In general, the histopathologist will only be able to offer comment on the significance of lesions when an appropriate range of specimens has been submitted, together with a good history and a description of the clinical and necropsy findings.

Standard Containers and Equipment

(Neutral) Buffered Formalin

Buffered formalin solution is the recommended fixative because of its stability, longer life of preserved tissues and a negligible fixation deposit. It is prepared as follows:

Buffer salts are available commercially in prepared packs.

* Commercial formalin is Formalin B.P. 34-38% formaldehyde. It should be used neat in this formulation.

Other fixatives

Other fixatives for special purposes are available from the laboratory.

Containers

These must have a tightly fitting (wadded) screw-topped lid to prevent leakage. Lids should be taped tightly closed with electricians' tape or similar to prevent them becoming loose.

The laboratory does not supply containers but will return non-disposable containers at cost

Containers must have sufficiently wide mouths to allow tissue to be easily withdrawn after fixation. Don't squeeze large tissue samples eg brain into containers.

Collection of Specimens

Tissues collected at necropsy should be preserved immediately.

If fresh organs are being submitted for bacteriological or gross pathological examination, then a small block of the organ (sliced 0.5 cm and no more than 1 cm thick) should be fixed in buffered formalin at the time of collection, for histopathology.

Always submit an adequate range of tissue specimens.

Preserved tissue should be submitted in at least 10 times its volume of fixative.

Decomposing or frozen tissues are not suitable for histopathological examination.

Organs

Organs smaller than 1 cm in thickness can be fixed whole.

Organs greater than 1 cm thick must be sectioned. Tissue for fixation should be sliced 0.5 cm (and no more than 1 cm) thick. They should include part of the lesion and adjacent apparently healthy tissue.

Gastrointestinal tract and other tubular organs

Ensure that the mucosal surface of tubular organs (eg gut, uterus, bladder) is exposed by excision of the wall before immersion in fixative.

Handle only the edges of delicate tissues so that histological detail is not lost.

Spinal cord

After exposure, the spinal cord should be lifted by grasping the dura with tissue forceps. Each spinal nerve root is then severed as it is exposed.

The dura is then opened longitudinally in the dorsal midline and the cord and dura submitted whole in a large container of buffered formalin. To prevent fixation of the cord in a curled position, transect the cord at 10 to 20 cm intervals, leaving a part of the dura intact at each transection site, to keep the cord segments connected. The cord will then be fixed in short, straight segments. Open the dura to allow better penetration of the fixative.

Brain

There are several methods of exposing the brain for removal. Convenient methods for large animals (eg longitudinal and transverse craniotomy) are used in the National TSE Surveillance Program

It is preferable to submit the brain whole in a wide mouth container with 10 times its volume of buffered formalin solution. After 24 hours fixation, the formalin solution can be changed to hasten fixation.

Ideally, bovine and equine brains should be immersed in at least 6 litres of buffered formalin solution (e.g. in plastic buckets) for at least 48 hours, then submitted in a large container of fresh buffered formalin solution.

It is essential to avoid distortion of the brain during fixation. Compression of brain surface resting on the base of the container can be avoided by adding neat formalin until the brain floats.

If the brain cannot be submitted whole, it may be cut transversely into two or three pieces. Always cut transversely so that all brain sections can be examined (grossly and histologically) for bilateral lesions. Do not cut the brain longitudinally.