3.11.
MORPHOLOGICAL FEATURES OF ALDEHYDE FIXATION
61
3.11. Morphological Features of Aldehyde Fixation
Sectioned material which has been well fixed in aldehyde and, perhaps,
subsequently stained, has an unconventional appearance (Fig. 4). It is
reminiscent of frozen-dried material from which the lipids are extracted
during dehydration and embedding procedures. Thus the proteins, including the nucleoproteins, are preserved and stainable. But, the lipids
are not rendered insoluble in organic solvents and, thus, the spaces they
originally occupied are finally visualized as negative images. The extraction of lipids of course involves all of the cytomembrane systems of the
cell. Structures such as mitochondrial cristae, however, may easily be
visible by virtue of the relatively dense matrix protein which remains to
outline them in negative contrast. It is evident, though, that the lipids
do remain in situ so long as the tissue is maintained in an
aqueous
medium. This is demonstrated by the fact that the lipids can still be
"fixed" by osmium tetroxide weeks and months after the initial aldehyde
fixation. If the tissue has been properly stored in the meanwhile, glycogen
similarly is retained, and is demonstrated by post-osmication.
Proteins which have been fixed solely in aldehyde seem often to have
somewhat different staining properties than after osmium tetroxide.
Indeed, sometimes they can be much more intensely stained. This is
particularly true of nucleoproteins, but a casual glance at Fig. 4 will
indicate that cytoplasmic proteins as well as those of the mitochondrial
matrix and of the red blood cells are intensely stained as well. A beginning has been made to explore these differences systematically (Chapter
7.9). Eventually this may yield new insights regarding the contents of
organelles.
When tissue that has been well fixed with an aldehyde is subsequently
post-fixed with osmium tetroxide, it probably will look almost as though
it had been fixed with the latter alone. However, there are some subtle
differences that are only partly understood at the present time. Perhaps
the most conspicuous and the most important is the visualization of
chromatin clumps in most nuclei, while after osmium fixation alone one
would expect the same nuclei to have an almost uniform appearance. I
think it is safe to say that we are achieving a better and more complete
negative images of cytomembranes. Note, however, the evident density of proteins in
the nucleus, mitochondrial matrix, pericanalicular cytoplasm (b), "microbodies" (b),
and the portion of a red blood cell in the upper right corner of (a).
MORPHOLOGICAL FEATURES OF ALDEHYDE FIXATION
61
3.11. Morphological Features of Aldehyde Fixation
Sectioned material which has been well fixed in aldehyde and, perhaps,
subsequently stained, has an unconventional appearance (Fig. 4). It is
reminiscent of frozen-dried material from which the lipids are extracted
during dehydration and embedding procedures. Thus the proteins, including the nucleoproteins, are preserved and stainable. But, the lipids
are not rendered insoluble in organic solvents and, thus, the spaces they
originally occupied are finally visualized as negative images. The extraction of lipids of course involves all of the cytomembrane systems of the
cell. Structures such as mitochondrial cristae, however, may easily be
visible by virtue of the relatively dense matrix protein which remains to
outline them in negative contrast. It is evident, though, that the lipids
do remain in situ so long as the tissue is maintained in an
aqueous
medium. This is demonstrated by the fact that the lipids can still be
"fixed" by osmium tetroxide weeks and months after the initial aldehyde
fixation. If the tissue has been properly stored in the meanwhile, glycogen
similarly is retained, and is demonstrated by post-osmication.
Proteins which have been fixed solely in aldehyde seem often to have
somewhat different staining properties than after osmium tetroxide.
Indeed, sometimes they can be much more intensely stained. This is
particularly true of nucleoproteins, but a casual glance at Fig. 4 will
indicate that cytoplasmic proteins as well as those of the mitochondrial
matrix and of the red blood cells are intensely stained as well. A beginning has been made to explore these differences systematically (Chapter
7.9). Eventually this may yield new insights regarding the contents of
organelles.
When tissue that has been well fixed with an aldehyde is subsequently
post-fixed with osmium tetroxide, it probably will look almost as though
it had been fixed with the latter alone. However, there are some subtle
differences that are only partly understood at the present time. Perhaps
the most conspicuous and the most important is the visualization of
chromatin clumps in most nuclei, while after osmium fixation alone one
would expect the same nuclei to have an almost uniform appearance. I
think it is safe to say that we are achieving a better and more complete
negative images of cytomembranes. Note, however, the evident density of proteins in
the nucleus, mitochondrial matrix, pericanalicular cytoplasm (b), "microbodies" (b),
and the portion of a red blood cell in the upper right corner of (a).