Alan Harvey wrote:
could someone explain the difference between
"fixation" and "preservation"? This must be an embarrassingly elementary
question, but I've had a hard time finding a definition of the term
"fixation" that is explicit and relevant to collection studies. E.g., I
vaguely recall the phrase "cross-linking of proteins" in connection with
fixation, but don't know why that's beneficial to specimen preparations.
I can't help you with the semantic question, but you can look at chemical
fixatives as a couple of different categories:
1) coagulants - such as Ethanol, which attracts water such that the proteins
form bonds that coagulate tissue. This is very unsatisfactory at the
microscopic level (shrunken, distorted tissues), but is used a lot for
freshwater invertebrates, for example, that will be examined at low
magnifications.
2) non-coagulants or "additive" fixatives - such as formaldehyde and
glutaraldehyde, which react with amino groups of proteins to form aldemine
bonds. Formaldehyde has a single bonding site, whereas glutaradehyde is a
dialdehyde, so it can form bridges between active sites on proteins.
Other non-coagulants may operate differently (osmium tetroxide reacts with
unsaturated bonds in lipids, for example) [this chemical is very dangerous
but routinely used for TEM or SEM preparations].
The reason that these chemicals are dangerous to work with is that they fix
us too (lungs, eyes, etc.), but we still use them for the high-quality
preservation we can expect, with enzymes inactivated and spatial
relationships fairly predictible in our specimens.
--John
John Kingston
U.S. Geological Survey - National Water Quality Lab
5293 Ward Road / MS426
Arvada, CO 80002
==================
John C. Kingston
Biological QA Unit
6 messages in edu.ku.nhm.mailman.taxacomSpirit Collections
