Making a Limb

Question 1

Limbs are _________ to the body plan

Answer 1

Appendages

Question 2

Which organism is a good model for studying limb development?

Answer 2

Chick

Question 3

What are limb buds?

Answer 3

The site for limb growth

Question 4

Limb buds have what kind of information?

Answer 4

Positional and axes info

Question 5

What processes occur at the limb bud?

Answer 5

Cell differentiation, growth, morphogenesis

Question 6

What are Hox genes?

Answer 6

A group of related genes that control the body plan of an embryo

Question 7

Which gene can be knocked out of mouse making forelimbs develop at a more anterior level?

Answer 7

Hoxb5

Question 8

What does the Hoxb5 gene do and to what organism?

Answer 8

If knocked out of mice it causes the forelimbs to develop at a more anterior level

Question 9

How does knocking out a Hoxb5 gene cause the forelimbs to develop at a more anterior level?

Answer 9

Restricts expression of fibroblast growth factors (FGFs) to limb-forming regionss

Question 10

In chicks is FGF required for limb bud formation?

Answer 10

Yes

Question 11

FGF signalling is ________ for limb bud formation

Answer 11

Required/Sufficient

Question 12

In chicks is FGF sufficient for limb bud formation?

Answer 12

Yes

Question 13

What does the homozygous knockout of FGF10 result in?

Answer 13

Embryos lacking limb buds

Question 14

Which gene can be homozygously knocked out to cause a lack of limb buds in chicks?

Answer 14

FGF10

Question 15

Local application of FGF4 induces what?

Answer 15

Ectopic (abnormally located) limb buds

Question 16

Which gene can induce ectopic limb buds in chicks?

Answer 16

FGF4

Question 17

Limb bud structure

Answer 17

Apical Ectodermal Ridge (AER)
Progress Zone (PZ
Zone of Polarising Activity (ZPA)

Question 18

What is the AER?

Answer 18

Apical ectodermal ridge

- Ridge of thickened epithelium around the tip of the limb bud

Question 19

The AER rigid, why?

Answer 19

Keeps the limb bud flattened

Question 20

What does the AER maintain?

Answer 20

Cell proliferation in the PZ

Question 21

What does removal of the AER result in?

Answer 21

Truncation of the limb

Question 22

Does the time of removal of the AER affect the truncation of the limb?

Answer 22

If removed at 3 days, only proximal elements of the arm formed. At days, more distal elements formed

Question 23

Can you induce normal development after removing the AER?

Answer 23

Yes, replaced the AER with beads soaked in FGF4

Question 24

What happens if you remove the AER and replace with beads soaked in FGF4?

Answer 24

Normal development of the limb

Question 25

What is the PZ?

Answer 25

Progress Zone | - Distal region of limb bud mesenchyme (mesodermal cells)

Question 26

Is the PZ the distal or proximal region of the limb bud mesenchyme?

Answer 26

Distal

Question 27

What are the cells like in the PZ doing?

Answer 27

Undifferentiated and rapidly dividing

Question 28

What does the length of time in the PZ determine?

Answer 28

Cell fate

Question 29

How is cell fate in the limb bud determined?

Answer 29

Length of time in the PZ

Question 30

Along which axis does the length of time in the progress zone determine cell fate?

Answer 30

Proximal-distal axis

Question 31

What is the ZPA?

Answer 31

Zone of Polarising Activity | - Region of limb bud mesenchyme which is an organising centre to pattern the anterior-posterior axis

Question 32

Which axis does the ZPA pattern?

Answer 32

Anterior-posterior axis

Question 33

Where can the ZPA be transplanted to induce symmetrical duplications of normal limb elects about the anterior-posterior axis?

Answer 33

Underneath the anterior AER

Question 34

What happens when the ZPA is transplanted underneath the anterior AER?

Answer 34

The ZPA induces symmetrical duplications of normal limb elements about the anterior-posterior axis

Question 35

Which gene is expressed in a very discrete zone of the limb bud?

Answer 35

Sonic Hedgehog (SHH)

Question 36

What acts a morphogen in the limb bud?

Answer 36

SHH

Question 37

What does SHH pattern by acting as a morphogen?

Answer 37

The anterior-posterior axis

Question 38

What can alternatively be done instead of transplanting the ZPA underneath the anterior AER to induce symmetrical duplications of limb elements about the anterior-posterior axis?

Answer 38

Implant an SHH-soaked bead as it mimics the action of ZPA

Question 39

Which class of genes are also expressed in overlapping zones of the limb bud?

Answer 39

Hox genes

Question 40

What roles do Hox genes have in the limb bud?

Answer 40

- May specify the ZPA - Deletion of Hox genes --> loss of SHH expression in ZPA (lead to limb truncation) - Transplantation of ZPA establishes new domains of the gene expression in the limb bum

Question 41

What does the deletion of Hox genes lead to?

Answer 41

Loss of SHH expression in the ZPA

Question 42

What do transplant experiments demonstrate about the limb bud?

Answer 42

That the mesoderm of the limb bud determines the pattern of cell death

Question 43

What regulates the pattern of cell death in limb buds?

Answer 43

Hox genes