Chapter 19: Tetrapod Limb

Question 1

What are the different parts/axes of the tetrapod limbs?

Answer 1

Stylopod: Humerus
Zeugopod: Ulna + radius
Autopod: Carpals + digits

Question 2

What are the different parts of the limb/field bud?

Answer 2

Epaxial myotome bud
Myotome
Spinal cord
Sclerotome
Notochord
Pronephron
Endoderm
Endoderm
Lateral plate mesoderm
Central dermatome
Hypaxial myotome bud
Limb muscle precursors
Limb bud
Limb skeletal precursors

Question 3

How do Hox genes contribute to the proximal/distal patterning of the tetrapod limb?

Answer 3

Different Hox genes are expressed in specific regions along the axis creating a molecular gradient that provides positional cues for limb bud cells

Question 4

How do Limb buds “know” where to go? What factors are involved?

Answer 4

Somite level at which they arrive, position is constant with respect to the level of Hox gene expression along a/p axis
example: chick by Tbx4 and Tbx5

Question 5

What are the steps in the induction of the limb bud?

Answer 5

1) making mesoderm permissive for limb formation
2) specifying forelimb and hindlimb
3) inducing EMT
4) establishing 2 feedback loops

Hox proteins-> retinoic acid->tbx factors -> FGF10 mesoderm -> Fgf8 ectoderm

Question 6

How does a limb become a forelimb versus a hindlimb?

Answer 6

Tbx4 +Pitx1 -> hindlimb
Tbx5-> forelimb
example: in mice positive feedback loop

Question 7

Positive feedback loop example in mice

Answer 7

Tbx -> Fgf10 (mesoderm) -> Wnts upregulates -> Fgf8 (ectoderm) back to Fgf10

Question 8

Initiation of forelimb bud in mice

Answer 8

• proliferating
• non specific
• plastic
• as limb grows out they can be anything

Question 9

Positive feed back loop

Answer 9

A change in one direction leads to further changes in the same direction

example: fgf10-> wnt3-> fgf8-> fgf10

Question 10

What are the main functions of the AER?

Answer 10

1) maintain mesenchyme underneath a plastic, proliferating state to enable p/d growth
2) maintains expression of molecules that generate the a/p axis
3) interacts with proteins specifying a/p and d/v axes so cells know how to differentiate

Question 11

What is the main molecule that could be used to substitute for the AER functions in inducing limb growth ?

Answer 11

FGF8 secreting beads can substitute for the AER functions in inducing limb growth.

Question 12

Experiments of AER

Answer 12

1) AER removed -> limb cuts off=nub
2) extra AER -> duplicated wing in distal one on top of the other
3) leg mesenchyme -> leg on wing
4) non limb mesenchyme -> AER regresses limb development -> need TBx
5) AER replaced by Fgf -> normal wing

Question 13

How is AIP axis of tetrapod limb patterned?

Answer 13

2 sources: Retinoic acid and FGF/wnts
S: meis Z: Hox11 a: Hox13
morphogen gradient + movement

Question 14

What is ZPA? What molecule is responsible for ZPA activity?

Answer 14

zone polarizing activity: small block of mesodermal tissue near posterior junction of young limb bud and body wall

zpa secretes (shh) -> duplicated digits emerge (mirror image)