Unit 3: Somitogenesis

Question 1

What are the 3 adult vertebrate derivates of somites ?

Answer 1

• Axial skeleton
• Skeletal muscle
• Dermis

Question 2

Which germ layer gives rise to somites ?

Answer 2

Paraxial mesoderm

Question 3

What are the 6 steps in somite generation ?

Answer 3

1) Gastrulation
2) Mesoderm
3) Paraxial mesoderm
4) Presomitic mesoderm
5) Somitomeres
6) Somites

Question 4

What part of the somite is epithelial ?

Answer 4

The outside

Question 5

What part of the somite is mesodermal ?

Answer 5

The inside

Question 6

What processes are important in somitogenesis (3) ?

Answer 6

• Cell division
• Mesenchymal to epithelial transition
• Fate restriction

Question 7

What can cell division in the presomatic mesoderm provide ?

Answer 7

It provides more tissue that is able to differentiate into somites

Question 8

Does the length of presomatic mesoderm stay constant during cell division ?

Answer 8

Yes it does

Question 9

What is Fgf8 ?

Answer 9

A signalling molecule that is expressed in cells around the node

Question 10

Where are low levels of Fgf8 expressed ?

Answer 10

Rostral/ anterior

Question 11

Where are high levels of Fgf8 expressed ?

Answer 11

Caudal/ posterior

Question 12

What would happen if Fgf8 expression was lower ?

Answer 12

The first somites would develop closer to hensens node

Question 13

Do adjacent tissues influence somitogenesis ?

Answer 13

No, environment is not important

Question 14

Is the number of cells in the presomitic mesoderm important ?

Answer 14

No, if you remove a slice of presomitic mesoderm somites still form at normal rate- only smaller in size

Question 15

What evidence shows that somite cell identity is already determined in the presomitic mesoderm ?

Answer 15

An experiment done shows that the identity of somites swapped when pre somitic mesoderm was inverted. e.g. posterior tissues formed posterior vertebrae

Question 16

What is homeotic transformation ?

Answer 16

The conversion of one body part to another (Usually more anterior)

Question 17

What controls WHEN somites are formed ?

Answer 17

Cycles of hairy expression

Question 18

What controls WHERE somites are formed ?

Answer 18

Fgf8

Question 19

What controls somite identity ?

Answer 19

HOX genes

Question 20

Are anterior or posterior somites older ?

Answer 20

• Anterior somites are older
• They are more developed and differentiated then posterior somites

Question 21

(In roman numerals) What is the most newly formed and most posterior site called ?

Answer 21

I

Question 22

What is a young somite (somites I-III) called ?

Answer 22

Somitocoel cell- An epithelial ball surrounding cells that remain mesenchymal

Question 23

What portion of somites undergoes EMT to become sclerotome ?

Answer 23

Ventral-medial somites (Somites IV-V)

Question 24

What forms dermomyotome ?

Answer 24

Dorsal somites- shaped like a fitted sheet

Question 25

When somites reach VI-X, where do they ingress and what do they form (2) ?

Answer 25

• Dermomyotome cells ingress from the dorsal medial lip and ventral lateral lip
• To form a new myotome layer between dermomyotome and sclerotome

Question 26

What type of cells do not divide ?

Answer 26

Myotomal cells

Question 27

What is patterning into somite derivatives driven by ?

Answer 27

Specific signals from the neural tube and notochord

Question 28

What gene is required for myotome formation ?

Answer 28

Want

Question 29

What tissue gives rise to vertebrae ?

Answer 29

Sclerotome