Large Animal Neurology. Joe Mayhew

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.

For a routine case with signs relating to a brain lesion, the entire brain should be harvested along with at least one trigeminal ganglion lying slightly lateral and projecting rostral to the optic chiasm. If vestibular signs are indicated, the skull should be sectioned transversely after brain removal at the level of the external acoustic meati for an inspection of the ear cavities. If no specific lesion site is indicated clinically or grossly, routine brain harvesting and fixation should proceed. The brain can then be cut into transverse 5–10 mm slices except for the cerebellum that should be sectioned initially on a median plane after sectioning its peduncles. The brain histopathology blocks would routinely be taken from the sites indicated in Figure 4.4, taking samples from the right side of the preserved brain as a routine unless otherwise indicated. These sites are selected based on sites of lesions for commonly occurring and for important diseases (e.g., rabies, cerebellar abiotrophy, and neuroaxonal dystrophy), and should be varied and added to depending on clinical signs, gross pathologic findings, funding available, and the regional experience of the neuropathologist.

Figure 4.4 Suggested levels for taking routine brain sections for histopathologic study. Selected brain regions to be routinely viewed (A). Sites for routine sectioning (B). Examples of final routine brain histologic sections (C).

With spinal cord cases where clinical documentation or gross findings indicate a specific site, then sections from this area should take priority. With no clinically or grossly documented focal lesion identified, then in addition to spinal cord, peripheral nerves (e.g., median and sciatic) and the brain should be harvested. Routine blocking of spinal cord should include two cervical, two thoracic, and one lumbar set of sections, each sampled on a transverse plane, a dorsal longitudinal plane, and a median longitudinal plane orientation (Figure 4.5). Again, a documented focal or multifocal lesion will dictate sampling sites. The remainder of the cord should be replaced in fixative such that the orientation of parts is still evident for future sub‐sectioning.

      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.

General histologic reactions of cells of the nervous system

Of some importance in histologic interpretation are occult and aging changes that can be quite impressive.^10–12 These include the presence of polyglucosan accumulations or Lafora bodies in cattle,¹³ cerebrovascular siderosis,¹⁴ mineralization,¹⁵ dense microspheres and axonal spheroids in adult, and especially aged animals¹⁶ and processing‐related neuropil vacuolation,¹⁷ all in the CNS. Peripheral nerves do show a depletion of large myelinated fibers with aging,^18,19 and normal equine nerves often contain a few to spectacular concentrations of Renaut bodies (Figure 4.6).^20,21 Although when profuse, these latter structures may indicate sites of pressure on, or tension of, nerves.²² Equine skeletal muscle can contain many nonpathological degenerative sarcoplasmic masses.²³

Figure 4.5 The recommended planes of sections to be taken for histologic examination at each selected spinal cord site are depicted here. [WM = white matter; GM = gray matter]. At the center is the standard transverse plane section. Additional routine histologic sections to be taken on the dorsal plane (left and right) and the sagittal plane (bottom) are strongly recommended. The major sensory (↑) and motor (↓) tracts that allow detection of where a lesion(s) is located are indicated by arrows.

Figure 4.6 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. They consist of loose connective tissue substances often arranged in loose whorls on transverse section, containing little collagen and few nuclear remnants. Usually they are in a subperineural location where they can occupy a large proportion of space in a fascicle (arrow heads). These normal structures in equine peripheral nerves do not appear to represent pathologic processes by themselves and should not be misinterpreted as pathologic in their own right on pathology reports. However, Renaut bodies can accumulate dramatically and appear to distend nerves undergoing changes such as pressure‐induced neuronal fiber atrophy as occurs with suprascapular nerve entrapment.

      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