A disadvantage of aqueous formalin fixation is that lipids are not chemically bound. Lipids are left free to dissolve in the xylene stage of dehydration. Nerve cells do not contain a particular abundance of lipid, but most of the volume of peripheral nerve is Schwann cell.
The myelin wrap of Schwann cells is mainly lipid. Without lipid preservation, such as by osmium tetroxide (OsO4), the regular wraps of lipid-rich plasma membrane that constitute myelin are severely distorted. The figures below demonstrate how a myelinated nerve appears when fixed with l0% BNF or OsO4.
The following slides are of dissected peripheral nerves:
80 - Teased, OsO4 - separated, large, myelinated nerve axons
81 - Equine, H&E and OsO4 - a mixture of large and small myelinated, and non-myelinated nerves
82 - Rodent, only OsO4 - mainly large, myelinated nerve axons
4 - Canine, only H&E - 50:50 small and non-myelinated nerve axons
Ideal comparison of OsO4 versus aldehyde fixation with H&E staining is achieved by having both on one slide as on slide 81. On this slide the XS portion of nerve has half of its epineurium and all of its peri- and endoneurium. In the LS portion of the nerve sample you should be able to locate clefts of Schmidt-Lantermann and nodes of Ranvier. Slide 80 has separated nerve axons, making identification of nodes and clefts even easier. |
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Slide 82 is of cross-sectioned, osmicated nerve with well preserved connective tissue
layers, excellent osmication of myelin and almost all large myelinated nerve axons
interspersed with a few small diameter myelinated nerve axons. Notice that connective tissue
is not highly osmicative.
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Slide 10 includes a lymph node and a nerve. In this sample, formalin was the fixative and H&E were used for staining. This sample has large, intermediate and small myelinated as well as non-myelinated axons.
At this point, you should be able to appreciate the artifactual changes that occur when nerve tissue is fixed in formalin, and recognize epineurium, perineurium and endoneurium, clefts and nodes, myelinated and non-myelinated nerves.
| Nerve somas (or bodies ... or perikarya) can be located in numerous places. Use: Slide 79 to locate multipolar neurons, with Nissl substance (RER) in the ventral horn of the grey matter in the spinal cord. |  |
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Slide 34 to locate visceral ganglia and plexi between the two layers of smooth muscle in the gut wall. The ganglia feature somas with very large, euchromatic nuclei and prominent nucleoli. You may think you are looking at cytoplasm at first, the nuclei being so large. |
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