Sorry Vinnie, better ask for a refund on your tuition- or was that a slip of 
the pen? The gas forming around a warm object inserted into liq N2 forms an 
insulating envelope (Leidenfrost). Freezing speed in liq N2 is slower than in 
near freezing point Iso-pentane.
You can stick a finger into liq N2 for a couple of seconds and have no harm, 
but don't try that in cold iso-pentane.
Faster freezing results in smaller (ultimately no) ice crystals. It's those 
crystals that destroy tissues.
For the light microscopist the goal is to have those crystals small enough to 
not cause tissue damage that is visible under the light microscope. There are 
numerous methods to attain that goal.
Cheers
Jim Darley
ProSciTech                 Microscopy PLUS
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Ph +61 7 4774 0370  Fax:+61 7 4789 2313  service@proscitech.com
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On Wednesday, October 04, 2000 7:59 AM, Greer, Bonnie 
[SMTP:Bonnie.Greer@stjude.org] wrote:
> I did muscle bx`s for a long time I used a dewar filled with liquid nitrogen
> and I used another container filled with the isopentane.... suspended inside
> until frozen. You thaw  just enough to freeze your tissue for 20 seconds. I
> was taught that the isopentane insulates the tissue causing lee artifact
> .Email me if I can help.
>
> -----Original Message-----
> From: Vinnie Della Speranza [mailto:dellav@musc.edu]
> Sent: Monday, October 02, 2000 9:51 AM
> To: histonet@pathology.swmed.edu
> Subject: Freezing tissue
>
>
> I would welcome your opinions and feedback.
>
> What is the optimal method of freezing tissues with liquid nitrogen? with or
> without isopentane and why?
>
> thanks,
> Vinnie
>
>
>
> Vinnie Della Speranza
> Manager for Anatomic Pathology Services
> Medical University of South Carolina
> 165 Ashley Avenue
> Suite 309
> Charleston, SC  29425
> ph:  (843) 792-6353
> fax: (843) 792-8974
> email: Dellav@musc.edu
>