4.7.
METHACRYLATE EMBEDDING
97
is the ideal embedment for phase contrast (and conventional) microscopy,
for good-sized sections can be obtained at any relevant thickness, which
can be stained easily with almost any ordinary dye (Chapter 7.12), either
with the embedment in place or after its removal. It is with these
thoughts in mind that what follows is carried over from the first edition
of this book into the present edition.
The hardness of the final blocks depends mainly upon the particular
methacrylate or mixture of methacrylates that is used. Polymerized butylmethacrylate is quite soft, and industry uses sheets of this material for
flexible things such as belts and handbags. It is hard enough, however,
to use in pure form to produce blocks which section satisfactorily. Ethyland methylmethacrylate, when polymerized, produce hard substances,
the Lucite and Plexiglas products of industry. These are altogether too
hard for sectioning purposes, but all three monomers are compatible, one
with another, and mixtures give plastics of varying and controllable
hardness. Most investigators find pure butylmethacrylate blocks to be
undesirably soft. They therefore add anywhere from 10 to 30% of one
of the smaller monomers to the butylmethacrylate before polymerizing.
In this writer's laboratory we routinely use 10-20% ethylmethacrylate
(with ultraviolet polymerization). Methylmethacrylate has been most
commonly used, presumably because of its cheapness and availability,
since this is the principal plastic of commerce. But it seems theoretically
desirable to use ethylmethacrylate instead. In the quantities used its
greater cost is without significance.
Several different catalysts are used commercially for polymerizing
methacrylates. The biologist wants one that is effective at relatively low
temperatures. For this reason a mixture known by the trade name of
Luperco CDB (the catalyst, 2,4-dichlorobenzoyl peroxide, with a plasticizer, dibutyl phthalate) achieved much popularity originally for it was
active at the lowest possible temperature (40°C). This mixture has to be
kept refrigerated, however, and moisture condenses on it as it is being
used, a most undesirable feature. Also it is in the form of a paste which
makes weighing difficult. Nowadays it seems altogether better to use
pure benzoyl peroxide (or 1,2-dichlorobenzoyl peroxide) as the catalyst
of choice. This material is available in dry granular form, and it is not
FIG. 7. The apical region of a cell of the proximal tubule of a rat kidney. This
material was fixed in buffered osmium tetroxide, blocked in butylmethacrylate, sectioned and mounted on a single supporting film, and lightly stained with phosphotungstic acid.
METHACRYLATE EMBEDDING
97
is the ideal embedment for phase contrast (and conventional) microscopy,
for good-sized sections can be obtained at any relevant thickness, which
can be stained easily with almost any ordinary dye (Chapter 7.12), either
with the embedment in place or after its removal. It is with these
thoughts in mind that what follows is carried over from the first edition
of this book into the present edition.
The hardness of the final blocks depends mainly upon the particular
methacrylate or mixture of methacrylates that is used. Polymerized butylmethacrylate is quite soft, and industry uses sheets of this material for
flexible things such as belts and handbags. It is hard enough, however,
to use in pure form to produce blocks which section satisfactorily. Ethyland methylmethacrylate, when polymerized, produce hard substances,
the Lucite and Plexiglas products of industry. These are altogether too
hard for sectioning purposes, but all three monomers are compatible, one
with another, and mixtures give plastics of varying and controllable
hardness. Most investigators find pure butylmethacrylate blocks to be
undesirably soft. They therefore add anywhere from 10 to 30% of one
of the smaller monomers to the butylmethacrylate before polymerizing.
In this writer's laboratory we routinely use 10-20% ethylmethacrylate
(with ultraviolet polymerization). Methylmethacrylate has been most
commonly used, presumably because of its cheapness and availability,
since this is the principal plastic of commerce. But it seems theoretically
desirable to use ethylmethacrylate instead. In the quantities used its
greater cost is without significance.
Several different catalysts are used commercially for polymerizing
methacrylates. The biologist wants one that is effective at relatively low
temperatures. For this reason a mixture known by the trade name of
Luperco CDB (the catalyst, 2,4-dichlorobenzoyl peroxide, with a plasticizer, dibutyl phthalate) achieved much popularity originally for it was
active at the lowest possible temperature (40°C). This mixture has to be
kept refrigerated, however, and moisture condenses on it as it is being
used, a most undesirable feature. Also it is in the form of a paste which
makes weighing difficult. Nowadays it seems altogether better to use
pure benzoyl peroxide (or 1,2-dichlorobenzoyl peroxide) as the catalyst
of choice. This material is available in dry granular form, and it is not
FIG. 7. The apical region of a cell of the proximal tubule of a rat kidney. This
material was fixed in buffered osmium tetroxide, blocked in butylmethacrylate, sectioned and mounted on a single supporting film, and lightly stained with phosphotungstic acid.