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Journal of Neuroscience, Vol 10, 3947-3959, Copyright © 1990 by Society for Neuroscience
The formation of terminal fields in the absence of competitive interactions among primary motoneurons in the zebrafish
DW Liu and M Westerfield
Institute of Neuroscience, University of Oregon, Eugene 97403.
To make specific synaptic connections, projection neurons extend neurites
to regions containing appropriate targets, then form synapses with the
correct type and number of target cells. To investigate the mechanisms
controlling this process, we have studied the formation of motoneuronal
terminal fields in live zebrafish embryos. The primary motoneurons of the
zebrafish are identifiable as individuals and innervate neighboring but
mutually exclusive territories. To study the first week of their
development, which includes embryonic and early larval stages, we labeled
identified motoneurons with fluorescent dyes and made sequential
observations of the axonal branches of individual neurons. We assessed the
roles of competitive interactions and synapse elimination in the formation
of specific synapses by identified neurons that innervate neighboring
territories. Our results demonstrate that primary motoneurons establish
their cell-specific terminal fields primarily by directed outgrowth of
branches and formation of neuromuscular junctions almost exclusively on
appropriate muscle fibers, rather than by overproduction and selective
elimination of inappropriate branches. Retraction of the few branches that
are inappropriately placed, though correlated in time with the ingrowth of
branches from appropriate motoneurons, occurs independently of the
influences of these other cells and when neuromuscular transmission is
blocked. We suggest that, similar to the way in which they pioneer
peripheral nerve pathways, primary motoneurons establish their cell-
specific terminal fields using mechanisms that operate independently of
activity and competition. The target or substrate interactions that are
likely to instruct directed growth-cone navigation may be similar to the
interactions that determine the locations of territorial borders and that
instruct the retraction of misplaced branches.
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