Neurulation Flashcards
1
Q
from what tissue is the notchord derived from?
A
the mesoderm- the dorsal mesoderm that makes up the initial SO
2
Q
once the neural tube has fused, what happens?
A
Neural crest cells delaminate
3
Q
when is neural folding generally finished?
A
after 25 days in humans, around 3-5 days in xenopus
4
Q
what are the 4 main mechanisms behind neurulation?
A
- convergence of the neural plate towards the midline
- elevation of the neural folds
- bending of the neural folds
- adhesion and fusion at the dorsal midline
5
Q
after gastrulation , how can the neural plate be viewed?
A
as a thickening of the ectoderm in the central dorsal region
6
Q
what type of tissue is the neural plate?
A
- columnar epithelium with apical and basal specialisations.
7
Q
how is the neural plate polarised?
A
along the apical to basal axis
8
Q
what does the convergence and extension of the mesoderm cooincide with?
A
convergence and folding of the neural plate
9
Q
what are the cell movements that occur as the neural plate undergoes convegence and extension?
A
the cells intercalate along the dorsal midline- they move in from the lateral to the medial region and move along the AP axis.
10
Q
how many hinge points are there
A
3
11
Q
where are the hinge points?
A
two dorso-lateral and one median
12
Q
how do the cels at the hing epoints change their shape?
A
they acquire a bottle like shape by undergoing constriction at their apical side while their basal side remains the same
13
Q
what is it that makes the hinge cells undergo apical constriction?
A
the interactions of actomyosin filaments and contractions- when they contract they put tension on the outside of the cell
14
Q
what happens during the adhesion/ fusion of the dorsal midline?
A
The site of fusion is characterised by cell protrusion. The protrusions
interdigitate, which leads to adhesion and fusion of the neural folds.
Apoptotic cell death, leads to disruption of the connection between
the non-neural ectoderm (green) and neuroepithelium on each side.
This will achieve continuity across the midline in both tissues.
15
Q
what type of actin is involved in to actinmyosin contractions during apical constriction?
A
Actin-F
16
Q
what connects the cells in the neural plate?
A
cadherin based adherens junctions
17
Q
what is secondary neurulation?
A
the caudal end of the neural tube (sacral and occygeal segments) forms by condensation and epithelialisation of mesenchymal cells, the lumen is formed by cavitation of a solid rod of cells by unknown mechanism
18
Q
how common are spinal chord defects?
A
1/1000
19
Q
what are three disorders that can occur from incomplete neural tube fusion?
A
ancephaly at the head, spina bifida at the bottom of the spin and craniorachischisis
20
Q
what types of mutations cause neural tube closure defects?
A
plana cel lpolairty genes
21
Q
name a specific gene which causes neural tube closure defects
A
Crash (flamingo/celsr1)
22
Q
what are the three closure points (where) in the embryo of the mouse and which do humans lack?
A
closure 1 is at the hindbrain cervical region
closure 2 is the forebrain midbrain boundary
closure 3 is the rota extremity of the forebrain. Humans lack closure 2.
23
Q
where do each of the neural tube defects affects?
A
spina bifida at the caudal end of the neural tube.
Lumbosacral spina bifida at the caudal neurpore
- ancephaly at the anterior neuropore (closure 3) and closure 2
- cranioraschisis at closure 1
24
Q
what pathway mediate planar cell polarity?
A
the WNT pathway - non canconical
25
what is the non canconical WNT pathway?
uses the DEP or PDZ domains of the dsh protein- it will either be PDZ- Damm1-Rho -Rock-actin
or
DEP-Rac-JNK-actin
26
what type of tissue is the neural plate?
neural epithelium
27
what happens in animals that deficient in PCP signalling?
they fail to form neural plate
28
what pathway mediates convergenet extension?
PCP pathway
29
are extneral forces importat for promoting the bending and folding of the neural plate?
- yes
30
how is the bending of the neural plate thouhght to occur on a molecular level?
Neuroepithelial
cells adhere to each other via cadherin-based adherens
junctions (AJs), which are located at the apical side of the neuroepithelial
layer. A scaffold protein,
Shroom3, localized around the AJs, recruits ROCKs to the AJs. ROCKs phosphorylate the regulatory light chains of myosin II, which is
associated with the circumferential actin cables lining the AJs,
leading to contraction of these actomyosin cables. Loss of Shroom3 impairs the closure of the neural plate
31
how is the PCP pathwya thought to mediate the convergence of cells to the midline during neural plate folding and convergence ?
Celsr1 distributes along a subpopulation of adherens junctions that is orientated towards the mediolateral axis of the neural plate on the apical surface
2. dsh is recruited to the celsr1-positive junctions with frizzled.
3. DAAM1 uniformly distributes at the AJs and is activated via the interaction with dsh.
4. activated daam1 then activates PDZ RHoa GEF via their physical interactionss
5. PDZ-RhoGEF recriots ROCKS to the sites
6. ROCKS acivate myosinII by phisphorylation of the myosin light chains.
7. actomyosin associated with these AJs contract alongth e mediolateral axis, organising into a cable across the multiple cells.
8. collectivley this results in cells relocating towards the neural midline
32
what is the simple sequence of protein -interactions involved in the PCP mediated progression of convergence to the midline that is needed for neural plate folding and convergence?
celsr1 distributes along AJs orientated toward the medolateral axis- dsh + frz recruited- DAAM1 is activated by DSH- PDZ-RhoGEF is ativated- ROCK is recruited- myosin II phosphorylated- actomyosin contracts
33
what is thought to be the reaison why constriction during neural folding doesnt occur at the basal side?
This
constriction should occur only at the ‘‘free’’ apical plane of the
plates because the basal surfaces of neuroepithelial cells are
structurally fixed to the extracellular matrices. Under this threedimensional
condition, the mediolateral contractile forces, operating
on the apical surface of the neural plate, are expected to
cause its polarized, inward bending
34
is it known how celsr1 becomes distributed to certain adherens junctions?
no
35
how was it shown that Celsr1 mediates the arrangement of F-actin in a mediolateral manner at the apical surface of the neural plate?
- staining for Celsr1 and F-action in WT chick embryos revealed that F-actin was highly preen tin WT but when Celsr1 was knocked down using MO injection, the polarity of the actin cables became reduced.
36
what is the phenotype of a celsr1 knock down?
much less folding of the neural plate, much less convergence
37
what is the phenotype o PCP mutants?
defective convergence and extension movements and shorter body axis- notochord is thicker and shorter
38
what happens to the hinge points when PCP is defective and so the neural plate is too wide?
- two median hinge points can form
- the median hinge point can be too wide
- there can be two median hinge points that are too fare away from each other
39
how are dorsolateral hinge points generated in mice?
Shh and BMP2 are the key players- normally BMP is inhibiting lateral hinge point formation.
- Noggin is a BMP inhibitor and BMP regulates noggin.
- Shh on the ventral side negatively regulates noggin too- it is erased from he floorplate
- when noggin is too far away to be inhibited by shh, it can inhibit BMP2 and allow hinge formation - this occurs in the lower spinal region
40
where are there dors lateral hinge points and why?
- in the lower spinal region where SHH levels are lower
41
where arent there dorsal lateral hinge points?
in the upper spinal region where SHH levels are high
42
where does the PNS come from ?
mostly neural crest cells
43
what type of tissue i sthe early neural tube?
pseudostratified neuroepithelium
44
what ca neural epithelial cells also be called?
neural progenitors
45
what are the 6 derivatives of the neural crest?
```
Dorsal root ganglia
Autonomic ganglia
Enteric nervous system
Peripheral glia (Schwann cells)
Adrenal medulla
Pigment cells
```
46
what levels of BMP and WNT do NC need o form in the ectoderm?
intermediate
47
where do NCs form?
at the border of the epidermis and ectoderm
48
what are the 4 stages of NC formation?
induction at the border, onset of NC specification, segregation, NC EMT and migration
49
what kind of transition do neural crest cells make?
EMT - loss of cadherins
50
how do differences occur between the fates of the NCs?
depending on where along the AP axis they are produced, they will form different structures
51
how was it shown that neural crest cells are derived from an interface between neural and non neural tissue?
transplant expiriment took tissue from the neural plate of a pigmented embryo and placed it into a non pigmented mebryo- they found that pigmented cells migrated from the transplant
52
how was it determined which signals were required for NC formation?
they took non neuronal ectoderm and palced in in a dish- they then applied levels of BMP and measured for neural crest markers- they found need both WNT and BMP
53
how is the mesoderm implicated in NC formation?
after gatsrulation, in the neurla, the intermediate mesoderm lies beneath the NC region- if this is emoved then you lose NC fate- it must be involved somehow
54
how is NC formed?
by BMP and WNT signals and by mesoderm
55
what is secondary neurulation?
a multi potent cell population in the tail bud differentiate to form neural cell fated cells. They then organise themselves in the dorsal part of the tail bud to create a neuroepithelium surrounding a central cavity: canalisation
56
what is interesting about closure 2?
mouse embryos that lack closure 2 achieve total cranial closure in around 80% of cases, indicating that closure 2 is not obligatory for brain formation, even in mice
57
how does neural tube closure progress in humans?
fusion progresses rostrally from closure 1 and caudally from closure 3 to close at a single anterior neuropore
58
what is craniorachischisis?
the failure to close closure 1- it results in an open neural tube encompassing the mid brain hind brain and entire spinal region
59
what is ancephaly?
when closure 1 is completed by he closure of the cranial neural tube in incomplete which effects the midbrain (meraoancephaly) or hindbrain (holoancephaly)
60
what is spin bifid a?
failure to close the posterior neuropore - can vary in severity
61
what does an open neural tube result in?
- exposure to the amniotic fluid environment in NTDs which leads to neuroepithelial degeneration
62
what treatment has been offered so far for neural tube defects ?
- folic acid
63
are NTDs thought to be genetics?
- it is accepted that the vast majority of NTDs show sporadic occurrence with very few reports of multi generational families occurrences
64
what are the non genetic factors thought to cause neural tube defects?
valproic acid increases the risk 10 -fold - it is thought that this is a histone deacetylase and leads to failure of the neural tube to close
- diabetes mellitus is also thought to predispose
- folate deficinecy
- B12 deficiency
65
why are mice good models for NTDs?
there are close similarities between the embryonic processes of neurulation in humans and mice
- there ar more than 200 mutant genes that cause NTDs in mice than cover spin dbifida, ancephaly and craniorachischisis
- good for transgenic studies
- can combine different alleles and environments easily in mice
66
what was the first gene o be identified as being associated to craniorachischisis?
Vangl2
67
what do all the genes associated with closure 1 failures have in common?
from the PCP pathway
68
name 54 PCP genes implicated in closure 1 defects
vangl1, Celsr, vangl2, scrb1
69
what is interesting about the combination of mutants that can give riser to NTD?
double gets one only one PCP gene and another PCP gene can give rise to NTDs- grhl3 and ptk7
70
what is interesting about the relationship between cytoskeletal and NTD that has been found?
cytochalasin D application has been shown o cause neural tube defetcs- implicating the actin cytoskeleton
71
how has the cell cycle and neurogenesis been linked to NTDs?
- some mutations that cause NTDs appear to be essential for cell proliferation and or prevention of premature neurogenesis - premature neurogenesis has been shown to perturb neurogenesis although the process mechanisms are not fully known
72
how is cell death associated with NTDs?
insufficient cells for NTC may also results in NTD- mutants genes for anti apoptotic genes - bcd-10
73
how is the ECM implicated in NTDs?
mutants for laminin and integrins have been implicated
74
what what is the shh pathway?
binding of shh to patched 1 receptor prevents the receptor from binding smoothened and inhibiting its activity, this allow stye Gli protein to enter the nucleus and act as transcription activators
75
what is the general role of shh thought to be in neural closure and what is the evidence for this?
mutations in proteins that have a negative influence on the Shh pathway lead to NTDS: patched mutants that allow over activity of smoothened cause NTDs but loss in function of mutations that inhibit Shh signalling do not have an effect- it is thought that shh signalling has a negative affect on neural closure- this is thought to be because it mediates the inhibition of the neural plate bedding in the dorsolateral region
76
why does the neural plate become thickened?
the cells become more columnar than their surrounding cells
77
what portion of the neural tube uses secondary neurulation?
beyond the lumbar-sacral region
78
what transition do the neural plate cells undergo during secondary neurulation?
mesenchymal to epithelial
79
how does neurulation occur in zebrafish?
the entree neural plate forms a solid rod and then hollows
80
what is the first exterior process of neural folding?
the dyes of the neural plate become raised above the plane of he epithelium forming two parallel neural folds with a depression - the neural groove- the neural folds eventually come together at the dorsal midline
81
what seems to stimulate convergent extension in the midline during neurulation?
signals from the underlying notochord
82
wha do the cells on the leading edges of the neural folds produce?
lamellipodia and filopodia
83
explain how the detachment of the neural tube from the overlying epidermis is thought to occur
neural plate ectoderm begins to express both N-cadherin and N-CAM whereas the adjacent ectoderm xpresses E-cadherin. Thes differences may help the neural tube to separate from the surrounding ectoderm and allow it to sink beneath the surface, he rest of the ectoderm forming a continuous layer over it
84
how are ephrin molecules implicated in the closure of the neural plate?
Eph-ephrin normally are involved repelling cells but when the EPH tyrosine kinase is lacking he binding can be adhesive. This is the case for neural tube closure in mice- a form of Epha7 lacking its kinase domain is produced in the cells as a result of alternative splicing. they also express ephrin A5 and the adhesive interaction between this two molecules is essential for neural tube closure- mouse mutants lacking A5 have NTDs
85
what is the mesoderm dependent and the mesoderm independent cause of neural tube closure defects?
too much Shh signalling can result in no dorso medial hinges
86
what does disruption of dishevelled signalling in xenopsy result in?
disturbed convergenet and extension - a short and broad neural plate that fails either totally or partially to complete neural tube closer. Significantly, such embryos do not lack neural folds; nor do the neural folds lack the ability to fuse across the midline in cases in which they achieve contact. The key defect is the abnormally wide spacing between the neural folds, which prevents contact being made across the dorsal midline.
87
how does neural tube closure proceed in xenopus?
all at the same time
88
what is the phenotype of a dishevlled double knockout?
craniorachischisis- the entire neural tube from midbrain to low spine remains open, as a result of failure of the initial neurulation event (so-called closure 1) at the hindbrain – cervical boundary. The neural folds at the site of closure 1 elevate normally but are spaced widely (Fig. 1e), preventing their apposition and fusion across the midline
89
what does the neural plate look like in a dsh KO?
The neural folds at the site of closure 1 elevate normally but are spaced widely (Fig. 1e), preventing their apposition and fusion across the midline
90
what induces the medial neural bend?
the notochord
91
what are the three neural chord defect phenotypes that could arise as a result of a lack of convergent extension in the notochord?
At this level of the body axis, bending progresses at the MHP, whereas more lateral regions of the neural plate remain straight during neural fold elevation, until the folds appose and fuse in the dorsal midline (arrows). (c,d) Hypothetical models to illustrate how diminished midline convergent extension (broken arrows) might lead to defective formation of the MHP. In (c), reduced convergent extension in the meso- derm yields an abnormally broad notochord that, secondarily, induces a defective MHP region. Bending occurs at two sites flanking the midline, producing neural folds that elevate but are spaced too far apart to fuse, as seen in the loop-tail and circletail mutant embryos (e). In (d), reduced neuroepithelial convergent extension yields an abnormally wide region of neural plate capable of responding to midline inducing signals from the notochord. The resulting disruption of MHP formation leads to a similar outcome, with failure of neural tube closure (e).
92
what are the different PCR mutants that cause neural tube closure defects?
- dsh
- flamingo
- celsr1
- scribble
