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Summary Development of the Nervous System
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The stages of cell growth and cell death described. Each stage detailed and summarised
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Chapter 9 – Development of the nervous systemFive phases of development
- Zygote: amalgamate ion of sperm and ovum. The zygote divides to form two daughter cells.
These continue diving into twos.
- Cells must differentiate so that we do not just become nothing. Some cells become muscles,
some bipolar neurons. Cells then align at specific sites to become specific structures. Then,
synaptic connections are established with other cells.
- A fertilised egg is called a totipotent. This can develop into any cell, but cell division causes
the egg to develop specific structures. When cell division occurs, and cells begin to specialise
to become other cells. These are pluripotent cells. Cells become multipotent when they
become specialised. Some of these cells become unipotent. This means they will only
develop into one type of cell.
Introduction of the neural plate
The tissue that becomes the nervous system is the neural plate. This develops three weeks after
conception. The neural plate is the ectodermal tissue on the dorsal surface of the embryo.
Embryonic cells have three layers.
- The ectoderm is the outside
- The mesoderm is the middle – this region sends signals to the neural plate to help
commence development.
- The endoderm is the innermost part
The cells in the neural plate are referred to as stem cells. These cells can differentiate into anything if
preserved, which is why they are used in cancer research. They differentiate into any cell possible, or
they can replicate. The two daughter nuclei produced by cell division turn into either a stem cell
(replicate) or another type of cell. The neural plate folds to form the neural groove, the lips of which
fuse to form the neural tube. The neural tube eventually keeps developing to form cerebral
ventricles and the spinal canal. Three swellings eventually form, which are the forebrain, midbrain,
and the hindbrain.
Neural proliferation
Once the neural plate folds into the neural groove, cells begin to generate quickly, which is known as
proliferation. Most cell division occurs in the ventricular zone, which is adjacent to the ventricles.
Proliferation follows specific sequences to develop the ridges and structures of the brain. Deficits in
the neural tube, therefore, prevent the ability to form developed structures. Proliferation is
controlled by chemical signals coming from the floor plate and the roof plate. The floor plate is a
specialised glial structure that spans from the anteroposterior axis of the midbrain to the tail region.
This region secretes chemical signals to stimulate development in the midbrain. The roof plate is on
the dorsal midbrain.
Migration and aggregation
Once cells are created in the ventricular zone, they migrate to specific locations. They are still
immature, lacking axonal and dendrite development. Radial migration is when migration occurs from
the ventricular zone extending out parallel to the outer tube wall. Tangential migration alongside the
neural tube. Many neurons develop from glial cells.
, Migration occurs in two ways:
- Somal translocation: The radial or tangential migration extends out to connect with
environmental cues around it, furthering the available connections in the neuronal tube.
- Glia-mediated migration: Providing a thick network of cells to further strengthen the neuron.
Only radial migration across the glia.
The neural crest: Dorsal of the neural tube. Cells that break off the neural tube form the neural crest.
These develop into neurons and glial cells in the peripheral nervous system.
Aggregation: migrated neurons align with other migrated neurons to further form the structures of
the nervous system. This is called aggregation. The cells mediating the migration and aggregation
processes are called cell-adhesion molecules. They can remember other cells and adhere to them.
Even one CAM being eliminated has devastating effects. Gap junctions have been prevalent during
brain development. The gaps are bridged by connexins. The cells can then exchange cytoplasm.
Axon growth and synapse function
Axon growth: Axon and dendrites grow on developing structures after aggregation. At the tips of
growing axon and dendrites, there are growth cones, which extend and retract cytoplasm fingers
called filopodia. These filopodia on the ends of growth cones help to direct migration and provide
energy along the way to travelling neurons. They are like traffic control officers.
Pioneer growth cones: first cones to travel across the nervous system, which set out the journey for
embarking neurons to follow, which allows a response to take place due to the abundance of cells at
one site (fasciculation). These cells know where to go due to the filopodia guiding them along the
way.
Cell regeneration was confirmed to be possible after the optic nerves of frogs were cut, and they
reacted the same way after growing back as initially. Retinal ganglion cells can regenerate. Chemo
affinity hypothesis was formulated, where interactions with specific markers act as the basis of initial
interaction with a target cell.
A single neuron can form an axon, but they must interact with other neurons to form a synaptic
junction. Synaptogenesis relies on the presence of astrocytes, a type of glial cell. More astrocytes
mean more cells growth, therefore there is more of a synapse abundance. These astrocytes are said
to play a role in transmitting and synthesising neuronal information, not just supplying fuel to the
process due to their high cholesterol. Cholesterol is needed to build healthy cells, but high
concentration in one organ can cause serious damage.
Neurons and synapses that do not function properly must die.
Neuron death and synapse rearrangement
Neurons are produced in abundance to ensure there are enough signals to help the brain and body
communicate. With an excess amount, there is guaranteed to be no shortage, as this would limit the
efficiency of transmission. Since there are so many, some happily die after they are found to be
inefficient to ensure there is not and over abundance of neurons. These cells do not die after
inadequate nutrition, but rather genetic codes cause useless neurons to commit suicide. This is
called apoptosis, but passive cell death is necrosis. Here, usually, useful cells die due to inadequate
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