Joe Mayhew

Large Animal Neurology


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cells. The latter may include cells such as hemosiderin‐laden macrophages as was the case here in a yearling foal that recovered completely from a perinatal hypoxic–ischemic event as a neonatal foal leaving porencephalic, fluid‐filled cysts such as the one shown here in one internal capsule on this transverse section of the fixed brain.

      If care is used in removing the spinal cord, areas of compression and discoloration associated with necrosis, hemorrhage, or inflammation can be detected easily. Obviously, one cannot section every part of the CNS, so an effort must be made to maintain orientation of the parts. If one suspects the presence of a gross lesion, then an examination of newly made cut surfaces of the fixed tissue should provide clarification.

       Blocking tissues

      If it is worth harvesting tissues at postmortem examination, then it is worth routinely blocking tissues in all cases. This will obviously be modified where there is a specifically known site (e.g., right cerebrum, left vestibular nuclei, right T6, etc.) and when collection of, and particularly blocking of, tissues will focus on those sites in the first instance.

Schematic illustration of suggested levels for taking routine brain sections for histopathologic study.

      As with the brain tissues, unless focal or multifocal sites of lesions are grossly very clear, a methodical approach of sectioning selected spinal cord segments should be taken. This enables the identification of sections being above and sections being below focal lesions so that such focal lesions can be pinpointed to assess possible cause. One system of such histopathologic sleuthing is depicted in Figure 4.5.

      In a neurologic case involving final neuronal pathways, especially when the lesion site(s) is unknown, proximal and distal major nerves along with nerve roots and selected dorsal root, paravertebral and autonomic ganglia, and selected muscle samples should also be harvested. Longitudinal as well as transverse sections of peripheral nerves and muscles should be clearly identified as to orientation for processing.

Schematic illustration of renaut bodies (arrows) may be described as often forgotten endoneural structures seen here on routine low power histologic view of a sciatic nerve fascicle of a horse.

      An understanding of the ways that cells of the nervous system respond to pathologic and artefactual insults is paramount to the clinical interpretation of pathology reports.()

      A précis of the recognized pathologic responses by cells in neural tissues thus follows.

       Neurons

      Artifactual changes in neurons are extremely common in microscopic specimens and are usually the result of fixation, sectioning, and staining procedures. Disappearance of neurons is good evidence of a lesion, but this is usually subjective. Neuronal changes, without associated glial or other cellular reactions, must be interpreted cautiously. Neuronal fiber (axon and myelin sheath) degeneration can be a useful sign of a neuronal defect even at a site distant from the