[Histonet] Cutting foams with microtome

[Monfils, Paul]

I'm not entirely sure what is meant by "foam", but I assume it is one of the
polymer materials which are formed into implantable "sponges", implanted in
animals, and later harvested?  If so, I section such materials frequently.
The first consideration of course is that the specific polymer is resistant
to the reagents used in processing. Some polymers are highly resistant to
typical hydrocarbon clearing agents and some are not. Some will undergo
various degrees of breakdown in such solvents, and some may dissolve
completely.  Some polymers, in order to get them into paraffin without
damage, must be processed with an alternative clearing agent.

Having cleared that hurdle, the principle challenge is getting the sponge
thoroughly infiltrated with paraffin. Some polymer sponges have a more
"open" configuration than others, that is to say something entering a given
pore of the sponge could find its way through to the other side of the
specimen without running into "dead ends".  The more open the structure is,
the more freely solvents can flow through the material.  But some such
materials have closed cells or dead end channels which make infiltration
more difficult. Also, such closed areas can trap water, or air. 

After fixation is complete I transfer such materials (in cassettes if they
will fit) to 70% ethanol, and place them in the vacuum oven (no heat, just
full vacuum) until bubbles stop rising from the specimens (15 or 20 minutes
is usually sufficient).  I then process them on a Tissue-Tek VIP processor,
using alternating vacuum and pressure on all stations, on a 40 hour cycle
(quitting time of day 1 until morning of day 3).  Then embed as usual, and I
find they section with little difficulty. 

I allow the sections to float on the water bath for 2 to 3 minutes before
picking them up, and I use a chrome gelatin product (Sta-On from Surgipath)
in the water bath to attach the sections to the slides.  Positively charged
slides won't work well on this kind of material because polymers don't have
the complementary net negative charge that tissues typically have.

Paul M.

> ----------
> From: 	histonet-bounces <@t> lists.utsouthwestern.edu on behalf of Fawn
> Jones
> Sent: 	Friday, March 10, 2006 9:35 AM
> To: 	histonet <@t> lists.utsouthwestern.edu
> Subject: 	[Histonet] Cutting foams with microtome
> 
> Dear histonetters,
> 
> I have someone using my lab who is cutting some paraffin embedded foam 
> infiltrated with cells.  She is having difficulty getting any sections 
> as the foam is falling apart.  Does anybody have a technique for this 
> type of material that they would like to share? 
> 
> Fawn Jones
> 
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