Somitogenesis

Question 1

Basics: how does the somite segmentation occurs?

Answer 1

molecular oscillator

opposing morphogen gradients

Question 2

Basics: which pathway is at the core of the oscillator?

Answer 2

Notch/Delta

Question 3

Basics: what is needed to pinch off somites?

Answer 3

cell-cell repulsion

Question 4

What develops from:
chordamesoderm
paraxial mesoderm
intermediate mesoderm
lateral plate mesoderm?

Answer 4

notochord
head and somites
kidney and gonads
circulatory system, body cavity, pelvis, limb bones, extraembryonic tissues

Question 5

What develops from the anterior part of the paraxial mesoderm?

Answer 5

forms head mesoderm
together with neural crest cells
forms skeletal muscles, skull, connective tissue and nerves of the head

Question 6

How is paraxial mesoderm specified on the level of TFs?

Answer 6

Noggin in paraxial mesoderm inhibits BMP-4 and specifies paraxial mesoderm

if you induce Noggin in lateral plate mesoderm = ectopic somites

Question 7

Define somites.

Answer 7

• repetitive segments/blocks of tissue that form from paraxial mesoderm
• bilaterally symmetrical
• form in A to P direction
• are transient structure
  important in organizing the segmental pattern of vertebrate embryos
• eventualy give rise to: vertebrae, rib cage, skeletal muscle, cartilge, tendons, skin
• are formed periodically during development (generally 33, every 4-5h; can be used for developmental timing)

Question 8

What defines periodicity of somite formation and their size?

Answer 8

oscillator

size/number proportional to the speed of the oscillator

Question 9

How is oscillatory gene expression achieved?

Answer 9

protein shuts its own transcription off
when it decays, promoter is on again

depends on half-life of the protein and mRNA

Question 10

Levels of which proteins oscillate in somitogenesis?

What is the read out of the clock?

Answer 10

bHLH TFs:
HER/HES proteins (e.g. Hes7, Lfng)

Hes7 and Lfng proteins are mutually exclusive

readout = Notch ICD

Question 11

How is oscillator regulated?

Answer 11

by negative-feedback loop

mRNA has to decay quickly
there must be a delay between mRNA and protein (transcription, expot/import into nucleus)

Question 12

Which model describes synchronization of cellular oscillators? Describe.

Answer 12

Three-tier model of the segmentation clock

bottom tier = single cell oscillators
middle tier = local synchroniation through intercellular communication (Notch/Delta)
upper tier = global control of slowing and arrest

Question 13

How is the wavefront created?

Answer 13

2 opposing gradients created by ligands of 3 signaling pathways:
RA = rostral to caudal
FGF and Wnt = caudal to rostral

Question 14

What happens at the wavefront?

Answer 14

oscillatory expression is halted
differentiation begins (pre-patterning of the somites; "cells begin to respond to the clock signals")

Question 15

How is FGF gradient created in the paraxial mesoderm?

Answer 15

FGF is transcribed only in the primitive streak or in the tail bud
mRNAs are progressively degraded
creating a gradient

Question 16

Describe Wnt signaling pathway.

Answer 16

CANONICAL
Wnt binds Frizzled
Frizzled activates Disheveled
Dsh prevents GSK-3 and APC from degrading B-catenin
stabilized B-catenin translocates to the nucleus
acts as a transcriptional coactivator of TFs of the LEF/TCF family

NON-CANONICAL
via PLC
leads to Ca release and PKC activation
eventual phosphorylation of NF-AT enables calcineurin to enter the nucleus and act as TF

Question 17

Describe FGF pathway.

Answer 17

FGF binding leads to dimerization of the FGF receptor
leads to cross-phosphorylation of Tyr residues in the IC domain of the receptor
phosphorylated residues are specificall bound by several intracellular signal transduction proteins
initiate several intracellular signaling pathways
e.g. Ras/ERK

cellular response:phosphorylation of ERK

Question 18

Describe RA signaling pathway.

Answer 18

retinol binds to retinol-binding protein in extracellular space
is transported into cytoplasm
binds CRBP
retinol –> retinaldehyde –> RA by RALDH2

RA can be degraded by cytochrme P450 26 or
act intercellularly and trigger target gene expression

Question 19

How is RA rostral to caudal gradient formed?

Answer 19

RALDH2 synthesizes RA in the anterior PSM and somites

CYP26 degrades RA
CYP26 active downstream of FGF
in the tail bud

Question 20

What is required for somite border formation?

Answer 20

high Notch activity

Question 21

Which protein is expressed in the whole presomitic mesoderm?

Answer 21

TBX-6

Question 22

Expression of which proteins is significant for S-1 segment?

Answer 22

TBX-6 +
NICD +
FGF -

Question 23

Expression of which protein is initiated in S-1 segment?

Answer 23

MESP
mesoderm posterior

master regulator of somite formation

Question 24

What is the role of MESP?

Answer 24

represses NICD formation by Lfng expression
induces TBX-6 degradation

MESP expression defines the size of the somite

Question 25

Expression of which proteins is significant for S0 segment?

Answer 25

MESP +
NICD -
TBX-6 -

Question 26

How is the caudal part of the somite specified?

Answer 26

Notch signaling:
inhibited in rostral part because MASP2 still present there

sidenote: rostral = anterior

Question 27

How are the somites pinched off (how is the fissure formed)?

Answer 27

Bidirectional Eph-Ephrin signaling

Eph and Ephrin are membrane bound

Mesp+ cells (A part of somite forming, Eph) vs Mesp- (P, Ephrin)

MET by downregulating snail TFs

Question 28

Which parts of the somites form the vertebrae?

Answer 28

rostral part of one segment + caudal part of the following one

Question 29

What do somites give rise to?

Answer 29

cartilages of ribs
vertebrae
tendons
muscles of rib cage, limbs, abdomen, tongue, back
skin
vascuar cells (dorsal part of aorta!)
meninges of the spinal cord

Question 30

As somite matures, which segments form?

Answer 30

notochord induces sclerotome formation (cartilage of ribs, vertebrae, major part of rib bone)

dermomyotome

• lateral (-> muscles)
• central (-> dermis)