Formation of the neural tube

When does formation of the neural plate begin ? When does it end ? What does the notocord cause ? Size ? What is is ectodermal restriction ? What are two physical features of the neural plate ?

• It begins in week 3

• Completed in week 4

• Formation of notocord induced thickening of the ectoderm

• It expands as the notochord regresses and disappears.

• Ectodermal restriction: the neuroectoderm (can only give rise to neural tissue, does not mix with the ectoderm as well as it used to.

• The neural grooves and the neural folds

Can you differentiate between neuroectoderm, ectoderm and neural crest cells ?

• Neuroectoderm: receives enough signal

• Ectoderm: receives no signal

• Neural crest cells: receives some signal.

What can you tell me about the growth of the neural plate ? and the neuroectoderm ?

• The development and rise of the neural plate causes embryonic folding.

• As the neuroectoder expands it goes up and up and as the two neuroectodermal folds meet they form a structure called the neural tube.

What happens after the neural plate is formed ? What day does this happen ? At the same time as what and where is it happening ?

• After the neural plate is formed, it starts folding.

• This happens day 22, around the 4 occipital lobes (at the higher neck, base of the skull), at the same time as the formation of the somites.

What direction does the neural folding take place ? What can you tell me about the closing of the neural tube ?

• It happens in both directions.

• The neural tube closes as the neural plate is still expanding to form the neural canal.

What about methods of closure of the neuropores ? How many of them ? Names ? What day ? How are they each closed ? Consequences of them not closing ?

• The cranial neuropore is closed bir-directionally (day 24) - like a stapling action.

• The caudal neuropore is closed in a cranial caudal fahion (like a zipper effect) on day 26.

• Failure to close these neuropores causes serious anomalies some of which are embryonic lethal.

Differentiate the difference between neuroectoderm, ectoderm and neural crest cells ?

• Neuroectoderm - receives enough signal

• Ectoderm - no signal

• Neural crest cells - a little signal.

What is the exclusion system?

• Neuroectoderm will merge with neuroectoderm only.

• Ectoderm will merge with ectoderm only.

• Both of these layers will exclude the neural crest cells.

What are neural crest cells ?

They are cells that detach from the lateral margin of the folding neural folds before the closure of the neuropore.

What can you tell me about cells that are caudal ? What does this mean ?

Caudal cells migrate from the caudal end of the prosencephalon to the caudal end of the spinal cord. This means that there is no NCC formed around the prosencephalon but from the midbrain down to the end of the spinal cord you will have NCC.

Does the location of the NCC matter ?

Yes, depending on their location they will give rise to different things.

What are 7 cranial neural crest ?

1. Components of the eye

2. Odontoblasts

3. Cranial nerve ganglia

4. dermal bones of skull

5. Dermis and hypodermis of face and neck

6. Pharanygeal arch cartilage

7. Truncococonal septum

What are the 4 trunk neural crest ?

1. Dorsal root ganglia

2. Chain ganglia

3. Preaortic (prevertebral) ganglia

4. Adrenal medulla

What are the 5 cranial and trunk neural crest:

• Schwann cells

• Enteric ganglia

• Melanocytes

• The inner meninges of the brain and the spinal cord.

• Glial cells of the peripheral ganglia.

What is the neural crest cells responsible for in terms of the nervous system ?

All of the sensory nervous system is derived from the neural crest cells.

Anomalies of the neural crest cells ? How many ? What are they called ?

There are over 300 anomalies associated with anomalies of the neural crest cells. They are called neurochristipathies,

What are the 4 Christopathies that you were made aware of ?

1. Waardenburg syndrome

2. Hirschsprung disease (anomalies in the migration of the neural crest cells in the formation of the enteric ganglia)

3. DiGeorge syndrome (anomalies in the development of the pharyngeal arches, problems with the cranial nerves and the bones that form the face and the neck)

4. Fibromatosis type 1 (problems with formation of Schwann cells)

Differentiate between neural tube anomalies ?

There is real neural tube defects in the closure of the neural tube then there is neural tube defects that are not necessary anomalies of the neural tube but they are anomalies associated with closed neural tube.

Give me two examples of neural tube defects ?

Total dysphraphism or Craniorachischisis: complete failure to close, neural tube is fused to surface, not viable with life

Partial dysphraphism: Partial failure to close, partially fused to the surface.

Lumbo-sacral: Myeloschisis (open spinal cord)

or spina bifida aperta (spinal cord split open

Cranial: meroencephly or acrania (absence of skull)

If a person has meroencephaly then it is recommended that they get an abortion as they won’t live long and quality of live isn’t good

what is another example of a neural tube defect ?

Encephalocele - a skin covered protrusion at the base of brain. Cele - means bag. If it is fluid-filled it is minor and it can get removed surgically.

How is the skin on the back formed ?

It is formed by pulling on the ectoderm

What is spina bifida occulta: in terms of % of population ? Visible defects ? What does it affect ? Signs ?

• Affects 2% of the population

• No visible defects on the surface.

• It usually only affects a single vertebral arch.

• Tuft of hair, a pigmented nevus ( a red - blotched stain ), andrioma (proliferation of blood cells) and lipoma (proliferation of fat cells) on the surface.

Spina bifida cystica: in terms of percentage ? Severity ? What is it ? What is it equivalent to ?

• 0.1% of live births

• More severe

• Form fluid-filled protrusions on the surface.

• It is equivalent to a Encephalocele (in the brain)

What is meningocele ?

• There is no nervous system

• The dura mater and the arachnoid mater are protruding, the closure of the ectoderm was not fast enough

• The spinal cord still remains in the vertebral canal.

What is the meningomyelocele ?

• The dura mater and the arachnoid mater are protruding.

• The spinal cord is protruding - a form of elongated roots that cannot be pushed back in—permanent impairment.

• Affects the development of the spinal cord and the nerves

• Affects the flow of the CSF.

So if everything goes right what should you have ? What is formed ? What is mesencephalic flexure?

If everything goes right you should have a closed neural tube.

Primary brain vesicles are formed - prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain) - elongated alomst segmented.

Mesencephalic flexure - is during the cranio-caudal folding the 90 degree rotation that occurs to bring the prosencephalon at an angle.

can you describe further development of the secondary brain vesicles ?

• So week 3 you have the spinal cord

• At week 4 the spinal cord splits into the prosencephalon, the mesencephalon, the rhombencephalon and the spinal cord.

• For the prosencephalon division, at week 5 it will split into the telencephalon (which at week 6 will form the neocortex, the paleocortex, and the corpus striatum), the diencephalon (at week 6 will form the infundibulum, the epithalmus, hypothalmus and the thalmus.

• The mesencephalon at week 4 will remain the mesencephalon at week 5 and then become the tectum, teletelum and the cerebral peduncles.

• The rhomboencephalon at week 4 will become at week 5 the metencephalon (will form pons and cerebellum at week 6) and the myelencephalon will form the medulla oblongata.

• The spinal cord will remain the spinal cord throughout the weeks.

• At week 6, the primordium of all the structures of the brain is formed.

give me 5 examples of cerebellar malformations ?

• Hypoplasia (underdevelopment)

• Dysplasia (abnormal development)

• Heterotopia (cells migrate to wrong place)

• Environmental

• Genetic

What are three examples of cerebral cortical malformations ?

• Lissencephaly (anomalies in the neuronal migration in months 2-5)

• Subcortical band heterotopia

• Microcephaly

Development of the PNS: where do the motor axons grow out? In response to what ?

• They grow out of the ventral spinal cord in response to developing muscles / targets.

what about NCC’s in the PNS ?

They migrate dorsally to the dorsal horn (dorsal part of spinal cord). The will extend axons or dendrites. They will extend one centrally to dorsal horn and one peripherally to motor root. Will then follow motor targets and will ultimately get seperated from motor innervation to innervate target.

Neural crest cells migrate to form structures derived from which germ layer lineage?

Ectomesenchymal (Ectodermal origin)

The notochord induces formation of the:
A. Neural crest
B. Neural plate
C. Neural canal
D. Neural tube
E. Neural groove

B: neural plate

Failure of closure of the entire neural tube is termed:
A. Myeloschisis
B. Spina bifida aperta
C. Craniorachischisis
D. Encephalocele
E. Anencephaly

C: craniorachiscis

The lumen of the neural tube becomes the:
A. Central canal of spinal cord and brain ventricles
B. Pericardial cavity
C. Neural crest canal
D. Vertebral canal
E. Subarachnoid space

A: the central canal of spinal cord and brain vesicles

The neural tube gives rise to:
A. Peripheral nervous system
B. Central nervous system
C. Skeletal muscles
D. Vertebrae
E. Epidermis

B: Central Nervous System