Journal of Neuroscience, Vol 16, 1780-1790, Copyright © 1996 by Society for Neuroscience
Synaptic remodeling revealed by repeated in vivo observations and electron microscopy of identified frog neuromuscular junctions
CP Ko and L Chen
Department of Biological Sciences, University of Southern California, Los Angeles 90089-2520, USA.
This work aimed to examine the mechanism of synaptic remodeling using
repeated in vivo observations, followed by electron microscopy, of
identified frog neuromuscular junctions (NMJs). Our previous light
microscopic studies suggested that extension of synaptic extracellular
matrix (ECM) precedes, and may play a role in, nerve terminal (NT) growth
during synaptic remodeling. To test this hypothesis, sartorius muscles were
double labeled with a fluorescent dye, 4-(4-diethylamino-
styryl)-N-methylpyridinium iodide, for NTs and rhodamine-conjugated peanut
agglutinin for synaptic ECM. The double-labeled NMJs were observed in vivo
with video-enhanced fluorescence microscopy. Two to three months after
nerve sprouting was induced by a nerve graft, the same NMJs were restained
and reexamined. After the final in vivo observations, the same NMJs were
examined with semiserial thin section electron microscopy. Light
microscopic observation of NMJs that showed synaptic ECM longer than the NT
was confirmed with electron microscopy. At junctional branches where
synaptic ECM extended beyond the NT, a Schwann cell process longer than the
NT was observed in one example, whereas a Schwann cell with the same length
as the NT was seen in other examples. In both cases, junctional folds were
absent at the extended ECM region. In contrast, junctional folds were
observed at the region vacated by a retracted NT. These results suggest
that extension of synaptic ECM and Schwann cell processes may lead, and
play a role in, the NT growth during the remodeling of adult synaptic
connections.
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