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Summary lectures Neurodevelopment (NWI-BB039C)
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A complete summary of lectures 1-15 of the neurodevelopment course. This was all material for the exam.
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Summary lectures neurodevelopmentLecture 1 + 2 – Neural induction
Notice that events happening in these lectures can overlap. While some cells undergo neurogenesis
others might already be differentiating for example. The back of the brain develops earlier than the
front of the brain.
Evolutionary perspectives
Throughout evolution most animals have used neurons. Also the features of these particular neural
cells are quite similar. It all started with electrical synapses and the synaptical transition is very
ancient. It did develop over time to be more and more specialized.
Also the very early building plan is very similar between
animals. This means that ontogeny (the origination and
development of an organism usually drom time of
fertilization of the egg to adult) recapitulates phylogeny
(the history of the evolution of a species or group). It
means that the building plan of all animal species is
similar and the building plan adds up for higher animal
species.
These differences between species are steered by need
to survive in the environment.
Derivation of neural tissue
After fertilization, the
early embryo develops
into a blastula, which is a
clump of cells with a
cavity (blastocoel) in the
middle. The blastula will
undergo gastrulation and
therefore develop into a
gastrula. Gastrulation is
the formation of three
germ layers. These three
germ layers are the
ectoderm, mesoderm and endoderm. The ectoderm is the layer that mostly develops into the central
nervous system. They also develop into the neural crest cells, which eventually end up making the
peripheral nervous system. Ectodermal development is called neurulation.
Hydra
In hydra, cells can wiggle out of the epidermis and can either become sensory cells or neurons. These
neurons will start to connect with each other in a nerve net. This means they will communicate, but
in a very simple manner.
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,C. elegans
When looking in c. elegans there first is a P0 blastomere. This will divide into an AB and a P1. This AB
lineage will give rise to all neurons as it keeps on dividing. These blastomeres will move to the
outside of the very early C. elegans. This outer ring of AB blastomeres will move to the inside and
form nerve rings.
Drosophila
In Drosophila, invagination of the mesoderm takes place at the
ventral surface at what’s called the ventral furrow. This brings the
neurogenic region closer to the midline. Now neuroblasts start to
enlarge and migrate towards the interior. These cells wiggle out
of the epidermis and start to form nerve cells. The proces in
which the neuroblasts of the Drosophila separate from the
ectoderm is called delamination. When the neuroblasts have
wiggled out of the ectoderm, neurons and glia will form ventral
nerve cords. These cells keep on dividing until there are enough.
A neuroblast gives rise to another neuroblast and a ganglion
mother cell. The next Nb gives rise to another Nb and a ganglion
mother cell, etc. This therefore is asymmetric cell division.
A hallmark of gastrulation is that the mesoderm has the capaity
to induce the neuroectoderm. If mesoderm is not present, there
would not be any nervous tissue.
Eventually there is a larval nervous system more complex than in
the C. elegans and in the hydra. Nb → neuroblast; GMC → ganglion
mother cell.
Xenopus
Here the gastrulation process is more advanced
compared to any other lower animal. When the egg gets
fertilized, there are the early cleavages. There are the
micromeres (on top) and the macromeres (at the
bottom) with a blastocoel inside in the blastula stage.
At the gastrulation level at the blastopore lip there is an
involuting marginal zone (IMZ). This is a cell layer
(mesoderm) that will move to the inside of the embryo
into the blastocoel and will stick to the inside of the
overlying cell layer (neurogenic region) and travel along
this top layer more ventrally. These cell layer is depicted
by the blue line in the picture. It is important that they
make contact, because this blue layer will become the
mesoderm through induction. It is important that the
mesoderm is close to the overlying ectoderm, because
the neuroectoderm needs to be induced by the dorsal
mesoderm. The neurogenic region will become a neural
plate and therefore nervous tissue.
When looking at the top view, it is visible that there is a
neural plate with a neural groove in the middle. This is already the early polarization of the embryo.
This stage is called the neurula stage. When a cross section is made through the neurula stage the
archenteron will also be visible. Only the top (dorsal) part of the mesoderm will induce ectoderm to
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,become neuroectoderm in the form of a neural plate. This is still a flattened surface. It is not yet a
tube. During neurulation the neural plate will become the neural tube. This happenes whenever the
neural groove closes.
The archenteron is a cavity. It is not the same cavity as the blastocoel as the blastocoel is being
replaced. The archenteron will become the future gut.
A specialization of the mesoderm is the notochord. The notochord defines the body axis. The
notochord makes sure the neural groove is correctly positioned.
Induction to the neural tissue
At the top picture, induction already takes place. The mesoderm
present at the top picture will form a rod, which is the notochord.
The notochord will define the first body axis. Allong the notochord
lies the neural groove. The neural groove will later form into a
neural tube. The neural tube will close, because the neural plate will
roll up. The ends of the neural tube will fuse at the dorsal side. The
red cells are the most dorsal and will form at the point of fusion of
the neural tube. They will form the neural crest cells. The neural
crest cells are able to migrate towards more peripheral positions to
become mostly the peripheral nervous system.
The notochord is not only important during the induction of
neurulation, but it is also very important in the polarity of the
neural tube itself (the dorsal ventral polarity). This dorsal ventral
polarity determines where the sensory and the motor neurons end
up.
Experimental evidence induction of the neural tissue
Experiments were done to show that without mesoderm
there is no induction of the neuroectoderm. They
removed the animal cap (dorsal cell layer) just before the
peak of gastrulation, it is visible in a dish that the tissue
will become normal epidermis as there is no induction of
mesoderm.
If the same experiment is performed a little bit later at the
peak of gastrulation where the animal cap is removed at a
stage in which mesoderm is underneath the ectoderm,
neuroectoderm will develop. This happens because the
isolated animal cap cells are already induced.
In another experiment is the transplantation experiment of Spemann and
Mangold. Here they transplanted parts of the dorsal blastopore lip from a
pigmented embryo to a non-pigmented host embryo into a ventral region.
They did this very early on during gastrulation. This resulted in the formation
of a secundary body axis. This happens because all these cells will be induced
by host tissue as there is a second environment in which a body axis is
induced. These individuals will have two spines and two heads. Because this
experiment was done by Spemann, the dorsal lip cells are also called the
Spemann organizer.
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, This experiment shows that whenever tissues are transferred
between early gastrulas, the presumptive neural tissue develops into
epidermis and only one neural plate is seen whenever it is
transplanted into a region in another embruo that normally
becomes epidermis. This because no induction has taken place.
When the same eperimebt is performed using late-gastrula tissues,
the presumptive neural cells form neural tissue, thereby causing two
neural plates to form on the host whenever the presumptive neural
ectoderm is transplanted into a region in another embryo that
normally becomes epidermis. They are committed and cannot go
back to becoming skin.
This concludes that early gastrula cells are uncommitted and late
gastrula cells are determined.
In this experiment an early Xenopus embryo is
investigated. The equatorial region will become the
mesoderm. The animal part will become the ectoderm
and the nervous tissue and the vegetal part will become
the endoderm. When the animal cap is isolated, and you
di this early on, nothing happens. There is still ectoderm,
but there will be no mesoderm. When at a later stage the
equatorial region is removed and the animal and vegetal
cap are placed together, mesoderm will be formed. This
means that interactions between the animal and vegetal cells of the amphibian embryo are
necessary for induction of the mesoderm.
When you have found a candidate mesoderm
inducer and you remove the animal cap, there will
be formation of the mesoderm as there is a
mesoderm inducer present. This mesoderm will
then induce the overlying ectoderm to become
neuroectoderm.
If there is a candidate neural inducer which will be
added to the animal cap, only neural tissue will be
formed as there is no mesoderm in the first place.
Not long ago, they found the molecules responsible for all this. These are soluble molecules. They
found that using UV light, there will be a ventrilized embryo. What happened is that cytoskeletal
structures are being destroyed by the UV resulting in a ventrilized embryo with no brain and no
nervous system.
When using lithium, there will be a hyperdorsalized embryo. These are embryos with a huge brain. If
the poly-A-mRNA is extracted from this hyperdorsalized embryo and put it in the UV treated
ventrilized embryo there sort of will be a normal embryo.
In this sort of normal embryo sequencing started. They found that there were several molecules
present which were able to induce ectoderm into neuroectoderm. They found out that one of the
inducers was noggin. When noggin cDNA was used in UV treated embryos, the result was a normal
embryo. When looking at the previous experiment, noggin was indeed the candidate neural inducer
and is able to induce without the presence of any mesodermal genes.
Besides noggin, chordin and other organizer molecules like for example follistatin are putative neural
inducers. They are mostly expressed at the dorsal lip of the blastopore.
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