Enlarged synaptic vesicles as an early sign of secondary degeneration in the optic nerve terminals of the pigeon.

The terminal degeneration of retino-tectal fibres was studied electron microscopically in the pigeon Synaptic vesicles seem to undergo systematic changes which can best be observed in aldehyde-fixed material. Initially (i.e. within 12-24 h) the vesicles begin to swell. The enlargement is clearly visible after 4 days (40 % increase in diameter) and reaches a maximum at 14 days (100% increase). At the latter stage, the enlargement is almost invariably associated with the well known opacity of degenerating terminals. In contrast, normal control tissue contains nerve endings with only a few enlarged and no ballooning vesicles. The conclusion seems warranted that the ballooning of synaptic vesicles is an early sign of terminal degeneration, it appears to precede vesicular disintegration.