Neural Crest Part 1

Question 1

What are the neural crest cells?

Answer 1

• They are a vertebrae specific population that arise during embryogenesis from the dorsal neural tube

Question 2

What do neural crest cells form from?

Answer 2

• The neuroectoderm will give rise to neural tissues, the non-neural ectoderm will form the skin and the neural plate border will give rise to the neural crest.

Question 3

What permits the movement of the neural crest cells?

Answer 3

• Neural crest cells at the dorsal tip of the neural tube undergo EMT transition which allows the cells to move out from the neural tube into the rest of the embryo by underneath the ectoderm or between the neural tube and somites.

Question 4

How is the neural ectoderm specified (recap)

Answer 4

• This process takes place during gastrulation and is regulated by signals from the organiser.
• BMP4 is widely expressed, however near the organiser BMP4 is inhibited by other signals shown in red. This allows the neural plate to develop.

Question 5

How is the neural crest cells specified in a developing embryo from the neural plate borders?

Answer 5

• Neural plate forms when BMP levels are low and ectoderm forms where BMP signals are high.
• Between these two regions and signals is the boundary region where there is an intermediate level of BMP4 signalling. This is necessary for the induction of the neural crest
• In addition neural crest induction requires signals from the Notch pathway, Wnt pathway, FGS and RA. These signals come from the neural ectoderm, non-neural ectoderm and somatic mesoderm.
• These signals induce the expression of a number of transcription factors which specify neural plate border tissue. These genes include pax3/7, dlx5/6 and msx1.

Question 6

How is the boundary between the neural plate and epidermis formed (in the middle is the crest cells)?

Answer 6

• The expression of these TFs forms part of a cascade of expression of transcription factors that leads to neural plate border formation. This forms a genetic network that regulates neural crest development.
• In the neural plate border these regulate each other in a positive manner to reinforce neural plate border and neural crest fates. They also repress neural crest fate in the adjoining neural plate and epidermis.

Question 7

What do TFs expressed at the border of the neural plate and epidermis do?

Answer 7

• The transcription factors expressed in the neural plate border in turn induce the expression of other transcription factors which regulate pre-migratory neural crest fate. These include genes such as sox9/10, twist ect. Some of these genes are also involved in EMT which is how epithelial cells leave the epithelial sheet that allows neural crest cells leave the tube so they can differentiate at tissue sites.
• As neural crest cells move from pre-migratory character towards a more migratory character they begin to express snail1/2 ect which enhance migratory character of a cell. This allows the crest to move away from the dorsal neural tube along the whole axis of the embryo to various position where they can receive a number of different signals and differentiate into a group of diverse cell fates.

Question 8

What induces the formation of the neural border itself?

Answer 8

• Neural crest is induced at the level of gastrulation by a number of secreted signals including BMP4, Wnts, Notch and FGS.
• This will induce a set of transcription factors in the early neurula which specify the neural plate border.
• These in turn lead to the activation of the second set of transcription factors which specify the pre-migratory neural crest which takes place in the late neurula.

Question 9

Once the neural crest cells have migrated what do they do?

What makes neural crest cells multipotentent?

Do neural crest cells having an increasing restriction of cell fate?

Answer 9

• As they migrate throughout the embryo and reach the final position at which they will differentiate, these migratory gene sets are turned off. Genes that cause neural crest to final differentiate are switched on.
• This ability to differentiate into a variety of cells types means that neural crest cells are multipotent. If we re-locate some of these early cells they will differentiate according to their new environment but later this is no longer possible, there is an increasing restriction of cell fate as they move through development.

Question 10

How many types of neural crest are there?

What enables them to give rise to different structures?

Answer 10

• The neural crest can be divided into 5 different types depending on the axial position that they arise from.
• Crest in each of these domains will be genetically patterned in a slightly different way and therefore form different cell types and form different anatomical structures.

Question 11

What is the most anterior region of neural crest cells? and where do they arise from?

What do they from?

Answer 11

• At the most anterior position is the cranial neural crest which arise from the 4 mid and anterior part of the hindbrain down to the level of rhombomere 5. These neural crest give rise to cartilage and bone, connective tissue, neurones and gilia. They form structures of the head and neck such as thyroid formation, skull bone formation, thymus, the teeth and innervation.

Question 12

What neural crest forms posteriorly to the cranial NCCs?

What is the anterior region of the vagal neural crest cells?

What does the caudal region of the vagal neural crest form?

What does the trunk crest and sacral crest give rise to?

Answer 12

• The vagal neural crest and runs from rhombomere 6-8 down to the level of somite 7. The most anterior region of the vagal neural crest is the cardiac neural crest. The cardiac neural crest migrate into the outflow region of the heart where they form muscle and connective tissue which is needed to divide the outflow tract into the aorta and pulmonary artery. The more caudal portion of the vagal crest (somite 2-7) are needed for the formation of sensory neurons and gilia which give rise to the parasympathetic ganglia of the gut and therefore control the contraction of the gut.
• The remaining caudal neural crest are called the trunk crest and somite 28 being called sacral crest. Trunk give rise to form a variety of derivatives such as melanocytes for the skin, sympathetic neurones and adrenal cells needed for the adrenal gland.

Question 13

What structures form the face?

Answer 13

the pharangeal arches which are numbered 1-6, athough there is no number 5.

Question 14

What are the pharyngeal arches and give an example

What is found in the centre of the pharyngeal arches and what are they surrounded by?

Answer 14

• They are transcient embryonic structures that form in a cranial-caudal fashion. They remodel to give rise to anatomical structures. Each pharangeal arch is made of different tissue: there is an aortic arch artery in the centre of each arch which is surronded by mesoderm. This is then surrounded by neural crest derived mesenchyme. These structures are lined with endoderm on the inside and ectoderm on the outside.
• From the diagrams we can see neural crest derievd tissue makes up a large portion of the pharngeal arches.

Question 15

How does the correct neural crest cells from the correct rhombomere enter the correct arch, cranial neural crest cell migration paths

Answer 15

• Eph receptor are tyrosine kinases and along with Ephrin ligands mediate contact depend communication between cells of the same or different types, controlling a variety of behaviours such as cell morphology, migration, adhesion, proliferation, survival and differentiation.
• In xenopus ephrin B2 ligand is expressed in even number rhombomeres such as 2 4 and 6 well as Ephrin receptor A2 is expressed in odd number rhombomeres such as 3 and 5

Question 16

As cranial neural crest cells only come from odd rhombomeres what stops them from migrating from even number rhombomere?

Answer 16

Inhibitory signals from odd rhombomeres keeps neural crest stream segregated. Eph and ERB B4 both perform this ventrally and dorsally.

This targets neural crest cells to the correct pharyngeal arch

Question 17

What are Hox genes and what do they do in terms of neural crest?

Answer 17

• The Hox gene family; evolutionarily conserved transcription factors which contain homeobox DNA binding motifs.

These are hombeobox-containing transcription factor gene clusters that are master regulators that direct the development of particular body structures and are evolutionarily conserved between species.

Question 18

What is the importance of hox genes being expressed along the AP axis?

Answer 18

• Hox genes expressed on the AP axis. The position of a gene on a chromosome is related to the axial position of expression. The more 3’ the gene is the more anterior the embryonic expression is.

Question 19

What do cranial hox genes do?

Answer 19

• In the hindbrain and cranial neural crest, nested domains of Hox expression provide a combinatorial Hox-code for specifying regional properties in the developing head
• Play important roles at multiple stages in NCC specification, migration, and differentiation

Question 20

Have a look at what hox genes are expressed in the different rhombomeres and what they give rise to

Answer 20

Page 11

Question 21

How do the different pharyngeal arches form?

Answer 21

Combinatorial hox gene expression that activates different transcription factors target genes specify the position the identity of the hox gene pharyngeal arches to form there specific derivatives

Question 22

What hox gene marks the formation of the lower jaw?

Answer 22

Hoxa2

• Here we can see how the hox code plays a role in jaw formation
• In the panels on the top left: hoxA2 expression is shown in green and anterior hox negative tissue is shown in pink
• In normal embryos the rostral hoxA2 boundary marks the position of lower jaw formation
• If hoxA2 expression is lost from the neural crest that expresses it, then the lower jax is duplicated.
• If the expression of this gene is modified above rhombomere 4 in regions that do not normally express hoxa2 then jaw formation is supressed.
• If we transplant hox negative neural crest into hox positive rhombomeres it does not affect jaw development
• But we if transplant hox positive crest into hox negative regions leads to abnormal jaw development
• Embryos from jawless species express hox genes in pharyngeal arch one unlike species that have developed jaws. This shows that the boundary between hox expression in rhombomeres is important in lower jaw formation.

Question 23

What are the cardiac neural crest cells derived from?

Where do they migrate to and populate?

What do they form?

Answer 23

• The cardiac neural crest are derived from the neural tube at the level of rhombomere 7 and 8 (caudal hindbrain) and somites 1-3.
• Neural crest cells from these regions migrate through pharyngeal arches 4-6 which then populate the outflow tract of the heart. They are essential for allowing the muscular septum of the outflow tract to form to form the aorta and pulmonary artery.
• They also contribute towards the pharyngeal arch arteries where they form smooth muscle

Question 24

What do the cardiac neural crest cells form?

What cell types are needed to maintain the aortic arch arteries?

Answer 24

• In the blue image we can see cardiac neural crest giving rise to the vascular smooth muscle required to support the remodelled great arteries such as the aortic and carotid arteries.
• The green stains shows the epithelium and red shows the smooth muscle of the arteries. The overlay shows that both are needed to maintain the aortic arch arteries.
• The cardiac neural crest will also need to help form the thymus which overlies the heart, the thyroid, parathyroid, brachial gland.

Question 25

Do the cardiac neural crest remodel the aortic arch into the great vessels?

Answer 25

• Cardiac neural crest from pharyngeal arch 4 is needed to maintain vascular smooth muscle which supports the formation and maintenance of aortic arch arteries.
• We can see the neural crest cells migrating to populate the caudal pharyngeal arch arteries which remodels them that leads to the formation of the aortic arch and great vessels.

Question 26

What experiments can be performed to show cardiac neural crest cells form the aortic arch ect

Answer 26

• Ablation experiments have shown that a lack of cardiac neural crest cells can impact the formation of the aortic arch and great vessels
• In the chick the neural tube and associated neural crest cells were removed between somites 3 and 6. The embryos were then cultured further and examined for cardiac defects: truncus arteriosus (the outflow tract does not separate and division, so blood mixes), double outlet right ventricle (heart does not rotate properly so the aorta and pulmonary trunk both exiting from the right ventricle, so there is no oxygenated blood from the left ventricle being pumped round the body), other defects include transposition of the great vessels and overriding aorta. Otgers include interrupted aorta.

Growth factors, TF, chemokines and signalling molecules can also give rise to form defects of the heart