Limb Development

Question 1

At what point of development does limb formation begin in humans?

Answer 1

End of the fourth week

Question 2

How does limb development begin?

Answer 2

Activation of a group of mesenchymal cells in the lateral plate mesoderm.

Question 3

Describe a limb bud.

Answer 3

Bulges from the body wall as a mass of mesodermal cells covered by a layer of ectoderm.

Question 4

What are the two portions of the limb bud?

Answer 4

Apical ectodermal ridge (AER)
Progress zone

Question 5

What is the apical ectodermal ridge derived from?

Answer 5

Ectoderm

Question 6

What does the apical ectodermal ridge interact with?

Answer 6

The underlying progress zone

Question 7

What makes up the progress zone?

Answer 7

Proliferating cells

Question 8

What is the progress zone derived from?

Answer 8

Two types of mesoderm

Question 9

From what types of mesoderm is the progress zone derived?

Answer 9

Lateral plate (bones)
Somites (muscles)

Question 10

How does the embryo decide where the place the limb field?

2

Answer 10

1. The somites send a signal to the lateral plate
2. The lateral plate mesoderm must be competent to receive this signal.

Question 11

What makes the lateral plate competent to receive signalling from the somites?

Answer 11

Absence of Fgf8

Question 12

What happens when different regions of the paraxial mesoderm are tranplanted into the limb field?

Answer 12

Changes in limb bud size.

Question 13

What determines what part of the body will form the limb bud?

Answer 13

Somites

Question 14

What determines where the forelimb buds will form?

Answer 14

Expression of Hoxc6 in the somites

Question 15

What genes pattern the somites?

Answer 15

Hox genes

Question 16

What directs the lateral mesoderm to form limb buds?

Answer 16

Somites

Question 17

What is RARE?

Answer 17

A reporter for retinoic acid expression.

Question 18

Where is RARE expressed?

Answer 18

Between Fgf8 in the heart and Fgf8 in the posterior progenitor zone

Question 19

What is a result of mutants that do not produce Retinoic acid?

Answer 19

Loss of Tbx5 expression

Question 20

What is the result of a loss of Tbx5 expression?

Answer 20

Expansion of Fgf8 expression and loss of the limb field.

Question 21

What two T-box transcription factors does retinoic acid from the paraxial mesoderm initiate the level-specific expression of?

Answer 21

Tbx5
Tbx4

Question 22

Where is Tbx5 expressed?

Answer 22

In the area of the future forelimb

Question 23

Where is Tbx4 expressed?

Answer 23

In the area of the future hindlimb.

Question 24

What do the Tbx genes activate and why is this important for limb development.

Answer 24

Activate Fgf10, which is essential for limb bud initiation.

Question 25

Outline the molecular initiation of limb buds.

Answer 25

RA expression *Tbx4/5* expression *Fgf10* expression Limb bud initiation

Question 26

Where is *Fgf10* expressed?

Answer 26

In the lateral plate mesoderm in the positions where limbs form.

Question 27

What determines if a wing, leg or chimera forms in a chick embryo?

Answer 27

Proximity of an FGF-10 bead to Tbx4 or Tbx5 expression.

Question 28

Is Fgf10 sufficient to induce an ectopic limb?

Answer 28

Yes

Question 29

What develops in a chick embryo if the FGF10 bead is closer to Tbx5 expression?

Answer 29

Wing

Question 30

What develops in a chick embryo if the FGF10 bead is closer to Tbx4 expression?

Answer 30

Leg

Question 31

What develops if the FGF bead in the chick embryo is equidistant between Fgf4 and 5?

Answer 31

Chimera

Question 32

What is a chimeric limb?

Answer 32

A limb containing anterior wing structures and posterior leg structures.

Question 33

What drives forelimb growth?

Answer 33

Feedback loops involving Tbx-5/FGF-10 and FGF-10/FGF-8

Question 34

Early in limb development, what is the primary bearer of the limb blueprint: The mesoderm or the ectoderm?

Answer 34

Mesoderm

Question 35

What does Fgf10 in the mesoderm induce?

Answer 35

Expression of Fgf8 in the overlying ectoderm

Question 36

What induces the apical ectodermal ridge?

Answer 36

Fgf-10 in the mesoderm

Question 37

What is secreted by the apical ectodermal ridge, and what does this do?

Answer 37

Fgf-8 promotes proliferation of the underlying mesoderm

Question 38

What happens if the apical ectodermal ridge is removed at any time of development?

Answer 38

Arrested distal outgrowth of the limb

Question 39

What is the consequence of extra apical ectodermal ridge in the forelimb mesenchyme of the chick embryo?

Answer 39

Wing duplication

Question 40

What is the consequence of the apical ectodermal ridge being in contact with the leg mesenchyme of the chick embryo?

Answer 40

Development of a wing attached to a leg.

Question 41

What is the consequence of the apical ectodermal ridge coming into contact with nonlimb mesenchyme in the chick embryo?

Answer 41

The AER regresses and limb development ceases.

Question 42

What develops when AER in the forelimb mesenchyme of the chick embryo is replaced with an FGF bead?

Answer 42

Normal wing

Question 43

What causes diplopodia in humans?

Answer 43

Formation of two AERs on the limb bud

Question 44

What happens when a ZPA is grafted to the anterior limb bud mesoderm?

Answer 44

Duplicated digits emerge as a mirror image of the normal digits.

Question 45

What is the morphogen secreted by the ZPA?

Answer 45

Shh

Question 46

What causes polydactyly in humans and cats?

Answer 46

Shh gain of function enhancer mutations

Question 47

What establishes the anterior-posterior axis of the limb?

Answer 47

Shh

Question 48

Where are limb buds located?

Answer 48

Bilaterally at the presumptive locations of the forelimbs and hindlimbs

Question 49

What is the Zone of Polarizing Activity? (ZPA)

Answer 49

Cells in the most posterior region of the progress zone.

Question 50

What is the maximum amount of limb buds a vertebrate embryo can have?

Answer 50

4

Question 51

Describe the transplantation that results in a larger forelimb bud.

Answer 51

Transplanting PSM from level of presumptive forelimb to flank

Question 52

Describe the transplantation that results in a smaller forelimb bud.

Answer 52

Tranplantation of PSM from the flank region to the level of the presumtive forelimb.

Question 53

Outline the four steps in the initiation of limb bud formation.

Answer 53

1. Making mesoderm permissive for limb formation 2. Specifying forelimb and hindlimb 3. Inducing epithelial-mesenchymal transitions 4. Establishing two positive feedback loops for limb bud formation.

Question 54

Describe how Shh specifies digit identity.

Answer 54

Concentration of Shh in a certain region of the limb bud and then duration of that area's exposure to Shh specifies the digit it will become.

Question 55

Describe how Shh helps shape the limb bud.

Answer 55

Shh stimulates proliferation and expansion of the limb bud mesenchyme, helping with its shape.