7: Neural crest cells, 8: Lateral plate meso and heart

What are the 4 main steps of NCC formation?

1. Neural crest specification @ border btw neural plate and ectoderm
2. Neural crest @ the apex of neural folds
3. Neural crests delaminate at point of closure
4. Migrate out of ectoderm

What type of cells do NCCs give rise to?

melonocytes, neurons, bone, cartilidge etc

What process do NCCs undergo when migrating from top of neural tube?

EMT

Describe the cranial Neural crests

enter pharyngeal arches to make teeth and bones

Cardiac NCCs

form muscular connective tissue for heart and melanocytes

vagal and sacral neural crest

parasympathetic ganglia of the gut

What are the two migration pathways of trunk neural crest

1. ventrolateral
2. Dorsolateral

how much BMP and WNT do the neural plate border cells express?

Int levels of BMP and WNT

If Wnt induces BMP but then remains active (Wnt + BMP) the border cells become

NCC

If border cells recieve Wnt only (BMP blocked by noggin) they become

trunk neural cells

If border cells receive both Wnt and BMP signalling from the beginning they become

epidermis

What are the 4 types of cadherin in epidermis, neural tube and NC?

e-cadherin, n-cadherin, cadherin-6b (in pre-migratory NCCS), cadherin 7 (in migratory NCCS)

What triggers EMT of NCCs?

Decreasing Noggin levels at Neural tube
and increase in BMP signalling = EMT

What is a core EMT regulatory factor?

Snail2

What does Snail 2 repress?

All cadherins

What does snail 2 activate?

- actin remodeling, migration and attachment of integrins to the surrounding ECM

What happens if you inject snail 2 into embryo?

overexpression of snail 2 = decreased cadherin = increased migration of neural crest cells

What forces cells to move away from eachother AND the ectoderm?

Contact inhibition

Describe how contact inhibition works

- 2 cells make contact
- at point of contact, they both depolymerize actin causing them to retract
-on the opposite side, the cells polymerize actin so cells will form new protrusions away from the point of contact
- this will cause the cells to move away from eachother -- a replusive cue

What is the differnce btw contact inhibition in pre-migratory vs migratory NCCs?

- premigratory cells do not undergo contact inhibition (they form a stable contact.) Cells overlap and intermingle
-migratory NC cells undergo contact inhibition, no overlap

What are the two pathways that trunk NCC take?

- dorsolateral (btw epidermis and mesoderm)
- ventral (through the sclerotome)

Which pathway is initially blocked to NCCs?

Dorsolateral Pathway. NCCs can only tale Ventral Lateral pathway

How are the NCs inhibited from entering the Dorsolateral pathway

by proteins in the ECM like ephrins (ex. Slit)
a repulsive cue

Where do the cells migrate instead when the DSL pathway is blocked?

The cells turn and migrate ventrally
1. either between somites (only early NC)
2. or through the somites (most NC)

How do the NCs migrate through the somites? (Segmental restriction of NC cells)

- migrate through the ANTERIOR portion of sclerotome
anterior region is rich in attractive proteins like laminin and fibronectin

What does the posterior part of the sclerotome contain (in terms of types of proteins)?

Contains proteins that exclude neural crest migration or inhibit migration through semaphorin and ephrin. Eph receptors on NCC bind to them and this blocks migration

How do the NCCs use laminin and fibronectin present in the anterior half of the sclerotome to move through it?

their integrin receptors bind with the attractive proteins (laminin and fibro) to mediate migration

How is it possible that late NCCs take the DL pathway if it has repulsive ephrin?

There must be upregulation of another set of attractive proteins in the DLP. (All Ncc express Eph receptors so it is not the loss of eph receptors on NCC)

Where do the NCC from the later pathway (DLP) go?

they remain above the neural tube and from cells like melanocytes

What does the chordamesoderm give rise to?

notochord (rigid rod like structure)
CHORDIN-MRNA

What does the paraxial/somatic mesoderm give rise to?

- paraxial --> somites (segmented blocks of meso) PARAXIS-MRNA
- somatic mesoderm --> head mesoderm (not segmented)
- flanks the notochord

What does the intermediate mesoderm give rise to?

parallel along paraxial meso (somites) PAX2-MRNA
forms kidneys, gonads, urogenital

What does the lateral mesoderm give rise to?

heart, blood vessels

Describe the Noggin and BMP activity along the mediolateral axis (mediolateral means centre to side)

High noggin @notochord, decreases towards outside
High BMP @lateral plate mesoderm, lower as goes in towards chordamesoderm

If you put noggin secreting cells into Lateral Plate Meso what will happen?

lateral plate mesoderm will alter into paraxial mesoderm - somites will form

What are the two layers that the lateral mesoderm horizontally splits into?

Dorsal layer is the somatic mesoderm (becomes head muscles)
ventral layer in the splanchnic mesoderm (becomes the heart)
The space between the two layers is a coelom

What gives rise to cardiogenic mesoderm?

the splanchnic mesoderm in region that has close contact to pharyngeal endoderm

How does close proximity to pharyngeal endoderm give rise to heart?

BMP2 in endoderm promotes heart development
ALSO BMP also induces FGF8 right underneath the cardiogenic mesoderm. FGF8 is critical for cardiac protein expression

What part secretes noggin and what role does noggin play in differentiation?

the notochord secretes noggin
Noggin inhibits BMP activity in center of embryo and at somites to prevent heart induction

What part secretes Wnt and what role does noggin play in differentiation?

The neural plate secretes Wnt
inhibits heart formation, but promotes blood formation

How are cardiac precursor cells induced (+/- Wnt/BMP)

With the presence of BMP
Without Wnt

Nkx2-5 and Mesp1 are activated in the presence of ?? without ??

BMP without Wnt .
These are two genes that are essential for heart specification

Is the localization of Nkx2 restricted or everywhere?

restricted - it only expressed at a specific location

Nkx2-5 expression and heart induction is only possible in the

anterior region - right over the pharyngeal endoderm

Describe the two major regions of the heart field

Primary - fuse at midline to form primary heart tube and scaffold
Secondary - adds cells to the anterior and posterior of the primary heart field

What is the RADH2 gene?

encodes the retinoic acid-synthesizing enzyme retinaldehyde dehydrogenase-2

How does RA organize the trunk?

it supresses FGF8 (FGF8 is expressed in anterior for heart and posterior for gastrulation) RA supresses this right in the middle in the somatic mesoderm

What will happen in a mutant without RA?

The heart field will expand posteriorly, small somites, abnormal head morphology
As well as shortened trunk region
no forelimb buds

What brings the two seperate heart primordia together? And what is it called when this doesnt happen?

The infolding of the primitive gut brings them together and they fuse forming the primary heart tube
If they dont fuse its called cardia bifida (2 seperate hearts will form)

Are somitic cells specified at first?

No at first they can become any type of somite derived tissue, but then their fates become specified.

What are the two main regions of the somite?

Sclerotome (closest to notochord, become mesencyme like tendons, vertebra, rib cartilidge)
and dermomyotome (make the myotome and dermatome)

What does the myotome give rise to?

muscles of back

How do the sclerotomes form vertebra?

The sclerotome separates into two parts, the rostral and the caudal segment. Then the spinal nerve will seperate them. The halves then fuse together to form vertebra (but not the two halves from the original one, they are two halves from diff sclerotomes)

Which direction is somite formation?

From anterior to posterior. Somites bud off from the pre somatic mesoderm (PSM) posteriorly

Describe somite boundary formation

ephrin -eph signalling promotes MET (mesenchymal to epithelial transition)
1. MESP (mesodermal posterior) becomes restricted to anterior half of a somitomere
2. MESP upregulates eph
3. eph upregulates ephrin in the posterior end of the previous somite
4. border of ephrin-eph = replulsive cue and triggers cells to seperate