An ultrastructural study of degeneration in axon terminals of the glomerular layer of the rat olfactory bulb is described, concentrating particularly on the sequence of degeneration in the olfactory nerve terminals and the long-term events in the degeneration process in several terminal types. Olfactory nerve terminal degeneration is divided into 5 stages, representing parts of the sequential changes taking place in the terminal after fibre section. The main features in the sequence are as follows: I. Swelling of the terminal and its vesicles. II. Initial shrinkage of the terminal, while vesicles remain swollen and some are distorted. III. Further shrinkage and darkening of the terminal, distortion of many of the vesicles and some mitochondrial swelling. IV. Extreme shrinkage of the terminal and loss of cytoplasmic detail; synaptic contact still intact or partially separated. V. Disappearance of the presynaptic terminal and persistence of the post-synaptic thickening.
The validity of the observation of the persistence of post-synaptic membrane thickenings is considered and evidence adduced in its favour, both qualitative and quantitative. Characteristics of the newly apposed profiles are described, including cisternae and alveolate vesicles; the extracellular material of the synaptic cleft is considered in the light of its association with persisting thickenings. The relevance of these associated Structures is discussed in terms of function and development. Spontaneous degeneration of olfactory nerves and the degeneration of the vomeronasal nerves in the accessory olfactory bulb are described, as well as Stage V in the degeneration of other terminal types of the glomeruli, following various lesions; persistence of post-synaptic thickenings after the degeneration of terminals showing symmetrical membrane thickenings is included. Observations on the persistence of post-synaptic thickenings in various sites in the central and peripheral nervous systems are reviewed in the context of the present study; the problems arising out of the morphological identification of apposition or reinnervations are discussed.
Newborn Neurons in the Olfactory Bulb Selected for Long-Term Survival through Olfactory Learning Are Prematurely Suppressed When the Olfactory Memory Is Erased
