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This is true for most PVA or Mowiol based aqueous mountants meant to be coverslipped.  They constrict a bit when curing, leading to air pockets along the edges of the coverslip.  The thicker the mount, the more pronounced the effect.  With just a drop under a coverslip, it isn't usually noticeable.  But for thick mounts, like yours, it is a problem.

The solution is to seal the edges with more of the same mountant, or to mount it such that there is excess outside of the edges.  That way, when it constricts, it just pulls in the excess.  If you want it to look "neat", then you would use a pen knife or razor blade edge to cut away the excess AFTER it has fully cured (a couple days later).

Nail polish or wax won't help since it won't "mesh" with the PVA or mowiol.

On a related note, if you see tiny bubbles appearing just on the tissue, then this is a different cause:  air trapped in the section is squeezed out when the mountant cures and constricts.  The solution to this is, just prior to mounting, have the slide and section in PBS in a Coplin jar in a vacuum bell and pull a low vacuum on it to gently pull out the air for ten minutes or so, then release the vacuum and mount.

Cheers,

Jason


-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Rusty Nicovich
Sent: Tuesday, February 28, 2017 5:21 PM
To: [log in to unmask]
Subject: Mowiol mounting -> bubbles

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We have a project that involves imaging tissue sections mounted in home-made Mowiol (PVA) mounting medium.  The coverslip is held away from the slide with an adhesive spacer, the aperture in the spacer filled with tissue and medium, and a coverslip applied.

These work great for about a month.  After that, we notice more and more often bubbles appearing in the mounting medium. These are mostly around the edge and immediately under the coverslip.

We've tried sealing with nail polish.  This seems to have little effect on the rate or the extent of bubble formation.

My theory is that the PVA is shrinking and pulling away from the coverslip and spacer.  If that's the case (or if not) is there a way to prevent this?  Would another sealing material do the job better? Any tips or tricks out there?

Thanks,
Rusty Nicovich