4.8.
POLYMERIZATION DAMAGE
101
(shrinkage) which occurs with polymerization, or which may accompany
an advancing front of polymerization. The end effect can be hard, indeed, to distinguish from poor fixation, or from decomposition which
may result when inadequately preserved material receives electron bombardment. [Morgan et al. (1956) have demonstrated the sensitivity of
formalin-fixed material to bombardment damage.] It is possible to prove
that polymerization damage truly exists, however, by suitably controlled
experiments. Samples of a single tissue embedded conventionally can be
compared with samples embedded with prepolymerized methacrylate
(Fig. 8), or after using epoxy or polyester embeddings.
Polymerization damage has often been likened to an "explosion."
There is always some swelling. Note that the magnification of Fig. 8(a)
is only half that of 8(b) so that the apparent size of the affected cell is
about twice that of the well preserved one. Cell and nuclear membranes
are apt to be interrupted. Mitochondria may be swollen and their membranes broken. Small vesicular structures, such as those associated with
the Golgi apparatus and endoplasmic reticulum, may become almost unrecognizable. The diffuse proteins of the cytoplasm and nucleoplasm are
apt to appear as a dispersed flocculent precipitate. It is usually obvious
that the whole cell is poorly preserved. Polymerization damage can be a
graded phenomenon so that varying degrees of explosion are recognizable.
It has been general experience that exposed cells on the surface of
tissue blocks are the ones particularly prone to be damaged. Below the
surface layer of cells, tissue ordinarily is unaffected by conventional embedding. Certain tissues are particularly prone to damage for inexplicable reasons. Thus, in this writer's experience, the endothelium of slabs of
an aortic wall is very sensitive [Fig. 8(a)]. Adjacent muscle is not affected
by conventional blocking, and exposed muscle on the opposite side of
the aortic wall is unaffected even though the adventitial coat is stripped
off prior to embedding. The endothelium of small blood vessels, blocked
in toto, is not sensitive to damage. Thus, different cell types are unpredictably variable, and are affected only under particular conditions of
exposure.
Polymerization damage has especially plagued investigators interested
in tissue cultures; for in these preparations almost all of the interesting
cells are on the surface of the blocks, and usually are sensitive to this sort
of damage. It also has caused much difficulty when efforts have been made
to handle free living cells such as protozoa, or free cells in suspension, as
POLYMERIZATION DAMAGE
101
(shrinkage) which occurs with polymerization, or which may accompany
an advancing front of polymerization. The end effect can be hard, indeed, to distinguish from poor fixation, or from decomposition which
may result when inadequately preserved material receives electron bombardment. [Morgan et al. (1956) have demonstrated the sensitivity of
formalin-fixed material to bombardment damage.] It is possible to prove
that polymerization damage truly exists, however, by suitably controlled
experiments. Samples of a single tissue embedded conventionally can be
compared with samples embedded with prepolymerized methacrylate
(Fig. 8), or after using epoxy or polyester embeddings.
Polymerization damage has often been likened to an "explosion."
There is always some swelling. Note that the magnification of Fig. 8(a)
is only half that of 8(b) so that the apparent size of the affected cell is
about twice that of the well preserved one. Cell and nuclear membranes
are apt to be interrupted. Mitochondria may be swollen and their membranes broken. Small vesicular structures, such as those associated with
the Golgi apparatus and endoplasmic reticulum, may become almost unrecognizable. The diffuse proteins of the cytoplasm and nucleoplasm are
apt to appear as a dispersed flocculent precipitate. It is usually obvious
that the whole cell is poorly preserved. Polymerization damage can be a
graded phenomenon so that varying degrees of explosion are recognizable.
It has been general experience that exposed cells on the surface of
tissue blocks are the ones particularly prone to be damaged. Below the
surface layer of cells, tissue ordinarily is unaffected by conventional embedding. Certain tissues are particularly prone to damage for inexplicable reasons. Thus, in this writer's experience, the endothelium of slabs of
an aortic wall is very sensitive [Fig. 8(a)]. Adjacent muscle is not affected
by conventional blocking, and exposed muscle on the opposite side of
the aortic wall is unaffected even though the adventitial coat is stripped
off prior to embedding. The endothelium of small blood vessels, blocked
in toto, is not sensitive to damage. Thus, different cell types are unpredictably variable, and are affected only under particular conditions of
exposure.
Polymerization damage has especially plagued investigators interested
in tissue cultures; for in these preparations almost all of the interesting
cells are on the surface of the blocks, and usually are sensitive to this sort
of damage. It also has caused much difficulty when efforts have been made
to handle free living cells such as protozoa, or free cells in suspension, as