On Tue, 25 Apr 2000, Paulo Faria wrote:

> ... Where are the old CO2 microtomes?  I thought 
> that before cryostats were readily available they were popular. Is that 
> true? Where are they? 

 One of them, made by Leitz, is in my lab. It's at least 40 years old,
 has served me well for the last 27, and is still in regular use. 

 Opinion: For getting thick sections of formaldehyde-fixed organs (and
   I mean FIXED, at least overnight and preferably for a week), you can't
   beat a CO2 freezer. Unless you're supremely skilful you can't get
   serial sections; for this you need accessories that maintain the 
   temperature of the chuck, either by circulating cold alcohol through
   its innards or by using a Peltier-effect device (which needs
   circulating tap water to carry the heat away from its "hot" 
   thermocouple junctions).

   A pathologist examining a frozen section from this traditional
   instrument must be able to evaluate sections very much thicker than
   the 4 to 7 um of routine paraffin histology. The screw advance
   on a typical CO2 freezer is calibrated from 0 to 100 um, and you 
   can waggle the handle (moving the blade) to do double or treble
   increments of thickness, but unless you can make use of occasional
   lucky scraps don't expect anything thinner than about 20 um. A
   freezing microtome is at its best with sections that are nominally
   40 to 80 um thick. The real thickness is variable, as is obvious
   at a glance to anyone who uses one. 
 
   Cryostats offer many advantages over the freezing microtome: thinner
   sections; fixation is optional, etc etc.  There are nevertheless
   plenty of circumstances when thicker sections are more informative
   than thin ones. It isn't easy to cut an 80 um section of anything,
   fixed or unfixed, in a cryostat. 

   Another good thing about freezing with carbon dioxide is that
   it's fast, and ice crystal holes are rarely visible in the
   sections.

 Perhaps some will disagree about these perceived virtues of the
 CO2 freezing microtome!

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1