4.8.
POLYMERIZATION DAMAGE
103
in blood or ascites fluid. In the latter instances the most successful microscopy has often been accomplished by centrifuging the cells to make
a pellet, which then is handled as though it were a solid block of tissue.
The possibility of polymerization damage presumably is augmented by
the degree of incompatibility of the different species of monomers, which
can be thought of as tending to polymerize with their own species at different rates. The damage can be expected to be most severe when relatively high concentrations of methylmethacrylate are used which start
polymerizing before the butylmethacrylate. The damage theoretically
should be less likely when the more compatible ethylmethacrylate is used
rather than methylmethacrylate, and, in particular, when its concentrations are kept minimal. This writer does not believe that polymerization
damage is a problem when well-fixed material is embedded as described
in the previous section, except in superficial layers of tissue blocks.
To avoid polymerization damage, Borysko (1956) advocated prepolymerizing methacrylate mixtures in bulk at 60°-80°C with repeated stirring until they reach the consistency of a thick syrup. The reaction may
take about a day at 60°C, and perhaps less than an hour at 80°C.
Since polymerization is an exothermic reaction, it may be hard to stop if
high temperatures are used. The reaction must be watched carefully. Refraction patterns can be seen in the mixture as polymerization begins. If
an Erlenmeyer flask is used for the polymerization, the reaction can be
slowed relatively quickly by dipping in cold water. Also small quantities
of cool monomer can be added safely to drop the temperature. Once substantial thickening is evident, however, by all means avoid stirring the
flask until it is cool. Otherwise polymerization is apt to be accelerated so
that the reaction quickly gets beyond control.
Prepolymerized syrup can be used at once by loading capsules and continuing polymerization at high temperature or with ultraviolet radiation,
or the reaction can be stopped by lowering the temperature. Then the
FIG. 8. Endothelial cells from a single rat aorta, (b) This sample was embedded in
"prepolymerized" methacrylate and showed uniformly a well-preserved endothelium,
(a) This material was embedded conventionally with a mixture of methacrylate monomers, and consistently showed serious "polymerization damage" in the tunica
intima.
This sort of damage suggests an "explosion." The cells appear swollen, in this case to
about twice the size of the control. Membranes of all sorts are interrupted. Cytoplasmic
and nuclear contents are dispersed, and have a
flocculent
appearance. This simulates
poor
fixation.
Ordinarily, severe polymerization damage is limited to the surface of
tissue blocks and freely exposed cells. Some cell types are more sensitive than others.
POLYMERIZATION DAMAGE
103
in blood or ascites fluid. In the latter instances the most successful microscopy has often been accomplished by centrifuging the cells to make
a pellet, which then is handled as though it were a solid block of tissue.
The possibility of polymerization damage presumably is augmented by
the degree of incompatibility of the different species of monomers, which
can be thought of as tending to polymerize with their own species at different rates. The damage can be expected to be most severe when relatively high concentrations of methylmethacrylate are used which start
polymerizing before the butylmethacrylate. The damage theoretically
should be less likely when the more compatible ethylmethacrylate is used
rather than methylmethacrylate, and, in particular, when its concentrations are kept minimal. This writer does not believe that polymerization
damage is a problem when well-fixed material is embedded as described
in the previous section, except in superficial layers of tissue blocks.
To avoid polymerization damage, Borysko (1956) advocated prepolymerizing methacrylate mixtures in bulk at 60°-80°C with repeated stirring until they reach the consistency of a thick syrup. The reaction may
take about a day at 60°C, and perhaps less than an hour at 80°C.
Since polymerization is an exothermic reaction, it may be hard to stop if
high temperatures are used. The reaction must be watched carefully. Refraction patterns can be seen in the mixture as polymerization begins. If
an Erlenmeyer flask is used for the polymerization, the reaction can be
slowed relatively quickly by dipping in cold water. Also small quantities
of cool monomer can be added safely to drop the temperature. Once substantial thickening is evident, however, by all means avoid stirring the
flask until it is cool. Otherwise polymerization is apt to be accelerated so
that the reaction quickly gets beyond control.
Prepolymerized syrup can be used at once by loading capsules and continuing polymerization at high temperature or with ultraviolet radiation,
or the reaction can be stopped by lowering the temperature. Then the
FIG. 8. Endothelial cells from a single rat aorta, (b) This sample was embedded in
"prepolymerized" methacrylate and showed uniformly a well-preserved endothelium,
(a) This material was embedded conventionally with a mixture of methacrylate monomers, and consistently showed serious "polymerization damage" in the tunica
intima.
This sort of damage suggests an "explosion." The cells appear swollen, in this case to
about twice the size of the control. Membranes of all sorts are interrupted. Cytoplasmic
and nuclear contents are dispersed, and have a
flocculent
appearance. This simulates
poor
fixation.
Ordinarily, severe polymerization damage is limited to the surface of
tissue blocks and freely exposed cells. Some cell types are more sensitive than others.