Re: From myoblast to mature muscle cell:

Sarah,

That's actually two questions, since it seems that the myoblasts can either
become mature muscle cells during embryonic development _or_ can wait as
"satellite cells" alongside muscle fibers and be activated later. 

In embryonic development, two mandatory genes are labeled MyoD and Myo5;
without at least one of those genes, muscle does not develop.

The formation of satellite cells seems to be mediated by the "Notch
Signaling" pathway which is described at http://en.wikipedia.org/wiki/Notch_signaling 

What causes the satellite cells to mature into muscle fibers (either new
ones, or additions to existing ones) isn't clear.  Mechanical deformation
of the fiber, or changes in the local environment, may release signals that
start the cascade of events.
 

Free references online:
 http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12587921 http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=17996437