86
4.
EMBEDDING
container and replaced with others. Of course, ordinary vials can be
used. Much better, however, are the short, wide-mouth weighing bottles
(Parr type, 4 ml, 23 X 24 mm) that have outside ground-glass caps
(illustrated in Fig. 1). Specimens ordinarily are fixed in one of these,
and stay in the original container until ready for final transfer to individual gelatin capsules just before polymerization. Small disposable
containers of relatively inert plastics which may serve these purposes
are beginning to appear on the market. It seems likely that these may
prove particularly useful when the embedding mixture is very viscous,
and when glassware therefore would be hard to clean (Chapter 4.5).
4.3. Transitional Solvents
Transitional solvents between the dehydrating agent and the embedding mixture are not necessary or commonly used when specimens are
to be embedded in methacrylate, except in the sense that monomeric
methacrylate plays that role when the embedding mixture has been
"prepolymerized" (Chapter 4.8).
Epoxy mixtures frequently employ large molecules in such viscous
solutions that diffusion mixing with alcohol is slow at best, even though
possible. In retrospect, undoubtedly much difficulty encountered in the
early days of epoxy embedding was due to the non-uniform impregnation of the variously sized molecules into the tissue specimens. Although
this was partly appreciated by various investigators who sometimes advocated very long impregnation times, it remained for Luft (1961) to
demonstrate that propylene oxide was an almost perfect transitional
solvent between alcohol and the resin mixture. It is a "reactive solvent,"
meaning that if traces remain in the block, its molecules will be incorporated into the structure of the final plastic. It also has the virtue of
being miscible with large quantities of water, so that the last stages of
dehydration actually can be completed with this compound. It is very
fluid, and so greatly lowers the viscosity of resin mixtures when they
are diluted with it. It is very volatile so that residual traces can be expected to evaporate easily. The only disadvantages that it seems to have
are its mild toxicity, and the fact that it is quite reactive. Thus, perhaps it acts somewhat as a fixative in its own right, and in combining
with reactive groups in the specimen, it might be expected to inhibit
some histochemical and staining reactions (Merrillees et ah, 1963). If a
4.
EMBEDDING
container and replaced with others. Of course, ordinary vials can be
used. Much better, however, are the short, wide-mouth weighing bottles
(Parr type, 4 ml, 23 X 24 mm) that have outside ground-glass caps
(illustrated in Fig. 1). Specimens ordinarily are fixed in one of these,
and stay in the original container until ready for final transfer to individual gelatin capsules just before polymerization. Small disposable
containers of relatively inert plastics which may serve these purposes
are beginning to appear on the market. It seems likely that these may
prove particularly useful when the embedding mixture is very viscous,
and when glassware therefore would be hard to clean (Chapter 4.5).
4.3. Transitional Solvents
Transitional solvents between the dehydrating agent and the embedding mixture are not necessary or commonly used when specimens are
to be embedded in methacrylate, except in the sense that monomeric
methacrylate plays that role when the embedding mixture has been
"prepolymerized" (Chapter 4.8).
Epoxy mixtures frequently employ large molecules in such viscous
solutions that diffusion mixing with alcohol is slow at best, even though
possible. In retrospect, undoubtedly much difficulty encountered in the
early days of epoxy embedding was due to the non-uniform impregnation of the variously sized molecules into the tissue specimens. Although
this was partly appreciated by various investigators who sometimes advocated very long impregnation times, it remained for Luft (1961) to
demonstrate that propylene oxide was an almost perfect transitional
solvent between alcohol and the resin mixture. It is a "reactive solvent,"
meaning that if traces remain in the block, its molecules will be incorporated into the structure of the final plastic. It also has the virtue of
being miscible with large quantities of water, so that the last stages of
dehydration actually can be completed with this compound. It is very
fluid, and so greatly lowers the viscosity of resin mixtures when they
are diluted with it. It is very volatile so that residual traces can be expected to evaporate easily. The only disadvantages that it seems to have
are its mild toxicity, and the fact that it is quite reactive. Thus, perhaps it acts somewhat as a fixative in its own right, and in combining
with reactive groups in the specimen, it might be expected to inhibit
some histochemical and staining reactions (Merrillees et ah, 1963). If a