Preservation of myelinated fibers for electron microscopy: a qualitative comparison of aldehyde fixation, microwave stabilisation and other procedures all completed by osmication.

A qualitative comparison was made of a variety of electron microscopic preservation methods for nervous tissue, especially with respect to myelinated fiber areas. The methods studied were aldehyde perfusion/immersion fixation, aldehyde-tannic acid immersion fixation (stimulated by either microwave or conventional heating), microwave stabilisation, saline treatment with conventional heating (all with secondary osmication), and primary osmication. For all methods three morphological aspects, the ultrastructural quality of myelin sheath and axon and the coherence between the two were judged separately. It appears that the best version of each method studied is capable of providing a good overall ultrastructural result but always shows a preference for one or two of the three separate morphological aspects. When aiming at good axon quality together with good axon/myelin coherence, aldehyde perfusion/immersion, saline treatment or primary osmication are almost equivalent. Microwave stabilisation, on the other hand, can be chosen when good myelin quality has to be combined with good axon quality. For more specific purposes the following examples can be given. When excellent myelin quality is needed both microwave-stimulated aldehyde-tannic acid fixation or microwave stabilisation can be considered. When the preservation of the axon quality has priority the aldehyde-perfused tissue should be further immersed in a heated aldehyde-tannic acid solution. Primary osmication guarantees excellent axon/myelin coherence. Despite the differences in detail, a remarkable correspondence is stressed between the overall results of sometimes extremely different methods of tissue preservation. Probably they all guarantee a reliable reflection of the in vivo situation. With respect to the use of microwave irradiation for tissue preservation, it appeared that stabilisation procedures are rather capricious. However, if successful, the results are not inferior to those of aldehyde fixation.