I guess most of them are still open question. We know that cells are able to
maintain there identity, such as a fibroblasts are normally divided into
fibroblasts but not epithelial cells or neurons. So why does that happen?
The unclear answer might be those existing proteins (such as cell-specific
proteins, gene transcription-related proteins etc), and the heritablity of
the epigenetic status, both of which help determine which genes are on or
off.