lecture 18 development: building a nervous system

Question 1

what does the brain begin as

Answer 1

the product of a single cell, the fertilized zygote

Question 2

why study development of nervous system

Answer 2

1) demystify adult brain by following formation

2) learn about what causes missteps in brain development

3) turning on developmental mechanisms in later life could hold the key to curing many brain disorders

4) plasticity in babies can hold additional potential for human mind to learn better

Question 3

parts of neural development

Answer 3

generate components
hard wiring
activity dependent tuning

Question 4

what is step 1 of neural development and what happens in it

Answer 4

neurulation: begins once the 3rd germ later (mesoderm) is created between the ectoderm (outer) and endoderm (inner) layers

these 3 germ layers make up all the tissues of the body

Question 5

embryology

Answer 5

studying how an embryo grows from egg cell to adult

Question 6

embryology with frog

Answer 6

vegetal pole

splits in half

becomes many different cells called blastula

gastrulation (pregenitor stage before nervous system starts)

blastopore (little pore created for intestinal system)

ectoderm and endodorm, mesodorm between

neurula, then nervous system

then free swimming tadpole

Question 7

when does the nervous system begin its development (neurulation)

Answer 7

after the gut is hollowed out starting from the blastopore (gastrulation)

Question 8

is human development faster, slower, or at the same pace as a xenopus frog

Answer 8

much slower (human neurulation occurs at 17 and 28 days after fertilization, frog neurulation occurs at 13-17 hours)

Question 9

three primary germ layers

Answer 9

endoderm
mesoderm
ectoderm

Question 10

endoderm (1st germ layer in neurulation)

Answer 10

gives rise to gastrointestinal tract, organs like liver and pancreas

Question 11

mesoderm (2nd germ layer in neurulation)

Answer 11

gives rise to muscles, bones, and a small structure that ends up in the vertebrae called notochord

Question 12

ectoderm (3rd primary germ layer in neurulation)

Answer 12

gives rise to skin and the nervous system via the neural plate

Question 13

what does neural tube become

Answer 13

central nervous system

Question 14

what does neural crest become

Answer 14

peripheral nervous system

Question 15

organizer experiment

Answer 15

hilde mangold and hans spemann

The dorsal lip of the blastopore now called “the organizer” normally gives rise to the mesodermal notochord, beneath the neural tube.

Spemann and Mangold
transplanted the dorsal lip of the blastopore from one
embryo into another (giving the recipient embryo a second organizer) at a site far from the original blastopore. The amazing result: It INDUCED a whole second body axis, including the neural tube

Question 16

what is step 2 of neural development and what happens in it

Answer 16

regionalization

segmentation and segmental identity

key concept: positional information

Question 17

key concept of regionalization: positional information

Answer 17

Studies of early embryonic development led to the concept that the position of a cell in an animal determines the identity it will acquire.

A chemical (“morphogen”) released from one end of a body axis in the embryo
forms a gradient, and the concentration of morphogen determines which genes
are activated. The pattern of gene expression, in turn, endows the cell (or tissue) with specific features.

Question 18

regionalization: dorsal ventral patterning

Answer 18

Morphogens that pattern the doral-ventral axis
include “sonic hedgehog” released from the
notochord at the ventral end of the spinal cord.
Others are released from the dorsal end.

(from ventral highest concentration to dorsal lowest concentration)

Question 19

Different types of neurons at
different distances from the
notochord

Answer 19

V1 interneurons
V2 interneurons
MN motor neurons
FP floor plate cells

Question 20

regionalization: dorsal ventral patterning (hindbrain)

Answer 20

“Cranial” nerves (with Roman
numerals) each with distinct
functions originate from different ”rhombomere”
segments of the hindbrain.

They are differentiated by
transcription factors (proteins)
whose function is to start gene transcription- all set up by
morphogen concentration

Likewise, morphogens
at the anterior and
posterior ends activate
transcription factors
such as “hox” genes
that determine
neuronal identity

Question 21

what is step 3 of neural development and what happens in it

Answer 21

neurogenesis

Question 22

why does brain size increase

Answer 22

during development, brains generate lots of neurons

Question 23

key concept of neurogenesis: stem cell

Answer 23

Stem cell - a cell that can
(1) divide to produce at least one daughter just like itself or

(2) turn into a more differentiated cell (including more stem cells with more restricted potential).

Examples: embryonic stem cell,
neural stem cell, motor
neuronal stem cell etc.

Question 24

key concept of neurogenesis: progressive restriction of cell fate

Answer 24

So called “European plan”: What you become depends on who your parents are.

So called “American plan”: Who you become depends on your surroundings

Question 25

“postmitotic”

Answer 25

Neuron can’t divide again

Question 26

Adult Neurogenesis

Answer 26

Until recently, it was thought that all neurons in the mammalian brain are
“born” (that is, have their last cell division and become irreversibly “postmitotic”) in embryos or right after
birth. So, people thought there were no neural stem cells left in the adult brain.

turns out to be not quite true: Neurogenesis continues at
low levels in a few parts of the adult human brain –especially the hippocampus but not cerebral cortex. Stem cells are in the subgranular zone
(SGZ) surrounding the dentate gyrus and from there they migrate and differentiate into neurons in the dentate gyrus. Thus, some neural stem
cells persist in the adult brain.
Exercise and behavioral enrichment increase their generation or survival, while stress decreases their survival.
Antidepressants may act in part by stimulation their production.

Question 27

what is step 4 of neural development and what happens in it

Answer 27

Neural progenitors are found in the neural tube near the
central cavity that becomes the ventricles in the brain. This
area is called the sub
ventricular zone.

Radial glia assist in radial migration

Cortex develops in an “inside out” fashion. Different layers are “born” at different periods. Shown by labeling DNA at last
division before neuron becomes postmitotic with radioactive nucleotide (thymidine)

Inhibitory neurons migrate in a different way (laterally
instead of vertically)

Gene mutations (e.g. of the Reelin gene expressed in radial
glia) affect neuronal migration and cause profound disorders
in brain development

Question 28

Neural progenitors are found in the neural tube near the
central cavity that becomes the ventricles in the brain. This
area is called the…

Answer 28

subventricular zone.

Question 29

what is step 5 of neural development and what happens in it

Answer 29

differentiation

Lineage and extrinsic factors determine:
Dynamic polarization (axons and dendrites)

Neurotransmitter type

Scientists take advantage of these key programming steps to induce neurons from stem cells (no need to memorize all these signals)

Question 30

Victor Hamburger

Answer 30

A student of Hans Spemann
(“organizer experiment”). He
left Germany in 1935 due to
rising antisemitism. Took job
at Wash U (St. Louis). There
he developed the chick
embryo as a model organism
to study neural development

Question 31

A role for the synaptic targets of neurons

Answer 31

When the synaptic target of a
neuronal type (such as a limb bud) is removed early in development before axons enter the limb, few neurons in ventral spinal cord are
present once development is
over…but only on the side of the spinal cord that is ipsilateral (same side) as the removed limb.

Question 32

what are extra limbs associated with

Answer 32

extra motor neurons on ipsilateral side of spinal cord

Question 33

Can you formulate a hypothesis that explains
how limb removal or conversely limb addition
would change the number of neurons in the spinal
cord on the ipsilateral side

Victor Hamburger
Rita Levi-Montalcini

Answer 33

In order to decide between these hypotheses they removed the hindlimb (leg of a very young chick embryos) by making a window in the egg and then at various times they counted the number of motor
neurons in the hindlimb part of the spinal cord to see whether the limb ablation inhibited motor neuron proliferation or motor neuron survival. The results were surprising!

First, on the limb-ablated side total number of motor neurons dropped precipitously by more
than 90% arguing that neurons were dying in the absence of limb.

Second on the normal
side neurons were also dying with the same time course as the death associated with limb
ablation. Subsequent studies showed that the contralateral cell death was naturally occurring and had nothing to do with limb ablation.

Third, it wasn’t just motor neurons, the dorsal root ganglion cells also died over the same time.

Question 34

what is step 6 of neural development and what happens in it

Answer 34

programmed cell death (apoptosis)

In an attempt to better understand how neuronal targets might affect the numbers of neurons that survived, one of Hamburger’s students, Elmer Beuker, discovered that a mouse
sarcoma (tumor) when implanted into the region of the hindlimb of a chick
embryo, stimulated nerve growth into the tumor. Levi-Montalcini noticed enlargement of the dorsal root ganglia (“SP” in figure) and the sympathetic ganglia (“SY”) but not extra motor neurons on the side where the tumor
was situated

Question 35

they set about to purify
the factor in the tumor that was causing the growth and greater numbers of sensory and sympathetic
neurons

Answer 35

Key to their success at purification was a “bioassay” that LeviMontalcini set up with cultured dorsal root ganglion cells. In the absence of the sarcoma extract the cells just sat there in the dish (upper panel). However with a small amount of sarcoma extract they grew out long processes (bottom panel). With the bioassay Cohen was
able to successively purify the active agent. A breakthrough came when Cohen had gotten the material purified to be either a nucleic acid or a protein and used a snake
venom to destroy the nucleic acid to see if that destroyed the activity seen in the bioassay (it did not- meaning the factor was a protein), but it turned out weirdly, that the venom was thousands of times more
powerful on its own than the sarcoma in causing nerve outgrowth. This led Cohen to look at salivary glands in mammals (as the analog of
the venom producing gland in a snake). Amazingly the male rat submandibular salivary gland was an even more plentiful source of the factor. And with such large quantities it was possible for him to
purify the Nerve Growth Factor (NGF) completely

Question 36

Nerve Growth Factor (NGF)

Answer 36

NGF is a peptide of consisting of 118 amino acids
It is produced by peripheral end organs innervated by
sympathetic and some sensory (especially pain-sensitive)
neurons. These particular neurons have a special receptor (TrkA) that binds to NGF and initiates a number of
downstream effects including survival and nerve
outgrowth. This “trophic factor” keeps sensory and
sympathetic neurons alive that reach their targets.
The concentration of NGF in the blood plasma is
significantly higher in individuals who have been in a
romantic relationship with another person for less than
12 months [227 pg/ml], than those who are either not in
a romantic relationship [149 pg/ml] or have been in one
for more than 12 months [123 pg/ml]

Question 37

The trophic factor NGF is part of a gene family

Answer 37

The trophic factor NGF is part of a gene family of Neurotrophins and their trk receptors. Each member of this family has selective effects on a different set of neurons. In the CNS Brain derived neurotrophic factor (BDNF) is most widely expressed and it has many functions including effects on synaptic plasticity.

Question 38

Gastrulation

Answer 38

the process by which an embryonic blastula (ball of relatively undifferentiated
cells) becomes a layered structure by the invagination and spreading of cells. Forms the
primary germ layers: endoderm (innermost layer, gives rise to the gut, liver and other organs),
mesoderm (middle layer, gives rise to bones, muscles, heart and circulatory system), and
ectoderm (outer layer, gives rise to skin and nervous system)

Question 39

Neurulation

Answer 39

the process that establishes the neural tube, which eventually develops into the
central nervous system and the neural crest cells, which eventually develop into the peripheral nervous system. Neurulation occurs when the notochord (specialized mesoderm cells) induce
the neural plate from the midline of the dorsal ectoderm. The lateral walls of the neural plate grow and close to form the neural tube. Neural crest cells form from the borders of the neural plate as the tube pinches off and becomes covered with a new layer of ectoderm cells

Question 40

Regionalization

Answer 40

the establishment of different compartments and patterning of axis within
the developing nervous system

Question 41

Morphogen

Answer 41

a diffusible signaling protein that can cause cells located at different distances from the source (different concentrations) to adopt different fates. The
morphogen “sonic hedgehog or shh” is released from the notochord near the ventral
edge of the spinal cord and helps pattern the dorsal-ventral axis. Similar programs
using “hox” genes determine neural identity in the rostral/caudal axis

Question 42

Transcription factor

Answer 42

a DNA binding protein that regulates transcription of target genes. Often, they are key regulators that help activate (or repress) programs that decide which genes are transcribed into RNA and subsequently translated into protein

Question 42

Neurogenesis

Answer 42

the process by which neurons are produced from stem cells (which can assume any fate) and neural progenitor cells (which have limited potential). As cells divide, their
progeny become more and more fate limited

Question 43

Adult neurogenesis

Answer 43

until somewhat recently, it was believed that all neurons in the
mammalian brain were “born” very early in development, However, some neurogenesis
continues in a few parts of the adult brain: the dentate gyrus of the hippocampus and
olfactory bulb. These neurons can be integrated into the established adult circuits,
giving hope for therapeutics and replacement of neurons due to injury or degeneration

Question 44

Migration

Answer 44

neurons are “born” in specific areas containing stem cell and neural precursor populations, and must travel to their final position in the brain. Most of the principle, excitatory
neurons are born near the ventricles (ventricular zone) and migrate radially (usually along radial glia) to form the layers of the cortex. The cortex develops in an “inside out” fashion. Different layers are “born” a different periods, this the deepest layers (5/6) born first and the upper layers
born later and then migrate past the other neurons. Inhibitory neurons are born near the ventral
edge of the ventricles and must migrate horizontally around to their final positions in the cortex

Question 45

Differentiation

Answer 45

the progressive specialization of developing cells until they establish their final
fate (identity). Both lineage and extrinsic factors determine what type of neurotransmitter is released by the cells, how their axons and dendrites are organized and probably many other properties

Question 46

Apoptosis

Answer 46

programed cell death. Most types of neurons are generated in approximately 2-
fold excess and about half die later in development, AFTER migration, differentiation and
sending out axons

Question 47

Neurotrophic hypothesis

Answer 47

the idea that the survival of developing neurons depends
on neurotrophins (a family of secreted signaling proteins) that regulate the survival and
morphology/physiology of target neurons through binding to specific receptors on the
target neurons). The developing neurons have an intrinsic pathway towards cell death unless that pathway is inhibited by trophic factors. Neurotrophins are typically produced by the post synaptic targets

Question 48

1. List and be able to briefly describe the 9 different steps in neural development

Question 49

2. Understand the concept of induction, especially as it applies to the notochord and
   regionalization

Question 50

2. Understand the concept of induction, especially as it applies to the notochord and
   regionalization

Question 51

3. State the trophic theory and explain how programmed cell death regulates development of the nervous system

Question 52

4. Connect a developmental disorder (or description of one) to dysregulation at a specific stage in early neurodevelopment.