The duration of aldehyde fixation as a “flattening factor” of synaptic vesicles

Summary Synaptic vesicle flattening can be induced in the excitatory mossy fibre endings of the rat cerebellum by prolonged immersion in aldehyde during fixation (with or without perfusion). The flattening is found in a greater percentage of vesicles if perfusion has been omitted before the prolonged immersion. This is discussed in relation to the various other factors that are thought to cause flattening and the important problem of the classification of different types of synapse.The author wishes to express his thanks to Prof. E.G. Gray, for advice and constant help; to Mrs. H. Samson and M. Lourdes Brito for technical assistance, and to the Instituto de Alta Cultura, Lisbon, for a grant.