Early Brain Development

Question 1

What happens during embryo gastrulation?

Answer 1

Gastrulation precedes development of the nervous system.

During gastrulation,

The ectoderm, mesoderm and endoderm form.

Midline, dorsal-ventral & anterior-posterior axes are established

The primitive streak forms and cells from the primitive streak form the notocord, which establishes where the nervous system develops, the neural plate (ectoderm over the notocord).

Formation of the neural plate = neurulation

Question 2

At 20 days, what happens for the neurulation of the mammalian embryo?

Answer 2

The neural plate invaginates to form the neural tube, which will eventually form the brain and spinal cord.

The cells that make up the neural tube are stem cells called neural precursor cells that will form the cells of the CNS (neurons, astrocytes & oligodendrocytes).

The neural precursor cells eventually form non-dividing neuroblast cells that will eventually differentiate into neurons.

Question 3

During the formation of the neural tube, what specializations occur? What do the specialized areas do?

Answer 3

During formation of the neural tube, two regions deviate from becoming neural precursors; instead they become the floorplate and roofplate.

These structures provide inductive signals that help determine precursor cell fate; e.g. neurons close to the floorplate eventually become motor neurons whereas neurons far from the floor plate eventually become sensory neurons (remember spinal cord morphology).

The somites are mesoderm-derived structures that form musculature and skeletal structures.

Question 4

How do cells decide what sort of neuron to differentiate into?

Answer 4

“Local signals specify sensory relay neurons, interneurons, and motor neurons”

What sort of neurons the neural precursor cells differentiate into is determined by signals expressed specific elements of the neural tube (floorplate, roofplate, notocord).

These signals ultimately activate specific transcription factors in the target precursor cells and these transcription factors determine the ultimate cell fate.

The spatial pattern of these signals/transcription factors determines the spatial arrangement of what type of neurons are where in the CNS (above example is the spinal cord).

Question 5

What is the neural crest?

Answer 5

Develops into peripheranl NS

The neural crest is a group of precursor cells that develops on either side of where dorsal midline of the neural tube.

The neural crest cells migrate away from the neural tube and form elements of the peripheral nervous system, i.e. sensory neurons, Schwann cells, neurons that form the autonomic & enteric nervous system, adrenal glands.
Some neural crest cells have non-neural fates as well.

Question 6

Outline what happens in the later stages of neurulation.

Answer 6

During later stages of neurulation the anterior neural fold will begin to form what will become the brain.

The neural tube and somites will form the spinal cord.

The sensory ganglion just outside the spinal cord are derived from the neural crest cells and will eventually form the DRG.

Question 7

What are the 3 regions of the developing brain

Answer 7

Prosencephalon (forebrain)

Mesencephalon (midbrain)

Rhombencephalon (midbrain)

Question 8

What will the prosencephalon develop into?

Answer 8

Prosencephalon (forebrain)

Will divide into the

• telencephalon , which gives rise to the cerebral cortex, hippocampus, basal ganglia and other forebrain nuclei
• the diencephalon, which forms the thalamic nuclei, hypothalamus and what will eventually become the retina (optic cups)

Question 9

What will the mesencephalon develop into?

Answer 9

Mesencephalon (midbrain)

Dorsal portion will form superior & inferior colliculi.
Ventral portion will form the midbrain.

Question 10

What will the rhombencephalon (hindbrain) develop into?

Answer 10

Will divide into two sections:

one that will becomes the pons and the cerebellum

the other will become the medulla

Question 11

Segmentation (definition, components, and maintenance signals)

Answer 11

– a process of dividing up the body into discrete, repeating units that help to establish specification of body regions along the anterior-posterior axis.

• Neuromeres – individual segments observed in the neural tube.
• Rhombomeres – individual segments observed in the developing hindbrain

What maintains this segmentation, is partly regulation of cell mobility (cells do not cross the boundary that separates segments) and partly cell adhesion (cells prefer to stick to/associate with cells from the same segment).

The segments, which start out as being nearly identical to one another, provide the substrate for regionalization and specialization of parts of the body.

Question 12

How does specialization arise? What are Hox and Homeobox genes?

Answer 12

This specialization arises from expression of genes that essentially establish the identity of a specific region of the CNS. Does this by the regulating the expression of other genes.

For example Homeobox (in Drosophila) are expressed in a segment-specific pattern that determines segment identity (head, abdomen, thorax) – mutations or duplications of these genes can result in the repetition of certain segments or there deletion.

In mammal, Hox genes are expressed in a segment-specific manner within the neuromeres that determines identity of spinal cord segments (or rhombomeres). Disruption of the expression pattern can lead to deletion or ectopic expression of cranial nerves & nuclei.

Hox and homeobox genes are similar genes that have been conserved over evolutionary time.

There are other genes that regulate development of the forebrain and midbrain and these, too are evolutionarily conserved.

Question 13

When does neural differentiation begin? When does it complete?

Answer 13

Differentiation of neural precursor cells into neurons and glia begins after the initial development of the brain/spinal cord morphology and is completed just prior to **end of gestation. **

Differentiation of occurs in the ventricular zones (innermost regions)

Rate of mitosis is very high, 250,000 new neurons/minute

Neural precursors express both neural and glial markers

Question 14

Neural differentiation: What happens when neural precursor cells undergo mitosis?

Answer 14

When precursor cells undergo mitosis they either divide symmetrically or asymmetrically. When they divide symmetrically, both cells becoming/remain pluripotent stems cells (actually express glial markers).

Question 15

When neural precursor cells divide assymetrically, what do the daughter cells become?

Answer 15

When they divide asymmetrically, one cell becomes a neuroblast (immature neuron) cell and the other continuing to have stem cell-like properties and re-entering the mitotic cell cycle.

Regardless of what kind of division, precursor cells undergo stereotypic movements between the ventricle and pial surface along their own cytoplasmic extensions (see next slide).

The neuroblasts are now committed to a neural fate.

The remaining stem cell (referred to as a progenitor cell in the next slide) is able to undergo a limited number of asymmetric divisions to produce more neuroblasts (called transit amplifying cells because it continues to make more neuroblasts).

Question 16

Describe the mechanisms that guide neruonal and glial differentiation in neural ectoderm (very detailed).

Answer 16

Differentiation results from a sequential series of inductive signals (usually cell-cell interactions but also signal gradients) that direct a cell down one pathway or another.

Inductive in this sense means the turning on or off of specific transcription factors in the cell, thereby inducing a specific cell fate.

1. Neural induction (conversion of ectodermal cells into neuroectoderm cells & ultimately the neural tube) is the result of noggin/chordin blocking BMP (Bone Morphogenic Protein) signals.
2. TGF-β and SHH establish roofplate and floorplate identities and consequently neural precursor cell identities (e.g. motorneurons, sensory neurons, interneurons in the spinal cord).
3. Neurogenesis and the establishment of neural vs. glial fates are regulated by a balance of signals.

Following the “birth” of a neuroblast, an excess of bHLH over notch signaling promotes a neural fate.

An excess of olig 1/2 over bHLH promotes an oligodendrocyte fate.

An excess of notch of bHLH promotes an astrocyte fate.

Undifferentiated cells next to the ventricles are ependymal cells, which retain stem cell properties in some cases.

Question 17

How is neurogenesis and neuronal birthdating tracked?

Answer 17

1) Radioactive thymidine or BrDU (a thymidine analog that can be visualized) label cells that are synthesizing new DNA (S phase). Can effectively label post-mitotic cells because the label does not get diluted by further cell divisions. Allows one to track the birthdate of cells.
2) Injection of stable tracers involves injecting into a stem cell that can still be visualized in daughter cells after multiple rounds of mitosis (e.g. dextran-conjugated rhodamine or fluorescein). dilutes in cell
3) Molecular mapping refers to the use of antibodies that recognize a protein unique to a specific type of cell.

Sometimes want to label neural vs. glial cells. Can also label glutamtergic vs. GABAergic neurons or even different types of GABAergic neurons.

Question 18

Why is neural migration important?

Answer 18

Movement precursor cells and neuroblasts is a critical part of neurodevelopment; gets neurons/glia to the right place and in the right numbers.

Question 19

Where are neural crest cells originally located? where do they migrate to?

Answer 19

Neural crest cells, which are initially located around the anterior of the neural tube (spinal cord and hind brain),

travel very long distances in the embryo to form the various elements of the peripheral nervous system (and other structures).

Question 20

How do neural crest cells begin migration?

Answer 20

neural crest cells express proteins (transcription factors) that turn off genes that maintain cell-cell adhesion.

This allows the neural crest cells to disengage and migrate.

Transition from epithelial state (a type of tissue where cells are bound together) to a mesenchymal state (cells that weakly attached to each other or can move freely).

Question 21

What is the location of neural crest cell migration influenced by?

Answer 21

Where neural crest cells migrate to (and therefore their eventual function) is influenced by their initial position along the anterior-posterior axis of the spinal cord/hind brain.

Remember that there is a spatial distribution of Hox and other genes that can influence the neural crest cell migration/fate.

The paths that neural crest cells take are fairly steroetypic, this allows for other signaling interactions, e.g. brief cell-cell signals that occur as cells transiently make contact with one another (e.g. somites).

Question 22

What are the signals that neural crest cells encounter during migration?

Answer 22

The signals that neural crest cells encounter along their paths are a mixture of secreted molecules, cells surface signals (e.g. adhesion proteins) and proteins that form the extracellular matrix.

Question 23

neural crest progenitor cells signal (migrate?) to what areas?

Answer 23

dorsal root ganglia

adrenergic neurons and cholinergic neurons (autonomic nervous system)

chromaffin cells

melanocytes

Question 24

Describe the radial migration in the developing cortex

Answer 24

Many neurons in the cortex undergo radial migration; they’re born in the deepest layers of the cortex (next to the ventricles) and travel up/outward to form the cortical layers.

The path the neurons take is guided by radial glia that have extensions reaching from the ventricles to the pia.

The neuroblasts moving along the radial glia processes can move through layers of already differentiated neurons until reaching their target layer (migrating characteristic morphology).

They then disengage and then begin extending processes that will eventually become the axons and dendrites.

The adherence of neurons to the radial glia and the migration up the glia are regulated by a number of different proteins that regulate cell-to-cell contact (e.g. integrins, laminin).

Sets up the 6 layers of the cortex

Question 25

Reelin

Answer 25

regulates when neurons disconnect from radial glial cells

Question 26

radial glial cells

Answer 26

big long process, act as scaffolding that neurons can climb up

Question 27

generation of cortical neurons during the gestation

Answer 27

In cerebral cortex, these cells migrate to form the 6 layers (lamina) characteristic of most regions of the cortex.

The earliest to differentiate neurons migrate to the lowest layers (i.e. they migrate the shortest distance) and each progressively later generation of cells populate the next layer up.

What layer a neuron ends up in plays a considerable role in its’ ultimate fate/role

In the brainstem, thalamus & spinal cord, neuron fate is determined by when they were “born” (became post-mitotic) and by local factors expressed by surrounding tissue.

Question 28

How can the mutations in genes influence neuronal migration and cause malformations in the cortex?

Answer 28

Reelin is expressed in the radial glia and regulates the disengagement of neurons from the glia. Mutations in reelin disrupt migration of cortical neurons during development.

Ventricles are expanded (symptomatic of reduced numbers of cortical neurons)

White matter is reduced (fewer neurons to contribute axons)
Sulci and gyri are disrupted

tend to over-migrate, pile up in cortex

DCX (doublecortin) is protein in the neurons that controls the cytoskeleton and allows the neuron to move up the radial glial (interacts with microtubules)

Mutations in DCX lead to Lissencephaly (smooth brain) in which all the gyri and sulci are absent (note the ventricular expansion as well.

• really smooth brain*
• Expressed in neurons, regulates cytoskeleton*
• failure to completely migrate, fewer cortical layers*

Question 29

Induction factor: Noggin

Answer 29

ectoderm to neuroectoderm

Question 30

Induction factor: TGF- B

Answer 30

induces roof plate from neural tube

Question 31

induction factor: sonic hedgehog

Answer 31

induces floor plate from neural tube

(motor neurons)

Question 32

induction factors: bHLH

Answer 32

induces neurogenesis from stem cells

Question 33

induction factors: Olig1/2

Answer 33

induces oligodendrocyte formation from neural stem cells

Question 34

Induction factors: Notch

Answer 34

induces astrocytes from neural stem cells