Lecture 10- Neuronal migration in brain development I (cortex-pyramidal) Flashcards
What does this picture show?
- development of the human brain
- undegroes large morphological changes
- requires birth of many neurons and their migration to target regions to achieve the final product
What are the 4 stages of development we have touched upon in class so far?
- Neural induction
- Neurulation
- Patterning of the neural tube (A-P and D-V)
- Neuronal migration
Where do neurons migrate and what is this process guided by?
- during development neurons migrate from the site of production (where progenitors reside) to a final position in neuronal circuits
- this process relies on cues in the environment
What are the NMDs and why do they occur?
- neuronal migration disorders (NMDs)
- need cues in the environment to guide neuronal migration, when these are absent it can result in structurally abnormal or missing brain regions.
- usually one of the proteins or genes involved in the migration process is mutated, this causes the migratory defect
What is Lissencephaly?
- neuronal migration disorder
- means smooth brain in Greek, called that as brain has no folds in these patients
- results from a mutation in LIS1, which is a downstream target of RELN (gene coding for Reelin)
- also get abnormalities in the cerebellum and the hippocampus
What are the 4 regions we will discuss in neuronal migration?
- Cortical hemispheres
- Cerebellum
- Hippocampus
- Rostral migratory stream
What is cortex derived from and what must happen for that to be achieved?
- cortex is derived from the Telencephalon
- to have a mature cortex there must be:
1. generation of the appropriate number of neurons (proliferation)
2. movement of newly generated neurons to a final position (migration)
3. Create cortical connections between neurons (differentiation and maturation)
How many layers are there in the cortex?
6
How do the cortical layers differ at first glance?
- each layer has different density of cells
- highly ordered
What are the cortical layers also called?
-cortical lamination
What are the two main neuronal types in the brain?
- Excitatory (pyramidal)
- Inhibitory
What are the characteristics of excitatory cortical neurons? (shape, neurotransmitter, circuits, morphology, % of cortex)
- also called excitatory pyramidal or projection neurons
neurotransmitter: Glutamate
morphology: pyramidal shape soma, layer specific aroborization
circuits: project both long distance and locally - comprise 80% of the cortex
What are the characteristics of inhibitory cortical neurons?
neurotransmitter: GABA (gamma-aminobutyric acid)
morphology: diverse, layer specific arborization
cicruits: project locally
- called inhibitory interneurons (exception excitatory spiny stellate cells)
How much of the cortex is made up of inhibitory cortical neurons and what is their main function?
- 20% of the cortex
- all cortical output is mediated through pyramidal neurons (80%) and the major role for interneurons is to fine tune this output
- modulate cortical output
What does a mouse brain look like? (adult)
- smooth brain, olfactory bulbs are large
- even here have lamination (so can use rodent as an animal model for humans)
- morphology of the neurons is very similar
What does an embryonic mouse brain look like?
have only 2 layers:
- ventricular zone= neurons born there
- cortical plate= where the neurons will go
What are the cortical domains during development (mouse)?
- during development the cortical wall can be divded into distinct domains
- ventricular and subventricular zone= germinal zone where neurons are born
- intermediate zone= migration of neurons occurs here
- cortical plate and marginal zone= where neurons terminate their migration
remember: subplate is much smaller in rodents than in humans
What happens to the cortical wall during develeopment?
- expansion of the cortical wall over time
- starts off as a thin structure, develops into a thicker and thicker structure
- lot of cells moving and growing
- picture is the cortical wall of a mouse from E11-E18 (E19 is term for a mouse)
What is corticogenesis?
- process in development when the cortex is created
- includes the birth and migration of neurons
When does corticogenesis occur in mice?
- occurs between embryonic day (E) 11-18
- term is around E 19
What technique is used to determine how the embryonic brain develops 6 layers?
-cortical birth dating analysis
What is the cortical birth-dating analysis? How is it done in rodents?
- neuronal birth is the time of final division as differentiated neurons do not divide and are referred to as post-mitotic
- can identify the birthdate using DNA analogues that incorporate into the genome during the S phase of the cell cycle (eg. 3H-thymidine, BrdU, EdU)
- S-phase markers are permanent and can be detected at later stages
- stain the mice at day 11, 13 and 15
- look at the adult brain layering and see which neurons were born on which day and can identify their location
Which layer of the cortex is born first and which last?
- the neurons in the lowest layer are born first
- neurons in the upper layers are born last
- inside out migration
How does neuronal migration occur?
- migration of neurons from the ventricular zone into the cortical plate creates the characteristic 6 layers
- it is called inside out becuase the first born are in the lower layers and the last born are in the upper layers
- the neurons leap frog over the existing layers to get higher up in the cortex
- one layer forms and then one forms on top of it and so on and on
What are radial glial cells and their main functions?
- bipolar-shaped cells that span the width of the cortex in the developing central nervous system(CNS)
- serve as primary progenitors capable of generating neurons, astrocytes, and oligodendrocytes
- during development, newborn neurons use radial glia as scaffolds, traveling along the radial glial fibers in order to reach their final destinations
Why are radial glia called radial?
-they are at a 90 degree angle to the ventricular surface
How do you observe neuronal migration?
- to visualize cell movement, neurons are labelled using fluorophores and then monitores using confocal microscopy
- e.g. Green Fluorescent Protein (GFP) encoded in viral DNA (retrovirus-GFP)
What are the four phases of radial glia daughter cells’ migration?
Phase 1: Radial movement of pyramidal neurons from the ventricular zone (VZ) to the subventricular zone (SVZ)
Phase 2: Conversion to multipolar migration, pause in migration in intermediate zone (IZ)
Phase 3: Retrograde motion towards the ventricle
Phase 4: Radial migration to the cortical plate
-some daughter cells go through a multipolar phase (no longer have a bipolar shape), some will move down but ultimately all go towards the cortical plate
What are the 2 distinct sequences that neurons can be generated in in the cortex?
- Direct sequence: radial glial cell generates post-mitotic neuron which migrates to the cortical plate
- Indirect sequence: radial glia cell generates an intermediate progenitor cell which undergoes additional rounds of proliferation (subventricular zone= SVZ) to produce a post mitotic neuron
How can you identify distinct populations of cells during the migration period?
-can be identified by differing transcription factors, cells turn on different genes depending which zone they are in and how mature they are
examples:
- Pax 6 homeobox TF expressed by progenitor cells in the ventricular zone (VZ)=RED
- Tbr2 T-Box TF expressed by intermediate progenitor cells=GREEN
- NeuroD basic helix-loop-helix TF expressed by post-mitotic migrating neurons=BLUE
- Tbr1 T-box TF expressed by maturing pyramidal neurons=ORANGE
What transcription factor do progenitor cells in the VZ express?
-Pax 6 homeobox (RED in picture)
What transcription factor do intermediate progenitor cells express?
-Tbr2 T-box (GREEN in picture)
What transcription factor is expressed by post-mitotic migrating cells?
-NeuroD basic helix-loop-helix (BLUE in the picture)
What transcription factor is expressed by maturing pyramidal cells?
-Tbr1 T-box (ORANGE in the picture, the one on top)
How does the cortex differ in different mammals? (evolution-wise)
- disproportionate expansion of the cortex in large-brain mammals
- increase in the degree of cortical folding (convolutions)
What did the cortical birthdating studies in non-human primates reveal?
- unlike in rodents, the layer IV is subdivided into three distinct layers 4a, 4b and 4c
- neurons in the deepest layers (layer VI and V) are the earliest to be born (oldest or early born)
- neurons in the superficial layers are the latest to be born (youngest or late born)
What are the differences in a primate brain when compared to a mouse brain?
- cortical domains are in different proportions compared to the rodent cortex during development
- subventricular zone is expanded to include and inner SVZ and outer SVZ
- intermediate zone (fibre) layer is separated into inner FL and outer FL
- suplate is disproportionately enlarged
- cortical plate is enlarged
- so have same domains in mice and primates but differing relative sizes
(PICTURE: upper is human, lower is mouse)
When does layering of the cortex happen in humans?
-at gestation week 17
What does the cortical area look like at GW (gestation week) 15? (humans)
-pictures show cytoarchitecture and proliferation at gestation week 15
What are outer radial glial cells?
- another type of radial glial cells that is in the human cortex
- differs from the “normal” radial glial cell (those found in mice for example) in that its soma is in the outer subventricular zone (OSVZ)
- attachment of fiber to the pial surface not the ventricular surface
- outer radial glal (RG) cells are positive for SOX2 indicating progenitor status
What is the model for human cortical development (when compared to rodents)?
- extended proliferative zones
- extended subplate region (fucntion unclear)
- Fibres do not span the entire cortical wall from ventricular to pial (instead have the outer glial cells as well)
- Two sets of progenitors attached at the ventricle (vRG- inner glial cells) and the other attached at the pial surface (oRG- outer glial cells)
How is the expansion of the human cortex achieved?
- there is a secondary round of proliferation (from the outer glial cells) and this most likely is the driving force behind the fast and large expansion of the cortex during development
- expansion of the human cortey is achieved from additional zones of proliferation
(picture shows the different radial glial daughter cell fates depending on in which animal)
What does Reeler cause?
(this is the mutant of Reelin)
- causes cytoarchitectonic abnormalities in the cerebral and cerebellar cortices
- with normal Reelin expression, radial glia are at a 90 degree angle to the ventricular surface, with Reeler these are a bit bent so the patient gets disorganised cortical plate
- failure to split the preplate
What are the migratory deviations in Reeler?
- cells labelled in the proliferative zones show altered migratory patterns compared to wild type cortex
- loss of radial migration (the radial glia are bent)
How is the layering disrupted in reeler mutants?
- birth dating analysis revealed that the layers are inverted
- the oldest neurons were positioned in the superficial layers
- the youngest neurons were positioned in the deep layers
- the neurons cannot “leap-frog” so they just push the layer above them further, that is how you get the inverse lamination
What is the Double Cortex Syndrome?
- brain disorder arising from a mutation in the gene doublecortin (DCX)
- DCX is required for normal migration of neurons from the ventricular region to the cortical plate, the cells carrying the DCX mutation only migrate about halfway to the cortex and then arrest migration
- DCX is also downstream of Reelin!
- on MRI it shows up as an extra band of gray matter underlying the normal outer aspect of the cerebral cortex
