Lecture 5 (2b) Somites & Segmentation In Other Organisms Flashcards
In vertebrates the … form the somites
paraxial and somitic mesoderm
Somites are
blocks of mesodermal cells on either side of the neural tube
Somites form
muscle and connective tissues of the back
eg dermis, vertebrae, ribs
Somites form one by one from the
paraxial mesoderm
• somites become compacted and spit apart as fissure split them
The number of somites is
species specific
• chicks - 50
• humans - 33
• snakes - some 500
Mesoderm forms
muscles and bones
Presomatic mesoderm
cells more compact then split into individual somites
Transplantation of boundary region to non-boundary region
when tissue put in isn’t meant to form a somatic boundary, no boundary forms
Transplantation of non-boundary region forms somites if
Notch signalling is activated
Notch signalling instructs cells to form
boundaries/somites (required)
• communication between cells in many types of tissues
• amplifies differences between cells
• activates some genes that then repress others
• this info sent to neighboring cells
Notch signalling is used for
communication between cells
Somite borders are formed at the interface of
Notch expressing and Notch non-expressing areas
Sequential molecular and cellular events of somitogenesis
- Notch aligns the cells immediately posterior to the presumptive border
- Notch activates Mesp
- Mesp suppresses Notch activity at presumptive border
- Mesp expression is maintained in the anterior half, uses Ephs
Notch aligns cells
posterior to presumptive border - sync gene expression
• activates Mesp that suppresses notch expression
• maintained in anterior
Ephs activated
induces expression of ephrins
• separate signal induces shape change for boundary formation
starts with NOTCH
Ephrins induce cells across the border to create
the fissure
• Ephs induces the expression of Ephrins
• a separate signal induces changes required for border formation
(mesenchymal-epithelial transition)
Segmentation is driven by a
clock-and-wave mechaniism
time and number of segments is driven by it
Notch is required for segmentation in
the spider
Notch not just specific for vertebrates
• also arthropods
new segments at growth zone
• in spiders, engrailed at posterior
Notch not functioning
affects segments
• segmentation is affected in Notch loss of function mutations
The expression of the Notch effector gene also affects
Hairy
Follow-up investigations showed an involvement of Notch in
myriapods and insects
Segments form sequentially in
crustaceans
In crustaceans, pair rule genes
are in every segment = not actually pair-rule genes
not every other
Snail expressed in
intersegmental furrow
in crustaceans
In the water flea (crustacean), Hunchback is only in
2 segments
In crustaceans, Notch is expressed
in stripes
Notch expression correlates with
formation of segments
To show Notch has early expression in segments
stage x segment x morpho. vis. forms
• T1 first
• gap and pair-rule genes expressed first with notch
Members of the Notch signalling pathway are expressed
at the same time as pair-rule genes
DAPT
inactivates Notch signallng
• interferes with border formation –> irregular stripes
• did a transient inhibition so still forms border/segment
2 possible functions of Notch in arthropod segmentation
- Vertebrate clock-and-wave model
* Drosophila model - mutual restriction of pair-rule expression
Notch in arthropods
- synchronizes gene expression at border formation –> sharp stripe
- mutual repression of pair-rule stripes
Notch is in border formation where
segments form
