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BIPN 140: FINAL REVIEW
LECTURES 14-16
LECTURE 14: NEURAL INDUCTION +
PATTERNING
•  Cytoplasmic determinants
•  Inductive signals
•  Neural tube development
•  Spemann Mangold experiment
•  Neural induction
•  Morphogens – Shh and BMP
•  Cortical differentiation
•  Neuronal survival
Cytoplasmic Determinants Begin to Differentiate Cells
WHAT INFLUENCES
DIFFERENTIATION?
from the First
Mitotic Division
• 
• An egg’s cytoplasm contains RNA, proteins,
and other substances
that are distributed
Cytoplasmic
determinants
unevenly in the unfertilized egg
•  during first mitotic division, due to
• Cytoplasmic determinants are maternal
unequal
distribution
of cytoplasmic
substances
in the egg that
influence early
determinants
development (maternal substances
in• the
egg
that
influence
early
As the
zygote
divides
by mitosis,
cells
development)
contain different cytoplasmic
determinants, which lead to different gene
expression
•  Inductive signals
•  signaling molecules that direct the
expression of specific genes that
determine a cell’s differentiation
•  Can be secreted or cell-tethered
Unfertilized egg cell
Sperm
Fertilization
Two different
cytoplasmic
determinants
Zygote
Mitotic
cell division
Two-celled
embryo
Campbell, Biology, Fig. 18.15a
Gastrulation: specification of the three major germ layers
Summary of Neural Tube Formation in Vertebrates
Eye
Neural folds
Neural
fold
Somites
Tail bud
Neural plate
SEM
1 mm
1 mm
Notochord
Neural
crest
cells
Coelom
Somite
Neural tube
Neural Neural
fold
plate
Neural crest
cells
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
Archenteron
(digestive
cavity)
Outer layer
of ectoderm
Neural crest
cells
(c) Somites
(a) Neural plate formation
Neural tube
(b) Neural tube formation
Campbell Biology, Fig. 47-12
SPEMANN-MANGOLD
EXPERIMENT
Inductive signals control cell differentiation (“cell fate”):
The Spemann-Mangold discovery of an ‘organizer region’
transplanted mesodermal tissue caused a dramatic change in the fate of the host ectoderm
Inducers: signaling molecules (diffusible or cell-tethered) that direct the
expression of specific genes that determine a cell’s differentiation
Competence: the ability of a cell to respond to inductive signals;
determined by its repertory of receptors, transduction molecules,
transcription factors
INDUCTIVE SIGNALS CONTROL CELL
DIFFERENTIATION
•  What determines competence?
•  ability of a cell to respond to inductive signals,
determined by presence of receptors, transduction
molecules, transcription factors
•  If a cell is incompetent to an inductive signal, will there
be an effect?
•  No, because it does not have the machinery capable to
induce the desired effect.
•  What was the main discovery of the Spemann Mangold
Experiment?
•  Organizer regions, which influence the differentiation of
other regions
Neural Induction Involves Inhibition of Bone Morphogenic Protein Signals
Default pathway for ectoderm is to become neural tissue, unless
it receives a BMP signal
Once neural induction has occurred, two systems pattern the dorsoventral and
rostral-caudal axes
Sonic hedgehog (SHH) and BMP are Morph
MORPHOGENS
signals that act in a gradient to pattern the
tube along its dorsoventral axis
How do morphogens act?
BMP
•  A type of inductive signal,
that is secreted and diffused
in a gradient fashion
•  Ligand secreted from cell in
gradient, binds to receptor
on another cell, induces
signal transduction pathway
to alter gene transcription,
allows for differentiation
SHH
•  Used in?
•  Patterning the neural tube
along D-V axis
Neuronal Birthdating Reveals the ‘Inside-Out’ Development of the Cortex
oldest neurons are deepest
there is a systematic relationship between cortical layers and the time of neuronal origin
Neurons Migrate Past Older Neurons to Reach the Pial Surface
•  What drives the upward migration and nice layering
of the cortical neurons?
•  Reelin secreted from the Cajal-Retzius cells in
marginal zone
•  What would happen if Reelin were knocked out?
•  Layering would be messed up, causing
lissencephalies, or regions of smooth brain
NEUROTROPHIC FACTOR HYPOTHESIS
t Tissues Supply Signals that Regulate Neuronal Survival
•  What are trophic
factors?
•  Molecules secreted by
target tissues, essential
for neuronal survival
•  Creates competition
for essential resources
•  What would happen if
a limb bud were
added? Taken away?
APOPTOSIS – PROGRAMMED CELL
DEATH
•  Why is apoptosis the preferred method of cell
death?
•  Very neat and orderly. Components of cell are
chopped and packaged into vesicles that are
digested. This prevents damaging enzymes from
leaking out and affecting neighbors.
A balance between TrkA & p75NTR signaling mediates the balance
between death & survival
LECTURE 15: AXON AND DENDRITE
DEVELOPMENT
•  Cellular morphogenesis of neurons
•  Growth cones
•  Actin
•  Myosin
Stages in Cellular Morphogenesis of
Neurons
Neurons undergo distinct stages of morphogenesis that are recaptiulated in cell
culture
Stage 1-2. Neurite initiation (condensation of lamellipodia, process
extension)
Stage 2-3. Establishment of polarity (axonogenesis)
Stage 3-4 Dendrite maturation (elongation & branching
Stage 4-5 Extensive branching of axons & initiation of Synaptogenesis
Stage 5-6 Maturation of synapses (e.g., emergence of dendritic spines at
glutamate synapses; stabilization)
modified from Dotti et al 1988
Example: cultured rat hippocampal neurons
1
2
3
4
‘minor neurite’
nascent axon
0.25
0.5
1.5
5
6
Stage
dendrites
4
>7
>14 days in vitro
GROWTH CONES
What is a growth cone?
•  Specialized structures
found at the tip of
extending axons that
enable axon guidance
and neurite development
by detecting and
responding to signaling
molecules
•  Made of actin and
microtubules
What imparts motility and
changes in cell shape?
•  Dynamic instability
•  Ability of the cellular
components
(microtubules and
actin) to polymerize
and depolymerize or
grow and shrink
Domains & Structures of the Growth Cone
Filopodia = F-actin bundles
Lamellipodia = F-actin meshwork
Actin arcs = contractile bundles
Microtubules
Actin Filaments assemble from actin monomer subunits
Microtubules assemble from tubulin dimer subunits
The Growth Cone ‘Wrist’ is an Important Region for
Organizing MTs into Bundles
actin arcs use myosin II to compact MTs into bundles; MAPs also are important,
both for crosslinking and for coupling regulatory molecules to the cytoskeleton
LECTURE 16: AXON GUIDANCE
•  Sperry’s experiment
•  Chemoaffinity hypothesis
•  Chemotropic gradients
•  Target selection in axon guidance
•  Guidance cues
Topographic representation in the visual system
Eye rotated
SPERRY’S EXPERIMENT
What conclusions did he
draw?
Is this experience
dependent? Why?
•  Chemotropic gradients
present in the tissues
that drive neural
connectivity
•  Specifically found in
topographic mapping
of the retina to code
spatial specificity
•  No, even after rotating
the eye the pattern of
connectivity is still the
same.
•  What signal mediates
retinotopic mapping?
•  Eph/Ephrin signals
Chemotropic Gradients Guide Axons to Targets with Spatial Specificity
Eph receptors reside on
retinal neurons that
project to the tectum
neurons from nasal
(anterior) retina
Ephrins are supplied in a
gradient by tectal tissue
neurons from temporal
(posterior) retina
Example: Eph/ephrin signals in the tectum repel axons from the temporal but not nasal retina
Finding Your Way Home:
an analogy for the stages of target selection in axon guidance
1. Long-distance navigation: follow the correct highway
2. Coming off highway
3. Turning into your area
4. Turning into your street
5. Find the lock on the door
Axons often travel complex paths to their destinations
example: retinotectal axons
retinal ganglion cells
1. axons make ‘choices’ at multiple steps in their journey
2. different axons make different choices
Holt movie of RGC outgrowth
Axon guidance molecules for brain wiring
Reference: Kolodkin and Tessier-Lavigne, Cold Spring Harb Perspect Biol. 2010
Advancing Growth Cones Encounter a Variety of Guidance Cues
‘static’
‘soluble’
guidance cues can be either soluble or ‘static’ (fixed to ECM or cell surfaces)
guidance cues can be either ‘attractive’ or ‘repulsive’