Vertebrate Limb Bud Formation

Question 1

What are the chick equivalent of the following bones?

a) humerus
b) ulna/radius
c) carpals/digits

Answer 1

a) Stylopod
b) Zeugopod
c) Autopod

Question 2

What digits do chicks have?

Answer 2

2, 3 and 4

Question 3

Where do limb buds first appear?

Answer 3

From the flank of the embryo along the AP axis in precise locations

Question 4

How do we know that the limb field is molecularly defined to form the limb?

Answer 4

Through transplantation studies eg take mesoderm from the prospective limb field and transplant ectopically means a limb will form

Question 5

What are the two transcription factors which specify limb identity?

Answer 5

tbx 5 and tbx 4

Question 6

Where is tbx 4 expressed?

Answer 6

The hind limb

Question 7

Where is tbx 5 expressed?

Answer 7

The fore limb

Question 8

What does tbx 5 inhibit through negative feedback?

Answer 8

Tbx 4

Question 9

What happens if you express Tbx 4 in the fore limb?

Answer 9

The fore limb will become a leg

Question 10

The position of the limb along the AP axis is dictated by what?

Answer 10

Hox genes

Question 11

Which Hox gene determines the position of the forelimb?

Answer 11

hox 6

Question 12

How are the hox genes and tbx genes related?

Answer 12

Hox gene expression provides a permissive enviroment that allows the synthesis of retinoic acid in the lateral mesoderm where the limb buds form
Retinoic acid induces the expression of Tbx

Question 13

What occurs if beads soaked in FGF4 are placed along the AP axis in lateral mesoderm?

Answer 13

Placing beads in a flank region not destined for limb bud formation would develop an ectopic limb
The identity of the extra limb would depend on which limb the bead was closer to

Question 14

What initiates FGF expression?

Answer 14

Tbx

Question 15

What does Wnt3a specify?

Answer 15

The apical ectodermal ridge

Question 16

What is the AER?

Answer 16

A thickening of ectoderm which lies over the limb bud which has a role in proximo-distal development of the limb bud

Question 17

What are the parts of the limb bud?

Answer 17

Apical ectodermal ridge
Progress Zone
Polarising Region

Question 18

What happens when the AER is removed at:

a) 3 days
b) 3.5 days
c) 4 days

Answer 18

a) no distal elements
b) no digits
c) nearly full compliment (digits slightly truncated)

Question 19

What are the two models for the proximo-distal patterning of the vertebrae limb?

Answer 19

The progress zone model

Two Signal model

Question 20

What is the progress zone model?

Answer 20

States that if a cell spends little time dividing in the progress zone it will be more proximal compared to if it spends a long time in the progress zone
This model was in place for a number of years but is not correct - fate mapping did not match these predictions

Question 21

What is the two signal model?

Answer 21

Proposes there are two precursor cells for all proximal, intermediate and distal elements and cells will differentiate according to the signals they are exposed to

Question 22

What are the two signals in the two signal model?

Answer 22

Proximal signal = retinoic acid

Distal signal = FGF

Question 23

Retinoic acid drives the expression of which homeobox transcription factor?

Answer 23

Meis

Question 24

What is Meis?

Answer 24

Specifies proximal elements

Question 25

Which Hox genes are distal in the limb bud?

Answer 25

Hoxx 11 and Hox 13

Question 26

The anterior graft of the ZPA causes what?

Answer 26

A mirror image duplicate of autopods - if the graft is performed early enough the ulna will also be duplicated also

Question 27

Which gene is expressed in the ZPA?

Answer 27

The Shh

Question 28

What happens if you lower the amount of Shh transplanted?

Answer 28

The weaker gradient makes a partial image duplication

Question 29

What happens in a Shh mutant limb?

Answer 29

There is a complete loss of the distal most skeletal elements
Loss of identity of zeugopods
This can be seen in mice
Remnant of digit 1 only