Lecture 5 (2b) Somites & Segmentation In Other Organisms

Question 1

In vertebrates the … form the somites

Answer 1

paraxial and somitic mesoderm

Question 2

Somites are

Answer 2

blocks of mesodermal cells on either side of the neural tube

Question 3

Somites form

Answer 3

muscle and connective tissues of the back

eg dermis, vertebrae, ribs

Question 4

Somites form one by one from the

Answer 4

paraxial mesoderm

• somites become compacted and spit apart as fissure split them

Question 5

The number of somites is

Answer 5

species specific
• chicks - 50
• humans - 33
• snakes - some 500

Question 6

Mesoderm forms

Answer 6

muscles and bones

Question 7

Presomatic mesoderm

Answer 7

cells more compact then split into individual somites

Question 8

Transplantation of boundary region to non-boundary region

Answer 8

when tissue put in isn’t meant to form a somatic boundary, no boundary forms

Question 9

Transplantation of non-boundary region forms somites if

Answer 9

Notch signalling is activated

Question 10

Notch signalling instructs cells to form

Answer 10

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

Question 11

Notch signalling is used for

Answer 11

communication between cells

Question 12

Somite borders are formed at the interface of

Answer 12

Notch expressing and Notch non-expressing areas

Question 13

Sequential molecular and cellular events of somitogenesis

Answer 13

1. Notch aligns the cells immediately posterior to the presumptive border
2. Notch activates Mesp
3. Mesp suppresses Notch activity at presumptive border
4. Mesp expression is maintained in the anterior half, uses Ephs

Question 14

Notch aligns cells

Answer 14

posterior to presumptive border - sync gene expression
• activates Mesp that suppresses notch expression
• maintained in anterior

Question 15

Ephs activated

Answer 15

induces expression of ephrins
• separate signal induces shape change for boundary formation
starts with NOTCH

Question 16

Ephrins induce cells across the border to create

Answer 16

the fissure
• Ephs induces the expression of Ephrins
• a separate signal induces changes required for border formation
(mesenchymal-epithelial transition)

Question 17

Segmentation is driven by a

Answer 17

clock-and-wave mechaniism

time and number of segments is driven by it

Question 18

Notch is required for segmentation in

Answer 18

the spider

Notch not just specific for vertebrates
• also arthropods
new segments at growth zone
• in spiders, engrailed at posterior

Question 19

Notch not functioning

Answer 19

affects segments

• segmentation is affected in Notch loss of function mutations

Question 20

The expression of the Notch effector gene also affects

Answer 20

Hairy

Question 21

Follow-up investigations showed an involvement of Notch in

Answer 21

myriapods and insects

Question 22

Segments form sequentially in

Answer 22

crustaceans

Question 23

In crustaceans, pair rule genes

Answer 23

are in every segment = not actually pair-rule genes

not every other

Question 24

Snail expressed in

Answer 24

intersegmental furrow

in crustaceans

Question 25

In the water flea (crustacean), Hunchback is only in

Answer 25

2 segments

Question 26

In crustaceans, Notch is expressed

Answer 26

in stripes

Question 27

Notch expression correlates with

Answer 27

formation of segments

Question 28

To show Notch has early expression in segments

Answer 28

stage x segment x morpho. vis. forms
• T1 first
• gap and pair-rule genes expressed first with notch

Question 29

Members of the Notch signalling pathway are expressed

Answer 29

at the same time as pair-rule genes

Question 30

DAPT

Answer 30

inactivates Notch signallng
• interferes with border formation –> irregular stripes
• did a transient inhibition so still forms border/segment

Question 31

2 possible functions of Notch in arthropod segmentation

Answer 31

• Vertebrate clock-and-wave model

* Drosophila model - mutual restriction of pair-rule expression

Question 32

Notch in arthropods

Answer 32

• synchronizes gene expression at border formation –> sharp stripe
• mutual repression of pair-rule stripes

Question 33

Notch is in border formation where

Answer 33

segments form