2301 final topic 10

Neurobiology of Development

Both brain and behaviors develop quickly
In early development

Embryonic Vertebrate Nervous System

Embryo nervous systems share similarities between species
Forebrain

Neural plate

Thickened region of early neural tissue that allows neural grooves to develop/raise up
Groove curls to then become neural tube

Neural tube

Structure in the early stage of brain development where the brain and spinal cord develop

Major Events in Development

7 weeks
Embryo begins to resemble a mini person
14 weeks
Brain looks distinctively human
7 months
Gyri and sulci begin to form
9 months
Brain looks like an adult brain

Origin of Neurons and Glia

Originate from stem cells which can come from different sources and can differentiate into many different cell types

Natural stem cell

Self renewing

Progenitor cells

Precursor cells derived from stem cells that can produce neuroblast or glioblast

• neuroblast: product of progenitor cells that give rise to different types of neurons
• glioblast: product of progenitor cells that give rise to different types of glial cells

Subventricular zone

Lining of natural stem cells around the ventricles found in adults

Weiss et al (1996)

Discovered that stem cells are capable of producing neurons and glia cells not only during development but also in the adult brain

How stem cells develop:
Neurotropic Factor

Chemical compound that signal cells to develop into neurons
Help keep neurons alive during adulthood

How stem cells develop: Epidermal growth factor (EGF)

Stimulates progenitor cell production
Stem cell → progenitor cell

How stem cells develop: Basic fibroblast growth factor (bFGF)

Stimulates neuroblast production
Progenitor cell → neuroblast

How stem cells develop: Human brain requires ~10 billion cells to form the cortex that blankets a single hemisphere
250k neurons/min at the peak of prenatal brain development
Brain prunes unnecessary cells and connections and sculpts itself according to the persons experiences and needs

Stage 1 of Brain Development:
Cell birth (neurogenesis

gliogenesis)

Stage 2 of Brain Development: Cell Migration

Begins shortly after first neurons are generated
Radial Glial Cells
Path making cells that a migrating neuron follows to its appropriate location

Stage 3 of Brain Development: Cell differentiation

Glial fibers extend from the ventricular zone to the cortical surface all throughout the brain
Cells migrate to inner layers first and then to outer layers
6 layers (migration: 6 → 1)

Stage 4 of Brain Development: Neuronal Maturation

Begins 20 weeks into development and continues long after birth
After migration and differentiation neurons mature in 2 ways:
Dendritic growth/arborization (branching): grow dendrites to provide surface area for synapses to form with other cells
Growth of dendrites from single protrusions to complex branches
Slower than axonal growth
1000x slower
micrometers/day
Axonal growth: extends axons to targets to initiate synapse formation and allow neural communication
Connections that axons need to make in the developing brain may be millimeters or meters away
Growth cone: growing tip of an axon
As it extends → they send out shoots
Responds to cues from 2 types of molecules
Tropic molecule: signaling molecule produced by the target that attracts/repels growth cones
Cell adhesion molecule (CAM): a chemical to which growth cones can adhere and be pulled towards their target
Filiopodia: processes at the end of a developing axon that reaches to search for a target or to sample environment for messages about location

Stage 5 of Brian Development: Synaptic Development

More than 100

Stage 6 of Brain Development: Cell Death and Synaptic Pruning

Keeping all neurons and synapses that we are born with is inefficient on a metabolic level
Neural Darwinism
Hypothesis that cell death and synaptic pruning are the outcome of competition among neurons for connections and metabolic resources similar to natural selection
If not enough resources are availablethe cell will be triggered to undergo apoptosis (cell self destruction)
Surviving neurons and synapses are pruned in an experience dependent manner
Optimize the brains efficiency by reducing the excess connections that dont function as well as others
Leave only the most useful and productive neurons remaining

Stage 7 of Brain Development: Myelinogenesis

Oligodendroglia forms myelin in the CNS
Myelination of the cortex begins after birth and continues long after birth
Areas with simpler functions → myelinated earlier and darker
Areas with complex functions → myelinated later and lighter