Brain Bee Chapter 6 - The Developing Brain

layers of embryonic development

ectoderm (outer)
mesoderm (middle)
endoderm (inner)

stages of brain development

formation and induction, proliferation, migration

neural induction

signals from mesoderm trigger ectoderm cells to become nerve tissue

3 weeks gestation

human brain begins to form as neural tube

neural tube

ventricular zone = inner surface
intermediate zone = middle
marginal zone = outer surface

4 weeks gestation

individual sections of brain (forebrain, midbrain, hindbrain) are recognizable

7 weeks gestation

signs of eyes and hemispheres appear

6 months gestation

ridges of brain can be observed

sonic hedgehog

signaling molecule for differentiation of cells

proliferation

division of neural stem and progenitor cells
includes symmetric and asymmetric division

symmetric divison

results in two identical daughter cells able to keep dividing

asymmetric division

results in one daughter cell that keeps proliferating and one that becomes differentiated

microencephaly

caused by premature switch to asymmetric divisions
severe reduction in brain size

megalencephaly

caused by excessive proliferation
abnormally large brain

migration

neurons journey to long-term location in brain, 3-4 weeks after conception
neurons move from ventricular zone (inner surface of neural tube) to marginal zone (outer surface)
accumulate in intermediate zone
guided by guidance mechanisms

radial glia

most common (90%) guidance mechanism for migration
neurons use this glia as scaffolding, projecting from intermediate zone to cortex

factors influencing migration

alcohol exposure, cocaine, radiation → intellectual disability, epilepsy (+ gene mutations associated with migration)

growth cone

where an axon extends from, guided by molecular cues (attractive or repellent)

signaling molecules

families of proteins like netrin, semaphorin, ephrin
attractive or repellent for axon’s growth cone
same proteins in all mammals

netrin

signaling molecule for connections
discovered in worms

synapse

connection between axon and dendrite

synaptic anchoring

axon and dendrite produce proteins and molecules that mediate target recognition to anchor the synapse together

astrocytes

provide scaffolding and passive support to neurons
important for synapse development, connection, and function
aid in synaptic pruning

acetylcholine

neurotransmitter for motor neurons (movement)

neurotransmitter type for neurons

influenced by location of synapse

nodes of ranvier

regularly spaced gaps within the myelin sheath that allow electrical signals to jump down axon faster

saltatory conduction

phenomenon of electrical signals jumping down the nodes of ranvier / myelin sheath
(“saltatory” = leaping)

neurons that survive to adulthood

½ generated during development

apoptosis

programmed cell death
activated if neuron does not receive enough trophic factors
method that neurons are paired back / removed

trophic factors

life-sustaining chemical signals
produced by target tissues

nerve growth factor

type of trophic factor
for survival of sensory neurons

synaptic pruning

removal of excess connections, dependent on activity of neuron’s connection