Development of the nervous system

Question 1

Name the three primary germ layers / embryonic layers

Answer 1

Endoderm
Mesoderm
Ectoderm => nervous system

Question 2

In terms of nerve tissue, what do neural tube/neuroepithelium and neural crest cells differentiate into?

Answer 2

Wall of neural tube => CNS

Neural crest => PNS, melanocytes

Question 3

Briefly outline the process of neurulation

Answer 3

• Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar
• Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube
• The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis - neural crest cells

Question 4

What regulates the proper location and formation of the neural tube?

Answer 4

Notochord cells

Question 5

What do neuroblasts differentiate into?

Answer 5

All neurons with cell bodies in the CNS

Question 6

What do glioblasts differentiate into?

Answer 6

Astrocytes (e.g. help to form B-B barrier, scaffold for other cells, removal and degradation of neurotransmitters etc.)

Oligodendrocytes (Myelin forming cells of the CNS)

Question 7

What do ependymal cells differentiate into?

Answer 7

Cells lining ventricles and central canal

Question 8

Neural crest cells differentiate into what kinds of cells?

Answer 8

• Sensory neurons of dorsal root ganglia
• Postganglionic autonomic neurons
• Schwann cells
• Non-neuronal derivatives e.g. melanocytes

Question 9

Are motor neurons produced from neuroepithelium?

Answer 9

Yes – although most of their axons are outside the CNS, their cell bodies are within the spinal cord

Question 10

Describe the differentiation of the neuroepithelium

Answer 10

• Cells withdraw from the outer membrane and undergo mitosis near the inner membrane => mass proliferation
• One daughter cell stays attached to the inner membrane and the other daughter cell will move away from the inner membrane and become a neuroblast
• The neuroblast will develop processes and one will become the axon which is directed away from the inner membrane

Note that the membranes are basal laminae

Question 11

What are the three layers formed by the differentiation of the neuroepithelium?

Answer 11

Ependymal (germinal layer) Grey matter (mantle layer)
White matter (marginal layer)

Question 12

Glioblasts show a similar pattern of differentiation to neuroblasts. State one difference.

Answer 12

Glioblasts can migrate into the white matter

Question 13

What controls migration and differentiation?

Answer 13

Signalling molecules which interact with receptors on neuroblasts

Question 14

Which factors are important with regards to signalling molecules?

Answer 14

Their concentration gradient and timing

Question 15

What can the grey matter of the neural tube be divided into?

Answer 15

Alar plates (located dorsally; => dorsal horns)
Basal plates (located ventrally; => ventral horns)

Question 16

Cells in the alar and basal plates become which types of neurones?

Answer 16

Alar plate – interneurons

Basal plate – interneurons and motor neurons

Question 17

What are the neural pores and when do they close?

Answer 17

The developing tube zips of rostrally and caudally, leaving only the top and bottom ends open - anterior and posterior neuropores.
Supposed to close at the end of the fourth week.

Question 18

The wall of the anterior neural tube differentiates to form three primary vesicles. Name these primary vesicles and state when this occurs.

Answer 18

Prosencephalon (future forebrain)
Mesencephalon (future midbrain)
Rhombencephalon (future hindbrain)

• From approx. 4th week

Question 19

Describe the changes that occur to these three vesicles in the week or so following their formation

Answer 19

The prosencephalon divides into two new vesicles - telencephalon (from alar plate) and diencephalon (from basal plate).

The rhombencephalon similarly divides into the pons and medulla

Question 20

What important tightly folded structure begins to appear quite late in development, where does it appear from and when?

Answer 20

Cerebellum
Appears as an out pouching from the back of the pons
Around 8 weeks

Question 21

What also occurs at approx. 8 weeks

Answer 21

Neural tube centre develops to become the ventricular system - two lateral ventricles (fore), midline third ventricle (fore), aqueduct (mid), and fourth ventricle below cerebellum (hind)

Question 22

Initially the three primary vesicles are separated by 3 flexures. Name them (rostral to caudal)

Answer 22

Cephalic
Pontine
Cervical

Question 23

How do neuroblasts migrate from the inner membrane to the outer membrane in the brain?

Answer 23

They attach themselves to radial glial cells and climb up them towards the outer membrane
Radial glial cells have their cell bodies anchored to the inner membrane and have a single long process to the outer membrane

Question 24

What happens to neuroblasts that undergo short migration?

Answer 24

They become the basal ganglia

Question 25

How many cell layers are there in the cerebral neocortex?

Answer 25

SIX

Question 26

A deficiency in what can lead to neural tube defects?

Answer 26

Folic acid

Question 27

Give a summary for each of the different main neural tube defects

Answer 27

• Anencephaly = anterior neuropore not closing
• Encephalocele = herniation if the meninges
• Iniencephaly = Occipital and spine defects with extreme retroflexion of the head
• Spina bifida = posterior neuropore not closing
• Closed spinal dysraphism = deficiency of at least two vertebral arches
• Meningocele = Protrusion of meninges (CSF filled)
• Myelomeningocele = open spinal cord