What makes each of our limbs different and how do they form? Limb development occurs at the limb bud and contains so much diversity and that diversity is provided by morphogen gradients. Morphogens are chemicals that change a type of cell depending on how much of that chemical is present at certain times. High amounts are found where these chemicals are made. For instance, if Chemical A is made on Side 1 and Chemical B was made on Side 2, the amount of Chemical A is greater on Side 1 than Side 2. Different amounts of the chemical make different types of cells. A large amount of the molecule forms a different identity for the area than a low amount of that molecule. There are intermediate amounts that also form different identities.
Shh and finger formation
More specifically, Shh is one of these chemicals and it forms your different fingers by using this gradient model. A high amount of Shh makes the pinky, while no Shh makes the thumb. Amounts of Shh in between make the middle three fingers. High amounts of Shh generally occur on the posterior (bottom) side of the limb bud. Implanting Shh to the other side of the limb bud showed the formation of a mirror imaged limb; this is the equivalent of having a pinky on both sides of a hand (Bartlett 2017).