Brain development

1
Brain development

• Nature and nurture
• From
• The University of Western OntarioDepartment of
  Psychology
• Psychology 240B Developmental Psychology
• http//www.ssc.uwo.ca/psychology/undergraduate/psy
  ch240b-2/

2
Outline

• Part 1 Brain development A macroscopic
  perspective
• Part 2 The development of the cerebral cortex
• Part 3 Nature and nurture

3
Part IBrain development A macroscopic
perspective
4
3-4 Weeks
5
3-4 Weeks
Neural Groove
6
3-4 Weeks
Neural Groove
Neural Tube
7
3-4 Weeks
Neural Groove
Neural Tube
Neuroepithelium
8
3-4 Weeks
Neural Groove
Neural Tube
Neuroepithelium
Brain
Spinal Chord
9
5 to 6 Weeks
Nervous system begins to function Hind-, mid-,
and forebrain are now distinguishable
10
5 to 6 Weeks
11
5 to 6 Weeks
12
5 to 6 Weeks
Forebrain
13
5 to 6 Weeks
Forebrain
Telencephalon
14
5 to 6 Weeks
Forebrain
Telencephalon
Diencephalon
15
5 to 6 Weeks
Forebrain
16
5 to 6 Weeks
Forebrain
Midbrain
17
5 to 6 Weeks
Forebrain
Midbrain
Hindbrain
18
7 Weeks

• Neurons forming rapidly
• 1000s per minute

19
7 Weeks
Division of the halves of the brain visible
14 Weeks
20
7 Weeks

• Nerve cell generation complete
• Cortex beginning to wrinkle
• Myelinization

6 Months
14 Weeks
21
7 Weeks
9 Months
5 Months
14 Weeks
22
7 Weeks
Telencephalon C-shaped growth Cortex Folding
9 Months
5 Months
14 Weeks
23
7 Weeks
Telencephalon C-shaped growth Cortex Folding
9 Months
5 Months
14 Weeks
24
9 Months
25
9 Months
26
9 Months
Medulla Hindbrain Pons Cerebellum
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9 Months
Medulla Hindbrain Pons Cerebellum
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9 Months
Medulla Hindbrain Pons Cerebellum
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9 Months
Medulla Hindbrain Pons Cerebellum
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9 Months
Controls respiration, digestion, circulation,
fine motor control
Medulla Hindbrain Pons Cerebellum
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9 Months
Midbrain
32
9 Months
Basic auditory and visual processing
Midbrain
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9 Months
Thalamus
Hypothalamus
Diencephalon
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9 Months
Sensory relay station Intersection of CNS and
hormone system
Thalamus
Hypothalamus
Diencephalon
35
9 Months
Telencephalon? 2 Cerebral hemispheres Forms a
cap over inner brain structures

36
9 Months
Cross-sectional view
37
9 Months
Cerebral Hemispheres
Cross-sectional view
38
9 Months
Cerebral Hemispheres
Thalamus
Hypothalamus
Cross-sectional view
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9 Months
As the telencephalon develops, it connects both
with itself, and with the diencephalon
Cross-sectional view
40
9 Months
As the telencephalon develops, it connects both
with itself, and with the diencephalon
Corpus Callosum
Internal Capsule
Cross-sectional view
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9 Months
Hippocampus

Telencephalon
42
9 Months
Formation of long-term memory
Hippocampus

Telencephalon
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9 Months
Thin layer of cells covering both hemispheres
Hippocampus
Cortex
Telencephalon
44
Cortex
High-level visual processing
Visual Cortex
45
Cortex
Auditory visual processing Receptive language
Visual Cortex
Temporal Cortex
46
Cortex
Sensory integration Visual-motor processing
Visual Cortex
Temporal Cortex
Parietal Cortex
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Cortex
Higher-level cognition Motor control Expressive
language
Visual Cortex
Temporal Cortex
Parietal Cortex
Frontal Cortex
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Cortical Development Begins prenatally Continues
into late adolescence
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II The development of the cerebral cortex

• A microscopic view

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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Dendrite
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Dendrite
Cell body
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Dendrite
Cell body
Axon
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Dendrite
Cell body
Axon
Synapse
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Dendrite
Cell body
Axon
Synapse
Transmit information through the brain
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Outnumber neurons 101 Nourish, repair,
mylenate neurons Crucial for development
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Outnumber neurons 101 Nourish, repair,
myelinate neurons Crucial for development
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Outnumber neurons 101 Nourish, repair,
myelinate neurons Crucial for development
Eg. Oligodendroglia
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Development of the Cortex

• 2 types of cells
• Neurons
• Glial cells

Outnumber neurons 101 Nourish, repair,
myelinate neurons Crucial for development
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8 stages of cortical development

• Neural proliferation
• Neural migration
• Neural differentiation
• Axonal growth
• Dendritic growth
• Synaptogenesis
• Myelination
• Neuronal death

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1. Neural proliferation

• Begins with neural tube closure

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1. Neural proliferation

• Begins with neural tube closure

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1. Neural proliferation

• Begins with neural tube closure
• New cells born in ventricular layer

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1. Neural proliferation

• Begins with neural tube closure
• New cells born in ventricular layer
• 1 mother cell produces 10,000 daughter cells

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1. Neural proliferation

• Begins with neural tube closure
• New cells born in ventricular layer
• 1 mother cell produces 10,000 daughter cells
• All neurons (100 billion in total) are produced
  pre-natally

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1. Neural proliferation

• Begins with neural tube closure
• New cells born in ventricular layer
• 1 mother cell produces 10,000 daughter cells
• All neurons (100 billion in total) are produced
  pre-natally
• Rate of proliferation extremely high
  thousands/minute

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2 Cellular migration

• Non-dividing cells migrate from ventricular layer

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2 Cellular migration

• Non-dividing cells migrate from ventricular layer
• Creates a radial inside-out pattern of development

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2 Cellular migration

• Non-dividing cells migrate from ventricular layer
• Creates a radial inside-out pattern of
  development
• Importance of radial glial cells

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2 Cellular migration

• Non-dividing cells migrate from ventricular layer
• Creates a radial inside-out pattern of
  development
• Importance of radial glial cells

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3. Cellular differentiation

• Migrating cells structurally and functionally
  immature

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3. Cellular differentiation

• Migrating cells structurally and functionally
  immature
• Once new cells reach their destination,
  particular genes are turned ?growth of axons,
  dendrites, and synapses

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4. Axonal growth

• Growth occurs at a growth cone

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4. Axonal growth

• Growth occurs at a growth cone

Growth cone
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4. Axonal growth

• Growth occurs at a growth cone
• Axons have specific targets
• Targets often enormous distances away
• Some axons extend a distance that is 40,000 times
  the width of the cell body it is attached to
• Finding targets ? ? chemical electrical
  gradients, multiple branches

77
5. Dendritic growth

• Usually begins after migration
• Slow
• Occurs at a growth cone
• Begins prenatally, but continues postnatally
• Overproduction of branches in development and
  resultant pruning
• Remaining dendrites continue to branch and
  lengthen

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Human Brain at Birth
14 Years Old
6 Years Old
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6. Synaptogenesis

• Takes place as dendrites and axons grow
• Involves the linking together of the billions of
  neurons of the brain

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6. Synaptogenesis

• Takes place as dendrites and axons grow
• Involves the linking together of the billions of
  neurons of the brain
• 1 neuron makes up to 1000 synapses with other
  neurons
• Neurotransmitters and receptors also required

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Overproliferation and pruning

• The number of synapses reaches a maximum at about
  2 years of age
• After this, pruning begins
• By 16, only half of the original synapses remain

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7 Myelinization

• The process whereby glial cells wrap themselves
  around axons

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7 Myelinization

• The process whereby glial cells wrap themselves
  around axons
• Increases the speed of neural conduction

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7 Myelinization

• The process whereby glial cells wrap themselves
  around axons
• Increases the speed of neural conduction
• Begins before birth in primary motor and sensory
  areas
• Continues into adolescence in certain brain
  regions (e.g., frontal lobes)

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8 Neuronal death

• As many as 50 of neurons created in the first 7
  months of life die
• Structure of the brain is a product of sculpting
  as much as growth

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III Nature and nurture in brain development
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III Nature versus nurture

• The adult brain consists of approximately 1
  trillion (surviving) neurons that make close to 1
  quadrillion synaptic links
• Functionally highly organized, supporting various
  perceptual, cognitive and behavioural processes
• Perhaps the most complex living system we know

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Question

• Of all the information that is required to
  assemble a brain, how much is stored in the
  genes?
• Nature view argues that most of the information
  is stored in the genes
• Nurture view brain is structurally and
  functionally underspecified by the genes ?
  emerges probabilistically over the course of
  development

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Nature View

• (1) Not much is left to chance

90
Nature View

• (1) Not much is left to chance
• (2) Brain a collection of genetically-specified
  modules

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Nature View

• (1) Not much is left to chance
• (2) Brain a collection of genetically-specified
  modules
• (3) Each module processes a specific kind of
  information works independently of other
  modules

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Nature View

• (1) Not much is left to chance
• (2) Brain a collection of genetically-specified
  modules
• (3) Each module processes a specific kind of
  information works independently of other
  modules
• (4) In evolution modules get added to the
  collection

93
Nature View

• (1) Not much is left to chance
• (2) Brain a collection of genetically-specified
  modules
• (3) Each module processes a specific kind of
  information works independently of other
  modules
• (4) In evolution modules get added to the
  collection
• (5) In development genes that code for modules
  are expressed and modules develop according to
  these instructions

The grammar genes would be stretches of DNA
that code for proteins that guide, attract, or
glue neurons together into networks that are
necessary to compute the solution to some
grammatical problem.
94
The nature view Evidence

• Neurogenesis
• Neuroblasts give rise to a limited number of
  daughter cells
• Cells have a genetically mediated memory that
  allows them to remember how many times they have
  divided

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The nature view Evidence

• Genetics and migration
• Mutant or knock-out mice

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The nature view Evidence

• Genetics and migration
• Mutant or knock-out mice
• Cannot produce a class of proteins called cell
  adhesion molecules (CAMs)
• Migration is disrupted because cells cannot
  attach to and migrate along glia

97
The nature view Evidence

• Growth of dendrites and axons
• Undeveloped neuron needs to establish basic
  polarity which end is which?

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The nature view Evidence

• Growth of dendrites and axons
• Undeveloped neuron needs to establish basic
  polarity which end is which?
• Involves specific proteins

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The nature view Evidence

• Growth of dendrites and axons
• Undeveloped neuron needs to establish basic
  polarity which end is which?
• Involves specific proteins
• Axons Affords a sensitivity to chemical signals
  emitted by targets

100
The nature view Evidence

• Growth of dendrites and axons
• Undeveloped neuron needs to establish basic
  polarity which end is which?
• Involves specific proteins
• Axons Affords a sensitivity to chemical signals
  emitted by targets

101
The nature view Evidence

• Formation of synapses
• Knock-out mice

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The nature view Evidence

• Formation of synapses
• Knock-out mice
• Staggered
• Neurons in the cerebellum make contact, but
  receptor surface does not develop
• Thus, a single gene deletion can interfere with
  the formation of synapses in the cerebellum

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The nature view Evidence

• Cell death
• Cells seem to possess death genes
• When expressed, enzymes are produced that
  effectively cut-up the DNA, and kill the cell
• Similar mechanism may control the timing of
  neuronal death

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Nurture view

• (1) Brain organization is emergent and
  probabilistic not pre-determined
• (2) Genes provide only a broad outline of the
  ultimate structural and functional organization
  of the brain
• (3) Organization emerges in development through
  over-production of structure and competition for
  survival

105
Nurture view

• Gerald Edelman Neural Darwinism
• Overproliferation of structures sensory
  experience produce Darwinian-like selection
  pressures in development
• Structures that prove useful in development win
  the competition for survival
• The rest are cast off

• (1) Brain organization is emergent and
  probabilistic not pre-determined
• (2) Genes provide only a broad outline of the
  ultimate structural and functional organization
  of the brain
• (3) Organization emerges in development through
  over-production of structure and competition for
  survival

106
The nurture view Evidence

• Does experience affect developing structures and
  functions?
• Is the pruning of brain structures systematic?
• Do developing brain regions competitively
  interact?

107
Hubel Weisel
The nurture view Evidence

• Raised kittens but deprived them of visual
  stimulation to both eyes (binocular deprivation)
• No abnormality in the retina or thalamus
• Gross abnormality in visual cortex
• Disrupted protein production caused fewer and
  shorter dendrite to develop, as well as 70 fewer
  synapses
• Effects only occur early in development, but
  persist into adulthood
• Example Surgery on congenital cataracts in adult
  humans

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Hubel Weisel
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired

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Hubel Weisel
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• One effect Monocular deprivation disrupted the
  establishment of ocular dominance columns

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The nurture view Evidence
Development of mammalian visual system
Adult structure
Cortex
Thalamus
Eyes/Retinas
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The nurture view Evidence
Development of mammalian visual system
Adult structure
Cortex
Thalamus
Eyes/Retinas
112
Hubel Weisel
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• Sensory input competes for available cortex
• With input from one eye eliminated, no
  competition
• Therefore, input from uncovered eye assumes
  control of available visual cortex and disrupts
  the establishment of ocular dominance columns

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Hubel Weisel
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• Sensory input competes for available cortex
• With input from one eye eliminated, no
  competition
• Therefore, input from uncovered eye assumes
  control of available visual cortex and disrupts
  the establishment of ocular dominance columns

Findings point to the importance of stimulation
from the environment
114
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired

115
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• A second effect Residual function of the
  deprived eye competitively inhibited by strong
  eye

116
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• A second effect Residual function of the
  deprived eye competitively inhibited by strong
  eye
• Deprived one of experience and then removed
  strong eye

117
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• A second effect Residual function of the
  deprived eye competitively inhibited by strong
  eye
• Deprived one of experience and then removed
  strong eye
• Prior to surgery, stimulation of deprived eye
  elicited activity in only 6 of cortical neurons

118
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• A second effect Residual function of the
  deprived eye competitively inhibited by strong
  eye
• Deprived one of experience and then removed
  strong eye
• Prior to surgery, stimulation of deprived eye
  elicited activity in only 6 of cortical neurons
  After surgery? 31

119
Kratz, Spear, Smith
The nurture view Evidence

• Early monocular deprivation
• After restoring stimulation, vision in this eye
  is severely impaired
• A second effect Residual function of the
  deprived eye competitively inhibited by normal
  eye
• Deprived one of experience and then removed
  normal eye
• Prior to surgery, stimulation of deprived eye
  elicited activity in only 6 of cortical neurons
  After surgery? 31

Findings point to the importance of competitive
interaction between developing brain regions
120
Impoverished Environments
The nurture view Evidence

• Animal raised in impoverished environments have
  brains that are 10 to 20 smaller than animal
  raised in normal environments. Why?

121
Impoverished Environments
The nurture view Evidence

• Animal raised in impoverished environments have
  brains that are 10 to 20 smaller than animal
  raised in normal environments. Why?
• Decreased glial cell density
• Fewer dendritic spines
• Fewer synapses
• Smaller synapses

122
Sor
The nurture view Evidence

• Cortical surgery
• Severed connection between optic nerve and the
  occipital cortex as well as the connection
  between auditory nerve and auditory cortex
• Reconnected optic nerve to auditory cortex
• Animals developed functionally adequate vision

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The nurture view Evidence

• Daphnia A crustacean easily cloned
• Simple nervous system consisting of several
  hundred neurons
• Connection patterns can be studied directly
• Genetically identical individuals show different
  patterns of neuronal connectivity

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Nurture view Summary

• Order in the brain is not highly specified by the
  genes
• Instead, structures and functions emerge
  probabilistically in development through the
  combined influence of initial over-production of
  structure, neural competition, and experience