The average brain has:
- Approximately 100 billion neurons,
- 1000 to 5000 billion glial cells, and
- Approximately 60 trillion synapses in the cerebral cortex.
neural crest cell
Neural crest cells (NCC) are multipotent cells induced at the border of the neural plate that subsequently migrate throughout the embryo and later differentiate into multiple cell types contributing to most of the peripheral nervous system and the cranio-facial cartilage and bones, as well as pigment and endocrine
Neural tube defect
Neural tube defects are birth defects of the brain, spine, or spinal cord. They happen in the first month of pregnancy, often before a woman even knows that she is pregnant. … In spina bifida, the fetal spinal column doesn’t close completely. There is usually nerve damage that causes at least some paralysis of the legs
if the top region does not close brain does not develop properly
Somites are blocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryo. … As the somite matures, the outer cells transform from mesenchymal to epithelial cells, creating a distinct boundary between individual somites.
somites subdivide into the sclerotomes, myotomes, syndetomes and dermatomes that give rise to the vertebrae of the vertebral column, rib cage and part of the occipital bone; skeletal muscle, cartilage, tendons, and skin
Where do neurons come from?
• The developing embryo undergoes gastrulation.
• This creates a three layered embryo.
• Neural tissue develops from the ectoderm, the
top layer of the embryo.
What cells are capable of becoming neurons?
• Signals converge on the middle region of the ectoderm and induce it to become neural tissue.
The middle region will fold up to make neural tube.
The region on the border between ectoderm and the neural plate gives rise to neural crest which migrate away and produce some of the cell for peripheral NS
What is neurulation?
Neurulation – formation of the neural tube
• The neural plate rolls up into a tube.
• The center (lumen) of the tube is filled
with fluid. filled with CSF
Neural tube defects
The neural tube joins up first in the middle, and zippers
up to closure points.
Neural tube defects occur when the neural tube fails to close.
failure of closure the brain doesn’t develop properly
skull doesn’t develop properly
results in spina bifida. a birth defect that occurs when the spine and spinal cord don’t form properly. It’s a type of neural tube defect
Folic acid supplement to reduce the risk
somites = give rise to vertebrea
along the spin- segmental structure
= segmental vertebrea
Development of brain vesicles
Brain region develops earlier then the cortical region
forebrain, midbrain and hindbrain
Cortical folding, or gyrification, coincides with several
important developmental processes = increase surface
Progenitor cells proliferate to become neuroblasts.
Cells divide with nucleus at ventricular surface
Symmetric division produces two more progenitor cells.
the nucleus moves away from the ventricular surface and it comes down as the cell strarts
dividing –> m-phase
Progenitor cells undergo cell division to produce
progenitor cell = stem cell can produce neurons and
the glial cells
In vertebrates, a neuroblast or primitive nerve cell is a postmitotic cell that does not divide further, and which will develop into a neuron after a migration phase.
no quite neuron yet
How do the neurons get to the cortex?
The neuroblasts generated from apical progenitor cells migrate along the process of the apical
progenitor cell to the cortex.
• This is radial migration
Neurons migrate to form the cortex in an insideout manner.
• That is, newer/younger cells further out.
Neural tube - vesicles
Parts of the neural tube swell to become vesicles.
mediated through the process of patterning
• These vesicles form different parts of the brain.
• Signals (growth factors) produced in specific regions pattern the developing brain and spinal cord
How and when are the pyramidal neurons patterned?
• Progressive patterning over time.
• Result of patterning of cortex – different expression patterns of particular transcription factors in different
regions, and different layers.
emx2 KO reduction results is
reduction in size of visual and auditory cortex
• Pyramidal motor neurons need to synapse on motor
neurons in the spinal cord
• Extend out projection, called neurites.
• Growth cone on developing axon.
A growth cone is a large actin-supported extension of a developing or regenerating neurite seeking its synaptic target
Neurons extend a growth cone at the end of the
it follows the signals produced in the environment
to find their target
Environmental signals control axon guidance
• Importance of:
• Substrate (extracellular matrix, other cells)
• Guidance cues not that way, stop, this way
• Attraction/repulsion easy and smooth
path to migrate ?
role of growth cone in axon guidance
Growth cone expresses many receptors.
• Many ligands are expressed by environment.
• Intracellular response to these ligands can affect
behaviour of growth cone
ligand binding Very localized in growth cone- cuases extension or repulsion of the microfilament
binding of ligand to the receptor –> change dirceton
Environmental cues can be generally divided into
repulsive or attractive
can act Long range or Short range (direct cell contact) - contact repulsion or contact adhesion
receptor want to get closer to the attractive source
is the formation of synapses between neurons in the nervous system.
1- axon terminal and dendrite make contact
2- Contact stabilization (cell-cell interaction)- close interaction
3- Synaptic maturation: recruit synaptic machinery to site of immature synapse. ( developing more vesicles and releasing more neurotransmitter
• Neural activity regulates synaptic connections.
- pre-synaptic maturation followed by post -synaptic maturation
Synapses are plastic, and change over time
• Activity is critical in maturation of synapses, and
whether they are retained or eliminated.
- relative activity and the timing is important
• Plasticity and learning.
Where do glia come from?
Astrocytes and oligodendrocytes also develop from the neural plate.
Gliogenesis begins after neurogenesis is finished.as the signal changes
Astrocytes important in formation and development of
Oligodendrocyte important in myelinating the axon
where do microglia develop?
• Microglia develop from the immune system.
they migrate into the brain during brain development
Development of dendrites
• The pyramidal neurons extend an apical dendrite, and multiple basal dendrites.
• Postnatally, the length of these dendrites increases greatly.
dendrites develop more extensively postnatal
Development of dendritic spines
- Dendritic spines develop over time.
* There are different types of spine shapes.
synaptic connections change over time
• The number of synapses changes with age.
• Important part of development/ maturation.
-elimination - important part of the development and
Development of electrophysiological function
Electrophysiological properties develop
early, but undergo significant changes in maturation
initially simple type of firing, then Action potential, and more complex pattern of AP
changes in # of ion channel changes the electrophysiological response
Prenatal brain development
neurons develop neurons multiply neurons migrate neurons branch and form synapse pruning/ apoptosis synapse reorganize myelination
brain development after birth
Important processes in brain development
continue after birth.
• Development of brain can be influenced by
genes and environment