Module B - Structural and functional brain development Flashcards
How many neurons and glial cells are in the adult cortex?
10-20 billion neurons
50-200 billion glial (support) cells
Describe neural tube differentiation:
Radial, circumferential and longitudinal direction
Precursor cells to neural tissues in neural plate are neuroepithelial cells
4-5 weeks neuroepithelial cells rapidly proliferate 250,000/minute (main driver of brain growth and defines ventricular and subventricular zones) following neural tube closure
Describe the brain regions during development and what they develop into:
Telencephalon - hypocampus, cerebral cortex, basal ganglia
Diencephalon - thalamus (almost all circuits use to integrate)
Metencephalon - cerebellum and pons
Myelincephalon - medulla
What forms the cerebral ventricles?
The cavity within the neural tube for the ventricles and central canal of the spinal cord
When does the most rapid period of brain weight occur?
Immediately before and after 40 weeks’ gestation
From 28-40 weeks there is exponential brain growth (circuit formation)
When does neuralation occur?
3-4 weeks post-conception
When does cell birth occur?
6 weeks, continues in the majority of neurons until 12 weeks
Continues in fewer neurons (primarily in cortex) until a few months postnatal)
When does migration occur?
~8 weeks until 24 months
Fewer neurons until birth
When does axonal/dendritic outgrowth occur?
18 weeks until 6 months post-birth
Fewer neurons until 18 months
When does programmed cell death occur?
~20 weeks until post brith
When does synaptic production occur?
~26 weeks until 18 months post birth
When does myelination occur?
Birth until 20 years
When does synaptic elimination/pruning occur?
12 months until 16 years, fewer neurons until 24 years
When do subplate neurons exist?
Prenatal period
Describe the embryonic disk:
Endoderm, mesoderm and ectodern
Initial CNS development termed neuralation
2-3 weeks after conception
Describe phase 1 of cell genesis:
Proliferation
Radial glia and IPC
Describe phase 2 of cell genesis:
Neurogenesis and migration, ~8 weeks
Blood vessels and pericytes
Microglia
Glut+ neuron
Describe phase 3 of cell genesis:
Neurogenesis and migration, up to ~6 months
GABA+ njeuron
Ependymal cell
Describe phase 4 of cell genesis:
Gliogenesis and maturation from ~6 months
Astrocyte
Oligodendrocyte
Describe the relationship between postconceptual age and absolute cortical GM volume
Positive
3x increase in volume over 12 weeks (extensive brain growth)
When does neuronal migration occur?
Primary neuronal migration occurs weeks 8-20 of gestation (largely complete by week 25-29)
Describe migration (cell displacement):
Oldest cells located farthest away from proliferative zone
Short distance
e.g. thalamus, dentate gyrus, brain stem
Describe active migration:
Movement of younger cells past the older cells (inside out)
Long distance
e.g. cerebral cortex
In active migration, how do neurons know how to get there?
Via radial glial cells (act as guides)
Migrating cells are immature, lacking dendrites
Neurons propel along surface of glial cell
Neuron recognises by way of chemical signals that is has reached final destination and stops migrating
Cells finished migrating align themselves with other cells and forms structures (aggregation)
Describe radial migration:
From VZ/SVZ cortex in radial direction (radial glia)
90% of cells
Describe tangential migration (parallel):
GABAergic interneurons migrate from MGE and LGE into cortex
Last cells in cortex (~18-36 weeks complete)
Describe neuronal differentiation:
Neuroblasts have simple morphology
Begin to differentiate during migration and once reached their final location (~25 weeks until adolescence)
Describe the four main overlapping events of neuronal differentiation:
- Dendritic growth/arborisation (post-synpatic connections with axons of other neurons)
- Axonal growth and targeting (short and long range fibre pathways)
- Synapse formation (synaptogenesis)
- Neurotransmitter receptor expression
Describe human dendritic development:
3-fold increase in dendrite length 30 weeks to term
Describe axonal guidance:
Guidance cues (fixed or diffusible chemical) Attract or repel Netrins, Slits (secreted), ephrins, semaphorins (cell surface)
Describe highly motile growth cones (which detect cues in the ECM):
Dynamic extension of developing axon
Contains sensory, motor, intergrative and adaptive functions required for axonal growth and targeting
Describe synaptogenesis:
Axons (with growth cones on end) and dendrites form a synapse with other neurons or tissue
Neurobiological substrate of almost all cell-cell communication
Describe the density of synapses in the brain:
Each neuron forms thousands of synapses
~28 weeks - 2-3 years ~40,000 synapses per second
Describe neuronal connectivity/circuitry:
Set of integrated components that serve a specific function
Cognition, emotions and behaviour all controlled by brain circuits
Circuit formation started prenatally
What are the two key pathways in the brain:
Thalamocortical
Corticothalamic
What is the function of the TC pathway?
Relays sensory and motor information form receptors in retina, cochlea, muscle, or skin to the thalamus and cortex
What is the function of the CT pathway?
Completes feedback loop by transmitting information back to thalamus
