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Summary BBS2002 From Cradle to The Grave
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Condensed summary with all the necessary information for the BBS2002 exam. Containing a summary of each case and lecture within a 70 page document.
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Case 1: The developing nervous systemGastrulation and neurulation during embryonic development
Gastrulation: germ layer formation
During week 3, the two-layered embryonic disc transforms
into a three-layered embryo in which the primary germ
layers -ectoderm, mesoderm and endoderm -are present.
This process is called gastrulation. Gastrulation begins
when a groove with raised edges called the primitive streak
appears on the dorsal surface of the embryonic disc and establishes the longitudinal axis of
the embryo. Surface (epiblast) cells of the embryonic disc then migrate medially across the
other cells and enter the primitive streak. The first cells to enter the groove displace the
hypoblast cells of the yolk sac and form the most inferior germ later, the endoderm. Those
that follow push laterally between the cells at the upper and lower surfaces, forming the
mesoderm. As soon as the mesoderm is formed, the mesodermal cells beneath the early
primitive streak aggregate, forming a rod of mesodermal cells called the notochord. The cells
that remain on the embryo's dorsal surface are the ectoderm. Ectoderm fashions structures
of the nervous system and skin epidermis.
Among the stem cell lines that emerge during gastrulation are the neural stem cells. The
neural stem cells are capable of producing all of the different cells that make up the brain
and central nervous system, and for this reason the neural stem cells are usually called the
neural progenitor cells. Cells that migrate along the axial midline send molecular signals that
induce cells in the overlying epiblast layer to differentiate into neuroectodermal cells (red
band) which are the neural progenitor cells.
Specialization of the ectoderm
The first major event in organogenesis (formation of organs) is
neurulation, the differentiation of ectoderm that produces the brain and
spinal cord. This process is induced by chemical signals from the
notochord.
1. The ectoderm overlying the notochord thickens, forming the
neural plate.
2. The ectoderm starts to fold inward as a neural groove.
3. As the neural groove deepens it forms prominent neural folds.
4. By day 22, the superior margins of the neural folds fuse, forming a
neural tube.
The anterior end of the neural tube becomes the brain, and the rest
becomes the spinal cord. The associated neural crest cells migrate widely and give rise to the
cranial, spinal and sympathetic ganglia (PNS), to the chromaffin cells of the adrenal medulla,
to pigment of the skin and contribute to some connective tissues.
Development of the brain
During gastrulation, the primitive node signals all migrating cells to produce the proteins
that signal neural progenitor fate, but each successive wave of migrating cells also receives a
second signal that specifies a regional identity for the neural progenitors. Thus, primitive
node signals early migrating epidermal cells to produce molecular signals for the cells in the
,overlying layer to differentiate into neural progenitors capable of producing cells
appropriate for forebrain structures, while later migrating cells signal differentiation of
neural progenitors capable of producing cells appropriate for hindbrain or spinal cord
structures.
Formation of the vesicular system
When, during neurulation, the neural tube is complete, the
neural progenitors form a single layer of cells that lines the center
of the neural tube immediately adjacent to its hollow center. In
the embryo, the hollow center of the neural tube is cylindrical,
like the center of a straw. But as the brain becomes larger and
more complex, the shape of the hollow cavity also changes,
eventually forming the ventricular system of the brain. Because
the neural progenitors are located in the region that will become
the ventricles, the region is called the ventricular zone (VZ). The second, an only other, layer
of the brain at this state of development is known as the marginal zone (MZ). The neural
progenitor cells in the most rostral region of the neural tube will give rise to the brain, while
more caudally positioned cells will give rise to the hindbrain and spinal column.
Three and five vesicle state
By the end of the first month, the three primary brain vesicles (fore-,
mid-and hindbrain) are apparent that then develop into secondary five
vesicles. By the end of the second month, all brain flexures are evident,
the cerebral hemispheres cover the top of the brain and brain waves
can be recorded.
1. Prosencephalon, develops into: telencephalon ('large brain') and
diencephalon (division of the forebrain; in between
telencephalon and mesencephalon).
2. Mesencephalon, develops into: mesencephalon (midbrain)
3. Rhombencephalon, develops into: Metencephalon (pons and
medulla) and myelencephalon (medulla oblongata).
Proliferation of neurons
The human brain contains billions of neurons most of which are produced by mid-gestation.
The pool of neural progenitor cells that is specified at the end of gastrulation is far too small
to accommodate neuron production on this scale. Thus, the first step in neuron production
involves increasing the size of the neural progenitor cell population.
Five positions
During proliferation, each cell performs a characteristic 'dance'
as it divides, which contains of five steps:
1. Cell extends a process.
2. Cell's DNA is copied.
3. Two complete copies of DNA are present.
4. Cell retracts arm from pial surface.
5. Cell divides in two.
,Neural progenitor cells vs neurons
Neural progenitors are a mitotic population of cells, that is, they can divide to form new cells.
Neurons are post-mitotic cells; once formed they are no longer capable of dividing and
producing new cells. Most, if not all, of these progenitor (stem-like) cells that generate
neurons and astrocytes are radial glial cells. Generally speaking, first the neurons and layers
of the cortex are formed. After that, the glia cells are produced too.
Symmetric vs asymmetric division.
Symmetrical: From the end of gastrulation through approximately
E42 in humans, the population of neural progenitor cells divides by
what is described as a symmetrical mode of cell division.
Symmetrical cell division produces two identical neural progenitor
cells that remain in the ventricular zone to divide again.
Asymmetrical: Beginning on E42, the mode of cell division begins to shift from symmetrical
to asymmetrical. During asymmetrical cell division, two different types of cells are produced.
In neural progenitors, asymmetrical cell division produces one neural progenitor and one
neuron. The new progenitor cell remains in the proliferative zone and continues to divide,
while the post-mitotic neuron leaves the proliferative zone to take its place in the developing
neocortex. It is the daughter cell that is furthest away from the ventricle that ceases further
division and migrates away.
Neuron formation of the brain
Neuron production begins in the embryonic period on day 42 and extends through mid-
gestation. Soon after they are produced, neurons migrate away from the proliferative regions
of the VZ. The neurons that will form the neocortex migrate in an orderly fashion forming the
six-layered neocortical mantel. Once positioned in cortex neurons begin to differentiate
producing neurotransmitter and neurotrophic factors, and extending the dendritic and
axonal processes that form fiber pathways of the brain neural networks.
Migration of neurons
Most neurons are produced in the VZ and migrate radially from the VZ in the center of the
brain out to the developing neocortex.
Somal translocation
Very early in neocortical development the distances the neuron must traverse are small. Thus
the earliest produced neurons can use a mode of migration referred to as somal
translocation.
1. In somal translocation the neuron extends a long basal process, which is an extension
of the cell’s body, just beyond the edge of the VZ into the outer region of the brain
compartment.
2. The basal process attaches to the pial surface, which is the outer surface of the
developing brain.
3. The nucleus of the cell then moves through cytoplasm of the basal process.
4. As the nucleus moves up the process becomes shorter and thicker but remains
attached to the pial surface.
5. At the end of somal translocation the nucleus of the cell has moved out of the VZ and
into the embryonic cortex.
, Radial glial guides
As development proceeds, the brain becomes larger and the primary mode of neuronal
migration from the VZ changes. Because of the greater distances, neurons require what was
originally identified as a special population of cells within the VZ called radial glial guides to
support their migration.
1. Much like neurons migrating via somal translocation, radial glial guides extend a basal
process that attaches to the pial surface of the brain.
2. However, the nucleus of the radial glial cells remains in
the VZ, and the basal process forms a kind of scaffolding
along which neurons can migrate.
3. The migrating neurons attach themselves to the radial
glial guide and move along the cellular scaffold out into
the developing cortical plate.
4. Each glial scaffold can support the migration of many
neurons.
Inside-out development of the cortex
The first cells to migrate to the cortical plate are those that form
the subplate. As these differentiate into neurons, the neural
precursor cells destined to become layer VI cells migrate past and
collect in the cortical plate.
This process repeats again and again until all layers of the cortex
have differentiated. The subplate neurons then disappear. The six
layers of the cortex form the grey matter of the brain.
Myelination
The efficiency of information transmission in the
pathways is greatly enhanced by myelin which
ensheaths the axons. Myelin is a fatty substance that
is white in appearance, hence the name white matter.
The oligodendrocytes are responsible for the myelin
sheath in the CNS, whilst the Schwann cells are
responsible for the myelin sheath inn the PNS. In the
central nervous system (CNS), oligodendrocytes
myelinate multiple axons; in the peripheral nervous system (PNS), Schwann cells (SCs)
myelinate a single axon.
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