Sonic hedgehog (Shh) specifies the anterior-posterior axis during limb development. Which one of the following statements regarding it is correct?
1
Shh secreting cells undergo apoptosis after performing its function.
2
Descendants of Shh secreting cells become the bone and muscle of the anterior limb
3
When the genes for Shh and Gli3 are conditionally knocked out in the mouse limb, the resulting limbs do not form any digit
4
Specification of the digit is primarily dependent on the amount of time the Shh gene is expressed and to a small extent on the concentration of the Shh protein that other cells receive.