Liu D W, Westerfield M
Institute of Neuroscience, University of Oregon, Eugene 97403.
J Neurosci. 1990 Dec;10(12):3947-59. doi: 10.1523/JNEUROSCI.10-12-03947.1990.
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.
为了建立特定的突触连接，投射神经元将神经突延伸至含有合适靶标的区域，然后与正确类型和数量的靶细胞形成突触。为了研究控制这一过程的机制，我们在活的斑马鱼胚胎中研究了运动神经元终末场的形成。斑马鱼的初级运动神经元可作为个体被识别，并支配相邻但相互排斥的区域。为了研究它们发育的第一周，包括胚胎期和幼鱼早期阶段，我们用荧光染料标记已识别的运动神经元，并对单个神经元的轴突分支进行连续观察。我们评估了竞争性相互作用和突触消除在支配相邻区域的已识别神经元形成特定突触过程中的作用。我们的结果表明，初级运动神经元主要通过分支的定向生长和几乎仅在合适的肌纤维上形成神经肌肉接头来建立其细胞特异性终末场，而不是通过过度产生和选择性消除不适当的分支。少数位置不当的分支的回缩，虽然在时间上与来自合适运动神经元的分支向内生长相关，但独立于其他这些细胞的影响且在神经肌肉传递受阻时发生。我们认为，与它们开拓外周神经通路的方式类似，初级运动神经元利用独立于活动和竞争的机制来建立其细胞特异性终末场。可能指导定向生长锥导航的靶标或底物相互作用，可能类似于决定区域边界位置并指导位置不当分支回缩的相互作用。
