Development of the Nervous System

Question 1

What will the ectoderm form?

Answer 1

Entire nervous system- both peripheral and central

epidermis of the skin

placodes in the head region

Question 2

Two parts of the ectoderm

Answer 2

Midline columnar epithelium- will form neural plate

lateral cuboidal epithelium- epidermis and sensory placodes

Question 3

What is the first structure to form in the establishment of the nervous system? + stages

Answer 3

Neural plate- just cranial to the primitive node

1. Formation of the neural plate is induced by the organiser (the primitive node)
2. ectodermal cells differentiate into a thick plate of pseudostratified, columnar neuroepithelial cells- neuroectoderm
3. Forms at the cranial end then grows in the cranial to caudal direction

Question 4

How does the organiser induce neural induction?

Answer 4

Chordin secreted, which forms a secondary axes

secretes 10 transcription factors.

Also antagonises the Bmp signalling pathway, changing the formation surface ectoderm to midline neuroectoderm

Question 5

Organiser’s importance evidence

Answer 5

Transport primitive node to another organism, induces another neural plate- thus two are formed in the organism

Question 6

Neurulation definition

Answer 6

Formation of the neural tube

Question 7

Shape of neural plate before neurulation (day 22)

Answer 7

Wide cranial portion that will form brain

narrow caudal portion that will give rise to spinal cord

Question 8

Why does the caudal portion grow rapidly?

Answer 8

Somite pairs are continually, rapidly added so the spinal cord lengthens faster than the cranial neural plate

Question 9

4 stages of neurulation

Answer 9

1. formation of neural plate
2. shaping of the neural plate
3. bending of the neural plate
4. closure of the neural groove

Question 10

Stages of bending of neural plate

Answer 10

1. Neural folds formed at lateral edges, consisting of neuroepithelium and adjacent surface ectoderm
2. during folding neural folds elevate dorsally by rotating about a central pivot point overlying the notochord cells called the median hinge point
3. Neural groove- groove delimited by the bending neural plate- is formed
4. neural folds brought into apposition along the dorsal midline

Question 11

What occurs differently in the cranial region + why?

Answer 11

Neural plate at the future brain level is much broader

1. dorsolateral hinge points form, that allow the neural folds to come together in the dorsal midline

Question 12

What is the next stage?

Answer 12

Closure of the neural groove

Question 13

Stages of closure of the neural groove

Answer 13

Day 22

1. Neural crest cells are formed at the interface between the epithelial layers
2. arise from the neural folds by undergoing EMT
3. The neural folds adhere to one another. Begins in the cervical region then moves cranially and caudally
4. The cells rearrange in the folds to form two separate epithelial layers- roof plate of the neural tube and the overlying surface ectoderm
5. neural tube formed
6. neural crest cells then migrate into the mesoderm

Question 14

What are the ends of the neural tube called?

Answer 14

Cranial neuropore and caudal neuropore

Question 15

Example of a neural defect where the neural tube does not close properly

Answer 15

Anencephaly- absence of a major portion of the brain, skull and scalp due to the cranial neuropore failing to close

Question 16

What is required for the shaping of the neural plate + their origin?

Answer 16

Forces are generated by changes in cell behaviour, particularly cell shape, position and number.

Intrinsic neurulation forces- generated within the neural plate itself

Extrinsic neurulation forces- arise outside the neural plate

Question 17

What does the shaping of the neural plate include + explained briefly?

Answer 17

Convergent extension- transverse narrowing and longitudinal lengthening.

Cell elongation- Cells thicken apicobasally as they get taller, which requires the presence of paraxial microtubules. Contributes to narrowing, as cell diameters decrease to compensate for the elongation

Cell rearrangement- cells move from lateral to medial in the neural plate, thus increasing its cranial-caudal length

cell division- many cells orientated to place daughter cells longitudinally

Question 18

Structure of hinge points

Answer 18

localised regions of neuroepithelial cells that change their chai to wedge like and become firmly attached to an adjacent structure through the deposition of ECM

Thus median hinge point firmly attached to notochord and dorsolateral hinge points to the adjacent surface ectoderm of the neural folds

Question 19

Explain neural determination

Answer 19

Signals from notochord and mesoderm- secrete noggin, chordin and follistatin.

All factors bind BMP-4, an inhibitor of neuralation

Question 20

Explain patterning of neural tube

Answer 20

Wnt pathway involved in convergent extension

Shh, secreted by notochord, initiates the formation of the median hinge point

Notochord produces chorine, a Bmp antagonist which results in the induction neural crest cells

Question 21

What are neural crest cells?

Answer 21

Unique population of cells that arise from the dorsal part of the neural tube

Question 22

Migration of neural crest cell stages

Answer 22

1. undergo EMT
2. migrate from various craniocaudal levels of neural folds into the underlying mesoderm
3. Migrates via two pathways
4. different neural crest cells react different to morphogens, thus become repulsed or attracted by different distances

Question 23

Two pathways explained

Answer 23

Ventral- through anterior part of somites- become sensory/dorsal root autonomic ganglia, sympathetic and enteric neurones, Schwann cells and cells of adrenal medulla

Dorsal- through dermis, enter ectoderm through holes in basal lamina- form melanocytes

Question 24

How are neurones formed?

Answer 24

1. Neural tube is the embryonic precursor to the central nervous system
2. Shh is secreted by the notochord and forms a concentration gradient from the ventral cells
3. high concentrations induce floor plate
4. high conc- ventral neurons induced, lower conc, more intermediate, lowest conc, more dorsal neurones

Question 25

Secondary neuralation stages

Answer 25

1. Neural tube extends past the caudal neuropore, due to the level of closing neuropore being merged with the tail bud 2. tail bud undergoes morphogenesis to form the most caudal extent of the neural tube 3. Neural crest cells and somites develop in this caudal region 4. medullary cord- condensation of central tail bud formed 5. fuses with more cranial neural tube, after cavititation. 6. neural crest cells then arise and form caudal spinal ganglia 7. lateral tail bud cells undergo segmentation to form the caudal somites 8. grows into sacral and coccygeal regions

Question 26

Explain spina bifida

Answer 26

Incomplete formation of neural arch- neural groove not completely formed. Can cause a sac of fluid from a gap in the spine to protrude.

Question 27

What morphogens control the fates of the neural tube?

Answer 27

Shh secretion induces different ventral fates BMP secretion induces different dorsal fates

Question 28

Segmental segregation explained

Answer 28

Spinal nerve growth from the neural tube splits the sclerotome into rostral and caudal segments- known as resegmentation of the sclerotome. Caudal half and rostral half between one nerve fuse to produce the vertebrae

Question 29

Segmentation of the nerves definition

Answer 29

Develops the separate central nervous system and peripheral nervous system Guides the developing axons

Question 30

Segmentation of nerves process

Answer 30

1. Axons of motor and sensory neurones develop from the neural tube region and through each of the successive somite's anterior 2. The division of the scelotome into anterior and posterior parts enable the nerves to leave at different stages

Question 31

Where does the dorsal root ganglion originate from?

Answer 31

Neural crest cells

Question 32

What is neural patterning?

Answer 32

Neural tube becomes patterned in a dorsal- ventral axis

Question 33

Explain stages of neural patterning

Answer 33

1. Floor plate cells form at the ventral midline, as well as the more dorsally located motor neurones and interneurons- as a result of Shh secretion from the notochord 2. the different concentrations of Shh are responsible for the identity of different neuronal progenitor cells 3. at the dorsal end of the neural tube, BMPs are responsible for neuronal patterning- initially secreted by overlying ectoderm

Question 34

Where do the cranial ganglia originate from?

Answer 34

Neural ectoderm of brain and spinal cord

Question 35

How is the spinal cord structured and formed in relation to ventral root?

Answer 35

1. floor plate- thin region that overlies the notochord in neural tube secretes Shh and ventrally patterns spinal cord 2. basal plate- thick wall on either side of floor plate- Ventral horn develops here and extends axons out to innervate developing muscle

Question 36

4 main domains of neural crest cells + what they contain

Answer 36

Cranial- craniofacial mesenchyme- cranial ganglia Trunk- melanocytes, dorsal root ganglia (travel through anterior portion of each sclerotome) Vagal and sacral- ganglia of enteric nervous system and parasympathetic ganglia cardiac- melanocytes, cartilage, connective tissue, regions of the heart- Schwann cell

Question 37

Two ways in which neural crest cells migrate

Answer 37

Repulsive- move away from chemical produced Attractive- move towards

Question 38

Which way do neural crest cells migrate?

Answer 38

Rostral half of somites- sensory and sympathetic neurones Dorsolateral- between epidermis and dermamyotome