Flashcards in Lecture 2 - Early Development of the Nervous System Deck (141):
Invagination at specific site in the blastula leads to the formation of the three different tissue layers (ectoderm, mesoderm, endoderm)
Gastrulation defines the
midline, anterior-posterior and dorsal-ventral axes of the embryo
By the end of gastrulation
the midline of the embryo is defined, defined by formation of the notochord, critical for formation of all tissue including the CNS
When does early neurulation start?
At around 18 days
Early Neurulation is coincident with
gastrulation signaling events, the neural ectoderm is induced.
Notochord formation is
central to gastrulation
How is notochord formation central to gastrulation?
by defining the midline of the embryo and inducing the formation of neural ectoderm.
What is the very first event in neurogenesis?
Neural ectoderm are
the neural precursor cells
How do you go from ectoderm to epidermis?
How do you go from ectoderm to neuroectoderm?
Bone morphogenic protein (BMPs) subclass of the TGFβ (transforming growth factor beta) family
What are BMPs produced by?
surrounding tissue (mesoderm, etc.).
What do BMPs do?
they push ectoderm towards an epidermal state
Factors that inhibit BMP signaling are produced by
the notochord (Chordin, Noggin, etc.) and therefore block BMP signaling in ectodermal cells overlying the notochord (midline cells).
Blocking of BMP signaling does what?
induces cells to take on a neural fate
What is the default path for ectoderm?
neural fate can be seen as the default (in the absence of signaling cells will adopt a neural fate). For example if ectodermal pecursor cells are isolated and grown in a dish, they become neurons.
What do BMPs bind to?
receptor serine kinases and a SMAD complex
What happens after BMP binds to the receptor serine kinases?
The SMAD is transported to the nucleus to mediate transcription.
BMP activity drives formation of
Where are Chordin, Noggin and Follistatin produced?
in the notochord
What do Chordin, Noggin and Follistatin do?
they inhibit BMP signaling and lead to neural induction.
Neural inducers act as inhibitors of
BMPs, Nodal and Wnt signaling promote embryonic stem cell differentiation to committed neural stem cells.
Stimulation of what also induces neural stem cell formation.
retinoic acid (RA), fibroblast growth factor (FGF) and insulin-like growth factor (IGF)
What are required for neural induction?
Coordination of multiple signaling pathways
In neural induction, what comes first – FGF or BMP?
FGF signaling precedes BMP inhibition during neural induction.
fibroblast growth factor (FGF) stimulation increases production of Noggin, which in turn inhibits BMP signaling.
What happens after neural induction?
the lateral margins of the neural plate fold inward to form the neural tube. This proceeds very rapidly.
Cells that make up the neural tube are
neural stem cells.
What are the distinct areas that are noted in the formation of the neural tube?
the floor plate and neural crest
What happens as the neural plate closes to form the neural tube?
the neural crest pinches off (these will contribute to the formation of other cell types) and the roof plate forms.
From where does the neural tube close?
from the middle both anteriorly and posteriorly (think of a zipper running in both directions).
Neural tube closure is sensitive to
nutrition and environmental toxins
What is important to neural tube closure regarding nutrition?
Folic acid is particularly important. B-complex vitamins in the first few weeks of pregnancy decrease neural tube closure defects. The exact mechanisms by which folic acid works in unclear
Neural tube closure defects (NTDs) 1) spina bifida 2) Anencephaly and holoprosenchephaly
What is the most common NTD?
1 in every 1000 births worldwide (one of the most common birth defects), 3.5 out of every 10,000 in US)
What is spina bifida?
Failure of the posterior end of the neural tube to close (spinal cord).
Anencephaly and holoprosenchephaly
1) 1 in 68,000 and 1 in 16,000 respectively 2) Represents failure of the anterior neural tube to close. Lack prosencephalon (forebrain). 3) Typically deadly.
What happens to the Neural Crest as the neural tube closes?
the Neural crest pinches off giving rise to: 1) Cranial Neural Crest 2) Trunk Neural Crest 3) Vagal and Sacral Neural Crest 4) Cardiac Neural Crest
Cranial Neural Crest
Cranial ganglia, bones and cartilage in face and head
Trunk Neural Crest
DRGs, sympathetic ganglia, adrenal medulla, melanocytes
Vagal and Sacral Neural Crest
Cardiac Neural Crest
Cartilage, melanocytes and neurons of the pharyngeal arches, regions of the heart.
overlaying ectoderm into neural ectoderm (by inhibition of BMP signaling) and formation of the neural plate and neural groove (Shh dependent)
Neural tube closure
begins in the middle of the embryo and proceeds in both the anterior and posterior directions
Defects in neural tube closure leads to
anencephaly and spina bifida
Neural crest pinches off when
neural tube is formed and gives rise to cells in the peripheral nervous system
Dorsal Ventral Patterning
patterning makes cells in one different from cells in another area
(motor) is secreted Sonic Hedgehog (Shh)
(sensory) is secreted by TGF betas (mainly BMPs)
What leads to the neuronal diversity along the dorsal / ventral axis?
More complex combinations of signaling through convergence of signaling pathways contribute to the remarkable neuronal diversity along the D/V axis primarily involving FGF and RA signaling
How is cellular and structural diversity created on the dorsal-ventral axis in the CNS?
1) Ventral Signal (motor) is secreted Sonic Hedgehog. 2) Dorsal Signal (sensory) is secreted TGF betas (mainly BMPs). 3) More complex combinations of signaling through convergence of signaling pathways contribute to the remarkable neuronal diversity along
What produces dorsal-ventral polarity?
High sonic hedgehog (Shh) expression in the notochord and roof plate and its absence in the roof plate
What does Shh bind to?
In the ventral neural tube sonic hedgehog (Shh) binds to Patched (PTC) and relieves the PTC-dependent inhibition of Smoothened (SMO).
(smoothened) activates the Gli class of zinc finger transcription factors.
induces transcription and leads to a ventral (motor neuron) cell fate.
1) In the ventral neural tube sonic hedgehog (Shh) binds to Patched (PTC) and relieves the PTC-dependent inhibition of Smoothened (SMO). 2) SMO activates the Gli class of zinc finger transcription factors. 3) Gli induces transcription and leads to a ventr
What happens in the absence of Shh signaling?
it has devastating effects on the development of the brain. 1) the forebrain does not form and D-V polarity is disrupted. 2) Importantly dirsruptions in this pathway also can cause cancers such as medulloblastomas and basal cell carcinoma. 3) Disruptions
a Shh antagonist
polarity AND proliferation
Development of dorsal-ventral polarity in the spinal cord
1) Similar to neural induction, the precise pattern of different neuronal subtypes requires the convergence of a number of different signaling cascades. 2) In addition to BMPs and Shh pathways, retinoic acid (RA) and FGF signaling play key roles in develo
What is the importance of understanding factors control brain polarity and cell identity?
1) A number of developmental disorders are disorders of polarity and/or cell identity. 2) Understanding the molecular mechanisms that control cell fate decisions may in the future (very near future) harness embryonic stem cells and neural stem cells for t
Anterior-Posterior Patterning - A/P patterning overlaps with neural induction (which overlaps with gastrulation).
A/P patterning leads to:
Spinal Cord, Rhombencephalon (Metencephalon-future pons, Myelencephalon-future medulla), Mesencephalon-future midbrain, Prosencephalon (Diencephalon-future thalamus and retina, Telencephalon-future forebrain)
A/P patterning below the midbrain
posterior CNS - The KEY to metazoan bodyplans.
The Hox genes
Homeotic genes identified in flies
1) Specify segment identity along the A/P axis (in fly along the whole axis). 2) The proteins encoded by these genes are powerful transcriptional activators and repressors that turn on or off 1000s of other genes
posterior CNS patterning
Hox genes are involved in
defining segmental differences in the spinal cord, medulla and pons.
In vertebrates each segment of the posterior CNS patterning involves
combinations of multiple Hox genes expressed in complex patterns.
Hox genes work through
repressing and enhancing each other to create unique patterns of gene expression in each segment.
A/P in the CNS is largely accomplished through
Hox code in vertebrates?
More complicated and likely combinatorial
No Hox code for
prosencephalon and mesencephalon (other genes involved; EMX1, EMX2, OTX1, FGFs, WNTs)
Hox in the forebrain
not well understood
OTX2 knockouts show
complete loss of anterior polarity
OTX2 knockout embryos
completely lack forebrain neural structures
Cell proliferation and migration
1) coordination of symmetrical and asymmetrical proliferation regulates nervous system expansion 2) cell migration is required to organize distinct cell types into functional units
is the thin strip of cells surrounding the CSF-filled ventricles (remember the nervous system is a tube and the fluid filled regions within the tube persist as the brain develops).
Neural stems cells and neural progenitor cells in ventricular zone
divide and differentiate in this zone to give rise to all the cells in the CNS.
Early in development the neural stem cells divide
symmetric giving rise to two daughter cells
Describe the two daughter cells that result from the symmetric division of the neural stem cells.
they are both pluripotent neural stem cells (NSCs). Importantly these cells are capable of self-renewal.
What is the effect of the symmetric division of the neural stem cells?
this increases the size of the ventricular zone, which in turn increases the size of the brain.
How does the ventricular zone increase in size?
The thickness of the ventricular zone stays relatively constant so increased number of NSCs expands the ventricular zone laterally.
Asymmetric NSC division
As development proceeds NSCs begin to divide asymmetrically giving rise to one NSC and one neural precursor.
Neural progenitor will give rise to
neurons and glia.
Late in development NSCs will do what?
again divide symmetrically but give rise to two neural pecursors and therefore NSCs disappear.
Precursor cells can also divide
symmetrically and asymmetrically.
Which comes first; neurogenesis or gliogenesis?
Neurogenesis precedes gliogenesis
What is one of the major signaling pathways that controls neural cell differentiation
Notch and the proneural basic-helix-loop-helix (bHLH) transcription factors.
Notch and proneural bHLH transcription factors control
neural progenitor differentiation.
through Delta requires cell-cell contact.
At low/moderate levels of Notch stimulation
through Delta, the intracellular domain of Notch (NICD) is cleaved and goes to the nucleus to activate bHLH genes.
What does notch do in the nucleus and how does it do this?
It activates bHLH genes via cleavage of NICD
What does the activation of bHLH genes lead to?
Ultimately through a feed-forward circuit this leads to high expression of proneural bHLH proteins and this cell is primed to differentiate into a neuron.
bHLH activation also upregulates what?
Delta on these cells.
What happens to the cells surrounding one that has bHLH activated?
in surrounding cells Notch gets hyper-activated, which shuts off proneuronal bHLH genes and keeps them in their pluripotent NSC state.
Why are the surrounding cells of a bHLH activated cell kept pluripotent?
Very precise mechanism to control the number of cells that differentiate into neural progenitors.
Gliogenesis starts after what?
the peak of neurogenesis
Which comes first oligodendrogiosis or astrogliosis?
Astrogliosis precedes oligodendrogiosis
Are signaling pathways mutually exclusive?
signaling pathways are reused in different stages of development.
from neural progenitors is Notch dependent and conversely inhibited by bHLH genes.
is induced by other factors (Oligs and Nkx)
Its important to remember the impact of signaling pathways depend on what?
the “state” of a cell (in other words which specific receptors and pathways are active in those cells).
1) inhibited by Notch 2) stimulated by bHLH gene products
1) inhibited by proneural bHLH 2) stimulated by Olig1/2 Nkx2.1
1) inhibited by proneural bHLH 2) stimulated by Notch / Nrg
When is neurogenesis typically finished?
it is very early in human development. In most regions it is finished by the middle of the second trimester (most likely the 19th week)
Timeline of neurogenesis in a rodent versus humans.
Note specifically the neocortex in which neurons are still being produced in a rodent at birth, Neurogenesis is likely mostly finshed by the 19th week in humans.
Timing of gliogensis
shown here is not well established and certainly the majority of it happens after birth in humans.
Timeline for myelination in humans
For example there is almost no myelination in the human at birth and it continues to increase out to about 20 years old!
The basic shape of the brain is fully formed at
birth and the vast majority of the neurons are already generated.
When is primary neurulation complete?
Within about the first 3 weeks
What does defects in primary neurulation result in?
they are often deadly
What happens after primary neurulation?
Expansion of neural precursors and neuronal development begins coincident with and immediately thereafter primary neurulation and is very rapid
When are most neurons in the cerebral cortex produced?
Between the 1st and 4th month of pregnancy
The brain is extremely sensitive to
nutrition and environmental toxins during the first and fourth months of pregnancy 1) Vitamin A 2) drugs of abuse
Brain development and drug abuse
exposure to drugs of abuse anytime during pregnancy can lead to neural defects
Severe exposure to drugs of abuse on brain development.
For example severe alcohol exposure. The effect on the brain is quite dramatic and not particularly specific.
Less severe exposure to drugs of abuse on brain development
example with likely less severe exposure you can see the decrease in the size of gray matter (cells) in the cortex and caudate.
One method determining the timing of neurogenesis in experimental animals is
inject them with radio-labeled thymidine.
What happens if you inject an animal with radio-labeled thymidine?
This will only label the cells in S-phase at the time of labeling.
Curiously in experimental monkeys labeled with thymidine on different days what happens?
it appears that the cortex forms in an inside to outside manner.
What layers in the brain form first?
In other words the layers closest to the ventricular zone from first while the ones furthest away from the ventricular zone form last. And this appears to happen in a very orderly pattern.
How does the cortex form?
It forms in an inside out manner
What happens to first born cells in the ventricular zone?
they migrate from the ventricular zone to the pial surface and subsequent cells take the same radial migration route and therefore migrate above the previously born cells.
Radial migration of neurons depends on
Radial glia in the ventricular zone
have a process that extends from the ventricular zone all the way to the pial surface.
What are neuroblasts and what is its migration pattern?
Postmitotic cells called neuroblasts (will become neurons) migrate along these radial glial fibers until they reach the pial surface, at which point they detach from the fiber.
How does the cortex form?
So the cortex does form in an inside out fashion.
What are the radial glia?
they are in fact the neural stem cells of the developing nervous system
Radial glial cells function
both give rise to neurons and provide a scaffolding on which they can migrate to their appropriate destination.
What regulates radial migration?
What happens to radially migrating neurons from the VZ zone?
Radially migrating neurons from the VZ move along radial glia to populate the cortical plate with newly born cells always migrating past previously born neurons.
Mutation in the extracellular matrix protein reelin does what?
disrupts the process of migration and leads to a cortex which is essentially inside out (early born neurons superficial to later born neurons).
Mutations in genes that influence neuronal migration cause what?
What do interneurons do in terms of migration?
They migrate tangentially over long distances in cortical development
In contrast to the migration of projection neurons in the cortex, interneurons are
derived from a different location and migrate long distances tangentially into the developing cortex.
Interneurons are derived from?
the medial and lateral ganglionic eminences (MGE and LGE, respectively) and therefore cannot note migrate radial to their destinations.
How is the derivation of interneurons mediated?
This is mediated by distinct mechanisms and involves DLX1 and DLX2 and Mash1 transcription factors.