Good question.  My answer is NO.  The big question is do you count
the osmolarity of the aldehyde fixatives?  Their ability to penetrate
tissue and cross membranes has been questioned but most of the
molecules must not contribute since the osmolarity of a 10% solution
of formaldehyde is 1300 mOsm and of a 4% solution is 520 mOsm.
Adding glutaraldehyde increases it even further.  On the other hand,
the fix I use has salts (NaCl, HEPES, CaCl2) that is hypo-osmotic (I
forget but I think it is around 225 mOsm instead of physiological of
300-308) and yet my tissue looks excellent (in my humble
opinion.....).  I forget what the osmolarity of salts in Karnovsky's
fix is but I think I modeled my concentrations on his.  But it could
be what I (we) think is excellent fixation is artifactual.  The
typical text book views of brain tissue shows little or no
extracellular space between all the neurons, glia, etc.  But Van
Harreveld showed that quick-frozen brain tissue had lots of space.
The classic view of mitochondria with cristae and open space is a
result of chemical fixation - in quick-frozen tissues, it is
difficult to see cristae since the mitos are so condensed.  If you
add ox phos uncouplers prior to quick-freezing, the mitos look
"normal".  Some think fixation osmolarity is of little consequence
since fixation is "fast" but there have been nice studies of
electrophysiological recordings of miniature end plate potentials at
neuromuscular junctions showing the electrical signals changing over
a time course of minutes during fixation.  The bottom line is we
usually end up using what gives us a pretty picture but we should
always remember, it might not be a perfect reality.  My thesis
adviser always called it chemical "embalming" rather than fixation.

In an earlier posting of a similar question,
*************************************************************** Geoff
McAuliffe, Ph.D.
Neuroscience and Cell Biology
Robert Wood Johnson Medical School
675 Hoes Lane Piscataway, NJ 08854
voice: (908)-235-4583; fax -4029 e-mail: [log in to unmask]
************************************************************* said:


"In the theme of Diana van Driel's post check out Arborgh et al. "The
osmotic effect of glutaraldehyde during fixation". J. Ultrastr. Res.
56:339-350, 1976. A very interesting and through study".


I haven't gotten around to looking this article up yet.  Good luck.  Tom




>Are there generally accepted rules for selecting osmolarity of fixative
>solutions for TEM?
>Thanks group!
>
>Milton Charlton
>University of Toronto
>
>-----

--
Thomas E. Phillips, Ph.D.
Associate Professor of Biological Sciences
Director, Molecular Cytology Core Facility

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