Download nervous system development and histology

Photographs of Human Fetal
Brain Development
Lateral view of the human brain
shown at one-third size at several
stages of fetal development. Note the
gradual emergence of gyri and sulci.
Phases of brain development
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
Induction of the Neural Plate
• 2-3 weeks after conception
• A patch of tissue on the dorsal surface of the
embryo that will become the nervous system
• Development induced by chemical signals
“growth factors”: several chemicals produced
in developing and mature brain that stimulate
neuron development and help neurons respond
to injury
cephalic flexure
cervical flexure
~19 days
~23 days
Phases of brain development
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
2. Mitosis/Proliferation
Proliferation –
Generation of new cells
3 swellings at the anterior end
in humans will become the
forebrain, midbrain, and
hindbrain
•Occurs in ventricular zone
•Rate can be 250,000/min
•After mitosis “daughter” cells
become “fixed” post mitotic
Phases of brain development
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
3. Migration: slow movement to the “right place”
Only a soma and immature axon at
this point
-undifferentiated at
start of migration.
But, differentiation begins as neurons
migrate.
They develop
neurotransmitter making ability, action
potential
3. Migration
Radial Glia
Radial glial cells act as guide wires
for the migration of neurons
Migrating cells are immature,
lacking dendrites
Cells that are done migrating align
themselves with others cells and
form structures (Aggregation)
Growth Cones: tips of axons on migrating, immature neurons
Growth cones crawl forward as they elaborate the
axons training behind them. Their extension is
controlled by chemical cues in their outside
environment that ultimately direct them toward
their appropriate targets.
5 Phases of Neurodevelopment
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
4. Axon Growth/Synaptogenesis
Once migration is complete and structures have
formed (aggregation), axons and dendrites begin
to grow to their “mature” size/shape.
Axons (with growth cones on end)
and dendrites form a synapse with
other neurons or tissue (e.g. muscle)
Growth cones and chemo-attractants
are critical for this.
Synaptogenesis
• Formation of new synapses
• Depends on the presence of glial cells – especially
astrocytes
• Chemical signal exchange between pre- and
postsynaptic neurons is needed
5 Phases of Neurodevelopment
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
5. Neuronal Death
Between 40-75% neurons made, will die after
migration – death is normal and necessary !!
Neurons die due to failure to compete for
chemicals provided by targets
Neurotrophins –
promote growth and survival
guide axons
stimulate synaptogenesis
Synaptic rearrangment
Release and uptake
of neurotrophic
factors
Neurons receiving
insufficient neurotropic
factor die
Axonal processes
compete for limited
neurotrophic factor
Synaptic rearrangment, cont’d: Myelination
Time after synaptogenesis
Postnatal Cerebral Development Human
Infants
• Postnatal growth is a consequence of
– Synaptogenesis
– Increased dendritic branches
– Myelination (prefrontal cortex continues into
adolescence)
• Overproduction of synapses may underlie the
greater “plasticity” of the young brain
• Young brain more able to recover function after injury, as
compared to older brain
Neural Tube Defects (NTDs)
1- Spina Bifida
Oculta
Meningocele
Meningomyelocle cystica
•
•
•
•
Rachischhisis •
occulta
meningomyelocele
meningocele
myeloschesis
10 %of normal people
L5 or S1
Neural Tube Defects (NTDs)
1- Cranial Bifida
Cranial Meningocele
Meningoencephalocele
Meningohydroencephalocele
Anencephaly
•
•
•
•
•
Cranial Bifida
Meningoencephalocele
Cranial Meningocele
Meningohydroencephalocele
Histology of the Nervous System
The Nervous system has three major functions:
Sensory – monitors internal & external 
environment through presence of receptors
Integration – interpretation of sensory information 
(information processing); complex (higher order)
functions
Motor – response to information processed 
through stimulation of effectors
muscle contraction
glandular secretion
General Organization of the nervous system
•
Two Anatomical Divisions
–
Central nervous system (CNS)
•
•
–
Brain
Spinal cord
Peripheral nervous system (PNS)
•
•
•
All the neural tissue outside CNS
Afferent division (sensory input)
Efferent division (motor output)
–
–
Somatic nervous system
Autonomic nervous system
Histology of neural tissue
Two types of neural cells in the nervous system:
Neurons - For processing, transfer, and storage 
of information
Neuroglia – For support, regulation & protection 
of neurons
Neuroglia (glial cells)
CNS neuroglia:
• astrocytes
• oligodendrocytes
• microglia
• ependymal cells
PNS neuroglia:
• Schwann cells (neurolemmocytes)
• satellite cells
Astrocytes
create supportive framework •
for neurons
create “blood-brain barrier”•
monitor & regulate interstitial •
fluid surrounding neurons
secrete chemicals for •
embryological neuron
formation
stimulate the formation of •
scar tissue secondary to CNS
injury
Oligodendrocytes
create myelin sheath •
around axons of neurons in
the CNS. Myelinated axons
transmit impulses faster than
unmyelinated axons
Microglia
“brain macrophages”•
phagocytize cellular wastes •
& pathogens
Ependymal cells
line ventricles of brain & •
central canal of spinal cord
produce, monitor & help •
circulate CSF (cerebrospinal
fluid)
Peripheral neuroglia
• 1- schwann cell
• 2- satellite cell
Schwann cells
surround all axons of neurons in the •
PNS creating a neurilemma around
them. Neurilemma allows for
potential regeneration of damaged
axons
creates myelin sheath around most •
axons of PNS
Satellite cells
support groups of cell bodies of •
neurons within ganglia of the PNS
Neuron structure
Most axons of the nervous system are •
surrounded by a myelin sheath (myelinated
axons)
of Ranvier
The presence of myelin speeds up the •
transmission of action potentials along the
axon
Myelin will get laid down in segments •
(internodes) along the axon, leaving
unmyelinated gaps known as “nodes of
Ranvier”
Regions of the nervous system containing •
groupings of myelinated axons make up
the “white matter”
“gray matter” is mainly comprised of •
groups of neuron cell bodies, dendrites &
synapses (connections between neurons)
Classification of neurons
Structural classification based on number of
processes coming off of the cell body:
Anaxonic neurons
• no anatomical clues to
determine axons from
dendrites
• functions unknown
Multipolar neuron
multiple dendrites & single •
axon
most common type•
Bipolar neuron
two processes coming off •
cell body – one dendrite &
one axon
only found in eye, ear & •
nose
Unipolar (pseudounipolar)
neuron
single process coming •
off cell body, giving rise
to dendrites (at one end)
& axon (making up rest of
process)
Classification of neurons
Functional classification based on type of information &
direction of information transmission:
Sensory (afferent) neurons – •
transmit sensory information from receptors of PNS towards the CNS•
most sensory neurons are unipolar, a few are bipolar•
Motor (efferent) neurons – •
transmit motor information from the CNS to effectors (muscles/glands/adipose •
tissue) in the periphery of the body
all are multipolar•
Association (interneurons) –•
transmit information between neurons within the CNS; analyze inputs, •
coordinate outputs
are the most common type of neuron (20 billion)•
are all multipolar•
Conduction across synapses
In order for neural control to occur, “information” must
not only be conducted along nerve cells, but must
also be transferred from one nerve cell to another
across a synapse
Most synapses within the nervous system are
chemical synapses, & involve the release of a
neurotransmitter
The Structure of a Typical Synapse
Anatomical organization of neurons
Neurons of the nervous system tend to group together into
organized bundles
The axons of neurons are bundled together to form nerves in
the PNS & tracts/pathways in the CNS. Most axons are
myelinated so these structures will be part of “white matter”
The cell bodies of neurons are clustered together into
ganglia in the PNS & nuclei/centers in the CNS. These are
unmyelinated structures and will be part of “gray matter”
Neural Tissue Organization
Anatomical structure of Nerves
Fig. 14.6