A few NMJs in aging animals exhibit a sudden change in morphology occurring over a very short time period associated with a loss of AChRs throughout the synapse. Labels are described in legends of Figures 1 and 3. In the bottom row in each panel, there is an additional image displaced to the right. In these cases, the AChR labeling (first image in the row) was very dim, showing that most of these receptors had turned over during the time intervening between this image and the previous one. In these cases, reapplication of new bungarotoxin revealed the receptors that were newly synthesized and inserted (images labeled “new AChR”). A, Young-appearing junction that showed a loss of the first labeling of AChRs, insertion of significant new receptors, and a fragmentation of the AChRs that became more apparent with time as the fiber grew and expanded (rightmost image). In this case, the nerve terminal becomes varicose. B, Fragmented, old-appearing junction that showed a loss of the first labeling of AChRs, insertion of significant amounts of new receptors and additional fragmentation of the AChRs. This loss occurred between the 11th and 12th image in a sequence beginning at 16 months of age. Scale bar, 20 μm.