Re: Chemical Fixation (Davidson's) etc etc

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From:"J. A. Kiernan" <>
To:Rena Krol <>
Date:Fri, 21 May 1999 00:57:35 -0400 (EDT)
Content-Type:TEXT/PLAIN; charset=US-ASCII

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
   its address.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1


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