Neuronal progenitors are stem cells

1
Class 17 Vertebrate neuronal development
Neuronal progenitors are stem cells
dividing neuronal progenitor
postmitotic neuron
2
In the ventricular zone (cells right around the
lumen of the neural tube), cells migrate back and
forth between the apical and basal layer
Stages of cell cycle
G1 S G2
M

Basal surface
Lumen of neural tube (apical
surface)
12.14
3
There are two kinds of division within the
ventricular zone
Symmetric vertical division plane Both cells
stay in contact with apical surface, both
re-enter cell cycle
4
What determines the plane of division? Unknown!

• Cell that will migrate away and differentiate
  into a neuron expresses Notch
• --distinguishes it from a stem cell, allowing it
  to leave VZ?
• Cell that remains at apical surface retains
  expression of a
• protein called Numb (cytoplasmic factor)
• (Numb actually prevents expression of Notch)

5
Maturation of the cerebral cortex from the neural
tube a model system for studying how neuronal
cell fate is determined
Fig 12.15

12.15
6
Organization of the cerebral cortex
1 2 3 4 5 6
Project to other targets in cortex
Layers of the cortical plate
Project to subcortical targets (thalamus)
frontal
parietal
A
P
occipital
cerebellum
temporal
Brain stem
7
To study cell fate in the cortex, one needs to be
able identify a dividing cell early on, and then
later determine its identity (what it has
differentiated into). A common way to do so is
to inject tritiated thymidine into the dividing
area of the neural tube

• Tritiated thymidine (an analog of the DNA base
  thymidine) is
• Incorporated into a dividing cell, and retained
  only in that cell
• Incorporated into a dividing cell this cell and
  its progeny will have the label

8
Neurons migrate to form cortical layers
superficial
deep
(This example is from monkeys)

• What does this experiment tell you? (.5 pts)
• The superficial layers form first
• The deep layers form first
• The cells are committed to make a particular
  layer before they migrate
• The cells are not committed until after they
  migrate

9
Neurons migrate inside-out to form cortical
layers

• deep layer cells are born first
• superficial cells are born last

(these experiments used ferrets)
12.19
10
How are these cell fates determined?
To find out, do transplantations Move progenitor
from donor into a different environment (time)
heterochronic transplant Does the cell take on
its normal fate (intrinsic)? or Does it take on
the fate of the host cells (environment)?
11
Transplant precursor from ventricular zone of E29
donor (early) to ventricular zone of P1 host
(late)
Cell transplanted after S phase
Cell transplanted in S phase

• What does this experiment tell you?
• The precursor cells are committed to their
  laminar fate
• The precursor cells are not always committed to
  their laminar fate

12.19
12
A progenitor cell can respond to its environment
if still in S phase
Cell transplanted after S phase
Cell transplanted in S phase
13
Is this plasticity maintained over time?
Transplant precursor from P1 (post natal day 1)
donor to E29 (embryonic day 29early stage) host

• What does this allow you to conclude about
  cortical progenitor cells? (.5 pts)
• There must be some intrinsic change in the
  precursor cells over time
• There could be a change in the precursors
  environment over time
• The fate of a precursor cell is restricted over
  time
• All of the above

14
Time changes the potential of the precursor cells
(progressive restriction of cell fate)
--Probably due to different tx factors being
synthesized at different times --Also possibly
due to different signaling molecules in the
environment
Cell cannot change its fate goes to layer 2/3
even though host cells are going to 5/6
15
Read over the experiment on the handout. Work
with your group to answer the following
questions. Write your answers on a piece of
paper (as a group!) and turn them in for
participation credit. 1. What is significant
about the difference between conditions A and
B? 2. What do you conclude from this experiment?
16
E29 cells (early), labeled with 3H, removed and
cultured in different environments, then
implanted into VZ of P1 (late) host
Culture in low density
Culture as pellet
Culture in low density after allowing contacts in
vivo
Culture as an explant
1 2/3 4 5 6
1 2/3 4 5 6
1 2/3 4 5 6
1 2/3 4 5 6
3H Thymidine labeled cells
Superficial fates
Mostly Superficial fates
More deep fates
Deep fates
Conclusion cell-cell contacts are required for
deep layer fate Perhaps their default is actually
layer 2/3 (superficial)!
17
Summary Cortical neuronal progenitors are
influenced by both environment and intrinsic
factors
Environmental factors Environment changes over
time i.e. Cell contacts (signaling molecules)
Intrinsic factors S phase of division (changes
in chromatin?) Time (changes inside of the
cells, i.e. different levels of transcription
factors)
18
Neural crest cells unique cells of ectodermal
origin that ultimately populate many different
locations and tissues
19
The Neural cresta unique population of cells
20
Neural crest Cranial, Cardiac, Trunk Each type
makes specific derivatives and has a stereotyped
pathway of migration
Spinal cord
Somites
13.2
21
Some cranial neural crest derivatives
22
Cardiac neural crest derivatives
23
Migrating neural crest cells (stained with an
antibody, HNK-1) follow specific pathways to get
to specific locations
Neural tube
These neural crest cells make sensory neurons in
the dorsal root ganglion and non-neuronal
melanocytes
Anterior part of somite
Posterior part of somite
13.3
24
RhoB (green) HNK-1 (red)
13.4
25
Localized molecules direct the migration of
specific neural crest cells through the anterior
region of the somites
Permissive Extracellular matrix molecules in
anterior (like thrombospondin, fibronectin,
laminin)
Inhibitory Ephrins act to inhibit
migration Ephrins are only localized to the
posterior half of each sclerotome
These mechanisms of guidance for the cells
migration are very similar to the mechanisms of
guiding axonal outgrowth
13.5
26
Are the neural crest cells a homogeneous
population of pluripotent cells? Or are they a
heterogeneous population of already determined
cells?
Take a group of cells and transplant them to a
new location
27
The neural crest are migratory cells that have
multiple fates
radiolabeled donor
host
Melanocytes
Neural tube
Dorsal root ganglion
Sympathetic ganglion
Kaltoff, 13.25
28
Transplantation experiments
heterotopic
heterochronic

• Do these experiments indicate that individual
  neural crest cells are capable of changing their
  fate?
• Yes
• No

29
Clonal analysis labeling of individual neural
crest cells
13.5
30
Summary Neuronal progenitors and Neural crest

• Start out multipotent
• Some restriction exists initially (in neural
  crest,
• for example, trunk cannot make cardiac and
  cranial fates)
• Potency is restricted over timepossibly changes
  in transcription factors
• Fate is also impacted by molecules in the
• environment (primarily growth factors and
  signaling molecules, like Wnts and BMPs)