Brain Development Flashcards
State the cerebral hemisphere’s (cortex) function (3)
receives sensory inputs (e.g. visual & auditory)
controls voluntary motor outputs
association areas (higher mental activities)
Hypothalamus function (1)
essential for control of homeostatic processes
e.g. temperature, thirst, appetite
Pituitary gland function (1)
linked with endocrine system via hypothalamus
Cerebellum function (1)
coordinates muscle movements, maintains posture
Medulla function (1)
vital physiological processes of the autonomic nervous system e.g. breathing & heart rate
How can we study brain development? (5)
we don’t have direct access to the developing brain in a foetus = look at animals
-Basic brain organisation is the same in all vertebrates
-structurally different
- similar regions conserved
birds
reptiles
amphibians
mammals
bony fish
shark
lamprey
= animal models
fundamental processes + steps can be studied in diff model organisms
How do you make a brain? (5)
Step 1: Neural induction - v early embryo, after gastrulation, brain develops from this -> 3 primordial layers (ectoderm = brain, mesoderm = muscle, endoderm =gut)
Step 2: Neural tube formation - forms the whole CNS of an embryo
Step 3: Subdivision in specific domains along the anterior-posterior axis (A-P patterning) - specialised regions
Step 4: Neurogenesis: Generation and expansion of new neurons
Step 5: Connecting neurons
How can you monitor neural induction in an animal model? (4)
In chicks:
- use electron microscopy = view the disc using ISH too to view early embryo
- see diff gene pattern (neural in nature) = forms CNS
- SOX2 = early markers of development (neural tissue) -> very strongly expressed in medial part of embryo -> these are the cells that are induced to become CNS
- also look at Delta1
Describe neural tube formation (5)
1) flat disc undergoes neural induction
2) infolding occurs(structural alterations): the middle neural tissue folds into itself
3) This further invaginates into the embryo
4) Eventually connects at the top
5) Forms complete tube that has internalised into embryo (neural tube)
Name the neural tube subdivisions (4)
Embryo precursors -> Adult:
Prosencephalon -> forebrain
Mesencephalon -> midbrain
Metencephalon -> hindbrain
Myelencephalon -> spinal cord
Describe anterior posterior patterning (2)
Establishment of different brain regions occur, which overtime develop further into these specialised regions of CNS
Describe dorso-ventral patterning (2)
Different neuronal identities are established along the dorsal-ventral axis of the neural tube
- section of spinal cord -> immunostaining shows the different markers of the diff cell types
what are key regulators that cause patterning of the neural tube? (2)
Secreted morphogens from secondary organisers (= specialised cell types in the dev. embryo) establish signalling gradients that determine cell fate
Homeotic genes e.g. those encoding homeobox transcription factors impart regional/segmental identity and position secondary organisers
Define morphogen (1)
A secreted factor that functions at a distance from its source to directly influence cell fate in a dosage dependent manner
What is the “French flag model”? (4)
Each cell has the potential to dev as blue, white or red
position of each cell is defined by conc of morphogen
positional value is interpreted by the cells which differentiate to form a pattern
= french flag
they develop diff identities depending on how far they are from the morphogen -. eg closest receive high conc of morph and furthest receive lowest conc
Explain the morphogen SHH (3)
the SHH gene encodes for the sonic hedgehog protein
it’s secreted molecule produced from the ventral neural tube all along to the developed spinal cord
this is critical for DV patterning of CNS
describe SHH and eye positioning (3)
the splitting of the eye field allows for the formation of the 2 separate eyes
inhibition of SHH shifts eye position: immediately the eyes move medially (w/ decreased SHH) = this is due to a genetic mutation that disrupts SHH production/exposure to specific pharm/natural compounds that inhibit SHH signalling
Loss of SHH/ v v lowe levels = cyclopia and loss of ventral neuronal fates <- no SHH produced
How do we make specific neurons in the lab? (5)
- making neurons may be useful for regenerative med
- using ES cells -> we can configure the fate by treating them with either SHH or an SHH antag (Cyclopamine WNT)
- SHH = Ventral fate
- Cyclopamine = dorsal fate
- undergoes WNT signalling
- Gli3R is a biomarker of tissue growth and inactivation of it = mutant phenotypes
Describe A-P patterning of the embryo in regards to the structure of the head (5)
- Dkk1-/- embryos are head-less
- WNT antagonists are required for head formation -> = Wnt signalling = headless, thus the whole head structure is completely dependent on INHIBITION of WNT signalling by secreted WNT antagonist
- WNT antagonists e.g. Dickkopf
- low to high WNT + high to low antag. = forebrain, midbrain, (hindbrain), spinal cord
- Fly → Frog → Chick and mouse (highly conserved process)
How are different brain regions established along A-P axis? give an example (2)
- Homeotic genes encode transcription factors e.g.
Homeobox (HOX) transcription factors
-Homeotic genes in Drosophila: WT vs mutant w/ fully developed legs in the place of the antennae
Describe the specific homeotic genes in vertebrate neural tube (4)
Otx2 is essential for the Forebrain and Midbrain
Gbx2 is essential for Hindbrain and spinal cord
- Otx2 and Gbx2 work in a complementary pattern (antagonistically)
Cross-repressive activities establish and maintain a sharp expression boundary between the mesencephalon and r1
Explain the IsO regulatory network (4)
Otx2 - r1 = fore + mid brain
Gbx2 - ar1 = hindbrain + spinal cord
A secondary organiser, the IsO, that produces FGF8 is established at the mes/r1 boundary:
Otx2 inhib Fgf8 expression
Gbx2 <-> Fgf8 (maintain each other’s expression)
What do Gbx2-/- embryos lack? (1)
They lack the cerebellum
- seen in mice: midbrain still forms but lack cerebellum when the GBX2 gene is completely deleted
Name some secondary organisers and their functions (4)
Several secondary organisers refine the
anterior-posterior specification of neural tube territories
key organisers:
- ANR: Anterior neural ridge - produces FGF8
- ZLI: Zona limitans intrathalamica - expresses SHH
- IsO: Isthmus organiser - produces FGF8
What happens as a result of Fgf8 loss? (2)
Fgf8 loss results in defects in gastrulation and early embryonic lethality because it is also important in the cranial facial structure formation
(seen in The Isthmus (mid-hindbrain organiser))
How can we bypass these early defects to study the function
of Fgf8 during later stages of development? (2)
Cre/loxP technology!
- carries out deletions, insertions, translocations + inversions at specific sites in the DNA, at specific times in development = DNA modification that can be targeted by external stimuli
==> bypass the early embryo lethality probs - Cre: site-specific DNA recombinase
“molecular scissors
Describe Conditional knockouts: Cre/loxP technology (6)
1) Floxed gene
2a) Add Cre
2b) minus Cre
3a) + Cre = defective gene
3b) Transcription = Normal mRNA
4a) Transcription = mRNA encoding defective protein
Conditional knock-outs mouse eg (3)
1) Mouse carrying Cre recombinase gene controlled by tissue specific promoter X (Mes/r1-specific Promoter)
2) Mouse carrying conditional (floxed) alleles of gene Y (Fgf8 exon 3)
1 + 2 = Gene Y is inactivated by Cre in tissue X (Mes/r1-specific gene Fgf8 deletion)
Explain En1cre activity in early mes/r1 (3)
1) Rosa26 —-STOP—-lacZ
2) w/ Cre expression only in mes/r1
3) = lacZ expression in mes/r1 and derivatives
What is Fgf8 necessary for? (3)
Fgf8 is required for survival + development of the midbrain and
cerebellum
- seen in cre/lox mice:
- control: Cre expression only
in mes/r1 - Fgf8 cko: Fgf8 loss only in mes/r1, All other tissues maintain normal Fgf8 expression
What is Fgf8 is sufficient to induce? (2)
Fgf8 is sufficient to induce mes/r1 identity
= this was enough to induce the dev of structures resembling cerebellum when (ectopically applied to the diencephalon)
Summary 1(3)
A-P patterning of the neural tube establishes distinct territories
A-P patterning is coordinated by the action of
secreted morphogens, homeobox (and other) transcription factors
D-V patterning likewise establish distinct territories and neuronal identities along the D-V axis; established by the action of morphogens like SHH and transcriptional regulators like Gli3
Define Neurogenesis (1) and talk about NSC’s (2)
The generation/expansion of new neurons
- Neural stem cells are known as radial glial cells because they extend to the peel surface of dev CNS (you can see translocation of nucleus that undergoes cell division = interkinetic nuclear migration)
- NSCs self-renew (maintain NSC pool) through symmetric or asymmetric division
what are the 2 types of cell division in the neurogenic cell? (2 +1)
symmetric mitosis: radial glial cells expand the population of these cells = increases the pool of available neurogenic cells.
asymmetric mitosis: generate another radial cell (maintain the population of the radial glial cells) but then generate a neuronal precursor = postmitotic neuronal precursors that will migrate up towards the peel surface + essentially start differentiating into mature neurons
these neural stem cells essentially self renew so they can maintain the neural stem cell pool either through symmetric or asymmetric cell division.
Name the layers of the developing cortex (3)
inner surface = ventricular zone (next to ventricle)
just outside this = sub-ventricular zone
cortical plate outside that
Why is the ventricular zone important? (1)
neural stem cells that give rise to new neurons are localised here = neurogenic zone of the developing cortex
Name the 2 main cortical neurons (3)
Two major classes of neurons: excitatory (glutamatergic) and
inhibitory (GABA-ergic) neurons:
- Pyramidal neurons are Excitatory (glu)
- Interneurons are Inhibitory (GABA)
Explain how Glutamate- and GABA-ergic neurons are born in different parts of the embryonic brain (3)
- Glutamatergic neurons produced in the dorsal part of the forebrain (pallium) migrates radially into the developing cortex (using SHH)
- GABA-ergic neurons produced in the ventral part of the forebrain(subpallium) migrates tangentially to the pallium to integrate into the cortical circuitry
LGE: Lateral Ganglionic Eminence = produce interneurons
MGE: Medial GE
Explain Glutamatergic progenitors migration (+settling) (3)
Glutamatergic progenitors migrate radially and different
populations, born at different developmental time points, position themselves in an “inside-out” fashion
Late-born neurons settle in the outer layers
Early born neurons settle in the deep layers of the cortex
(‘inside out’)
- so they migrate up to the peel surface and place themselves above the already generated neurons
What happens to Reeler mutants? (1)
Reeler mutants (mouse mutant) fail to undergo “preplate splitting” and have an inverted cortical organisation
Summary 2 (4)
New neurons are generated from neuronal stem cells
Different classes of neurons are born in distinct embryonic regions e.g. glutamatergic neurons (forebrain pallium and cerebellar rhombic lip) and GABA-ergic neurons (forebrain subpallium and cerebellar ventricular zone)
Neuronal progenitors migrate to their final position
Migratory cues are essential for normal brain development and architecture
Name some facts about neurons (4)
- Adult brain has 86 billion neurons
- Each neuron makes 7,000 connections with other neurons
- Total complexity = 100-500 trillion connections for transferring
info - 100 billion galaxies x 200 billion stars = 2,000 billion stars
How does Axon guidance work and name some molecules (2)
Works through a process of attraction and repulsion of molecules to help guide developing axons to the necessary source
eg of molecules:
- slit
- netri
- semaphorin
- ephrin
- DCC
- Robo
- Plexin
- Met
- L1
- Neuropilin
- Eph
Define Synaptogenesis (3)
the formation of synapses between neurons in the nervous system. Although it occurs throughout a healthy person’s lifespan, an explosion of synapse formation occurs during early brain development, known as exuberant synaptogenesis
- defects in this development can cause many disorders
Name some developmental brain defects and their causes (6)
Neurulation - spina bifida
Micro- or macrocephaly
Transformation, Aplasia or hypoplasia of specific regions e.g. cerebellar vermis hypoplasia (patterning)
Lissencephaly and heterotopia (migration)
Axon tract agenesis or mis-routing (axonal transport)
Altered connectivity and/or excitatory:inhibitory balance ( altered connectivity = autism)
Name some Human syndromes with cerebellar defects (5)
Joubert syndrome
Dandy-Walker malformation
Ponto-cerebellar hypoplasia
Cerebellar vermis hypoplasia
Rhomb- encephalosynapsis
Explain CHARGE syndrome (4)
1/8,000- 1/12,000 incidence -caused by an autosomal dominant congenital malformation
*Coloboma of the eye
*Heart defects
*Atresia of the choanae
*Retardation of growth and development
*Genito-urinary abnormalities
*Ear abnormalities
Brain defects:
*Hypoplasia of the inferior cerebellar vermis, brain stem
*Cerebellar heterotopia
*Olfactory bulb hypoplasia
*High (estimated 27.5%) of patients are autistic
Explain Vermis defects in CHARGE
syndrome (3)
This is vermis hypoplasia = incomplete development or underdevelopment of cerebellar vermis
= all children w/ this = dev. delays (80% severe, 20%
moderate)
- Reduced FGF signalling from the IsO results in vermis hypoplasia
What do we see in CHARGE models? (2)
- Fgf8 expression is reduced in mouse models of CHARGE syndrome
- Chd7 interacts with Fgf8 to cause cerebellar vermis hypoplasia
Describe Lissencephaly (4)
Smooth cortex, lack of gyri (agyria), broad, thick gyri(pachygyria) and neuronal heterotopia (cells in abnormal positions)
Caused by abnormal neuronal migration
Mutations in LIS1, DCX (Doublecortin)
- Autosomal recessive lissencephaly with cerebellar hypoplasia: depicts severe abnormalities of the cerebellum, hippocampus + brainstem maps due to Mutations in RELN
What are Corpus callosum defects? (2)
a defect caused by axon guidance
- either incomplete formation of corpus callosum or not formed at all
Describe Autism spectrum disorders(6)
- 1% of US children (3-17yrs) diagnosed with ASD
- Fastest-expanding developmental disability (from
1/150 (2007) to 1/110 (2009)) - 40% of children with autism do not speak
- Neuroanatamical abnormalities observed include
cerebellar hypoplasia and expanded pre-frontal cortex - Complex genetic inheritance, more than 300 genes
implicated - Many genes are linked to synaptogenesis suggesting that synaptic dysfunction and defective connectivity might underlie a subset of ASD cases
What are some genes implicated in ASD? (3)
number of proteins thought to be highly likely to be involved in synaptogenesis problems -. causing autism
BUT also genes:
eg SCN2A, SHANK3, CH83
-Many high confidence ASD risk genes encode chromatin regulators
Summary 3 (1)
Neurodevelopmental disorders can result from developmental defects in brain patterning, neurogenesis and growth, neuronal migration, axon pathfinding and synaptogenesis
What is the future? (4)
Human brain development
The aetiology of many neurodevelopmental disorders remain unknown
Can we treat any of these disorders e.g. those caused by synaptogenesis defects?
How do environmental factors affect brain
development?
