Why do embryotic cells differentiate?

[bkmather]

When an embryo is developing, what causes a change in DNA sequence resulting in cells differentiating? I understand the concept of stem cells being specific to the heart, liver, etc. but why do the cells suddenly become different?

[j2buttonsw]

Differentiation is the heart of a multicellular organism's organisation (to mix metaphors). It is not the DNA which is changing and causing the cells to be different, it is which genes are being expressed in a particular cell which causes it to differentiate. I know what you are going to ask next: what causes the different gene expression? It probably has something to do with the biochemical environment the cells are in (with regards to growth factors or chemical messengers). BTW I thought stem cells were pre-differentiation and thus could be coaxed into differentiating into a cell of ones choice (but it has been a while since I studied all of this).