124
4.
EMBEDDING
produces a block with satisfactory sectioning qualities even without
blending with ordinary methacrylate. (This is not true of 2-hydroxyethyl
methacrylate which is usually blended with 30% butyl methacrylate.)
There is sufficient thermal stability so that supporting films are not
strictly necessary. Staining of embedded material is intense. The preservation of much endogenous enzymatic activity can be demonstrated in
thick sections at the level of conventional microscopy, although so far
these reactions have proved to be too weak to be apparent in ultrathin
sections.
Specimens to be embedded in the water-soluble methacrylate are put
directly into an 80% solution. The specimens quickly become very sticky,
and have to be separated from each other and from the container walls
from time to time during infiltration by stirring with a toothpick.
Mechanical stirring is not practical. Aldehyde-fixed specimens will be
partly "cleared" after infiltration is completed with the 80% solution,
and the tissue then can be transferred to pure methacrylate.
Final infiltration and embedding can best be in a "prepolymerized"
syrup of the 2-hydroxypropyl methacrylate. This is prepared in advance
as previously described (Chapter 4.8) for ordinary methacrylate, using
1-2% benzoyl peroxide (or Luperco) as the catalyst. (In our own experience the addition of the catalyst and the subsequent heating have
produced brown colors, which eventually have disappeared after complete polymerization.) We have completed polymerization in a 60°C
oven. Ultraviolet polymerization is said also to be possible, and may be
desirable.
Blocks prepared of pure 2-hydroxypropyl methacrylate are hard and
brittle, so much so that they are hard to trim, although they section
well if sufficiently small. A small amount of water deliberately added to
the prepolymerized syrup probably is desirable. We have added 10%,
and even more might be an advantage. However, Rosenberg et al. (1960),
in discussing glycol methacrylate, indicate that hardness (measured by
the Brinell scale) decreases sharply when the water content is increased
from 5 to 10%, and drops precipitously as more water is added. It would
be an advantage of ultraviolet polymerization if the water content of the
capsules could be controlled more accurately than when using high heat.
We recently have found that the cross-linking agent, divinyl benzene
(Chapter 4.15), greatly toughens blocks of 2-hydroxypropyl methacrylate
without any great effect upon their hardness. We expect their sectioning
4.
EMBEDDING
produces a block with satisfactory sectioning qualities even without
blending with ordinary methacrylate. (This is not true of 2-hydroxyethyl
methacrylate which is usually blended with 30% butyl methacrylate.)
There is sufficient thermal stability so that supporting films are not
strictly necessary. Staining of embedded material is intense. The preservation of much endogenous enzymatic activity can be demonstrated in
thick sections at the level of conventional microscopy, although so far
these reactions have proved to be too weak to be apparent in ultrathin
sections.
Specimens to be embedded in the water-soluble methacrylate are put
directly into an 80% solution. The specimens quickly become very sticky,
and have to be separated from each other and from the container walls
from time to time during infiltration by stirring with a toothpick.
Mechanical stirring is not practical. Aldehyde-fixed specimens will be
partly "cleared" after infiltration is completed with the 80% solution,
and the tissue then can be transferred to pure methacrylate.
Final infiltration and embedding can best be in a "prepolymerized"
syrup of the 2-hydroxypropyl methacrylate. This is prepared in advance
as previously described (Chapter 4.8) for ordinary methacrylate, using
1-2% benzoyl peroxide (or Luperco) as the catalyst. (In our own experience the addition of the catalyst and the subsequent heating have
produced brown colors, which eventually have disappeared after complete polymerization.) We have completed polymerization in a 60°C
oven. Ultraviolet polymerization is said also to be possible, and may be
desirable.
Blocks prepared of pure 2-hydroxypropyl methacrylate are hard and
brittle, so much so that they are hard to trim, although they section
well if sufficiently small. A small amount of water deliberately added to
the prepolymerized syrup probably is desirable. We have added 10%,
and even more might be an advantage. However, Rosenberg et al. (1960),
in discussing glycol methacrylate, indicate that hardness (measured by
the Brinell scale) decreases sharply when the water content is increased
from 5 to 10%, and drops precipitously as more water is added. It would
be an advantage of ultraviolet polymerization if the water content of the
capsules could be controlled more accurately than when using high heat.
We recently have found that the cross-linking agent, divinyl benzene
(Chapter 4.15), greatly toughens blocks of 2-hydroxypropyl methacrylate
without any great effect upon their hardness. We expect their sectioning