Development of limbs and fins

Question 1

What does the limb consist of?

Answer 1

• skeleton (bone)
• muscle

Question 2

Where does the musculature of the limb come from?

Answer 2

• comes from the somite

Question 3

Where does the bone of limb come from?

Answer 3

• mesoderm

Question 4

Where does the lateral plate mesoderm run?

Answer 4

• runs along the majority of the trunk

Question 5

Where does the lateral plate mesoderm sit in relation to the somites?

Answer 5

• sits lateral to the somites bilaterally

Question 6

Where is the forelimb always positioned?

Answer 6

• positioned at the cervical-thoracic boundary
• brachial plexus

Question 7

Where is the hind limb always positioned?

Answer 7

• at the lumbar sacral boundary
• pelvic plexuses

Question 8

What happens to knockout mice missing the Hoxb5 gene?

Answer 8

• has mal-positioned forelimbs (more anterior)

Question 9

What genes are the regulator for positioning of genes?

Answer 9

• Hox genes

Question 10

With limb positioning there are remarkable consistencies between species but there can be issues - what could these be?

Answer 10

• calf with additional limbs
• frogs with additional pair of limbs

Question 11

Limbs grow from a specific part of the LPM. They are induced to grow out by what signal?

Answer 11

• fibroblast growth factor 10

Question 12

When limbs are induced to grow by fibroblast growth factor 10 the tissue grows out to for a bud - what does this bud consist of?

Answer 12

• consists of both ectoderm and mesenchyme

Question 13

What triggers the tissue to grow out and form a bud?

Answer 13

• within the limb territories there are receptors for fibroblast growth factor and once this is stimulated this will trigger proliferation

Question 14

What is the apical ectoderm ridge?

Answer 14

• a special ectodermal structure at the apex of the limb bud is called the apical ectodermal ridge (AER)

Question 15

What does the apical ectodermal ridge control?

Answer 15

• controls the rate of cell proliferation in the underlying mesenchyme (proliferation and differentiation)

Question 16

What does the outgrowth of the apical ectodermal ridge correspond to?

Answer 16

• to proximo-distal growth

Question 17

The apical ectodermal ridge maintains genes that control what?

Answer 17

• maintains genes that control patterning of the antero-posterior axis (thumb - little finger)
• maintains genes that control the dorsal ventral axis (back of hand to palm)

Question 18

In proximal-distal outgrowth of the forelimb what is the most proximal bone?

Answer 18

• the humerus

Question 19

In the forelimb due to proximal- distal outgrowth what are the middle bones?

Answer 19

• the radius and ulna

Question 20

In proximal to distal outgrowth what is the most distal bones in the forelimb?

Answer 20

• the digits

Question 21

What does signals from the AER to the adjacent 200um of the mesoderm tell cells to do in the forelimb?

Answer 21

• become either humerus, radius/ulna or digits

Question 22

What is the region between the AER and the adjacent 200 um of mesoderm called?

Answer 22

• this region is called the progression zone

Question 23

The AER is crucial if its removed what will happen?

Answer 23

• proximal to distal limb growth wont happen

Question 24

Proximo-distal outgrowth is thought to be due to what?

Answer 24

• this is thought to be due to the amount of time mesoderm cells spend in the progress zone

Question 25

In forelimb proximo-distal outgrowth the cell exiting the progression zone first and last will become the what?

Answer 25

- first = humerus - last = digits

Question 26

Combinations of hox gene expression are key in controlling what?

Answer 26

- cell differentiations in the limb

Question 27

What Hox genes are required to make digits?

Answer 27

- Hoxa 9, 10, 11, 12 and 13

Question 28

What hox genes are required to make the humerus and femur?

Answer 28

* hoxa9 = humerus *hoxd9 = femur

Question 29

What is the anterior-posterior axis?

Answer 29

- this is the thumb to little finger axis

Question 30

When is the anterio-posterior axis specified and what is this due to?

Answer 30

- in very early development (almost before you can see a limb bud) due to to signals coming from AER

Question 31

The anterio-posterior axis requires a small piece of tissue situated at the posterior edge of the limb - what is this called?

Answer 31

- the zone of polarising activity (ZPA)

Question 32

What molecules does the ZPA express (in the anterio-posterior axis)?

Answer 32

- expresses a molecule called sonic hedgehog (Shh)

Question 33

The sonic the hedgehog molecule is a diffusible molecule, what does this mean for concentration?

Answer 33

- so high concentration at the posterior end and a low concentration at the anterior end

Question 34

High concentrations of Shh specifies what at the posterior end?

Answer 34

- specifies digit 5 at the posterior end

Question 35

Low concentration specifies what digit at the anterior end?

Answer 35

- digit 1

Question 36

What is required for the AER to ensure Shh is expressed in the ZPA?

Answer 36

- fibroblast growth factor (Fgf)

Question 37

What is the dorso-ventral axis regulated by?

Answer 37

- this is regulated by the ectoderm that covers the limb mesoderm

Question 38

In the dorso-ventral axis wnt7a is expressed where and induces what?

Answer 38

- expressed in the dorsal side of the ectoderm - it induces Lmx1 expression in dorsal mesenchyme

Question 39

In the dorso-ventral axis what gene is expressed on the ventral side?

Answer 39

- engrailed gene

Question 40

signals from the AER, including Fgfs ensure dorsal and ventral signals are what in the dorsol-ventral axis?

Answer 40

- ensure dorsal and ventral signals are in the correct position

Question 41

What does the limb first look like as it starts growing out?

Answer 41

- starts growing out as a paddle shape

Question 42

The paddle shape of the limb needs refining to produce a functional skeleton - what days does this take place?

Answer 42

- day 10-14

Question 43

Where does programmed cell death (apoptosis) occur in the forelimb?

Answer 43

- between the radius and ulna and at the end of the limb and between the digits

Question 44

Apoptosis between bones in the limb allows for what?

Answer 44

- this removes cells and allows separate skeletal elements to emerge

Question 45

What are the main group of molecules responsible for the apoptosis to sculpt limbs?

Answer 45

- are the BMP'S = Bone Morphogenetic proteins

Question 46

Evolution/natural selection doesn't generate a completely new structure - what does it usually do instead?

Answer 46

- usually it is a modification of structures already present

Question 47

Do all limbs have the same basic design?

Answer 47

- yes

Question 48

Fish have fins in different positions - where are the fins located?

Answer 48

- they have dorsal and ventral fins that are unpaired ( likely to be hox genes that encode for this) - they have paired pectoral and pelvic fins

Question 49

The duck has a webbed foot - what is this?

Answer 49

- soft tissue persisting between the digits

Question 50

Developmental studies conducted on the webbed foot found out what about cells death?

Answer 50

- there is no cell death between the digits - but there is normal cell death in other regions of the limb

Question 51

What would happen if you place BMP inhibitors in between digits in the developing chick?

Answer 51

- there is no apoptosis so they would have webbed feet

Question 52

What do changes in expression of BMP and or its inhibitors change?

Answer 52

- change foot morphology

Question 53

What about snake limbs? Do they have them? What can they have?

Answer 53

- the majority if snakes have complete lack of limb develop - some however, such as pythons have pelvic girdles and rudimentary femurs

Question 54

What is associated with absence of forelimb?

Answer 54

- changes in Hox gene expression within the LPM

Question 55

Expansion in the expression of certain Hox genes along the axis of the body can result in what?

Answer 55

- in the majority of the vertebrate being thoracic

Question 56

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

Answer 56

- no specific site therefore for the forelimb to develop

Question 57

Describe python hindlimb development?

Answer 57

- hindlimb buds do start to form in the python - the AER however doesn't form any further - no AER results in no P-D outgrowth and therefore no ZPA to create an A-P pattern - this results in the formation of a tiny spur - a very small femur