Specimens: CNS, PNS, Skeletal Muscle, and Ophthalmic Structures
Handling and interpreting diverse specimens
Each tissue type has its own processing rules and diagnostic pitfalls.
CNS specimens:
- Frozen vs permanent: avoid freezing tiny stereotactic samples if lymphoma is suspected (crush artifact); reserve tissue for molecular studies.
- Brain biopsies: sample the edge of lesions to capture transition zones.
PNS and nerve:
- Orientation matters - longitudinal vs cross-section; teased fiber preparations show axonal vs demyelinating patterns. Vasculitic neuropathy shows vessel wall inflammation with fibrinoid necrosis.
Skeletal muscle:
- Orientation: muscle must be oriented for cross-section; avoid clamping artifact. Frozen sections are needed for enzyme histochemistry (ATPase, NADH) and most myopathies.
- Recognize neurogenic atrophy (group atrophy, target fibers) vs myopathic change (fiber size variation, necrosis, regeneration, internal nuclei).
Ophthalmic:
- Enucleation specimens require orientation (optic nerve margin), and intraocular tumors (retinoblastoma, uveal melanoma) need specific handling. Retinoblastoma shows Flexner-Wintersteiner rosettes.
Traps:
- Freezing muscle intended for enzyme studies incorrectly, or fixing tissue needed fresh.
- Misreading crush artifact as malignancy in small biopsies.
- Forgetting optic nerve margin status in enucleation.
Rule of thumb: Ask what ancillary test will be needed before choosing fixation. Fresh/frozen for enzymes and some molecular work; formalin for routine histology.