Chemical Fixatives - Most commonly and routinely used - 2 types - Coagulative and
Coagulative - The original killing agents used in light microscopy: FAA, Farmers,
Carnoy's, Bouins, CRAF, alcohols.
Generally low pH, unbuffered.
Coagulate cellular contents, much like frying an egg or cooking meat.
Most also contain high alcohol conc., and therefore also dessicate tissue.
It's long been known that these fixatives destroy much of the fine structure of cells,
making them unsuitable for transmission electron microscopy.
In many cases, an SEM is used as a "glorified" dissecting scope. An image may not
be magnified more than a few times; therefore the level at which the damage
occurs (the ultrastructural level) is not seen.
For SEM, they have in many cases proven quite useful, particularly in the case of
specimens that are hard or durable to begin with (insects, some plant material) and
will be observed at relatively low magnification.
They also tend to greatly harden tissue. This is often a bonus in SEM, because there
tends to be less shrinkage and distortion during the drying process.
These fixatives are commonly used to preserve or 'pickle' museum specimens, and
if this is the only way a specimen is available, then often the individual must make
do with this type fixation. - It is often quite sufficient.
Non-Coagulative - The fixatives most commonly associated with transmission E.M. Includes most aldehydes at low
concentration (formald, acrylic, glutar,) osmium tetroxide.
MODE OF ACTION:
(1) react chemically with tissue macromolecules, forming inter and intra-molecular crosslinks which
increase the structural stability of the molecules.
(2) inactivate autolytic enzymes and
(3) make tissue more resistant to enzymatic degradation from
autolysis and microbial activity (a recapitulation).
These fixatives normally consist of at least 2 components: 1 or more active ingredients (fixatives) and a vehicle, often a buffering system. Some optional components in the vehicle may include:
metabolic poisons (azide, cyanide)
divalent cations (Ca, Mg)
surfactants (detergents - ie TWEEN)
electrolytes or non-electrolytes added to adjust osmolarity