[Histonet] Re: GMA resin i.e. Technovits 7100

[Gayle Callis]

You wrote:
Hello, I am a newby in the histology business and working with anchovy
gonads in the daily egg production method. We have to cut the gonads
at 3 micron thick, so we use technovit 7100 resin for the blocks. We
have a leica rm2145 microtome equipped with a tungsten-carbide knife.

Until now, all my little knowledge about histotechnique has been
adquired with the experience and with the old trial-and-error method,
but I would like to ask you about severals aspects of the work.

What's the correct angle for cutting this kind of plastic?
How should be the maximum witdh of the block to be cutted?
What's the "life expectance" of the edge of a tungsten-carbide knife
when cutting resin?
I think I've read something about lubricating the cut with water to
ease the cut, but I have experienced that even when in some cases I
have got some moisture over the knife, the contact of the slice with
the moisture ruins it. But also I have experienced that a little
breathing over the block just before the cut eases it and prevents
from rolling.

I would like to know about good books in spanish or in english about
these particulars, but unhappily i do not have good access to
libraries and this stuff, but anyway i'd appreciate comments about it.

I have a lot more of questions to ask you, but i think it is enough
now. Thank you very much and excuses for my bad english

Jorge Tornero
IEO - Cádiz

Technovits 7100 is glycol methacrylate, and it will not be necessary to wet 
the block face with anything.  In general, this plastic is soft enough to 
be sectioned with a tungsten carbide blade, although Linda Jenkins has 
great success with disposable microtome blades. I am not sure what brand 
she uses.

The plastic that requires wetting the block face for sectioning is methyl 
methacrylate (MMA) a much harder, more hydrophobic plastic than GMA. If you 
get the GMA block face too wet with water, it may make the sectioning more 
difficult, causing section to bunch up in a gooey mess.  Some people have 
terrible problems with too much humidity when sectioning GMA, so just 
section without any wetting. Breathing on block face IS providing warm 
moist air.

People section GMA with a tungsten carbide blade, but it must be very 
sharp, as with any blade. Have you reconditioned recently?  If you are 
cutting very thick sections, it may contribute to rolling.  We never cut 
GMA any thicker than 2 to 3um .  It may be knife angle and sharpness of 
blade that contributes to rolling. I have heard of some people reversing a 
d profile tungsten carbide blade so you do not cut with largest angle 
facing you, rather the smaller angle on what is considered the back of the 
knife.  There are also c profile tungsten carbide blades made, from DDK 
(Delaware Diamond Knives.  We sometimes captured the section with forceps 
as it starts to come over blade, to insure it does not roll or do funky 
flyaway things.

We rarely had what you experience, but we preferred glass triangular 
knives, freshly broken and extremely sharp for all our GMA sectioning, with 
a new knife for every block.