Thank all of you have replied to my dilemma.  For everyone's information.  For
fixation I use a 4% paraformaldehyde and have used the same concentration on
all of my other mouse brains with no problems.  I also use a 4 hour processing
program.

I used someone elses machine and I am beginning to think that possibly the
tissue did not get infiltrated properly.  Is there a fix without damaging the
tissue.  I am doing immuno on it.

Thanks all
Cindy