Neural Development

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How do cells become nerve cells?– Environmental factors

• Inductive events

– Genetic factors• Competence

Cell lineage

Timing

Focus on the cortex

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The cortex has 6 layers---how do they develop?

superficial

deep

3

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

ventricular zone

intermediate zone

cortical plate

marginal zone

radial glia

4http://www.stanford.edu/group/skmlab/movies/qtmovies/rad_migr.mov

precursor

neuron

precursor

Cell cycle

S

M G1

G2

At mitosis, precursor self-renews or differentiates

Choice point for the precursor

precursor

precursor

5

precursor

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

6

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.

time3H-thymidine

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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)

timevirus

Basic Techniques in Neurobiology:

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CP

IZ

VZ

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3456

Expt: Use lineage tracing to label single neurons and follow their progeny.

Result: Single cells generate neurons in multiple layers.

This result disqualifies the lineage model.

Are there layer-specific progenitors?

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1

2/3

4

5

6

Born Last

Born First

“Inside First-Outside Last” development of cortical layers

Different layers form at different developmental times

Post-natal Day 2 (P2)

Embryonic Day 26 (E26)

Dev

Tim

e

10

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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?

Most are layer 4 Most are layer2/3 Most are layer 2/3

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?

Does the environment instruct the layer fate?

Early cells can take on later fateModel: Environment influences cell-fate

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E36 = Early P2 = Later Early into Later

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?

Does the environment instruct the layer fate?

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Early Earlier Early in to earlier

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

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Both the environment and the progenitor determine cell fate

A) the age of the environment restricts cell-fateB) the age of the progenitor restricts cell-fate

MODEL OF PROGRESSIVE RESTRICTION

Events inside the cell and outside influence layer decision

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

One extracellular cue that influences cortex formation

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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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precursor

neuron

precursor

Cell cycle

S

M G1

G2

Decision point for the precursor

18

precursor

neuron

precursor

Cell cycle

S

M G1

G2

Choice point for the precursor

precursor

precursor

5

precursor

General Principle:An intracellular timing mechanism regulates cell fate decisions

• At mitosis, precursor continues to self-renew or differentiates

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A Timing Mechanism for Neural vs Glial Fate

precursor

Glial markers

Neural markers

A single precursor cell will first produce neurons then glia

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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

Transcriptionfactors

“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.”

Every cell in C elegans is determined by lineage

Sydney Brenner 2002 Nobel PrizeGenetics of Development in C elegans

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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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ADAL AB.plapaaaapp Ring interneuronADAR AB.prapaaaapp Ring interneuron ADEL AB.plapaaaapa Anterior deirid, sensory neuron ADER ABprapaaaapa Anterior deirid, sensory neuron ADFL AB.alpppppaa Amphid neuron ADFR AB.praaappaa Amphid neuron ADLL AB.alppppaad Amphid neuron ADLR AB.praaapaad Amphid neuron AFDL AB.alpppapav Amphid finger cell AFDR AB.praaaapav Amphid finger cell AIAL ABplppaappa Amphid interneuron AIAR AB.prppaappa Amphid interneuron AIBL AB.plaapappa Amphid interneuron AIBR AB.praapappa Amphid interneuron AIML AB.plpaapppa Ring interneuron AIMR AB.prpaapppa Ring interneuron AINL AB.alaaaalal Ring interneuron AINR AB.alaapaaar Ring interneuron AIYL AB.plpapaaap Amphid interneuron AIYR AB.prpapaaap Amphid interneuron AIZL AB.plapaaapav Amphid interneuron AIZR AB.prapaaapav Amphid interneuron ALA AB.alapppaaa Neuron, sends processes laterally ALML AB.arppaappa Anterior lateral microtubule cell ALMR AB.arpppappa Anterior lateral microtubule cell

Neurons 1-25 in C. elegans

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Name Cell divisions Cell fate

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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 celldetermine its fate

3. The time in which you are born will determine what you become