2.1 Phases of Neurodevelopment

Question 1

What happens to the egg shortly after fertilisation?

Answer 1

begins to divide

Question 2

What is another name for fertilised egg?

Answer 2

zygote

Question 3

What happens to the egg on day 4?

Answer 3

• solid ball of cells (morula) forms 16 cells
• this stage = morulation

Question 4

What happens to the egg on day 5?

Answer 4

• cells separate into 2 groups
  1. inner cells mass (embryo)
  2. encompassing sac
• this is a blastocyst
• this stage = blastulation
• cells of the inner cell mass form 2 layers
  1. epiblast (embryo)
  2. hypoblast

Question 5

What happens to the egg on days 13-19?

Answer 5

• gastrulation
• formation of a groove - the ‘primitive streak’
• becomes more like an embryo

Question 6

What is the end result of the egg?

Answer 6

• production of 3 cell layers:
  1. ectoderm
  2. mesoderm
  3. endoderm

Question 7

What is the function of the endoderm?

Answer 7

• gives rise to digestive, respiratory systems etc

Question 8

What is the function of the ectoderm?

Answer 8

• gives rise to the whole of the nervous system

Question 9

What is the function of the mesoderm?

Answer 9

• gives rise to muscle, skeletons etc

Question 10

What forms the neural plate?

Answer 10

• thickening of the ectoderm

Question 11

What becomes the midline of the embryo?

Answer 11

• uneven rates of cell division form a grove known as the ‘neural groove’ which becomes the embryos midline

Question 12

What happens to the egg on days 22?

Answer 12

• tops eventually come together to form the ‘neural tube’
• interior becomes the fluid filled ventricles of the brain and central ventricles of the spinal cord

Question 13

What happens to the egg on days 24?

Answer 13

• three major divisions of the brain are discernible:
  1. prosencephalon
  2. mesencephalon
  3. rhombencephalon

Question 14

How does a tube become a brain?

Answer 14

• channel between spinal cord and brain partially closes
• fluid is pumped in to swell the brain
• differential rates of cell growth determine regions of enlargement
• regions of swelling demarcate future brain regions

Question 15

What happens to the egg at 3-4 weeks?

Answer 15

• the tube doesn’t remain in a straight line
• the telencephalic tube expands and grows over the rest of the brain to form the cerebral cortex
• as it travels, it forms the occipital and temporal lobes

Question 16

Where do neurons come from?

Answer 16

• during gastrulation, cells in the ectoderm around midline receive a chemical signal from the notochord
• this causes some of them to develop into neural precursor cells
• neural stem cells: divide
• proliferation (symmetrical)

Question 17

What happens following neuralation?

Answer 17

• as the precursors divide they produce a primitive neuron (neuroblast) or a glial cell (glioblast)

Question 18

What happens during migration?

Answer 18

• most neurons/glial cells are produced in the tubes ventricular zone
• neuroblasts/glioblasts migrate to their final destination with help of guidance mechanisms

Question 19

What determines what type of neuron/glial cell produced?

Answer 19

• local chemical interactions with neighbouring cells

Question 20

What happens to immature neurons?

Answer 20

• initially produce a number of small extensions
• eventually commits to one of these being the axon and the others dendrites
• once the axon has been specified it navigates to find its target

Question 21

How do axons find their targets?

Answer 21

• use specialised terminals, known as growth cones
• they consist of lamellipodia (sheet-like expansion) and filopodia (fine processes)
• receptors in filopodia sense attractant or repellant chemical cues in the environment

Question 22

How do axonal growth cones navigate?

Answer 22

using…
1. diffusible agents: float about in liquid and can be detected
- chemoattractant
- chemorepellant

2. non-diffusible agents: sits on tissue that axon is moving across
   - contact attractants
   - contact repellants