4.19.
PREPARING THE BLOCK FOR SECTIONING
129
erine had been added. The latter worker used prolonged intervals to
achieve infiltration of a 40% solution. Temperatures were maintained
at 40° or 37°C, respectively.
Following gelatin impregnation, Fernandez-Moran and Finean solidified their gelatin by chilling it after pouring it into a Petri dish, and then
they cut out the blocks they wanted. These were next placed in a
vacuum desiccator and dried for 5-6 hours, the pressure finally reaching
10"
3
mm Hg. The blocks so obtained apparently were very hard and
difficult to section.
Gilev used only air drying at 37°C to harden his blocks. This was
deliberately done slowly by keeping a moist atmosphere. He describes
his final blocks as "hard but not brittle." He regards as desirable some
traces of water in the blocks at the time of sectioning, yet there must not
be too much humidity. He flattened his sections by floating them on
water, or a 0.5% solution of potassium-chrome-alum (subsequently washed
out), or even 2% acetic acid. These treatments undoubtedly removed
much of the gelatin, and hence the support, from the specimen, and so
must be regarded as a dangerous and probably undesirable step.
4,19
Preparing the Block for Sectioning
Tissue samples embedded in plastic and molded in a gelatin capsule
of course require at least some trimming before they can be sectioned in
a microtome. How this is done will depend partly upon the embedment.
Some, such as methacrylate, are sufficiently soft so that they can be
whittled to final size with a razor blade (Fig. 13). Most of the newer
cross-linked plastics, however, are so hard that only the final trimming
can be done this way, when thin shavings can be trimmed without too
seriously turning the edge of the razor. A jeweler's saw with fine teeth
probably is the best tool for the rough cutting of these harder plastics,
although in some laboratories high speed dental burrs or grinding
wheels are used for this purpose.
There is a temptation to warm the plastic so that it will be softer
when it is cut than it would be otherwise. This is a somewhat dangerous
procedure, however, if sectioning is to follow immediately, for some
plastics, at least, return to a proper hardness only very slowly once they
have been softened by heat. This is one reason why it is dangerous even
to remove the gelatin capsule in a hot water bath before trimming the
PREPARING THE BLOCK FOR SECTIONING
129
erine had been added. The latter worker used prolonged intervals to
achieve infiltration of a 40% solution. Temperatures were maintained
at 40° or 37°C, respectively.
Following gelatin impregnation, Fernandez-Moran and Finean solidified their gelatin by chilling it after pouring it into a Petri dish, and then
they cut out the blocks they wanted. These were next placed in a
vacuum desiccator and dried for 5-6 hours, the pressure finally reaching
10"
3
mm Hg. The blocks so obtained apparently were very hard and
difficult to section.
Gilev used only air drying at 37°C to harden his blocks. This was
deliberately done slowly by keeping a moist atmosphere. He describes
his final blocks as "hard but not brittle." He regards as desirable some
traces of water in the blocks at the time of sectioning, yet there must not
be too much humidity. He flattened his sections by floating them on
water, or a 0.5% solution of potassium-chrome-alum (subsequently washed
out), or even 2% acetic acid. These treatments undoubtedly removed
much of the gelatin, and hence the support, from the specimen, and so
must be regarded as a dangerous and probably undesirable step.
4,19
Preparing the Block for Sectioning
Tissue samples embedded in plastic and molded in a gelatin capsule
of course require at least some trimming before they can be sectioned in
a microtome. How this is done will depend partly upon the embedment.
Some, such as methacrylate, are sufficiently soft so that they can be
whittled to final size with a razor blade (Fig. 13). Most of the newer
cross-linked plastics, however, are so hard that only the final trimming
can be done this way, when thin shavings can be trimmed without too
seriously turning the edge of the razor. A jeweler's saw with fine teeth
probably is the best tool for the rough cutting of these harder plastics,
although in some laboratories high speed dental burrs or grinding
wheels are used for this purpose.
There is a temptation to warm the plastic so that it will be softer
when it is cut than it would be otherwise. This is a somewhat dangerous
procedure, however, if sectioning is to follow immediately, for some
plastics, at least, return to a proper hardness only very slowly once they
have been softened by heat. This is one reason why it is dangerous even
to remove the gelatin capsule in a hot water bath before trimming the