4.16.
WATER-SOLUBLE RESIN EMBEDDING
125
qualities to be improved by this addition. So far we have employed a
5% addition of the divinyl benzene to the prepolymerized syrup, which
does not alter the storage qualities of the latter. It is not yet known
whether this will affect histochemical properties of embedded tissue.
Sectioning 2-hydroxypropyl methacrylate blocks does not seem to present any particular difficulties if a glass knife is used. The hydrophilic
tendency of this plastic encourages wetting of the block face, and therefore pure water in the trough seems best. We have been quite unable to
get satisfactory sections with a diamond knife, however, apparently because the sections do not float freely away from the diamond edge.
Thus, they are thrown into myriads of folds.
Sections of tissue embedded in 2-hydroxypropyl methacrylate can be
collected directly on grids, and observed in the microscope without added
support. However, in our experience there has been an overwhelming
mortality when we have made efforts to stain such naked sections. This
was true even when unusually fine (400 mesh) supporting grids were
used. Thus, we have felt it necessary to abandon attempts to use unsupported sections except for extraordinary purposes. Not much support
is needed, though, and a thin layer of carbon evaporated on the backs
of the sections (through the grids) suffices (Chapter 6.5). The sections
then can be stained by floating the grids face down on drops of stain
(Chapter 7.3).
Since 2-hydroxypropyl methacrylate does not melt during electron
bombardment, there is no apparent collapse of the tissue fine structure
during observation. In that sense, this material does not behave like
conventional methacrylate even though both lack cross-linkages. It
would seem that the only danger to the specimen lies in the damage
that may occur during polymerization. Fortunately, the use of prepolymerized syrup with the water-soluble methacrylates is as effective in
preventing this as when conventional methacrylates are used. It must be
emphasized, though, that work with these plastics is still in a developmental stage and possibly unforeseen difficulties will arise.
McLean and Singer (1964) have introduced yet another embedment
with water-soluble monomers. This is a cross-linked polyampholyte, a
vinyl type plastic which is highly ionic in character. The polymers are
said to be "studded with positive and negative charges, with a small net
charge, and in this respect resemble protein molecules." Their polar
character should have not only less tendency to extract non-polar sub-
WATER-SOLUBLE RESIN EMBEDDING
125
qualities to be improved by this addition. So far we have employed a
5% addition of the divinyl benzene to the prepolymerized syrup, which
does not alter the storage qualities of the latter. It is not yet known
whether this will affect histochemical properties of embedded tissue.
Sectioning 2-hydroxypropyl methacrylate blocks does not seem to present any particular difficulties if a glass knife is used. The hydrophilic
tendency of this plastic encourages wetting of the block face, and therefore pure water in the trough seems best. We have been quite unable to
get satisfactory sections with a diamond knife, however, apparently because the sections do not float freely away from the diamond edge.
Thus, they are thrown into myriads of folds.
Sections of tissue embedded in 2-hydroxypropyl methacrylate can be
collected directly on grids, and observed in the microscope without added
support. However, in our experience there has been an overwhelming
mortality when we have made efforts to stain such naked sections. This
was true even when unusually fine (400 mesh) supporting grids were
used. Thus, we have felt it necessary to abandon attempts to use unsupported sections except for extraordinary purposes. Not much support
is needed, though, and a thin layer of carbon evaporated on the backs
of the sections (through the grids) suffices (Chapter 6.5). The sections
then can be stained by floating the grids face down on drops of stain
(Chapter 7.3).
Since 2-hydroxypropyl methacrylate does not melt during electron
bombardment, there is no apparent collapse of the tissue fine structure
during observation. In that sense, this material does not behave like
conventional methacrylate even though both lack cross-linkages. It
would seem that the only danger to the specimen lies in the damage
that may occur during polymerization. Fortunately, the use of prepolymerized syrup with the water-soluble methacrylates is as effective in
preventing this as when conventional methacrylates are used. It must be
emphasized, though, that work with these plastics is still in a developmental stage and possibly unforeseen difficulties will arise.
McLean and Singer (1964) have introduced yet another embedment
with water-soluble monomers. This is a cross-linked polyampholyte, a
vinyl type plastic which is highly ionic in character. The polymers are
said to be "studded with positive and negative charges, with a small net
charge, and in this respect resemble protein molecules." Their polar
character should have not only less tendency to extract non-polar sub-