding. Cajal-Retzius
cells (red) release reelin
(orange).
Each wave of migrating cells travel past their predecessors forming layers in an inside-out manner,
meaning that the youngest neurons are the closest to the surface. It is estimated that glial guided
migration represents 90% of migrating neurons in human and about 75% in rodents.
Pioneer neuron is a cell that is a derivative of preplate in the early
stages of corticogenesis of the brain. Pioneer neurons settle in the marginal
zone of the cortex and project to sub-cortical levels. In the rat, pioneer
neurons are only present in prenatal brains. Unlike Cajal-Retzius cells, these
neurons are Reln-negative.
Pioneer neurons are born in the ventricular neuroepithelium all over the
cortical primordium. In the rat cortex, they appear at embryonic day (E) 11.5
in the lateral aspect of the telencephalic vesicle and cover its whole surface
on E12. These cells, which show intense immunoreactivity for calbindin and
calretinin, are characterized by their large size and axonal projection. They
remain in the marginal zone
one after the formation of the cortical plate;
plate they
the
project first into the ventricular zone, and then into the subplate and the
internal capsule. Therefore, these cells are the origin of the earliest efferent
pathway of the developing cortex.
Tangential migration Most interneurons migrate
tangentially through multiple modes of migration
to reach their appropriate location in the cortex.
An example of tangential migration is the
movement of interneurons from the ganglionic
eminence to the cerebral cortex. One example of
ongoing tangential migration in a mature
organism, observed in some animals, is the
rostral migratory stream connecting subventricular
zone and olfactory bulb.
Oh
Others
modes
d off migration
i
i There
Th
is
i also
l a
method of neuronal migration called multipolar
migration. This is seen in multipolar cells, which
are abundantly present in the cortical intermediate
zone. They do not resemble the cells migrating by
locomotion or somal translocation. Instead these
multipolar cells express neuronal markers and
extend multiple thin processes in various
directions independently of the radial glial fibers
Tangential migration of
interneurons from
ganglionic eminence
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Neurotrophic factors
The survival of neurons is regulated by survival factors, called trophic factors. The neurotrophic
hypothesis was formulated by Victor Hamburger and Rita Levi Montalcini based on studies of
the developing nervous system. Victor Hamburger discovered that implanting an extra limb in the
developing chick led to an increase in the number of spinal motor neurons. Initially he thought
that the extra limb was inducing proliferation of motor neurons, but he and his colleagues later
showed that there was a great deal of motor neuron death during normal development, and the
extra limb prevented this cell death. According to the neurotrophic hypothesis, growing axons
compete for limiting amounts of target-derived trophic factors and axons that neurons that fail to
receive insufficient trophic support die by apoptosis. It is now clear that factors produced by a
number of sources contribute to neuronal survival.
Nerve Growth Factor (NGF): Rita Levi Montalcini and Stanley Cohen purified the first trophic
factor, Nerve Growth Factor (NGF), for which they received the Nobel Prize. There are three
NGF-related trophic factors: BDNF, NT3, and NT4, which regulate survival of various neuronal
populations. The Trk proteins act as receptors for NGF and related factors. Trk is a receptor
tyrosine kinase. Trk dimerization and phosphorylation leads to activation of various intracellular
signaling pathways including the MAP kinase, Akt, and PKC pathways.
CNTF: Ciliary neurotrophic factor is another protein that acts as a survival factor for motor
neurons. CNTF acts via a receptor complex that includes CNTFRα, GP130, and LIFRβ.
Activation of the receptor leads to phosphorylation and recruitment of the JAK kinase, which in
turn phosphorylates LIFRβ. LIFRβ acts as a docking site for the STAT transcription factors. JAK
kinase phosphorylates STAT proteins, which dissociate from the receptor and translocate to the
nucleus to regulate gene expression.
GDNF: Glial derived neurotrophic factor is a member of the TGFb family of proteins, and is a
potent trophic factor for striatal neurons. The functional receptor is a heterodimer, composed of
type 1 and type 2 receptors. Activation of the type 1 receptor leads to phosphorylation of Smad
proteins, which translocate to the nucleus to activate gene expression.
Nobel Prize Winner in 1993
Synapse formation
Neuromuscular junction Much of our understanding of synapse formation comes
from studies at the neuromuscular junction. The transmitter at this synapse is
acetylcholine. The acetylcholine receptor (AchR) is present at the surface of
muscle cells before synapse formation. The arrival of the nerve induces clustering
of the receptors at the synapse. McMahan and Sanes showed that the
synaptogenic signal is concentrated at the basal lamina. They also showed that the
synaptogenic signal is produced by the nerve, and they identified the factor as
Agrin. Agrin induces clustering of AchRs on the muscle surface and synapse
formation is disrupted in agrin knockout mice. Agrin transuces the signal via MuSK
receptor to rapsyn.
rapsyn Fischbach and colleagues showed that receptor subunits are
selectively transcribed from nuclei next to the synaptic site. This is mediated by
neuregulins.
In the mature synapse each muscle fiber is innervated by one motor neuron.
However, during development many of the fibers are innervated by multiple axons.
Lichtman and colleagues have studied the process of synapses elimination. This is
an activity-dependent event. Partial blockage of the receptor leads to retraction of
corresponding presynaptic terminals.
CNS synapses Agrin appears not to be a central mediator of CNS
synapse formation and there is active interest in identifying signals that mediate
CNS synaptogenesis. Neurons in culture develop synapses that are similar to
those that form in vivo, suggesting that synaptogenic signals can function properly
in vitro. CNS synaptogenesis studies have focused mainly on glutamatergic
synapses. Imaging experiments show that dendrites are highly dynamic during
development and often initiate contact with axons. This is followed by recruitment
of postsynaptic proteins to the site of contact. Stephen Smith and colleagues have
shown that contact initiated by dendritic filopodia can develop into synapses.
Induction of synapse formation by glial factors: Barres and colleagues made the
obser ation that factors in glial conditioned media ind
observation
induce
ce ssynapse
napse formation in
retinal ganglion cell cultures. Synapse formation in the CNS is correlated with
astrocyte differentiation suggesting that astrocytes might provide a synaptogenic
factor. The identity of the astrocytic factors is not yet known.
Neuroligins and SynCAM as synaptogenic signals: Sudhof, Serafini, Scheiffele
and colleagues have shown that neuroligins and SynCAM can act as factors that
will induce presynaptic differentiation. Neuroligins are concentrated at the
postsynaptic site and act via neurexins concentrated in the presynaptic axons.
SynCAM is a cell adhesion molecule that is present in both pre- and post-synaptic
membranes.
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Growing axons have a highly motile structure at the
growing tip called the growth cone, which "sniffs out" the
extracellular environment for signals that instruct the axon
which direction to grow. These signals, called guidance cues,
can be fixed in place or diffusible; they can attract or repel
axons. Growth cones contain receptors that recognize these
guidance cues and interpret the signal into a chemotropic
response. The general theoretical framework is that when a
growth cone "senses" a g
g
guidance cue,, the receptors
p
activate
various signaling molecules in the growth cone that eventually
affect the cytoskeleton. If the growth cone senses a gradient of
guidance cue, the intracellular signaling in the growth cone
happens asymmetrically, so that cytoskeletal changes happen
asymmetrically and the growth cone turns toward or away from
the guidance cue.
Axon guidance in the Drosophila embryonic ventral nerve cord. From SanchezSoriano et al., 2007
A combination of genetic and biochemical methods (see below) has led to the
discovery of several important classes of axon guidance molecules and their
receptors:
Netrins: Netrins are secreted molecules that can act to attract or repel axons by
binding to their receptors, DCC and UNC5.
Slits aka Sli: Secreted proteins that normally repel growth cones by engaging
Robo (Roundabout) class receptors.
Ephrins: Ephrins are cell surface molecules that activate Eph receptors on the
surface of other cells. This interaction can be attractive or repulsive. In some
cases, Ephrins can also act as receptors by transducing a signal into the
expressing cell, while Ephs act as the ligands. Signaling into both the Ephrinand
dE
Eph-bearing
h b i cells
ll iis called
ll d "bi
"bi-directional
di
i
l signaling."
i
li "
Semaphorins: The many types of Semaphorins are primarily axonal repellents,
and activate complexes of cell-surface receptors called Plexins and Neuropilins.
In addition, many other classes of extracellular molecules are used by growth
cones to navigate properly:
Developmental morphogens, such as BMPs, Wnts, Hedgehog , and FGFs
Extracellular matrix and adhesion molecules such as laminin, tenascins,
proteoglycans, N-CAM, and L1
Growth factors like NGF
Neurotransmitters and modulators like GABA
Synapse elimination
Several motorneurones compete for each neuromuscular junction, but only one
survives till adulthood. Competition in vitro has been shown to involve a limited
neurotrophic substance that is released, or that neural activity infers advantage
to strong post-synaptic connections by giving resistance to a toxin also released
upon nerve stimulation. In vivo it is suggested that muscle fibres select the
strongest neuron through a retrograde signal.
What is neuronal determination?
What is neuronal differentiation?
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Cellular determination
Cellular determination is a concept from developmental
biology describing the process by which cells determine to
differentiate and acquire a "type". The morphology of a cell
may change dramatically during differentiation, but the genetic
material remains the same.
What make these deterministic changes?
Deterministic: determine by cell lineage or cell fate
Deterministic Dev. Potential = Dev. Fate
Or alternatively, the cell interactions (the surrounding
environment) are so reproducible that specific cells always
choose the same fate.
Progressive Reduction of the Dev. Potential of a cell or its
progenitors
Two general sources that have taken on a
number of different names:
names:
Intrinsic
Extrinsic
Nature
European
Within cell
Mosaic
Autonomous
Potential = Fate
Nurture
American
outside cell
Regulative
Non-autonomous
Potential > Fate
How do we begin to gain mechanistic explanations of these
processes?
First is the mechanism cell intrinsic or extrinsic?
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The fates of some neurons, particularly those of
invertebrates, are the product of particular lineages. The
fates of others, particularly those in vertebrates, appear
to depend more on the local environment.
Sidney Brenner suggested that neurons are either European
or American. A neuron is European if its fate is largely the
result of who its parents were. For American neurons, it is
more about the neighborhood
g
where they
yg
grew up.
p
When one looks closely, however, it turns out that fate is not
strictly controlled by either lineage or environment alone.
alone
Usually, it is the mixture of the two that is essential; the
adoption of a particular fate is a multi-step sequential process
that involves both intrinsic and extrinsic influences.
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A number of experiments and observations in several
different systems have led to the conclusion that cellular or
neuronal determination takes place by progressive and
successive restriction in potency and potential as progenitor
cells develop and divided.
There is immense variation in the role of lineage versus
environment in neuronal determination.
The general rule is that in the neuronal determination in
invertebrates are more dominated by lineage mechanisms,
mechanisms,
while in the vertebrates are more dominated by diffusible
signals and cellular interactions.
interactions.
Each determination pathway, however, usually brings its own
mix of lineage-dependent and lineage-independent
mechanisms
The last phases of determination involve each neuron
interacting with its synaptic targets, which may provide the
final differentiative signals for the maturing neurons.
At the end of the process, the neuron becomes an
individual cell with its own biochemical and morphological
properties and its unique set of synaptic inputs and outputs.
Of the transcription factors, Basic-Helix-Loop-Helix (bHLH)
factors of the proneural class help tell cells to become
neurons and are antagonized by the Notch pathway which
favors which favors the late differentiation of glia.
Homeobox and paired domain transcription factors are often
used to restrict neurons to certain broad classes linked to
their position or coordinates of origin.
Finally, POU, LIM, and ETS domain transcription factors
may restrict cellular phenotypes even further.
Of the signaling molecules, there are particularly important
roles for Bone-Morphogen-Proteins (BMPs), FibroblastGrowth-Factors (FGFs), and Sonic-Hedgehog-Proteins
(SHH-Ps)
Cellular differentiation
From Wikipedia, the free encyclopedia
Cellular differentiation is a concept from developmental
biology describing the process by which cells acquire a
"type". The morphology of a cell may change dramatically
during differentiation, but the genetic material remains the
same,with few exceptions.
A cell that is able to differentiate into many cell types is known
as pluripotent. These cells are called stem cells in animals
and meristematic cells in higher plants. A cell that is able to
differentiate into all cell types is known as totipotent. In
mammals, only the zygote and early embryonic cells are
totipotent, while in plants, many differentiated cells can
become totipotent with simple laboratory techniques.
In most multicellular organisms, not all cells are alike. For
example, cells that make up the human skin are different from
cells that make up the inner organs. Yet, all of the different cell
types in the human body are all derived from a single fertilized
egg cell through differentiation
differentiation.
Differentiation is the process by which an unspecialized cell
becomes specialized into one of the many cells that make up
the body, such as a heart, liver, or muscle cell.
During differentiation,
differentiation, certain genes are turned on, or
become activated, while other genes are switched off, or
inactivated. This process is intricately regulated. As a result, a
differentiated cell will develop specific structures and perform
certain functions. Differentiation can involve changes in
numerous aspects of cell physiology; size, shape, polarity,
metabolic activity, responsiveness to signals, and gene
expression profiles can all change during differentiation.
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Finite amount of genetic material; infinite amount of
complexity
For the nervous system alone the estimate is the human nervous system is
composed of 100 billion neurons, 100 trillion synapses, a variety of
neurotransmitters, thousands of different fates in our nervous system, the blue print
for this lies in the genes
Epigenetic theory of development—
development—Waddington
Determination , Differentiation and Pattern Formation
Developmental
p
Potential;; Fate Maps
p
C. elegans lineage figure as an example of a fate map
Deterministic dev. Pot.= Dev. Fate or
Alternatively the cell interactions (the surrounding environment) are so reproducible
that specific cells always choose the same fate.
Progressive Reduction of the Dev. Potential of a cell or its progenitors
Differentiation is what a particular cell is. A variety of measures: Structural
measures, functional measures, biochemical...
Concept Map: Mechanistically the events responsible for cell specification
(determination and differentiation) are no different for neurons than other cell types
but the difference is the complexity of the nervous system.
Two general sources that have taken on a
number of different names:
names:
Intrinsic
Extrinsic
Nature
European
Within cell
Mosaic
Autonomous
Potential = Fate
Nurture
American
outside cell
Regulative
Non-autonomous
Potential > Fate
How do we begin to gain mechanistic explanations of these
processes?
First Is the mechanism cell intrinsic or extrinsic?
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Some of the basic techniques: Transplantation
Transplantation, a
progenitor from a donor animal is transplanted to a different
part of a host animal.
If the fate of the cell is unaltered by putting it in this new
environment , then the cell is “Autonomously determined”.
If, however, the cell adopts a new fate , consistent with the
position to which it was transplanted, then the fate at the time
of transplantation is still flexible and can be “Determined
Determined
non--autonomously”.
non
Putting cells into tissue culture is another valuable
technique: By isolating a cell from the embryo entirely, it is
possible to assay the state of determination of a cell in the
absence of all interaction. An advantage of this experimental
system is that the culture medium and substrate can be
controlled. In this way, potential extrinsic cues can be added
and assayed for their effect on the fate choice.
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A very informative approach for studying the process that
lead neurons down particular differentiation pathways, at
least in terms of identifying the factors that influence
determination is genetic manipulation, such as mutational
and transgenic analyses.
analyses
Mutations in particular genes can alter the fate of certain
types of neurons.
Genetics combine with transplantation or culture can
reveal whether normal phenotypes are extrinsically
regulated by the gene in question, as in the case of gene
that codes for a secreted factor,
factor or intrinsically
regulated, as in the gene that codes for the receptor to
such a factor.
factor
With the genetic approach it is possible not only to show
where and when the normal fate decisions are made but
also to identify the gene product in question.
question
Forward genetics uses random mutagenesis to define new
genes that have effects on neural differentiation, while
reverse genetics uses molecular engineering to knock out or
over--express particular genes that are candidates for roles in
over
neuronal fate determination and differentiation.
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Transcriptional Hierarchies in Invariant Lineage:
Time-lapse studies of the development of the nervous system
of nematode C. elegans show that every neuron arises from
an almost invariant lineage (Fate map). In this system, the
progenitors are uniquely identifiable by their position and
characteristic pattern of division.
Ablation of one of these p
progenitors
g
usually
y leads to the loss
of all the neurons in the adult animal that arise from that
progenitor, indicating neighboring cells cannot fill in the
missing fates. This is called “Mosaic development”
Nematode, C. elegans
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Complete lineage map of C. elegans
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Asymmetric cell divisions and Asymmetric
fate
-stem cell division just described.
How do you think daughter cells can realize different fates?
Extrinsic scenario different environments different fates,
cell-cell interactions…
Intrinsic scenario… in this case the two daughters would
have to inherit different intrinsic determinants.
How do daughters get different intrinsic determinants?
Epithelial cells and neuroepithelial cells are polarized, an
apical face and an basal face
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Conclusion:
Spatial and Temporal Coordinates of Determination:
There is a hierarchical pathway, rich in transcription
factors that operate through the specific lineages. The
factors regulate other intrinsic transcription factors in a
molecular cascade where by the lineage, the specification,
the differentiation, and finally the physiological properties of
the neurons are established through a successive stages.
stages
This molecular strategy is also used in the determination of
neurons in most other species.
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Controlled Interference what do you think that means?
Fig. 1 A Describes one type of controlled interference: Cell transplantationheterotopic
B. Cell culture and conditioned media to identify
C. a mechanistic explanation for B. Regulation
Genetic Approaches offer an alternative means for controlled interference. The
book describes work performed by Marty Chalfie and his colleagues at Cambridge
England Columbia University on the nematode C. elegans
Amenable for integrating genetic approaches and cell biology
biolog
Touch sensory neurons: Light Touch, mechanosensory neurons—AVM
Fig. 3 & 4 Default fates…
Important conclusions: Hierarchical pathway, rich in transcription factors
that operate through the specific lineages. Molecular cascade whereby the
lineage, the specification the differentiation and functional properties are
progressively established.
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Asymmetric cell divisions and Asymmetric fate:
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Epithelial cells and neuroepithelial cells are polarized, an apical face and an
basal face
At the apical pole of the NB is the Inscuteable complex (inscuteable and Bazooka)
The role of this complex is to bind the spindle with polar microtubules and orient the
spindle so that the mitotic is oriented vertically, cytokinesis is perpendicular
At the basal face of the cell membrane is a protein Miranda, Miranda binds all of
the Numb in the cytoplasm.
Notch signalingàmaintains the NB fate however in the presence of Numb. Notch
entry into the nucleus is blocked (brakes) and creates a permissive condition;
Prospero is also partitioned to the basal side the positive influence activating genes
necessary for GMC differentiation.
The compound eye of insects; Composed of an array of simple eyes, each is called
an ommatidium. A complex of 20 cells of which there are 8 Photoreceptors 18. These were named based upon position
R8 Hedgehog Notch signaling lateral inhibition; a regularly spaced pattern of R8s,
which then produce atonal causes the recruitment of 2 & 5, and they produce
rough, which recruits 3 & 4; another cascade of signaling molecules and
transcription factors à1 & 6 and finally 7.
Genetic approach to crack this complex sequence began with mutagenesis that
produced animals that were insensitive to UV. Seven or a cone
In addition to Notch, R8 produces an EGF-like signal that activates RTK paths.
Sevenless also genera
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Spatial and Temporal Coordinates of Determination
A molecular coordinate system in which the NBs are made unique based upon
their A/P position and their D/V position.
Fig. 5 A Neuroblasts labeled with an antibody to Snail
5B NB labeled with three different antibodies to the different NB specific proteins
HB, Eagle and Castor showing first the similarity, and second the unique position
specific and segmental pattern.
An important point is that every one of the Nbs has a unique postion in an X-y
coordinate system and a unique state of transcriptional activity.
Fig. 4.5 (or ppt 6)
D&E
Each NB undergoes several rounds of a special type of division called stem cell
division, each one regenerates the NB and a GMC. Controlled
Interference: necessary
Sufficient
Expts have shown that the expression of the successive tc factors is linked to the
cell cycle, which functions as a kind of clock, since blocking the cell cycle blocks
the succession and reactivating it reactivates the succession.
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Specification and Differentiation Through
Cellular Interactions and Interactions with the
Local Environment:
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Competence and Histogenesis:
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Interpreting Gradients and the Spatial Organization of
Cell Fate:
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The Interplay of Intrinsic and Extrinsic Influences
in Histogenesis:
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