1. Somitogenesis and Somite differentiation Flashcards
(18 cards)
How do different levels of BMP define the three ectodermal regions?
High BMP = surface ectoderm e.g. hair, nails, lens, cornea, mouth epithelium
Moderate BMP = neural crest e.g. PNS, adrenal medulla, melanocytes, facial cartilage
Low BMP = neural plate/tube e.g. brain, neural pituitary, spinal cord, retina, motor neurones
What’s the difference between primary and secondary neurulation?
Primary = neural plate is initially flat then cells surrounding direct NP cells invaginate into the body, separate from the surface ectoderm.
Secondary = aggregation of mesenchyme cells into a solid core that subsequently hollows out (cavitation)
Describe what happens to neural crest cells.
Specified at border of the neural plate and found at apex of neural folds.
Epithelial to mesenchymal transition resulting from downregulation of cadherin-6b and hence NCC adhesion. Delamination upon neural tube closure and migration out of ectoderm tissue.
Vagal NC (S 1-7) and Sacral neural crest (posterior to S28) form parasympathetic nerves of the gut.
NCC of the trunk (S6 through tail) make sympathetic neurones of PNS and pigment cells (melanocytes)
Trunk NCC (s18-24) form medulla of adrenal gland. Non-equivalence of trunk and cranial NC due to Hox gene expression
Why are radial glia cells counted as stem cells?
Multipotent (give rise to neurons and glia)
Self-renew
What are somites?
transient mesodermal metameric structures along AP axis either side of the NT. Segmental pattern preserved in tissues generated. Number is species specific: chick 50, mice 65, zebrafish 33, snakes up to 500
What type of experiments show that specification along the AP axis occurs early?
Heterochronic transplantation in chicks.
Transplant from PSM of thoracic vertebra (thorax) into PsS of cervical (neck) vertebra of slightly younger embryo –> get vertebrate developing from donor tissue, therefore presomitic mesoderm has acquired positional identity before somites have formed
What is Mesenchymal to epithelial transition (MET)?
Outer cells form an epithelium, inner cells remain mesenchymal.
Start with mass of disordered mesenchymal cells with no boundaries or obvious morphological features.
Cell shape changes
Cell adhesion changes producing intersomitic boundaries
Mesp becomes restricted to rostral (anterior) half > induces expression of Ephrin receptor (Eph-A4) which induces expression of ligand (Ephrin B2) in adjacent domain > Separation occurs at boundary of cells expression Ephrin B2 and Eph-A4
What does MET require?
Expression of the transcription factor Mesp (mesodermal posterior) – zebrafish, high levels of Mesp in defined regions at the time
somitogenesis occurring in very near to the future somite boundaries
Eph and their ligand Ephrins – ephrinB2 (ligand), ephrinA4 (receptor) – both transmembrane, see alternating bands of ligand and receptor as the somites are forming
What are the mechanisms of MET?
- Pattern of gene expression sets up changes in the cell shape so there are signals leading to molecules like CDC42 (GTPase) influencing cell shape)
- Downstream is changes in cell surface molecule expression like integrins (responsible for binding of extracellular matrix - some cells become stronger binders of ECM and less able to bind their fellow mesenchymal cells
What is the clock-wavefront model?
Cooke and Zeeman (1976)
Clock: This represents a molecular oscillator within the presomitic mesoderm cells. These cells exhibit periodic cycles of gene expression, switching between on and off states. Key genes involved in this oscillation include members of the Notch, Wnt, and FGF signaling pathways.
Wavefront: This is a gradient of signaling molecules, such as FGF and Wnt, that moves from the posterior to the anterior of the embryo. The wavefront marks a threshold of activity that interacts with the oscillating cells.
Interaction: As the wavefront moves anteriorly, it encounters cells in the presomitic mesoderm that are in a permissive state due to the clock’s oscillation. When these cells intersect with the wavefront, they undergo a mesenchymal-epithelial transition, forming a new somite boundary.
How do FGF and RA interact within the clock-wavefront model?
Opposing gradients
RA limits fgf8 expression
FGF highly expressed and made in posterior of embryo, and forms gradient lower towards anterior
Retinoic acid highly expressed at anterior – RA limits FGF expression
FGF activity maintains presomatic mesoderm in an immature state, not competent to form
boundary
At lower FGF conc, PSM becomes competent to form boundary (i.e. activation of Mesp)
Gradient of Fgf8 becomes progressively displaced caudally during tail-bud extension, moving determination front more posteriorly and sequential formation of somites
Where is the determination front and how was it found?
Dubrulle et al. (2001)
Invert sections of PSM at differ positions along the AP axis and examine effects on caudal/rostral pattern
At position 0 (very close to future somite formation), kept original information, so DF below 0
At S-3 (3 somites time), sometimes cells expressed original information labial (DF about -3)
At S-6, gene expression fully consistent with new position (didn’t know if they’d form boundary or not at this position)
Determination front lies at position S-IV
Describe the evidence that fgf8 levels are critical to the determination front
Overexpressing chick Fgf8 by in vivo electroporation of DNA constructs strongly perturbed somitogenesis – formation of smaller somites or complete absence of somites
Fgf8 is sufficient to maintain caudal identity of PSM cells
Suggest that downregulation of FGF signalling at the level of the determination front is required for PSM cells to proceed further with their segmentation program and respond to the ‘clock’
Generated local perturbations in the Fgf8 gradient by implanting Fgf8 soaked beads next to the PSM > altered somite size
Fgf8 gradients are critical to the determination front
How is notch involved in somitogenesis?
Wave of expression of Notch targets that travels anteriorly
Time taken to cross the PSM correlates with time taken to form a somite
Mesp is activated by Notch thereby terminating the clock and causing MET
As boundary is formed, somite also gains rostral/caudal polarity
Remember it’s waves of notch signalling, notch itself can’t diffuse
More posterior PSM cells do not undergo MET because they are not competent to respond due to high levels of FGF signalling.
Normally notch-delta signalling lead to lateral inhibition and not oscillations
Unstable negative feedback loop: one notch downstream target is lunatic fringe which modulates the ligand affinity (and activity) of the Notch receptor
Notch signals lunatic fringe (Lfng) > Lfng signals to downregulate notch activity > Lfng goes off because it’s downstream of Notch > Notch signalling can go up again
Describe embryonic segmentation in drosophila
D and other ‘long-germband’ insects have no posterior growth zone
Segments are generated almost simultaneously by the subdivision of an existing field of cells, using a cascade of patterning interactions that are triggered by maternal gradients. Growth and segmentation are uncoupled
What is the molecular mechanism in the sclerotome?
Sclerotome progenitors express TF Pax6 and loose N-cadherin expression.
Undergoes EMT, establishing migratory population of cells.
Sclerotome migrations around notochord and NT to build vertebral column is directed by chemoattractant epimorphin (expressed by cells surrounding NT and acts extracellularly) with dorsal migration attracted by PDGF.
Each sclerotome splits into rostral and caudal segment.
Spinal neurones grow outward to innervate muscles derived from myotome.
Rostral segment of each sclerotome combines with the caudal segment of the next anterior sclerotome to form a vertebral rudiment (resegmentation) - Somites are out of sync with final adult vertebrae because of resegmentation.
Neural crest cells can migrate through rostral but not caudal regions of the sclerotome because of the effect of ephrin on neighbouring tissues
What is the mechanism in the dermomyotome?
Myotome > muscle
Dermatome > dermis, muscle, muscle stem cells, brown fat
Dorsolateral half of the somite, maintains much of its epithelial structure (unlike sclerotome).
Dorsomedial and ventrolateral lips progenitor zones that generate the myotome that lies beneath the dermatome
Cells delaminate from the VLL to give rise to the myogenic progenitors of the limbs that migrate into the LPM
