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Chapter 2: Brain Development
TJHSST Neuroscience Society
Prepared by Usnish Majumdar
• First neurons must be created (differentiated
• Neurons must then migrate
• Axon-dendrite migration and connection
3 Layers of Embryo
• Ectoderm (“ecto” latin = outside)
– Skin
– Neural Tissue
• Mesoderm (“middle” latin = middle)
• Endoderm (“endo” latin = within)
Neural Tube
• Formed 3-4 weeks
after conception
• Two parallel ridges
form from ectoderm
• Hollow
• Forms spinal cord
and Brain
Cell Differentiation
• Most embryonic cells are pluripotent stem cells
• A variety of chemicals signal cells to turn into
specialized cells
• Ectodermal cells are inhibited by molecules,
signalling a development into neural cells and not
skin cells
• After neural cell determination:
Sonic hedgehog – protein secreted from mesoderm
Higher concentration = glial cell
Lower concentration = motor neuron
Even lower concentration = interneuron
More Cell Differentiation (Yes, this is
• Signals help determine the specific
neurotransmitters that can be used by a
• If neurons are cultured by themselves =
• Cultured with cardiac tissue = acetylcholine
• Based on different genes turning off and on
Neuronal Migration in The Brain
•Neurons produced in center of tube,
migrate outward
•Ventricular zone -> Marginal zone
•Migration prominent in cortex
•Neurons migrate along fibers to destination
•inhibitory interneurons migrate tangentially
•External forces that can disrupt proper
neuronal migration:
•Alcohol, Cocaine, Radiation, etc.
• Axons travel long distances until they find
• In the case of a motor neuron, axon may travel
from spinal cord to foot
Axon Growth
• Controlled by Growth Cone
• Contains receptors for certain signaling
• Signaling proteins (actually classes of proteins)
– Netrin
– Semaphorins
– Ephrins
• Possible effects
Move forward
Change Direction
• Axon meets Dendrite and forms synapse
– Additional molecules help with target recognition
• Last process is the wrapping of axon in myelin
Paring Back
• Neural network is ‘pruned’ to create a more
efficient and accurate system (too many
connections at first)
• Apoptosis: programmed cell-death
– AP Biology: regulated by several proteins
• Neuron doesn’t receive enough trophic factor
to deter these proteins, it dies
• Each trophic factor supports a specific group
of neurons
– Ex: nerve growth factor = sensory neuron survival
Critical Periods
• Important period when nervous system must
obtain certain critical experiences.
• “use it or lose it”
• Enriched environments bolster brain
Development Continues
• Even in your late teens, your brain is still
• Frontal lobe is last to become connected with
myelinated axons
– Judgement, insight, and impulse control
– …college students …
Quiz: 5 minutes, 15 Questions