Neural tube formation

Question 1

LO

Answer 1

• Explain the mechanisms of neural plate induction
• Describe the steps of primary neurulation
• Explain the molecular mechanisms of hinge point formation, neural tube closure, fusion and separation during primary neurulation
• Explain where and how the neural crest originates
• Explain the molecular and cellular mechanisms of neural crest cell migration

Question 2

What are the three major derivative of the ectoderm germ layer?

Answer 2

1. Surface ectoderm (primarily epidermis)
2. Neural tube (brain and spinal cord)
3. Neural crest (peripheral neurons, pigment, facial cartilage)- pluripotent stem cells

Question 3

What is the ‘default rate’ of the ectoderm to become?

Answer 3

The neural tissue

Question 4

Name 3 proteins that bind to BMPs and what is their role?

Answer 4

Chordin, Noggin and Follistatin

bind directly to BMPs (Bone Morphogenetic Proteins) to block their interaction with BMP receptors- mesodermal cells release these and are antagonists to BMP signaling pathway

Question 5

Where is Chordin, Noggin and Follistatin expressed?

Answer 5

In the dorsal mesoderm

Question 6

Neural ectoderm neural induction

Answer 6

Implicated in neural induction (mechanism: inhibition of TGFβ (transforming growth factor beta) signalling in the ectoderm)

Question 7

What has BMP4 signalling been identified in?

Answer 7

BMP4 signalling has been identified in ectoderm cells differentiating into epidermis skin cells

Question 8

The role of Chordin, noggin and follistatin is to inhibit BMP signalling and induce neural plate formation

Question 9

BMP ligands are members of what superfamily?

Answer 9

TGF-beta

Question 10

What do BMP ligands bind to?

Answer 10

Ligand binds to a TGF-β type II receptor (serine/threonine receptor kinase)

Question 11

Tell me about BMP signalling

Answer 11

• Member of TGF-β signalling superfamily
• Ligand binds to a TGF-β type II receptor (serine/threonine receptor kinase)
• Forms dimer with type 1 receptor and the kinase phosphorylates Type 1 receptor
• Downstream phosphorylation of cytoplasmic SMAD proteins follows
• SMAD complex enters nucleus and regulates transcription of target genes

Question 12

Neural tube formation in an amphibian embryo

Answer 12

Question 13

Tell me about the control of neural specification by levels of BMPs

Answer 13

(A) Control embryo with neural folds stained for the expression of the neural gene Sox2

(B) Lack of neural tube and Sox2 expression in an embryo treated with the morpholinos against three BMP inhibitors (chordin, noggin, follistatin)

(C) The neural tube, visualized by Sox2 staining, is greatly enlarged in embryos treated with antisense morpholinos that destroy BMPs 2, 4, and 7

(D) Complete transformation of the entire ectoderm into neural ectoderm (and loss of the dorsal-ventral axis) by inactivation of BMPs 2, 4, and 7

Question 14

The soluble protein Noggin dorsalises the amphibian embryo

Localization of Noggin mRNA within organizer and subsequently notochord (this example is from the frog)

Answer 14

(B) Localization of noggin mRNA in the organizer tissue, shown by in situ hybridization

At gastrulation (i), noggin mRNA (dark areas) accumulates in the dorsal marginal zone.

When cells involute (ii), noggin mRNA is seen in the dorsal blastopore lip

During convergent extension (iii), noggin is expressed in the precursors of the notochord, prechordal plate, and pharyngeal endoderm,

Which (iv) extend beneath the ectoderm in the center of the embryo

Question 15

Label this cross section of late neurula

Answer 15

Question 16

Primary neurulation in the chick embryo (part 1)

• The neurulating chick embryo (dorsal view) at about 24 hours
• The cephalic (head) region has undergone neurulation
• The caudal (tail) region is still undergoing gastrulation

Answer 16

Question 17

How can cells of the neural plate be distinguished?

Answer 17

(A, 1a) Cells of the neural plate can be distinguished as elongated cells in the dorsal region of the ectoderm

Question 18

Where does folding in the neural tube begin and where does it move towards?

Answer 18

(B, 1b) Folding begins as the medial hinge point (MHP) cells anchor to the notochord and change their shape while the presumptive epidermal cells move toward the dorsal midline

Question 19

The neural folds are elevated as the presumptive epidermis continues to move toward what?

Answer 19

(C, 2a) The neural folds are elevated as the presumptive epidermis continues to move toward the dorsal midline. Asymmetric constriction of actin on the apical side changes cell shapes to promote MHP bending (B, C, 2b)

Question 20

Where does convergence of the neural folds occur at?

Answer 20

(D, 3a) Convergence of the neural folds occurs as the cells at the dorsolateral hinge point (DLHP) become wedge-shaped and the epidermal cells push toward the center. (D, 3b) Similar apical constriction occurs at the DLHP

Question 21

Tell me the final step of primary neurulation in the chick embryo and where the neural folds are brought into contact with?

Answer 21

(E, 4) The neural folds are brought into contact with one another. The neural crest cells disperse, leaving the neural tube separate from the epidermis (notochord remains in these last steps and helps formation of neural tube and also helps release key molecules which helps the process)

Question 22

Primary neurulation in the chick embryo (part 2)

Answer 22

Question 23

Neurulation in a mouse embryo

Answer 23

Live imaging of a 15-somite stage embryo using a transgenic CAG:Venusmyr mouse to visualize all cell membranes

Optical dorsoventral (cross) sections show DLHP formation (curving of white line on left fold) to the point of near closure (decreasing size of double arrow)

Question 24

What does activated BMP signalling lead to?

Answer 24

Neural tube defects

Question 25

What does a loss in noggin result in?

Answer 25

failure of neural tube closure

Question 26

Tell me the following about **noggin**: Affiliated with, uniquely expressed in, positively regulates, binds to what and the effect it has

Answer 26

**Loss of Noggin results in failure of neural tube closure** * Affiliated with the dorsal blastopore lip during gastrulation * Uniquely expressed in the neural folds * Positively regulates neural tube closure * Binds directly to and inhibits BMP4

Question 27

Where are the following expressed: * BMPs * Noggin * Shh

Answer 27

* **BMPs** are expressed by the **surface ectoderm** (green) * **Noggin** is expressed in the **dorsal neural folds** (blue) * **Shh** (sonic hedgehog) is expressed ventrally in the **notochord and floor plate**

Question 28

The regulation of the hinge points revolves around BMP and acts as an antagonist to what?

Answer 28

Both DLHP and MHP formation

Question 29

Tell me about the regulation of hinge points around BMP...

Answer 29

1. Shh is required for the specification of floor plate, while additional signals from the notochord induce MHP (medial hinge point) morphology 2. Noggin directly inhibits BMP ligands, thus alleviating BMP repression of the hinge points. 3. The DLHPs, however, only form at the correct size and dorsal-ventral position, which is based on Noggin’s distance from inhibitory Shh gradients ascending from the floor plate.

Question 30

What cells experience apical constriction?

Answer 30

Apical constriction occurs only in those cells experiencing low enough concentrations of both BMP (MHP and DLHP) and Shh (DLHP) morphogens

Question 31

Name an inhibitor of noggin

Answer 31

Shh

Question 32

If theres no Shh present, what can noggin do?

Answer 32

No Shh, noggin can lead to **more DLHP** (dorsal lateral hinge point) which can impair the primary neurulation process. So need to make sure its localized at this point

Question 33

Tell me the effects caused by either low or excessive BMP signalling

Answer 33

Dosage is important: High levels of BMP signaling inhibit MHP formation while low levels promote excessive folding at the midline

Question 34

During neural tube closure at the midbrain region, what do non-neural ectodermal cells establish?

Answer 34

Question 35

What can Larval ciona be used as a model to study?

Answer 35

Neural tube formation

Question 36

What direction does fusion of the neural folds zippers in?

Answer 36

A posterior to anterior direction

Question 37

What is the essential roles of differential adhesion regulation?

Answer 37

* Cadherin family of CAMs * facilitate homophilic binding * Keep cells expressing cadherin type and amount adhered together

Question 38

What does differential E- and N-cadherin expression pattern allow for during neurulation?

Answer 38

Question 39

Name two disorders that can come about for defects in neural tube closure in the mammalian embryo

Answer 39

Exencephaly Anencephaly

Question 40

Whats exencephaly?

Answer 40

Question 41

Whats Anencephaly?

Answer 41

Question 42

Early brain development and formation of the first brain chambers

Answer 42

Question 43

**Summary**

Answer 43

* **Neural plate (thickened region in the ectoderm) forms (induced by underlying mesoderm)** * **Bending of the neural plate begins at the medial hinge point (notochord-dependent)** * **Medial push of epidermal ectoderm and cell shape changes at the dorsolateral hinge points cause convergence of the elevated neural folds** * **Neural folds give rise to neural crest cells, which disperse, and the neural tube separates from the epidermis** * **The neural tube develops three primary vesicles, and further into five secondary vesicles, from which the brain develops**