7.9.
STAIN DEPENDENCE UPON FIXATION
243
may suffice. Grids with sections up are placed on a piece of filter paper.
About 5 cm of chromyl chloride vapor are drawn up in a Pasteur pipette,
and then the vapor is slowly expelled over a grid. Staining occurs as an
almost immediate reaction.
In our own laboratory we have had only a limited experience with this
stain. However, it is evident that the timing and concentration of the
stain is critical, and hard to standardize. Not infrequently sections have
been spoiled by deposits of tiny dense droplets. Chromyl chloride also
reacts with the copper of the grid wires if the latter are not protected
by a supporting film. It seems evident, too, that this substance reacts with
at least some embedments so that unsupported stained sections may not
survive electron bombardment.
Basically chromyl chloride is to be regarded as a general stain. Its
toxicity, nuisance of preparation, and uncertainty of action argue against
its common usage, but the fact that it is a gas suggests that it may have
applications of special importance in staining frozen-dried or frozen-substituted tissue, thus avoiding contact with water (Chapters 3.14 and 3.15).
7.9. Stain Dependence upon Fixation
Marinozzi and Gautier (1961) and Marinozzi (1963) have emphasized
that staining reactions often are quite dependent upon the character of
the fixation, and may be altered by suitable pretreatment of the sections
prior to staining. They have tried to study some of these reactions in a
controlled and logical manner and to interpret and systematize their
results. In principle one cannot argue with their approach for surely
any staining reaction ultimately involves chemical reactions which one
would like to know about specifically, and which then might be controlled advantageously.
A principal interpretation of Marinozzi (1963) is that the alkaline lead
hydroxide stains react mainly with such reduced osmium as may be
present in the sections, but also stain RNA in an altogether different and
specific way. Thus, after osmium tetroxide fixation, these compounds act
as general stains, whereas after formalin fixation their dramatic effect is
limited to RNA. He finds that if reduced osmium tetroxide is oxidized
before lead staining, the density of all tissue components except nucleoproteins is greatly decreased. Thus, such pretreated sections come to have
much the appearance of aldehyde-fixed sections.
STAIN DEPENDENCE UPON FIXATION
243
may suffice. Grids with sections up are placed on a piece of filter paper.
About 5 cm of chromyl chloride vapor are drawn up in a Pasteur pipette,
and then the vapor is slowly expelled over a grid. Staining occurs as an
almost immediate reaction.
In our own laboratory we have had only a limited experience with this
stain. However, it is evident that the timing and concentration of the
stain is critical, and hard to standardize. Not infrequently sections have
been spoiled by deposits of tiny dense droplets. Chromyl chloride also
reacts with the copper of the grid wires if the latter are not protected
by a supporting film. It seems evident, too, that this substance reacts with
at least some embedments so that unsupported stained sections may not
survive electron bombardment.
Basically chromyl chloride is to be regarded as a general stain. Its
toxicity, nuisance of preparation, and uncertainty of action argue against
its common usage, but the fact that it is a gas suggests that it may have
applications of special importance in staining frozen-dried or frozen-substituted tissue, thus avoiding contact with water (Chapters 3.14 and 3.15).
7.9. Stain Dependence upon Fixation
Marinozzi and Gautier (1961) and Marinozzi (1963) have emphasized
that staining reactions often are quite dependent upon the character of
the fixation, and may be altered by suitable pretreatment of the sections
prior to staining. They have tried to study some of these reactions in a
controlled and logical manner and to interpret and systematize their
results. In principle one cannot argue with their approach for surely
any staining reaction ultimately involves chemical reactions which one
would like to know about specifically, and which then might be controlled advantageously.
A principal interpretation of Marinozzi (1963) is that the alkaline lead
hydroxide stains react mainly with such reduced osmium as may be
present in the sections, but also stain RNA in an altogether different and
specific way. Thus, after osmium tetroxide fixation, these compounds act
as general stains, whereas after formalin fixation their dramatic effect is
limited to RNA. He finds that if reduced osmium tetroxide is oxidized
before lead staining, the density of all tissue components except nucleoproteins is greatly decreased. Thus, such pretreated sections come to have
much the appearance of aldehyde-fixed sections.