Neurogenesis

Question 1

Stages of neurogenesis

Answer 1

birth, migration, differentiation

Question 2

Where does neurogenesis occur?

Answer 2

It occurs at
the ventricular surface of
the developing brain

During early develop this region is very thin

Ventricular zone – contacting ventricle
Marginal zone (dorsal)

Question 3

Where are neural progenitors?

Answer 3

Neural progenitors are in
the ventricular zone.

Progenitor cell for
neurons and astrocytes of
CNS

Known as radial glial cells

Question 4

What are the steps of neurogenesis?

Answer 4

1. Radial glial cell extends process to reach towards the pia at the surface of the brain
2. Interkinetic nuclear migration – DNA is replicated
3. Radial glial cell retracts its apically extending arm
4. Cell division
5. Migration of neuroblast from ventricular surface

Question 5

What is 1-3 interkinetic
nuclear migration?

Answer 5

Process by which the nucleus of a radial glial cell migrates away and back towards the ventricular zone.
Necessary for DNA replication and ultimate cell division.

Question 6

Describe symmetrical division

Answer 6

Happens early in development
Expands progenitor population
During symmetric divisions no neuroblasts are formed

Question 7

Describe asymmetrical division

Answer 7

Happens later in development
Maintains progenitor pool size
Produces one neuroblast and one progenitor from each division

Majority of neocortical neurons born between 5 weeks and 5 months

Peak neurogenesis rate: 250,000 new neurons per minute

Question 8

Schematic of the development of the
embryonic neocortex

Answer 8

• VZ – ventricular zone
• iSVZ – inner subventricular zone
• oSVZ– outer subventricular zone
• IZ – intermediate zone
• CP – cortical plate
• MZ – marginal zone
• aRG – apical radial glial cells
• bRG - basal radial glial cells
• IP – intermediate progenitor

Question 9

Subventricular zone

Answer 9

Specific to primates
Second layer of proliferative progenitors
Gives rise to upper cortical layers
Hypothesized to increase connectivity between diverse neural areas through bridging neuronal projections that concentrate in the upper layers

Question 10

What cells maintain
the ability to divide?

Answer 10

Daughter cells from an asymmetric division will
never divide again
Some progenitor cells are maintained into
adulthood
Vast majority of neurons you are born with are
what you have to work with your whole life

Stem cells are a new and fascinating area of investigation

Question 11

Originally thought no new neurons were born after adulthood

The discovery of
adult
neurogenesis

Answer 11

1965: Altman and Das provided evidence that adult neurogenesis occurs in mammals
1983: Goldman and Nottebohm showed adult
neurogenesis in canaries
Radiolabeled thymidine (T) incorporates into DNA during replication. .Slice brains and expose to film. Radiation will work like light to create “exposure” of the film
1996: Kuhn, Dickinson-Anson, and Gage showed adult neurogenesis in mammals again, this time using BrdU.

Question 12

Can be incorporated instead of thymidine into replicating DNA

How does BrdU work?

Uridine analog

Answer 12

Add BrdU during fixed time period
Animals – inject
Cells – bath
BrdU incorporates into DNA during replication as a thymidine analog
Antibodies again BrdU for immunolabeling of DNA that includes BrdU

Question 13

Nobel Prize in chemistry 2022: Click chemistry

Using EdU instead of
BrdU

Modernized DNA labeling

Answer 13

Copper catalyzes binding of substrates.
Fluorescently label DNA that has EdU incorporation.

Question 14

Neurogenesis in the human brain

Postmortum brains of humans were analyzed

Nuclear bomb tests released Carbon-14 into the
environment, essentially labeling biological materials
from the fallout

Answer 14

Using carbon dating, Spalding, Frisèn, and colleagues
found cortex from these people did not show labeling
but hippocampus did.

700 neurons are turned over in the hippocampus each day (~2%/year)

Question 15

DNA transcribed to RNA in nucleus

RNA spliced and exported to cytoplasm

How do cells know whether to be
a progenitor or a daughter?

RNA translated to protein via ribosomes
Ribosomes seated on Rough ER

Protein is packaged and actively moved to correct location.

Answer 15

During division, asymmetric compartmentalization
of particular proteins (and probably RNAs) leads to
fate determination

Transcription, translation, and compartmentalization govern

One radial glial progenitor can produce neurons and glia

Question 16

Cell migration in the developing cortex

Once a neuroblast is born, how
does it get where it needs to go?

Answer 16

Cortical migration from ventricular zone of dorsal telencephalon to cortex which gives rise to pyramidal
neurons and astrocytes.
Tangential migration from ventricular zone of the ventral telencephalon to cortex which gives rise to inhibitory interneurons and oligodendrocytes

Question 17

Migration of neuroblasts occurs along radial glial fibers

Cortical migration

Have a leading process, nucleus, and trailing process

Nucleus is pulled to leading process as it migrates up radial glial
fiber to cortex

Answer 17

Involves the Subplate:
Transient structure
Contains a variety of neurons
Most are glutamatergic
Responsible for early motor behavior
Role in defining cortical regions for sensory processing

First cells to migrate from the ventricular zone are the subplate cells

Activity onset 9-10 weeks post-conception in humans. Thickest 28-34 weeks post-conception. Most active fetal brain
area. Subplate is gone by 3 months post-birth

Question 18

Inside out cortical assembly

Next cells to migrate make up layer VI
Then layers V,IV,III,II, I

Answer 18

Inner layers are set up first and cells migrate past them to form the next layer

Mutations can disrupt this assembly: Reelin mutant

Question 19

Specific to late born cortical neurons

What does Reelin do?

An extracellular matrix glycoprotein.
Protein bound by oligosaccharides

Secreted by Cajal-Retzius neurons in the marginal zone

Answer 19

Binds to very low density lipoprotein receptor (Vldlr) and Apoer2

Vldlr – reenlin binding is a stop signal for migration
Apoer2 – reelin binding is a go signal for migration

Question 20

How was reelin discovered?

Spontaneous mutation in a mouse line

Answer 20

Named reelin because of its abnormal gate.
Identified in Edinburgh in 1951

Histopathological analysis showed cortical laminar organization deficit

Abnormal reelin expression linked to a variety of psychiatric disorders: Schizophrenia, autism, Bipolar disorder, Alzheimer’s.
Whether there is a causal link is unclear.

Question 21

Tangential migration

Gives rise to inhibitory interneurons and oligodendrocytes

Answer 21

Does not use radial glia
Neurons maintain a stellate shape and are highly individually motile
Also some evidence for chain migration
Migrate from the ganglionic eminences

Question 22

Medial ganglionic eminence

Answer 22

Produces GABAergic interneurons that migrate to cortex

Question 23

Lateral ganglionic eminence

Answer 23

Produces GABAergic interneurons that migrate in the rostral migratory stream to the olfactory bulb

Question 24

Caudal ganglionic eminence

Answer 24

Also produces inhibitory interneurons
that migrate to cortex

Question 25

Occurs immediately following neural precursor (neuroblast) migration

Neuronal differentiation

Answer 25

Layer IV neurons differentiate before Layer III neurons migrate through.
Stepwise: Neurite outgrowth, Axon and dendrite specification, Target selection and stabilization, and Synapse formation.
Differentiation is regulated by intracellular and extracellular signals.

Cultured hippocampal neuron after 8 days in vitro (DIV) expressing green fluorescent protein (GFP)

Question 26

Radial Unit hypothesis

Answer 26

The idea that progenitor cells in the ventricular zone give rise to columns of cortical neurons with connected fates

This would mean that a cortical ”protomap” exists in ventricular zone

1/3 of all neurons do not migrate up radial glia

Question 27

Transcriptional patterning of
cortical areas

profiles

Neurons in discrete cortical regions have transcriptional

Answer 27

Emx2 and Pax6 example
* Anterior cortex expresses Pax6
* Posterior cortex expresses Emx2
* This is essential for cortical patterning

Altering expression alters cortical patterning

Emx2 mutant
* Expand motor and reduce visual. Somatosensory moves
posterior

Pax6 mutant
* Expand visual and reduce somatosensory and motor. Anterior shift of auditory and somatosensory

Question 28

Normally, LGN of thalamus projects to visual cortex

What happens if you eliminate LGN input?

Projections also help to pattern the cortex

Input is clearly necessary for cortical differentiation

Answer 28

Reduce the size of visual cortex
Arrows point to the edges of visual cortex in primate with and without LGN input

Typically ventral posterior thalamus projects to somatosensory cortex

In rats, somatosensory cortex is arranged in “barrels”
Barrels correspond to whisker fields

Question 29

Rat cortex to address the effect of innervation on cortical patterning

Cortical transplants address
sufficiency

Answer 29

In rats, thalamic fibers do not enter cortex until after birth
Pealed off somatosensory cortex (area 3) and replaced it with visual cortex (area 17)

Transplanted visual cortex develops barrels

Evidence that cortical architecture can be shaped by thalamic projections