Histopathology continues to be a powerful, yet inexpensive,
part of veterinary diagnostics.
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Specimens should represent typical lesions, including active
margins and adjacent (normal) tissue, rather than lesion
cores or curetted debris. Autolysis, freezing, mutilation
(forceps crushing or tearing), or removal of small samples
by electrocautery will make samples unsuitable for proper
evaluation.
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Multiple specimens (from different sites or types of
lesions) should be identified individually by size, suture
tags, or separate containers. Samples thicker than 1/2 cm
should be sliced to allow adequate fixation. Brain and eyes
are exceptions; they should be fixed whole
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Nearly all diagnostic histopathology can begin with tissue
fixed in 10% buffered neutral formalin.
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Formula
| 37-40% formaldehyde
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100 ml
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| distilled water |
900 ml
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| sodium phosphate, monobasic
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4.0 gm
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| sodium phosphate, dibasic |
6.5 gm
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Fixative volume should be 10-20 times specimen volume. After
12-24 hours, specimens can be transferred to just enough
formalin to keep them moist during shipment. There is no
need to pay for transport of excess fixative. Formalin will
freeze at extremely low temperature, damaging the tissues.
Adding 1 ml of ethanol to each 9 ml of 10% formalin will
prevent such freezing.
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Wide-mouth bottles or Whirl-Paks are preferred containers.
Narrow-mouth bottles often have to be broken or cut to
release fixed tissues. Plastic bottles are better than
glass; anticipate rough handling during shipment, and
package accordingly. Most bottle lids will leak; if in
doubt, tape the lid, or place the bottle and accession form
in separate Whirl-Pak bags to avoid contamination or damage.
Label container(s) adequately (owner, animal, veterinarian,
site). Full interpretation of histopathologic findings
hinges on complete clinical histories.
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Submit specimens from all major organs, including brain, if
in doubt about which tissues to collect or if there are no
gross lesions.
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Fresh tissue, handled gently and fixed adequately in 10%
buffered neutral formalin will yield excellent results. Some
pathologists, however, have advocated using Bouin's fixative
for endometrial and endocrine specimens. The advantages, in
our opinion, of Bouin's do not outweigh the disadvantages of
extra reagents and processing steps. Tissues should be fixed
in Bouin's no longer than 18 hours, as they become hard and
brittle. Specimens must be washed 4-6 hours in several
changes of alcohol to remove any picric acid (yellow), then
stored/shipped in 70% alcohol. Over-fixation with Bouin's
results in poor histologic staining.
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Cytologic examination of fluids, smears, or imprints is done
by the Clinical Pathology Laboratory, Washington State
University Veterinary Clinic, (509) 335-0745.
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Containers or formalin can be purchased from WADDL.
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Duplicate glass slides of specimens can be prepared for
practitioner use and files. The cost of these slides is
listed in the fee schedule.
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