Development of limbs and fins

Question 1

What two components make up the limb?

Answer 1

Skeleton (bone) and muscle

Question 2

From where does the musculature of the limb originate?

Answer 2

Somite

Question 3

Where do the bones of the limb come from?

Answer 3

Mesoderm

Question 4

What runs along the majority of the trunk and sits lateral to the somites?

Answer 4

Lateral Plate Mesoderm (LPM)

Question 5

At which boundaries are the forelimb and hindlimb positioned?

Answer 5

• Cervical-thoracic boundary
• Lumbar-sacral boundary

Question 6

What happens to forelimbs in mouse Hox (Hoxb5) gene knockouts?

Answer 6

They are mal-positioned (more anterior)

Question 7

What induces the outgrowth of the limb bud?

Answer 7

Fibroblast Growth Factor 10

Question 8

What does the limb bud consist of?

Answer 8

Ectoderm and mesenchyme

Question 9

What is the special ectodermal structure at the apex of the limb bud called?

Answer 9

Apical Ectodermal Ridge (AER)

Question 10

What does the Apical Ectodermal Ridge control?

Answer 10

• Rate of cell proliferation in underlying mesenchyme
• Proximo-distal growth
• Genes for antero-posterior axis patterning
• Genes for dorsal-ventral axis patterning

Question 11

What are the proximal and distal components of the limb?

Answer 11

• Humerus (most proximal)
• Radius and ulna (middle)
• Digits (most distal)

Question 12

What is the region adjacent to the AER that influences limb differentiation called?

Answer 12

Progress zone

Question 13

How does the timing of mesoderm cell exit from the progress zone affect limb structure?

Answer 13

Cells exiting first become humerus; those exiting last become digits

Question 14

Which Hox genes are required to make digits?

Answer 14

Hoxa 9, 10, 11, 12, and 13

Question 15

Which Hox gene is required to make the humerus?

Answer 15

Hoxa 9

Question 16

What is the anterior-posterior axis in limb development?

Answer 16

Thumb to little finger axis

Question 17

What tissue is crucial for specifying the anterior-posterior axis?

Answer 17

Zone of Polarising Activity (ZPA)

Question 18

What molecule does the Zone of Polarising Activity (ZPA) express?

Answer 18

Sonic hedgehog (Shh)

Question 19

What is the concentration gradient of Shh in the limb?

Answer 19

High concentration at posterior end and low concentration at anterior end

Question 20

What is the role of Fgf in relation to Shh in the ZPA?

Answer 20

Ensures Shh is expressed in the ZPA

Question 21

Which gene is expressed on the dorsal side of the ectoderm?

Answer 21

Wnt7a

Question 22

What does Wnt7a induce in dorsal mesenchyme?

Answer 22

Lmx1 expression

Question 23

What gene is expressed on the ventral side of the limb?

Answer 23

Engrailed

Question 24

What happens to the limb shape as it grows out?

Answer 24

It starts growing out as a ‘paddle’ shape

Question 25

What process refines the limb to produce a functional skeleton?

Answer 25

Programmed cell death (apoptosis)

Question 26

What are the main molecules responsible for sculpting the limb?

Answer 26

Bone Morphogenetic Proteins (BMPs)

Question 27

What is the role of evolution in limb morphology?

Answer 27

Evolution is normally a modification, doesn't generate a completely new structure.

Question 28

What is the basic design shared by all limbs?

Answer 28

All limbs have the same basic design.

Question 29

What types of fins do fish have?

Answer 29

Fish have dorsal and ventral fins that are unpaired, and pairs of pectoral and pelvic limbs.

Question 30

What structures do fish fins consist of?

Answer 30

Fish fins consist of structures similar to humerus, radius, and ulna.

Question 31

What is the main difference between fish fins and tetrapod limbs?

Answer 31

Fish fins don't have structures similar to digits.

Question 32

What is the Hox gene expression pattern in fish compared to tetrapods?

Answer 32

Fish have the same Hox gene expression proximally but are missing the distal Hox expression seen in tetrapods.

Question 33

What type of foot does a duck have?

Answer 33

The duck has a webbed foot with soft tissue persisting between digits.

Question 34

What does developmental studies show about cell death in duck feet?

Answer 34

Developmental studies show no cell death between digits.

Question 35

What happens if BMP inhibitors are placed between the digits of a developing chick?

Answer 35

There is no apoptosis, and a chick with webbed feet can be created.

Question 36

What changes in gene expression affect duck foot morphology?

Answer 36

Changes in expression of BMP and/or its inhibitors change foot morphology.

Question 37

What is a common characteristic of snake limb development?

Answer 37

Most snakes have a lack of limb development.

Question 38

Which snakes have rudimentary limbs?

Answer 38

Some snakes, such as pythons, have pelvic girdles and rudimentary femurs.

Question 39

What genetic changes are associated with the absence of forelimbs?

Answer 39

Changes in Hox gene expression within the LPM are associated with the absence of forelimb.

Question 40

What results from the expansion of certain Hox genes along the body axis?

Answer 40

The majority of vertebrae being thoracic.

Question 41

What does the lack of a cervical-thoracic boundary result in?

Answer 41

No specific site for the forelimb to develop.

Question 42

What happens during hindlimb development in pythons?

Answer 42

Hindlimb buds start to form, but the AER doesn't form further.

Question 43

What is the consequence of not having an AER in pythons?

Answer 43

No AER results in no P-D outgrowth and therefore no ZPA to create an A-P pattern.

Question 44

What is formed as a result of the lack of proper hindlimb development in pythons?

Answer 44

The formation of a tiny spur, a very small femur.