Somitogenesis Flashcards
Basics: how does the somite segmentation occurs?
molecular oscillator
opposing morphogen gradients
Basics: which pathway is at the core of the oscillator?
Notch/Delta
Basics: what is needed to pinch off somites?
cell-cell repulsion
What develops from: chordamesoderm paraxial mesoderm intermediate mesoderm lateral plate mesoderm?
notochord
head and somites
kidney and gonads
circulatory system, body cavity, pelvis, limb bones, extraembryonic tissues
What develops from the anterior part of the paraxial mesoderm?
forms head mesoderm
together with neural crest cells
forms skeletal muscles, skull, connective tissue and nerves of the head
How is paraxial mesoderm specified on the level of TFs?
Noggin in paraxial mesoderm inhibits BMP-4 and specifies paraxial mesoderm
if you induce Noggin in lateral plate mesoderm = ectopic somites
Define somites.
- 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)
What defines periodicity of somite formation and their size?
oscillator
size/number proportional to the speed of the oscillator
How is oscillatory gene expression achieved?
protein shuts its own transcription off
when it decays, promoter is on again
depends on half-life of the protein and mRNA
Levels of which proteins oscillate in somitogenesis?
What is the read out of the clock?
bHLH TFs:
HER/HES proteins (e.g. Hes7, Lfng)
Hes7 and Lfng proteins are mutually exclusive
readout = Notch ICD
How is oscillator regulated?
by negative-feedback loop
mRNA has to decay quickly
there must be a delay between mRNA and protein (transcription, expot/import into nucleus)
Which model describes synchronization of cellular oscillators? Describe.
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
How is the wavefront created?
2 opposing gradients created by ligands of 3 signaling pathways:
RA = rostral to caudal
FGF and Wnt = caudal to rostral
What happens at the wavefront?
oscillatory expression is halted differentiation begins (pre-patterning of the somites; "cells begin to respond to the clock signals")
How is FGF gradient created in the paraxial mesoderm?
FGF is transcribed only in the primitive streak or in the tail bud
mRNAs are progressively degraded
creating a gradient
Describe Wnt signaling pathway.
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
Describe FGF pathway.
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
Describe RA signaling pathway.
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
How is RA rostral to caudal gradient formed?
RALDH2 synthesizes RA in the anterior PSM and somites
CYP26 degrades RA
CYP26 active downstream of FGF
in the tail bud
What is required for somite border formation?
high Notch activity
Which protein is expressed in the whole presomitic mesoderm?
TBX-6
Expression of which proteins is significant for S-1 segment?
TBX-6 +
NICD +
FGF -
Expression of which protein is initiated in S-1 segment?
MESP
mesoderm posterior
master regulator of somite formation
What is the role of MESP?
represses NICD formation by Lfng expression
induces TBX-6 degradation
MESP expression defines the size of the somite
Expression of which proteins is significant for S0 segment?
MESP +
NICD -
TBX-6 -
How is the caudal part of the somite specified?
Notch signaling:
inhibited in rostral part because MASP2 still present there
sidenote: rostral = anterior
How are the somites pinched off (how is the fissure formed)?
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
Which parts of the somites form the vertebrae?
rostral part of one segment + caudal part of the following one
What do somites give rise to?
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
As somite matures, which segments form?
notochord induces sclerotome formation (cartilage of ribs, vertebrae, major part of rib bone)
dermomyotome
- lateral (-> muscles)
- central (-> dermis)
