4.9.
LIMITATIONS OF METHACRYLATE
107
macromolecular geometry presumably is unaffected by surface tension
forces. The polyester, Vestopal W, apparently decomposes to some extent
under electron bombardment, but sublimes in such a fashion that no
liquid phase is created. Again there is little or no chance for surface tension to destroy fine structure. Subsequent to these original studies of
"cross-linked" plastics, useful ways of handling other epoxy and polyester
plastics have been worked out. It has even proved possible to cross-link
methacrylate in tissue blocks to be sectioned. This work of recent years
has amply reenforced the realization that the usefulness of methacrylate
is limited.
An additional important reason for using cross-linked plastics is to
avoid "polymerization damage" which seems to be a danger only with
methacrylate embedding (discussed in the previous section of this chapter). Since material preserved with permanganate or formalin fixation is
particularly prone to this, the investigator should use these newer plastics
whenever he is using material preserved in some other way than with
osmium tetroxide.
These important technical gains are not achieved without serious difficulties. In general, only those biological systems can be studied which can
be "stained" to give them a contrast greater than osmium tetroxide fixation can provide. Also, sectioning generally becomes more difficult, and
one necessarily has to content himself with smaller sections than with
methacrylate.
Finally, gelatin as an embedding medium deserves mention. Its advantage is that organic solvents are avoided entirely, so that substances are
retained that might otherwise be lost. No doubt there are histochemical
possibilities in the use of this method of embedding and sectioning, but
these so far have not been exploited. These embedding media are considered in the next sections of this chapter.
FIG. 9. This section of the wall of a glomerular capillary was exposed first to electron
bombardment in an electron microscope, and subsequently obliquely shadowed with
chromium in the direction of the arrow. It is apparent from the shadows that the surface was contoured, and that much of the supporting of methacrylate plastic had
disappeared, leaving the specimen in relief, and thus subject to surface tension forces.
Specimens embedded in Araldite or Vestopal W do not show this sort of sublimation.
Sandwiching sections between two supporting films (see Chapter 6.10 and Fig. 24)
also is designed to minimize possible effects of surface tension.
This micrograph is printed as a negative as is commonly done with shadowed material
so that the shadows appear dark. Legends are as follows: epi., epithelial cell; r.b.c, red
blood cell; cap., capillary; b.m., basement membranes; urin., urinary space.
LIMITATIONS OF METHACRYLATE
107
macromolecular geometry presumably is unaffected by surface tension
forces. The polyester, Vestopal W, apparently decomposes to some extent
under electron bombardment, but sublimes in such a fashion that no
liquid phase is created. Again there is little or no chance for surface tension to destroy fine structure. Subsequent to these original studies of
"cross-linked" plastics, useful ways of handling other epoxy and polyester
plastics have been worked out. It has even proved possible to cross-link
methacrylate in tissue blocks to be sectioned. This work of recent years
has amply reenforced the realization that the usefulness of methacrylate
is limited.
An additional important reason for using cross-linked plastics is to
avoid "polymerization damage" which seems to be a danger only with
methacrylate embedding (discussed in the previous section of this chapter). Since material preserved with permanganate or formalin fixation is
particularly prone to this, the investigator should use these newer plastics
whenever he is using material preserved in some other way than with
osmium tetroxide.
These important technical gains are not achieved without serious difficulties. In general, only those biological systems can be studied which can
be "stained" to give them a contrast greater than osmium tetroxide fixation can provide. Also, sectioning generally becomes more difficult, and
one necessarily has to content himself with smaller sections than with
methacrylate.
Finally, gelatin as an embedding medium deserves mention. Its advantage is that organic solvents are avoided entirely, so that substances are
retained that might otherwise be lost. No doubt there are histochemical
possibilities in the use of this method of embedding and sectioning, but
these so far have not been exploited. These embedding media are considered in the next sections of this chapter.
FIG. 9. This section of the wall of a glomerular capillary was exposed first to electron
bombardment in an electron microscope, and subsequently obliquely shadowed with
chromium in the direction of the arrow. It is apparent from the shadows that the surface was contoured, and that much of the supporting of methacrylate plastic had
disappeared, leaving the specimen in relief, and thus subject to surface tension forces.
Specimens embedded in Araldite or Vestopal W do not show this sort of sublimation.
Sandwiching sections between two supporting films (see Chapter 6.10 and Fig. 24)
also is designed to minimize possible effects of surface tension.
This micrograph is printed as a negative as is commonly done with shadowed material
so that the shadows appear dark. Legends are as follows: epi., epithelial cell; r.b.c, red
blood cell; cap., capillary; b.m., basement membranes; urin., urinary space.