Lecture 3 - vertebrate limb

Question 1

Is the thumb considered anterior or posterior?

Answer 1

anterior

Question 2

Is the pinky finger considered anterior or posterior?

Answer 2

posterior

Question 3

What type of limbs share homologous skeletal features along the proximal distal axis?

Answer 3

Tetrapod limbs

Question 4

Outline the stages of embryonic development

Answer 4

Early cleavage - gastrulation - somitogenesis - pharyngula - neurulation - limb development

Question 5

What is the first sign of limb development?

Answer 5

Limb buds form on either side of the embryo

Question 6

What occurs after the formation of a limb bud?

Answer 6

the limb bud elongates towards distal end.

Question 7

What occurs simultaneously to the growth of limbs?

Answer 7

differentiation of the skeleton

Question 8

What are the 3 distinct regions in the limb bud?

Answer 8

• ectoderm
• mesoderm
• endoderm

Question 9

What is found in the ectoderm of the limb bud?

Answer 9

AER - apical ectodermal ridge (epithelium)

Question 10

What is found in the mesoderm of the limb bud?

Answer 10

• progress zone (mesenchymal cells below AER) - lots of separation
• differentiating tissues

Question 11

Where is the AER (apical ectodermal ridge) found in terms of P/A & D/V?

Answer 11

Posterior dorsal axis

Question 12

When does growth occur in limbs?

Answer 12

When cells proliferate at the end of the limb bed.

Question 13

What occurs to cells when they leave the proliferative zone?

Answer 13

They specialise

Question 14

What is the consequence of the removal of the AER (apical ectodermal ridge)?

Answer 14

Tructation of the limb - meaning signals MAY BE instructive (telling them what to specialise) or external signals

Question 15

What is needed in the progress zone?

Answer 15

something secreted by the AER

Question 16

What can older AER be substituted for?

Answer 16

Younger AER

Question 17

How does the signal from AER impact the progress zone?

Answer 17

The signal is not instructing the progress zone on what to make, but it is permissive to growth.

Question 18

What is the name given to the molecules that permits growth?

Answer 18

FGFs - fibroblast growth factors

Question 19

How many FGFs are found in mice?

Answer 19

22

Question 20

What can FGF expression lead to?

Answer 20

Gravel proliferation (growth)

Question 21

What 2 FGFs are expressed strongly, at the same time and in overlapping domains?

Answer 21

Fgf4 & Fgf8

Question 22

What occurs as a result of a mutation in Fgf8?

Answer 22

Mild limb defect

Question 23

What occurs as a result of a mutation in Fgf4?

Answer 23

No limb defect

Question 24

What is redundancy?

Answer 24

The inclusion of extra components which are not strictly necessary to functioning

Question 25

What word describes the level of requirement Fgf8 & Fgf4 hold for limb development?

Answer 25

Redundant

Question 26

What does the fact that FGF8 soaked beads rescue AER ablation suggest about the use of AER?

Answer 26

suggests that AER is only required as a source of FGF

Question 27

What does ectopic expression of FHF in the flank lead to?

Answer 27

ectopic limbs (using beads of genetic methods)

Question 28

What can be said about FGF signalling as a result of ectopic limbs growing as a result of ectopic expression?

Answer 28

FGF signalling is sufficient for limb development

Question 29

What scientist researched anterior to posterior patterning?

Answer 29

Saunders

Question 30

What occurred when Saunders moved the limb bud part from the mesoderm from the posterior to anterior?

Answer 30

Mirror image was created, with most posterior digit formed in most anterior location

Question 31

What complex is involved in the process of A/P patterning?

Answer 31

ZPA - zone of polarising activity (cluster of mesodermal cells)

Question 32

What is the ZPA (zone of polarising activity) made of?

Answer 32

Cluster of mesodermal cells

Question 33

What occurred when the limb bud was removed, along with the mesoderm, but the ectoderm was left?

Answer 33

Toes develop distally but not A/P axis

Question 34

Hypothetically, how would you go about restoring an A/P axis?

Answer 34

addition of ZPA

Question 35

What leads to A/P axis polarity?

Answer 35

A signal is secreted by the ZPA

Question 36

What is the secreted morphogen proposed by Turing?

Answer 36

ZPA is secreting morphogen signal, which diffuses away from posterior or anterior, to establish a gradient.

Question 37

How do the different digits attached to a limb differentiate?

Answer 37

They differ in level of morphogen expressed - one digit will have high concentration with another containing intermediate levels.

Question 38

What other hormone expression correlates to the ZPA?

Answer 38

SHH (sonic hedgehog)

Question 39

What is SHH (sonic hedgehog)?

Answer 39

Secreted ligand - identified in Dosophilia

Question 40

How is SHH required for A/P?

Answer 40

SHH soaked beads induce posterior cell fates in a dosage dependent manner.

Question 41

What is AER required for?

Answer 41

AER is required for outgrowth and surgical removal results in loss of distal structures.

Question 42

What mimics the effect of AER?

Answer 42

FGF - and is a limb organiser

Question 43

How does FGF mimic the effects of AER and act as a limb organiser?

Answer 43

• expressed in the right place
• beads rescue the loss of the ZPA
• beads can induce reversed polarity in limbs
• mutant looses posterior limb structures