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

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Flashcards in Lecture 5 (2b) Somites & Segmentation In Other Organisms Deck (33):
1

In vertebrates the ... form the somites

paraxial and somitic mesoderm

2

Somites are

blocks of mesodermal cells on either side of the neural tube

3

Somites form

muscle and connective tissues of the back
(eg dermis, vertebrae, ribs)

4

Somites form one by one from the

paraxial mesoderm
• somites become compacted and spit apart as fissure split them

5

The number of somites is

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

6

Mesoderm forms

muscles and bones

7

Presomatic mesoderm

cells more compact then split into individual somites

8

Transplantation of boundary region to non-boundary region

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

9

Transplantation of non-boundary region forms somites if

Notch signalling is activated

10

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

11

Notch signalling is used for

communication between cells

12

Somite borders are formed at the interface of

Notch expressing and Notch non-expressing areas

13

Sequential molecular and cellular events of somitogenesis

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

14

Notch aligns cells

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

15

Ephs activated

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

16

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)

17

Segmentation is driven by a

clock-and-wave mechaniism
(time and number of segments is driven by it)

18

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

19

Notch not functioning

affects segments
• segmentation is affected in Notch loss of function mutations

20

The expression of the Notch effector gene also affects

Hairy

21

Follow-up investigations showed an involvement of Notch in

myriapods and insects

22

Segments form sequentially in

crustaceans

23

In crustaceans, pair rule genes

are in every segment = not actually pair-rule genes
(not every other)

24

Snail expressed in

intersegmental furrow
in crustaceans

25

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

2 segments

26

In crustaceans, Notch is expressed

in stripes

27

Notch expression correlates with

formation of segments

28

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

29

Members of the Notch signalling pathway are expressed

at the same time as pair-rule genes

30

DAPT

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

31

2 possible functions of Notch in arthropod segmentation

• Vertebrate clock-and-wave model
• Drosophila model - mutual restriction of pair-rule expression

32

Notch in arthropods

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

33

Notch is in border formation where

segments form