6A. The Developing Brain

Question 1

3 layers that form during embryonic development

Answer 1

Ectoderm (outer), mesoderm (middle), endoderm (inner)

Question 2

Nerve tissue arises from…

Answer 2

Ectoderm. Signals from mesoderm trigger this in a process called neural induction. Subsequent signaling interactions refine the nerve tissue into the basic
categories of neurons or glia (support cells), and then into subclasses of each cell type.

Question 3

Fate of a developing cell is determined by…

Answer 3

Its proximity to various sources of signaling molecules. The concentration of signaling molecules decreases farther from the source, creating gradients throughout the brain.

Question 4

Sonic hedgehog

Answer 4

Signaling molecule secreted from mesodermal tissue lying beneath developing spinal cord. When exposed to this signal, adjacent nerve cells are converted into a specialized class of glia. Cells farther away become motor neurons. Even lower concentration promotes formation of interneurons.

Question 5

Early divisions of cells during proliferation are…

Answer 5

Symmetric, resulting in two identical daughter cells that are both capable of further division

Question 6

Later divisions of cells during proliferation are…

Answer 6

Asymmetric, giving rise to only one daughter cell that keeps proliferating and a second that progresses towards its ultimate fate as a neuron or glia.

Question 7

After the initial proliferative process is complete…

Answer 7

Only a few neural stem and progenitor cells remain within the brain, and neurogenesis in adulthood is limited to a few regions of the brain, such as those involved with memory

Question 8

Proposed cause of microcephaly

Answer 8

Protein defects causing a premature switch from symmetric to asymmetric divisions

Question 9

Excessive proliferation causes…

Answer 9

Megalencephaly

Question 10

Migration - definition

Answer 10

New neurons travel from inner surface of embryonic brain, where they formed, to their long-term locations in the brain.

Question 11

When does migration begin?

Answer 11

3-4 weeks after conception.

Question 12

What happens 3-4 weeks after conception?

Answer 12

Ectoderm begins to thicken and build up along midline of embryo. Cells continue to divide, neural plate grows, followed by formation of parallel ridges that rise along either side of the midline. Ridges extend from the head end along the future spinal cord, eventually folding towards each other and fusing into a hollow neural tube. Head end thickens into 3 bulges.

Question 13

What happens 7 weeks after conception?

Answer 13

First signs of eyes and hemispheres appear. New neurons produced and move from neural tube’s ventricular zone (inner surface of tube) towards border of marginal zone (outer surface). After neurons stop dividing, they form an intermediate zone where they gradually accumulate as the brain develops.

Question 14

Most common guidance mechanism for migration

Answer 14

Radial glia. Accounts for 90% of migration in humans. Project radially from intermediate zone of cortex. Neurons use these glia as scaffolding, inching along glial projections until they reach their final destinations.

Question 15

Process of radial migration occurs in what manner?

Answer 15

Inside-out manner. Cells that arrive earliest (oldest ones) form deepest layer of cortex, while late-arriving (youngest) neurons form outermost layer.

Question 16

Which three stages of brain development occur internally during fetal development?

Answer 16

Induction, proliferation, migration

Question 17

Where are the longest human axons?

Answer 17

Periphery, extending from lower spinal cord to toes

Question 18

How does a developing axon grow?

Answer 18

By the extension of its growth cone, an enlargement at the tip of the axon that explores the environment to seek out its destination. Guided to its destination by molecular cues in its environment, some of which are on the surfaces of cells, while others are secreted into areas near the growth cone.

Question 19

Types of cues for growth cones

Answer 19

Attractive and repellent

Question 20

Families of proteins that are signalling molecules

Answer 20

Netrin, semaphorin, ephrin

Question 21

Signalling molecules are common to many organisms. True or false?

Answer 21

True. Common to worms, insects, and mammals. As a result, simpler animals are useful experimental models.

Question 22

Where was netrin first discovered, and how did that discovery apply to human research?

Answer 22

In worms, where it was found to guide neurons around the worm’s nerve ring. Later, vertebrate netrins were found to guide axons around the mammalian spinal cord

Question 23

Formation of synapse

Answer 23

On presynaptic side, axon terminal becomes specialized for releasing neurotransmitter, stocking itself with neurotransmitter packets and proteins that enable those packets to be held in place then released.

On postsynaptic side, receptors on dendrite that respond to neurotransmitters begin to dot membrane.

Astrocytes play an important role

Question 24

Possible cause of autism

Answer 24

Defects in molecules that promote proper synapse formation

Question 25

Neurotransmitter used by motor neurons

Answer 25

Acetylcholine (fixed)

Question 26

When certain immature neurons are maintained in a culture dish with no other cell types, they produce…

Answer 26

Norepinephrine

Question 27

When certain immature neurons are cultured with specific cells, such as cardiac tissue, they produce…

Answer 27

Acetylcholine

Question 28

Myelination increases the speed of signals by ___ times

Answer 28

100 times

Question 29

Name of gaps in myelin sheath

Answer 29

Nodes of Ranvier

Question 30

Saltatory conduction

Answer 30

Saltatory = leaping. Alternating pattern of insulation and nodes allows electrical signals to travel faster, jumping from one node to the next

Question 31

What fraction of the neurons generated during development survive in an adult, and why?

Answer 31

Half. Creates a more efficient system

Question 32

How are populations of neurons removed?

Answer 32

Apoptosis. Programmed cell death. Activated if a neuron fails to receive enough trophic factors. Each type of trophic factor supports the survival of a distinct group of neurons

Question 33

Nerve growth factor

Answer 33

Maintains survival of sensory neurons

Question 34

What is the pruning of excess connections dependent on?

Answer 34

Relative activity of each connection

Question 35

Role of astrocytes in pruning

Answer 35

Aid the formation of eye-specific connections by engulfing unnecessary synapses.