Ultrastructural features of transneuronal cell degeneration in the olfactory system

The electron microscope has been used to study transneuronal degenerative changes in the cells of the olfactory bulb of the rabbit and rat following removal of the olfactory mucosa. The principal features of this type of change are cell shrinkage, a concentration of cytoplasmic and nuclear constituents, an alteration in the organization of nucleic acid material, and a swelling of membrane-bound systems; some elements of cellular organization appear unchanged, including synaptic specializations. The changes are described for each part of theneuron.—soma, dendrite, axon and terminal—as well as the variants found in different cells. The degree of morphological alteration appears to be related to the extent of denervation for any individual cell and does not seem to be sequential. These changes are related to findings on altered impregnation properties of the same cells with the Golgi technique and are discussed in the context of the functional alteration produced by deafferentation. A new interpretation of some ‘dark neurons’ is also suggested on the basis of these findings.