Jon,
Your picture is a good example of bubbling artifact, often incorrectly called nuclear bubbles.
I daresay I know as much about them as anyone, which is to say, nothing. Heat has been mentioned, and years ago I thought I had cracked it when I caused their elimination by reducing heat at every stage of processing. When I moved to another place where they were bad (they are in most places) I could not reduce them in any way by heat reduction, so theory one went down the pan. (At that stage I thought it was due to little balls of molten wax indenting the tissue).
Some 12 years ago I spent several months of afternoons fiddling around trying to produce/reduce them. What a waste of time that was. 
Certainly, good fixation, particularly coagulatives, reduces the likelihood of seeing them. 
P.M. tissue seems resistant to their formation. 

Several explanations I have read, which I will not repeat for the sake of my own blood pressure, are plain ridiculous.

I have ranted here before about this phenomenon, and some who know me of old may be fed up with it (hi Russ).

No bigger service to histopathology can there be than to sort this artifact out. 

BTW, Agustin Venzano relates poor staining of mucosal nuclei in the same breath, and he is right. I call it the vanishing nucleus syndrome, and it goes hand in hand with bubbles.

Terry L Marshall B.A.(Law), M.B.Ch.B., F.R.C.Path
Consultant Histopathologist
Rotherham General Hospital, Yorkshire
terry.marshall@rgh-tr.trent.nhs.uk