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CSB331H1 Chapter Notes -National College Of Art And Design, Reassortment, Bone Morphogenetic Protein


Department
Cell and Systems Biology
Course Code
CSB331H1
Professor
Maurice Ringuette

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P. 250-251
FN and the Pathways for Mesodermal Migration
How are the involuting cells informed where to go once they enter the inside of the embryo?
Amphibians involuting mesodermal precursors migrate toward the AP on a FN lattice secreted by the cells of
the blastocoels roof
o Shortly before gastrulation, the presumptive ectoderm of the blastocoels roof secretes an ECM that
contains fibrils of FN which the involuting mesoderm travel along
FN-containing ECM appears to provide both a substrate for adhesion as well as cues for the direction of cell
migration
In xenopus, FN is similarly secreted by the cells lining the blastocoels roof -> band of FN lining the rood
Convergent extension pushes the migrating cells upward toward the AP
FN fibrils are necessary for the head mesodermal cells to flatten and to extend broad (lamelliform) processes in
the direction of migration
Studies using inhibitors of FN formation have shown that FN fibrils are necessary for the direction of mesoderm
migration, the maintenance of intercalation of AC cells, and the initiation of radial intercalation in the marginal
zone
Mesodermal cells are thought to adhere to a5b1
Mesodermal migration can also be arrested by the microinjection of Ab against either FN or a5 subunit
A5 appears just prior to gastrulation and it persists on the surfaces throughout gastrulation and disappears
when gastrulation ends
The integrin coordinated the interaction of the FFN on the blastocoels roof with actin filaments within the
migrating mesodermal cells -> this allows for increased traction and determines the speed of migration
The coordinated synthesis of FN and its receptor signals the times for the mesoderm to begin, continue, and
stop migration
P. 333-340
The Emergence of the Ectoderm: CNS and epidermis
A portion of the dorsal ectoderm is specified to become neural ectoderm, and its cells become distinguishable
by their columnar appearance
This region of the embryo is called the neural plate
The process by which the neural plate tissue forms a neural tube the rudiment of the CNS is called
neurulation, and an embryo undergoing such changed is called a neurula
The neural tube forms the brain anteriorly and the spinal cord posteriorly
Establishing the Neural Cells
Neural cells become specified through their interactions with other cells
At least 4 stages through which the pluripotent cells of the epiblast or blastula become neural precursor cells, or
neuroblasts:
1. Competence: multipotent cells can become neuroblasts if they are exposed to the appropriate combination
of signals
2. Specification: cells have received the appropriate signals to become neuroblasts, but progression along the
neural differentiation pathway can still be repressed by other signals
3. Commitment (determination): neuroblasts enter the neural differentiation pathway and will become
neurons even in the presence of inhibitory signals
4. Differentiation: the neuroblasts leave the mitotic cycle and express those genes characteristic of neurons
Formation of the Neural tube
2 major ways of converting the neural plate into a neural tube

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Primary neurulation: the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate,
and pinch off from the surface to form a hollow tube
Secondary neurulation: the neural tube arises from the coalescence of mesenchyme cells into a solid cord that
subsequently forms cavities that coalesce to create a hollow tube
In general, ant. portion is made by primary and post. is made by secondary neurulation
Complete neural tube forms by joining the tubes together
Primary neurulation
The events of primary neurulation divide the original ectoderm into 3 sets:
1. The internally positioned neural tube -> brain and spinal cord
2. Externally positioned epidermis
3. Neural crest cells
NC cells form in the region that connects the neural tube and epidermis, but they migrate to new locations
where they will generate the peripheral neurons and glia, the pigment cells of the skin, and several other cell
types
The process of primary neurulation appears to be similar in all vertebrates:
o Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds,
while a U-shaped neural groove appears in the center of the plate, dividing the future right and left sides
of the embryo
o The neural folds migrate toward the midline of the embryo, eventually fusing to form the neural tube
beneath the overlying ectoderm
o The cells at the dorsal most portion of the tube become NC cells
Primary neurulation can be divided into 4 distinct but spatially and temporally overlapping stages:
1. Formation and folding of the neural plate
2. Shaping and elevation of the neural plate
3. Convergence of the neural folds, creating a neural groove
4. Closure of the neural groove to form the neural tube
Formation and Shaping of the Neural Plate
The process of neurulation begins when the underlying dorsal mesoderm signals the ectodermal cells above it to
elongate into columnar neural plate cells
Their elongated shape distinguishes them from the flatter pre-epidermal cells surrounding them
As much as half the ectoderm is in the plate
The neural plate lengthens along the ant-post axis and narrows by convergent extension, intercalating several
layers of cells into a few layers
Divisions of the NP cells are preferentially in the ant-post direction
If the neural plate is isolated, its cells converge and extend to make a thinner plate, but fail to roll up into a
neural tube
If the border region containing both the presumptive epidermis and NP tissue is isolated, it will form small
neural folds in culture -> epidermis is important in shaping the plate
Bending and Convergence of the Neural Plate
The bending of the neural plate involves the formation of hinge regions where the neural plate contacts
surrounding tissues
o In birds and mammals, the cells at the midline of the plate form the medial hinge point
o The cells of the MHP are derived from the portion of the neural plate just anterior to Hensens node
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