Re: Chemical Fixation (Davidson's) etc etc
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|From:||"J. A. Kiernan" <firstname.lastname@example.org>|
|To:||Rena Krol <email@example.com>|
|Date:||Fri, 21 May 1999 00:57:35 -0400 (EDT)|
On Thu, 20 May 1999, Rena Krol wrote:
> I am looking for the original reference for Davidson's fixative. Can
> anyone help me?
There is an excellent essay about Davidson's fixative on
Bob Richmond's web page. Unfortunately I've lost the address,
but perhaps Bob, once a frequent HistoNet contributor, is
looking in and can supply it.
The short answer to your question is:
MOORE, K. L., GRAHAM, M. A. & BARR, M. L. 1953
The detection of chromosomal sex in hermaphrodites
from a skin biopsy.
Surgery, Gynecology and Obstetrics 96, 641-648.
Includes introduction of Davidson's fixative:
95% alc 30; formalin 20; Glac HOAc 10; water 30.
For tissues in which sex chromatin is to be examined.
Apparently Davidson never published it himself.
I don't know how it was brought to the attention of
my late colleague and friend Murray L. Barr (1908-1995).
MLB spent the 1950s investigating (with numerous
graduate students etc) the distribution of the sex
chromatin in numerous organs, and also studying
disorders in which there were abnormal numbers of
X chromosomes. Murray Barr was a modest man. He
preferred "sex chromatin" to the eponymous term
"Barr body," which was already well known to high school
kids like me doing high-school biology in Britain in
the late 1950s. His teaching subject was Neuroanatomy.
It's interesting that he favoured the use of eponyms in
this field, perhaps because another of his interests
was the history of medicine.
The cytological quality of the stained preparations of
objects fixed in Davidson's mixture is very high. I can
attest to this having seen the photomicrographs in the
PhD theses of Barr's students, including that of Keith
Moore (a former chairman of the Anatomy Department in
Toronto, perhaps best known for his textbooks of human
embryology). This is rather surprising, because the
fixative contains much more water than other AFA
(alcohol-formalin-acetic) mixtures. With about 33%
alcohol, are proteins adequately coagulated? (Gin, with
40% is supposed to be OK.) Perhaps the alcohol acts mainly
or partly to restrain the swelling of collagen, which would
otherwise be considerable, fast and injurious to micro-anatomy.
The covalent cross-linking by formaldehyde will eventually
toughen the tissue, but it takes a few days.
From the cytological viewpoint, the most important component
of Davidson's fixative is the acetic acid, which penetrates
quickly and promptly precipitates DNA within nuclei. This
provides a sharp pattern of nuclear chromatin distribution
that is valuable for recognizing different cell-types, even
though it may be largely artifactual. Neutral formaldehyde
fixatives provide bland nuclei with much less variation
among different cell-types.
HistoNet correspondents during the last few years seem to
like Davidson's fixative because it provides congenial
appearances and textures for dissecting large, fatty
specimens. This is probably because of the moderate
alcohol content. If you dissect something in a real
alcohol-acetic fixative (70%+ alcohol), evaporation and
drying are a real problem. If you dissect after aqueous
formaldehyde, the fat is just as wobbly and intrusive as
it is in an unfixed specimen. Perhaps the combined alcohol
and acetic acid of Davidson's mixture extract enough fat
to facilitate dissection, while leaving behind enough
water to keep everything comfortably damp.
All this is no substitute for the account given in Dr
Richmond's web page. Tomorrow I'll try again to find
John A. Kiernan,
Department of Anatomy & Cell Biology,
The University of Western Ontario,
LONDON, Canada N6A 5C1
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