4.1.
INTRODUCTORY REMARKS
83
It was work with epoxy resins, begun by Maal0e and Birch-Andersen
(1956), that disclosed the limitations of methacrylate embedding, for
these resins do not liquefy or decompose in the electron beam. Surface
tension forces are not active, and macromolecular structure is preserved
intact. In this situation the price one pays is a very low specimen contrast,
so low, indeed, that usually electron "stains" must be used to visualize
anything. Fortunately these have been forthcoming so that this is no
longer a handicap (see Chapter 7).
Initial difficulties with the original epoxy resins provoked much exploration and experimentation with a variety of plastics, and their
manner of formulation, in an effort to achieve the best possible combination of desirable features. The plastics all had one feature in common in that all were "cross-linked," unlike ordinary methacrylate. However, even methacrylate can be cross-linked too, and there are some
methacrylates that are thermostable even without cross-linking, so that
in these special forms it joins the other resins of current interest.
At the present time satisfactory methods of handling several different
epoxy and polyester plastics have been developed. The potential quality
of the micrographs that may be produced using these different embedments usually will be almost identical. One's choice from this variety
will surely depend principally upon secondary considerations such as
availability, and what seems to be ease of handling. Two particular
epoxy resins, Araldite and Epon, as well as one polyester, Vestopal W,
have been by far the most extensively used. Thus, it is fair to say that
there are by now fully dependable ways of handling these particular
resins. There is less certainty in handling those which have been proposed more recently, if only because there has not as yet been adequate
time to assess fully the problems associated with their use.
In spite of the advances in embedding technology associated with the
introduction of cross-linked plastics, "old-fashioned" methacrylate still
has its place. It will become apparent that there are ways of protecting
sections of this material from at least most beam damage. Methacrylate
has compensating advantages which continue to make it useful. Therefore, in the sections that follow, methacrylate embedding will be discussed first, then the cross-linked plastics will be considered, with the
best known ones given preference. Finally, there are resins which are
more or less miscible with water so that biological material can be
directly dehydrated in them. This has some advantages, particularly
from a histochemical point of view. These will be discussed last.
INTRODUCTORY REMARKS
83
It was work with epoxy resins, begun by Maal0e and Birch-Andersen
(1956), that disclosed the limitations of methacrylate embedding, for
these resins do not liquefy or decompose in the electron beam. Surface
tension forces are not active, and macromolecular structure is preserved
intact. In this situation the price one pays is a very low specimen contrast,
so low, indeed, that usually electron "stains" must be used to visualize
anything. Fortunately these have been forthcoming so that this is no
longer a handicap (see Chapter 7).
Initial difficulties with the original epoxy resins provoked much exploration and experimentation with a variety of plastics, and their
manner of formulation, in an effort to achieve the best possible combination of desirable features. The plastics all had one feature in common in that all were "cross-linked," unlike ordinary methacrylate. However, even methacrylate can be cross-linked too, and there are some
methacrylates that are thermostable even without cross-linking, so that
in these special forms it joins the other resins of current interest.
At the present time satisfactory methods of handling several different
epoxy and polyester plastics have been developed. The potential quality
of the micrographs that may be produced using these different embedments usually will be almost identical. One's choice from this variety
will surely depend principally upon secondary considerations such as
availability, and what seems to be ease of handling. Two particular
epoxy resins, Araldite and Epon, as well as one polyester, Vestopal W,
have been by far the most extensively used. Thus, it is fair to say that
there are by now fully dependable ways of handling these particular
resins. There is less certainty in handling those which have been proposed more recently, if only because there has not as yet been adequate
time to assess fully the problems associated with their use.
In spite of the advances in embedding technology associated with the
introduction of cross-linked plastics, "old-fashioned" methacrylate still
has its place. It will become apparent that there are ways of protecting
sections of this material from at least most beam damage. Methacrylate
has compensating advantages which continue to make it useful. Therefore, in the sections that follow, methacrylate embedding will be discussed first, then the cross-linked plastics will be considered, with the
best known ones given preference. Finally, there are resins which are
more or less miscible with water so that biological material can be
directly dehydrated in them. This has some advantages, particularly
from a histochemical point of view. These will be discussed last.