Brain Development Flashcards
Germinal Stage
first 2 weeks
- repeated cell division
- attaches to mother’s uterus about 10-14 days after conception
Prenatal Stage: Germinal period
- zygote divides forming a blastocyst, a hollow ball of 150 cells
- week 2, fully embedded in uterine wall and 250 cells
Embryonic Stage
2-8 weeks
- organogenesis from differentiated layers of blastocyst
- amnion and chorion form from outer layer of blastocyst
- cells from interior of blastocyst: ectoderm, mesoderm, endoderm
ectoderm
brain and spinal cord
mesoderm
muscles, bone, cartilage
endoderm
gastrointestinal tract
placenta
contains membranes that allow nutrients to pass from the mother to the umbilical cord
umbilical cord
contains blood vessels that carry nutrients and oxygen to the embryo, carries waste from embryo back to mother
28 day embryo
- recognisable head
- first formulation of a spinal cord (neural tube)
32 day embryo
brain is developing fast
44 day embryo
started to sprout arms and legs
Fetal stage
- 9 weeks to birth
- from parts of first trimester to last trimester
- muscles strengthen, bodily systems develop
age of viability
23 weeks
When survival outside the uterus is possible if the brain and respiratory system are sufficiently developed
cell body
the cell’s life centre
dendrites
receive messages from other cells
axon
passes messages away from the cell body to other neurons, muscles or glands
myelin sheath
covers the axon of some neurons and helps speed neural impulses
when do sulci and gyri develop
7 months
prenatal development and CNS
structural formation of CNS
-Interruptions, via genetic mechanisms or interuterine trauma or infections, likely to have significant impact on cerebral structure, so that brain’s morphology appears abnormal even at a macroscopic level
Postnatal development of the CNS
with elaboration of the CNS, in particular, dendritic arborisation, myelination, and synaptogenesis. Although still largely genetically regulated, these processes are thought to be more susceptible to the impact of neuronal activity and thus to environmental and experiential influences
sequence of brain development
- Hierarchical progression
- Cerebellar / brain stem areas mature first
- Posterior —> anterior
stages of brain development
- Formation of the neural tube (neurulation)
- Cell birth (neurogenesis; gliogenesis)
- Cell migration
- Cell differentiation
- Cell maturation (dendrite & axon growth)
- Synaptogenesis (formation of synapses)
- Cell death & synaptic pruning
- Myelogenesis
Formation of the neural tube
- The neural tube forms soon after conception: during the third week of gestation (embryonic period)
- Formation of a hollow tube that develops into CNS (i.e. spine and brain)
- Tube slowly closes over
- Inside tube neurons and glia develop and migrate
Spina bifida
part of the spinal cord is not fully encased in the protective covering of the spinal column
Anacephaly
- failure to close at the top of the neural tube
- fatal
Neurogenesis
neural stem cells line the neural tube
-Extensive capacity for self-renewal
-Give rise to progenitor (precursor) cells
-Eventually produce neuroblasts and glioblasts (nondividing)
-Begins early in gestation (approx. day 40)
Complete by midgestation
Proliferation
- Neurons multiplying during this period
- Number of neurons increases by hundreds of thousands every minute throughout pregnancy
- Concentrated period of proliferation occurring between 6 and 17 weeks after conception
- As a result, young infant has around 100 billion neurons
glial cells
- supportive and nutrient role in NS
- Enable regeneration of damaged nerves
Astrocytes
- Form blood-brain barrier
- Direct migration of neurons
- Clean up & plug injury sites
Oligodendrocytes
speed up transmission of neural impulses, coat axons with myelin
Microglia
clean up tissue around injury sites, especially grey matter
glial development
- Birth of glial cells begins after most neurons are born; continues throughout life
- Normal adult function is attained only after myelination is complete
- Myelination useful as a rough index of cerebral maturation
- MRI analyses used to look at myelin development
Cell migration
- Neurons move to particular locations throughout the brain where they will become part of specialized functioning units.
- Influenced by genetic instructions and the biochemical environment.
- Some neurons migrate passively (brain stem & thalamus), others actively (cerebral cortex).
when is brain most delicate?
Last 4 ½ months of gestation, the brain is especially delicate and extremely vulnerable to injury or trauma, including asphyxia.
The brain can more easily cope with injury during neuron generation than it can during cell migration and differentiation.
where are most neurons produced?
Most neurons are produced in the ventricular zone (VZ) and migrate radially from the VZ in the center of the brain out to the developing neocortex.
how to cells travel?
Cells travel along “roads” made of cells - radial glial cells - fiber extending from the ventricular zone to the surface of the cortex
The cells from a given region need to follow the glial road and they will end up in the right location
As the brain grows, the glial fibres stretch, but they still go to the same place
Lissencephaly
Brain fails to form sulci and gyri and corresponds to a 12-week embryo – can be caused by a virus, lack of blood supply or genetic causes
Cell Differentiation
The process in which neuroblasts become specific types of neurons.
Every neuron starts with the potential to become any specific type of neuron.
What each neuron becomes depends on where it migrates to.
essentially complete at birth but maturation continues for years
Neural Maturation
neurons must begin the process of growing dendrites to provide the surface area for synapses with other cells
Axons also extended to appropriate targets to initiate the formation of other synapses
Axons have specific targets that they must reach if the neuron is to survive and become functional
Process can be disrupted leading to behavioural disturbances
disruptions in neural pathways
- axons may be blocked
- axonal development can be disrupted if the axonal system’s target is damaged (system may degenerate)
Synapse formation
Synaptogenesis not equal throughout brain, or at same time
1st in primary motor and sensory areas
Then association areas; and prefrontal cortex (PFC)
This order is sensible to function – infant needs basic motor and sensory skills in place before more complex mental abilities can develop
Synaptic pruning
- elimination of synapses
- happens very rapidly
- lose up to 200 axons per second
Why this proliferation of synapses and then elimination?
Initial over-production allows brain to be maximally responsive to environment
Depending on this input, certain connections become important while others are irrelevant
Allows brain to fine-tune and specialize itself
Apoptosis
naturally occurring cell death
Myelination
First regions to be myelinated = regions NB for vital functions (eg spinal chord, medulla)
1st year after birth basic sensory and motor systems become myelinated
Later childhood myelination between brain regions occurs
Myelination of corpus collosum continues into early 20’s
Post natal development
Structural changes in both the major grey and white matter continue through childhood and adolescence.
Parallel changes in functional organization that are also reflected in behavior.
The brain increases four-fold during the preschool period, reaching approximately 90% of adult volume by age 5.
Brain lateralization
- The functions controlled by the two hemispheres diverge
- Typically, the left cerebral hemisphere controls the right side of the body
- -language processing
- Typically, the right hemisphere controls the left side of the body
- -understanding spatial information and for processing visual-motor information and the emotional content of information
- The hemispheres are connected by the neurons that make up the corpus callosum
- Signs of lateralization are evident at birth
- Direction the head is turned, grasp reflex, hemispheric response to speech sounds
Plasticity
the brain’s ability to change as a result of experience
-greatest in childood
“experience expectant” development
Captures the idea that the early experience of the organism plays an essential role in normal brain development, particularly in the early postnatal period and functional organization of the developing brain.
“experience dependent” development
Behaviour altered as a result of learning from the environment
The adolescent brain
- neural networks are streamlined, especially in prefrontal cortex and limbic system (emotions)
- upsurge of dopamine
dopamine
- Helps control the brain’s reward and pleasure centers
- Helps regulate movement and emotional responses, and it enables us not only to see rewards, but to take action to move toward them
- too much = psychotic
adult brain
- neural networks become more efficient
- Some loss of neurons, diminished functioning of neurons, and changes to related tissues, such as myelin
- Greatest loss in the areas that control sensory and motor activities
- Decrease in brain weight and volume
- Transmission of signals by atrophied neurons is less effective
- Declines in levels of neurotransmitters
- Formation of “senile plaques”
- Reduced blood flow to the brain
dementia
a gradual loss of cognitive abilities that accompanies abnormal brain deterioration and interferes with daily functioning
Left hemisphere functions
- sequential processing
- analytic thought
- logic
- language
- science and health
Right hemisphere functions
- simultaneous processing
- holistic thought
- intuition
- creativity
- art and music
Grey matter changes in adolescence
Volume of grey matter increases, peaks and then decreases through teen years
White matter in adolescence
Increases in a linear fashion as a result of steady progression of mylination of axons.
