Artifacts in Transmission Electron Microscopy - Sample Preparation Topic for Transmission Electron Microscopy - page 133

Techniques involved: osmic fixation.
Change in the natural contrast : Change induced by the addition of a heavy
metal.
Techniques involved: chemical fixation using osmium and “positive-staining”
contrast.
Change in the molecular bonds : Creation of new bonds between macro-
molecules, leading to interpretation errors of reactive sites after labeling.
Techniques involved: chemical fixation using glutaraldehyde.
Protein cross-linking and changes in their spatial conformation : Connection
of macromolecular chains by the creation of bridges or chemical bonds with
an exogenous compound, i.e., fixative. This leads to the formation of a net-
work with physical-chemical properties different from the initial material.
Cross-linking is an irreversible process that constitutes an obligatory artifact
resulting from chemical fixation, but which is indispensable to being able
to carry out dehydration. Cross-linking leads to a change or loss of reactiv-
ity of the primordial macromolecules. It also causes the formation of bonds
between molecules that were originally independent.
Techniques involved: chemical fixation and substitution-infiltration-embe-
dding at room temperature.
Change in chemical composition : Loss of compounds: saturated lipids, simple
sugars, ions, salts, loss or addition of chemical elements.
Techniques involved: chemical fixation, substitution-infiltration-embedding at
room temperature, and “positive-staining” contrast.
Residues : Deposits of matter extrinsic to the sample, which are chemical
residues resulting from preparation. They are superimposed on the thin slice
and impede observation. Residues are produced by chemical additions during
preliminary preparation or during thinning.
Techniques involved: electropolishing, chemical polishing, continuous support
film, holey support film, twin-jet electrolytic thinning, full-bath electrolytic
thinning, twin-jet chemical thinning, full-bath chemical thinning, extractive
replica, chemical fixation, freeze fracture, “positive-staining” contrast, and
immunolabeling.
2.3.2 Secondary Thermal Damage Induced During Chemical Preparation
Microstructural change : Partial or total change of the structural organization
of microstructure components that can be caused by thermal effects dur-
ing chemical preparation. It can result in changes in morphology, phase
redistribution, crystal-structure changes, precipitation, chemical gradients,
formation of new phases, etc.