What triggers Embryonic Stem Cells to differentiate? That is, how do ESC’s differentiate at the right time and in the right place to produce a body with bits in all the right places? Additionally, why can early evolutionary stages be seen during the growth of a foetus (i.e, gill like things on human foetuses)?
-Oli
There are all kinds of signals that participate in embryonic development. It’s really its own branch of science, and an active one at that. Just as a rough estimate, I’d say that hundreds of pathways have been worked out, and there are probably about that many still to go.
You’re not really seeing early evolutionary stages. What you are seeing is that different organisms are using some of the same developmental genes. The reason is that some developmental genes (especially early ones) are difficult for evolution to alter, usually because they have many, many effects down the line. Alter the gene that creates gill slits and you mess up something else that is really important. It is easier to get rid of them later (or, more accurately, turn them into something else).
lots o’ stem cell info (pdf files a go go): http://www.nih.gov/news/stemcell/scireport.htm