Spirit Collections

[John@Mizzou1.Missouri.edu C@Mizzou1.Missouri.edu Kingston@Mizzou1.Missouri.edu, BIOLOGIST@Mizzou1.Missouri.edu, Denver@Mizzou1.Missouri.edu, CO (John Kingston)]

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



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John C. Kingston
Biological QA Unit