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Prenatal development of the nervous system (BBS1005) + peripheral vs central
Neural plate, neural tube
Ectoderm → nervous system
Neural plate → neutral tube (neurulation)
The brain develops from the walls of the five fluid-filled vesiccles, in the early stages consisting of 2
layers: (1) the ventricular zone and (2) the marginal zone.
1
,The neural plate forms from the ectoderm layer
and folds to form the neural tube. The caudal
region becomes the spinal cord, the rostral region
becomes the brain.
Neural development cells divide rapidly.
Differences in the rate of proliferation of cells in
rostral regions of the neural tube result in the
formation of thee brain vesicles: the forebrain
(prosencephalic) vesicle (→ becomes the
telencephalon and diencephalon), the midbrain
(mesencephalic) vesicle, and the hindbrain
(rhombencephalic) vesicle (→ becomes the
metencephalon and myelencephalon).
Generation and differentiation of brain cells + where?
cell proliferation, cell migration, cell differentiation, ventricular zone, marginal zone
Sensory Neurons
Sensory neurons are neurons responsible for converting external stimuli from the environment into
corresponding internal stimuli. They are activated by sensory input and send projections to other
elements of the nervous system, ultimately conveying sensory information to the brain or spinal
cord. Unlike the motor neurons of the central nervous system (CNS), whose inputs come from other
neurons, sensory neurons are activated by physical modalities (such as visible light, sound, heat,
physical contact, etc.) or by chemical signals (such as smell and taste).
Most sensory neurons are pseudounipolar, meaning they have an axon that branches into two
extensions—one connected to dendrites that receive sensory information and another that
transmits this information to the spinal cord.
Motor Neurons
Motor neurons are neurons located in the central nervous system, and they project their axons
outside of the CNS to directly or indirectly control muscles. The interface between a motor neuron
and muscle fiber is a specialized synapse called the neuromuscular junction. The structure of motor
2
,neurons is multipolar, meaning each cell contains a single axon and multiple dendrites. This is the
most common type of neuron.
Interneurons
Interneurons are neither sensory nor motor; rather, they act as the “middle men” that form
connections between the other two types. Located in the CNS, they operate locally, meaning their
axons connect only with nearby sensory or motor neurons. Interneurons can save time and
therefore prevent injury by sending messages to the spinal cord and back instead of all the way to
the brain. Like motor neurons, they are multipolar in structure.
The genesis of neurons
1. Cell proliferation
2. Cell migration
3. Cell differentiation
neuronal differentiation is followed by astrocyte
differentiation (peak at time of birth).
Oligodendrocytes are the last cells to differentiate
notch signaling regulates the fate of cells in the
developing cerebral cortex
Cell proliferation
The brain develops from the walls of the five fluid-filled vesicles. The walls of the vesicles consist of a
ventricular zone and a marginal zone. The ventricular zone lines the inside of each vesicle and the
marginal zone faces the overlying pia. Within these layers of the telencephalic vesicle, a cellular
ballet is performed that gives rise to all the neurons and glia of the visual cortex.
1. First position: a cell in the ventricular zone extends a process that reaches upward toward
the pia
2. Second position: the nucleus of the cell migrates upward from the ventricular surface
toward the pial surface; the cell’s DNA is copied
3. Third position: the nucleus, containing two complete copies of the genetic instructions,
setlles back to the ventricular surface
4. Fourth position: the cell retracts its arm form the pial surface
5. Fifth position: the cell divides in two
These neural progenitors (radial glial cells) give rise to all neurons and astrocytes of the cerebral
cortex. Radial glial cells also give rise to most of the neurons of the central nervous system.
Multipotent stem cells divide to expand the population of neural progenitors via a process called
symmetrical cell division (asymmetric cell division happens later in development).
3
, In marginal zone: DNA replication → back intro the ventricular zone → cell division
((a)/symmetrical)
Many precursor cells necessary when there is room for migrating, to form a structure (not when
the brain is already full). Deciding when and where to form neurons.
Cell migration
daughter cells migrate along the thin fibers
emitted by radial glial cells that span the
distance between the ventricular zone and the
pia. The immature neurons, called neural
precursor cells, follow this radial path from the
ventricular zone toward the surface of the brain
radial glial cells are in fact progenitor cells that
generate both neurons and astrocytes. In
addition to their role in neuronal migration,
they are able to undergo both asymmetric and
symmetric division + act as a source of neurons
and astrocytes.
First migrating cells are from the subplate.
Neural precursor cells destined to become layer
6 migrate past and collect in the cortical plate.
This process repeats until all cortex layers are
differentiated.
The subplate neurons then disappear!!
4
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