Dear Jennifer,
>From painful experience, I can attest to the fact that yu cannot leave
tissues in glut. fix for extended periods.  You will see at the EM level an
artifact known as "fixation pepper" (for an example, see Crang &
Klomparens, Artifacts in Biological Electron Microscopy,Plenum Press, 1988.
pg61).  Once in your tissue, it cannot be removed.  It looks much like the
peppering you can see with a lead staining gone bad, sometimes finer,
sometimes very coarse.  I agree with the previous recommendations about
storing is buffer.  I prefer to do it in the buffer after osmium for
routine EM.
Good luck,
Lee

Leona Cohen-Gould, M.S.
Sr. Staff Associate
Director, Electron Microscopy Core Facility
Manager, Confocal Microscopy Core Facility
Joan & Sanford I. Weill Medical College
of Cornell University
voice  (212)746-6146
fax (212)746-8175