Question: how do the stem cells know what will be the legs or arms or head

David Christensen answered on 19 Nov 2013:

That’s a tough question evewilcox!

There’s some complicated chemical signals that tell cells what to be during development. Basically where there is more of a chemical, then the cells will be told that they are one side of the organism and where there is less of the chemical, the cells become the other side of the organism – this type of thing works so that the left hand knows it is a left hand and the right hand is a right hand and also for front and back and head end vs feet end. To an extent, cells also know what to be depending on the cells next to them. If 1 cell is next to a cell that thinks it is part of a growing leg, then it is likely that it is also part of the growing leg. I hope this answers your question! I’ve never found this stuff easy to understand, so I might not be the best at explaining it!

Another question that I find harder to understand with the information that I’ve given already is how do cells know where something ends? If a cell thinks it is part of the lower leg because it is next to another cell that thinks that it is part of the lower leg, then when do we get the first cell that thinks it is a foot? Why does a cell become part of the ankle when it has cells that are part of the shin on 1 side and cells that are part of the foot on the other side? How do the cells know when to stop growing longer so that we don’t end up with much longer arms and legs than we do have? Sorry to be answering your question with more questions, but these are things that I genuinely don’t know and find really interesting!