Download 02/28 PPT - Molecular and Cell Biology

Neural Development
How do cells become nerve cells?
– Environmental factors
• Inductive events
– Genetic factors
• Competence
Cell lineage
Timing
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Focus on the cortex
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The cortex has 6 layers---how do they develop?
superficial
deep
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Cortex Development: Formation of layers
• neural progenitors rapidly divide in the ventricular zone
• they keep dividing to produce more progenitors OR
• they undergo terminal differentiation and migrate to
final cortical destination
• migrating neurons move along radial glia
radial glia
marginal zone
cortical plate
intermediate zone
ventricular zone
http://www.stanford.edu/group/skmlab/movies/qtmovies/rad_migr.mov
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At mitosis, precursor self-renews or differentiates
Choice point for the precursor
Cell cycle
M
G2
G1
precursor
neuron
precursor
precursor
precursor
precursor
S
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How do layers of the cortex form?
Are there different kinds of progenitors?
(layer 1-specific progenitor)
Does one progenitor make all cell layers?
LINEAGE MODEL
1
2/3
4
5
6
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Basic Techniques in Neurobiology:
Neuronal Birthdating with 3H-thymidine
• 3H-thymidine is incorporated into the DNA during replication
• It marks all mitotic cells
• It disappears in ~2-4 generations.
3H-thymidine
time
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Basic Techniques in Neurobiology:
Lineage Tracing
• Use retrovirus (incorporated into a cell’s genomic DNA but not
infectious)
• The virus DNA will be inherited by all the daughter cells (it
doesn’t become diluted)
virus
time
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Are there layer-specific progenitors?
Expt:
Use lineage tracing to label single neurons and follow
their progeny.
CP
IZ
VZ
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2
3
4
5
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Result: Single cells generate neurons in multiple layers.
This result disqualifies the lineage model.
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Different layers form at different developmental times
“Inside First-Outside Last” development of cortical layers
Born Last
1
Post-natal Day 2 (P2)
Dev Time
2/3
4
5
Born First
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Embryonic Day 26 (E26)
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OBSERVATION: A single progenitor first produces
layer 6 neurons, later produces layer 1
How does one progenitor produce different cell fates?
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Does the environment instruct the layer fate?
Expt: Transplant Progenitor cells from early stage to late stage
What would they normally become?
What do they become when the host environment is changed?
E36 = Early
Most are layer 4
P2 = Later
Most are layer2/3
Early into Later
Most are layer 2/3
Early cells can take on later fate
Model: Environment influences cell-fate
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Does the environment instruct the layer fate?
Expt: Transplant Progenitor cells from late stage to early stage
What would they normally become?
What do they become when the host environment is changed?
Early
Earlier
Early in to earlier
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Summary of many transplantation experiments
Put early progenitor into late host environment, takes on late fate
Put late progenitor into early host environment, takes on late fate
Both the environment and the progenitor determine cell fate
A) the age of the environment restricts cell-fate
B) the age of the progenitor restricts cell-fate
MODEL OF PROGRESSIVE RESTRICTION
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Events inside the cell and outside influence layer decision
One extracellular cue that influences cortex formation
Reeler mutant mice (missing Reelin protein)
• Identified because of severe movement disorders
• Mutation in an extracellular matrix protein (?)
• Cortex is built outside in
• 26 human disorders associated with cortex malformation
• One form of lisencephaly (smooth brain) is a Reelin mutant
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Cortex development in Reeler mutant mice
• Precursor cells never separate from the cortical plate
• Grows Top down instead of Bottom up
Normal
Reeler
Model: Reelin is an important extracellular cue that
separates verticular zone from marginal zone
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Decision point for the precursor
Cell cycle
M
G1
neuron
G2
S
precursor
precursor
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General Principle:
An intracellular timing mechanism regulates cell fate decisions
• At mitosis, precursor continues to self-renew or differentiates
Choice point for the precursor
Cell cycle
M
G2
G1
precursor
neuron
precursor
precursor
precursor
precursor
S
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A Timing Mechanism for Neural vs Glial Fate
precursor
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A single precursor cell will first produce neurons then glia
Neural
markers
Glial
markers
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A single neuroblast clone will produce many different cells
• In Drosophila, sequential expression of transcription factors
induces different fates
• Expression of transcription factors is controlled by cell cycle
Transcription
factors
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Every cell in C elegans is determined by lineage
Sydney Brenner 2002 Nobel Prize
Genetics of Development in C elegans
“In late 1962, Francis Crick and I began a long series of conversations
about the next steps to be taken in our research.
Both of us felt very strongly that most of the classical problems of molecular biology
had been solved and that the future lay in tackling more complex biological problems.”
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Why C. elegans?
• Reproduce sexually and asexually
• Each worm makes 200 eggs
• Develops from egg to larvae in 12 hrs
• Total number of cells is 1000
• Total number of neurons is 300
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Neurons 1-25 in C. elegans
Name
ADAL
ADAR
ADEL
ADER
ADFL
ADFR
ADLL
ADLR
AFDL
AFDR
AIAL
AIAR
AIBL
AIBR
AIML
AIMR
AINL
AINR
AIYL
AIYR
AIZL
AIZR
ALA
ALML
ALMR
Cell divisions
AB.plapaaaapp
AB.prapaaaapp
AB.plapaaaapa
ABprapaaaapa
AB.alpppppaa
AB.praaappaa
AB.alppppaad
AB.praaapaad
AB.alpppapav
AB.praaaapav
ABplppaappa
AB.prppaappa
AB.plaapappa
AB.praapappa
AB.plpaapppa
AB.prpaapppa
AB.alaaaalal
AB.alaapaaar
AB.plpapaaap
AB.prpapaaap
AB.plapaaapav
AB.prapaaapav
AB.alapppaaa
AB.arppaappa
AB.arpppappa
Cell fate
Ring interneuron
Ring interneuron
Anterior deirid, sensory neuron
Anterior deirid, sensory neuron
Amphid neuron
Amphid neuron
Amphid neuron
Amphid neuron
Amphid finger cell
Amphid finger cell
Amphid interneuron
Amphid interneuron
Amphid interneuron
Amphid interneuron
Ring interneuron
Ring interneuron
Ring interneuron
Ring interneuron
Amphid interneuron
Amphid interneuron
Amphid interneuron
Amphid interneuron
Neuron, sends processes laterally
Anterior lateral microtubule cell
Anterior lateral microtubule cell
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Lessons in Development
1. Lineage is an important factor in development
(Different cell fates can be determined by cell division)
2. The cortex develops through the
process of progressive restriction
cues in the environment and within the cell
determine its fate
3. The time in which you are born will
determine what you become
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