Processing animal tissues

From:Gayle Callis

Animal tissues in general can withstand the same processing schedules as
human tissues, particularly larger animals with more body fat, ie bovine
but with some caveats.  The instruments and solvents used are the same in
most laboratories. 

Rodent (rat, mouse, hamster) tissues, particularly when not mixed in with
larger animal tissues can be difficult to section due to overexposure to
dehydrants, clearing and long paraffin infiltrations.  These may require
shorter schedules. 

When doing veterinary diagnostic tissue processing, overnight runs, ie
longer schedules of 45 min to 1 hour/station could contain a mixture of -
chicken, wild bird, bovine, horse, sheep, goats, pigs, dog, cat, mouse,
rat, rabbit, deer, elk, wild sheep, buffalo, occasional reptile, any of
God's creatures other than human.

Different schedules were often not feasible for vet diagnostic setting.
All tissues, in general, cut well after a brief soak with ice water. Tiny
biopsies were often included, and if cassettes containing these tiny
tissues were interspersed between cassettes containing larger samples, they
were fine. I don't know if this was from some cushioning effect of other
cassettes impeding solvent flow??? other???  but biopsies never seemed
overly crunchy.

Wild birds, chickens, turkeys were always among the worst, requiring
patience and a good icy soak and probably would have done better with
shorter schedules, but not feasible. Bovines weren't difficult and never
left us in a bad mooooo'd! Sorry, couldn't resist punning! 

I think most problems occur (as with human,clinical specimens) when tissues
are not FIXED adequately.  If fixation is incomplete then alcohols in
dehydration finish the job, leading to nasty crispy critter cutting or just
the opposite, mush!  It is a good practice to put fresh NBF on processor
daily to insure replenishment of formaldehyde or hold tissues in fixative
until they are fixed.  

For research animal tissue, mouse and rat, we use a short schedule but have
the luxury of time for fixation before processing. All research processing
schedules start in dehydrant 50% or 70% alcohol rather than in fixative
since tissues are totally fixed before processing begins.  With bone, once
again for research, we perform custom processing to fit type of bone
(trabecular versus cortical), age of animal, size of bone, etc with
lonnnnggggg schedules for large bones. Sheep bone left a lot of fat behind
in solvents, these always seemed sooooo greasy, as did pig bone. 

Care was taken to never oversoak trimmed brain blocks from any species.  

If one is having difficulty with all tissues being consistently crunchy,
brittle, hard at sectioning, then the types of solvents, times and
temperatures, vacuum/pressure, etc used in processing should be reexamined
and adjusted accordingly with first look at FIXATION.  

Whew, what a lecture!  


Gayle Callis
Research Histopathology Supervisor
Veterinary Molecular Biology - Marsh Lab
Montana State University - Bozeman
19th and Lincoln St
Bozeman MT 59717-3610

406 994-6367 (voice mail)
406 994-4303 (FAX)


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