CONTROL OF SEGMENT-SPECIFIC PREGANGLIONIC AXON OUTGROWTH

胸中部交感神经节前轴突做出选择， 沿椎旁交感神经链向吻侧或向尾侧沿着下行 神经节;节前轴突不分叉项目在两个 方向，即使它们所源自的段提供了 神经支配到喙和尾神经节。既有一个 intersegment和intrsegment位置依赖于所做的选择。 段间位置依赖性反映在以下事实中： 几乎所有的节前轴突从吻胸段 突出喙，而那些从尾胸段项目 尾部有一个平滑的梯度， 沿沿着神经节前的主要向尾侧投射 细胞列。还有一个突出的内部梯度， 胸中段的位置依赖性：节前神经元 位于喙部内的节段突出于喙部，并且位于喙部内的节段突出于喙部， 向尾部突出。该提案提出了一系列实验， 旨在阐明导致特定片段的机制 节前轴突投射具体目标是： 1.确定体节是否影响轴突的选择 神经节前神经元的运动轨迹 2.为了确定胸交感节前神经元是否 脊髓本质上是分段的。 3.为了确定对维甲酸敏感的形态发生梯度是否 酸是轴突节间位置依赖性的基础 弹道 4.确定吻侧和尾侧投射是否是节前的 轴突根据谱系限制或选择性 肌束震颤 5.确定环境边界或促进 基质对节前神经元轴突轨迹选择的影响 . 所有这些目标都将在鸡胚中实现，因为它是 适合胚胎操作。鸟类的组织 交感神经系统与哺乳动物非常相似;因此， 这些实验将提供普遍适用于以下方面的信息： 交感神经发育

Midthoracic sympathetic preganglionic axons make a choice to either ascend rostrally or descend caudally along the paravertebral chain of sympathetic ganglia; preganglionic axons do not bifurcate to project in both directions even though the segment from which they derive provides innervation to both rostral and caudal ganglia. There is both an intersegment and an intrasegment positional dependence to the choice made. The intersegmental positional dependence is reflected by the fact that nearly all of the preganglionic axons from rostral thoracic segments project rostrally, while those from caudal thoracic segments project caudally. There is a smooth gradient from predominately rostral projections to predominately caudal projections along the preganglionic cell column. There is also a prominent intrasegmental gradient of positional dependence in midthoracic segments: preganglionic neurons located rostrally within a segment project rostrally and those located caudally project caudally. This proposal presents a series of experiments designed to illuminate the mechanisms that result in the segment-specific preganglionic axon projections. The specific aims are: 1. To determine whether the somites influence the choice of axonal trajectory by preganglionic neurons. 2. To determine whether sympathetic preganglionic neurons in the thoracic spinal cord are intrinsically segmented. 3. To determine whether a morphogenetic gradient susceptible to retinoic acid underlies the intersegmental positional dependence of axon trajectory. 4. To determine whether rostrally and caudally projecting preganglionic axons are grouped according to either lineage restrictions or selective fasciculation. 5. To determine whether environmental boundaries or facilitating substrates influence the choice of axonal trajectory by preganglionic neuro . All of these aims will be accomplished in the chicken embryo because it is amenable to embryonic manipulation. The organization of the avian sympathetic nervous system is quite similar to that in the mammal; thus these experiments will provide information that is generally applicable to sympathetic neuronal development.