An electron microscopic study of terminal degeneration in the inferior olive of the cat

Abstract An electron microscopic study has been made of terminal bouton degeneration in the inferior olive of the cat following interruption of afferent fibers to the olive. Bouton degeneration is found already on the second day, when the boutons show an increased density of the matrix and a close packing of synaptic vesicles. On the seventh day many mitochondria show marked changes. They disintegrate, and their fragments usually fill the bouton entirely. The synaptic vesicles in such boutons have lost their individuality. Most of the degenerating boutons found at later stages are similar to those found on the seventh day. Degeneration proceeds slowly in the boutons. Occasionally mitochondria and synaptic vesicles can be recognized even at the 112‐day stage. At this time the degenerating boutons often show vacuolization. Where a degenerating bouton participates in a synaptic complex, thickening of the postsynaptic membrane is found even at the 112‐day stage. The dendritic pits found at the 14‐ and 36‐day stages and described in a previous publication (Walberg, '63b) are even more prominent at the one‐hundred‐twelfth day. Glial activity has not been observed. The relation between degenerating boutons and glial profiles, even at the 112‐day stage, is similar to that found in unoperated animals. A very long period is obviously required for the lysis of the degenerating boutons.