130
4.
EMBEDDING
specimen. Additionally, some plastics have a surface layer which may be
permanently altered (softened) by exposure to a hot water bath which,
of course, is a most undesirable effect. This perhaps occurs only with
those epoxy resins which are not cured completely before their use, and
which thus may have residual components that remain soluble.
Sections will stick together best, and thus form long ribbons, if they
have maximal contact. This is achieved by having the leading and trailing
edges of the block as smooth and straight as possible. Thus the last steps
of block trimming should be completed with the removal of smooth
shavings with a sharp razor. Naturally, the front and back edges should
be parallel or the ribbon will bend.
The vertical sides of the block can also be parallel and truly vertical,
but it is a little better if a trapezoidal face is created such that the leading edge is slightly longer than the trailing one. Thus each new section
pushes upon the full width of the trailing part of the previous one, and
corners do not remain stuck to the knife edge.
The width of the block that can be sectioned successfully depends
much upon the character of the specimen and the embedment. With
the harder plastics one cannot expect to use a block appreciably more
than 0.5 mm wide. With "difficult" material it may have to be substantially narrower than this. (The most generally difficult material is that
containing dense bundles of collagen. Some secretory products as well as
osmicated lipid droplets and mineralized deposits also tend to cause
trouble.) Under the most favorable of circumstances, even with "easy"
material and a soft embedment, block faces cannot be made much over
1 mm wide. Sections from blocks wider than appropriate under the
circumstances are almost certain to show "chatter" effects (Chapter 5.22).
The length of the block face is not as critical as its width, although
extreme lengths are to be discouraged if straight ribbons are desired.
In general the length should not exceed twice the width.
When blocks are trimmed in the form of truncated pyramids it is inevitable that sections become wider and wider as one gets deeper into
the block. This prompts some people to advocate that side walls do not
slope, but rather be vertical. Actually, though, it is not difficult to learn
to perform minor trimming operations on the block without removing
it from the microtome.
It is almost necessary to have some device which will help hold capsules while they are being trimmed. The chuck-type holders of some
microtomes can be removed from the instruments and serve very well
4.
EMBEDDING
specimen. Additionally, some plastics have a surface layer which may be
permanently altered (softened) by exposure to a hot water bath which,
of course, is a most undesirable effect. This perhaps occurs only with
those epoxy resins which are not cured completely before their use, and
which thus may have residual components that remain soluble.
Sections will stick together best, and thus form long ribbons, if they
have maximal contact. This is achieved by having the leading and trailing
edges of the block as smooth and straight as possible. Thus the last steps
of block trimming should be completed with the removal of smooth
shavings with a sharp razor. Naturally, the front and back edges should
be parallel or the ribbon will bend.
The vertical sides of the block can also be parallel and truly vertical,
but it is a little better if a trapezoidal face is created such that the leading edge is slightly longer than the trailing one. Thus each new section
pushes upon the full width of the trailing part of the previous one, and
corners do not remain stuck to the knife edge.
The width of the block that can be sectioned successfully depends
much upon the character of the specimen and the embedment. With
the harder plastics one cannot expect to use a block appreciably more
than 0.5 mm wide. With "difficult" material it may have to be substantially narrower than this. (The most generally difficult material is that
containing dense bundles of collagen. Some secretory products as well as
osmicated lipid droplets and mineralized deposits also tend to cause
trouble.) Under the most favorable of circumstances, even with "easy"
material and a soft embedment, block faces cannot be made much over
1 mm wide. Sections from blocks wider than appropriate under the
circumstances are almost certain to show "chatter" effects (Chapter 5.22).
The length of the block face is not as critical as its width, although
extreme lengths are to be discouraged if straight ribbons are desired.
In general the length should not exceed twice the width.
When blocks are trimmed in the form of truncated pyramids it is inevitable that sections become wider and wider as one gets deeper into
the block. This prompts some people to advocate that side walls do not
slope, but rather be vertical. Actually, though, it is not difficult to learn
to perform minor trimming operations on the block without removing
it from the microtome.
It is almost necessary to have some device which will help hold capsules while they are being trimmed. The chuck-type holders of some
microtomes can be removed from the instruments and serve very well