Hollyday M, Morgan-Carr M
Department of Biology, Bryn Mawr College, Pennsylvania 19010, USA.
J Comp Neurol. 1995 Jun 26;357(2):254-71. doi: 10.1002/cne.903570206.
The development and distribution of neuronal projections to the developing chick wing was studied using anterograde transport of horseradish peroxidase (HRP). Small injections of HRP were made into motor or sensory neuronal populations in order to visualize individual axons and their associated growth cones. Motor growth cones were observed in different regions of the embryo at different stages, in a proximal-to-distal pattern of distribution which paralleled the process of axon outgrowth and nerve formation. Different growth cone morphologies were associated with differing regions of the developing projection. In the spinal nerves, axons destined for the limb were unbranched and terminated in simply shaped growth cones. As axons approached the developing limb and entered the plexus region, their growth cones became more complex and larger primarily because of widening, and they sometimes branched, producing processes which could extend tens of microns from a tricorne branch point on the parent axon. Both motor and sensory fibers showed similar morphological changes in the plexus region. A distinctively shaped growth cone expanded on its leading edge was observed, sequentially apparent in the distal spinal nerves, in the plexus region, in the loosely organized axonal sheets projecting to the uncleaved dorsal or ventral muscle masses, and where muscle nerves diverged from nerve trunks and within muscle nerves. It is likely that some of these are transitional growth cones preparing to branch, because complex and branched growth cones were also observed in these regions. Branched axons oriented along the anteroposterior axis were similarly observed in the plexus region and distal to the plexus when axons first projected to the limb bud. At somewhat older stages when the basic peripheral nerve branching pattern had formed, motor growth cones were observed in common nerve trunks and in individual muscle nerves, but they were no longer found in the plexus region. Branched axons were likewise restricted to these peripheral locations. Taken together, these observations suggest that one of the ways in which axons navigate is by exploration in the form of growth cone widening, and in some cases terminal bifurcation which may produce axon branches. Selection of the most appropriately directed growth cone process and/or precocious axonal branches may be one of the ways in which axons respond to specific growth cues which guide axons into the limb bud. Alternatively, this precocious branching may be an early neurotrophic response to developing muscle and play no significant role in axon navigation.
利用辣根过氧化物酶（HRP）的顺行运输，研究了发育中的鸡翼神经元投射的发育和分布。将少量HRP注射到运动或感觉神经元群体中，以观察单个轴突及其相关的生长锥。在胚胎的不同阶段，在不同区域观察到运动生长锥，其分布呈近端到远端的模式，这与轴突生长和神经形成的过程平行。不同的生长锥形态与发育中投射的不同区域相关。在脊神经中，伸向肢体的轴突没有分支，末端是形状简单的生长锥。当轴突接近发育中的肢体并进入丛区域时，它们的生长锥变得更加复杂和更大，主要是因为变宽，并且它们有时会分支，产生从母轴突上的三叉分支点延伸数十微米的突起。运动和感觉纤维在丛区域都表现出类似的形态变化。观察到一种独特形状的生长锥在其前缘扩展，依次出现在远端脊神经、丛区域、投射到未分裂的背侧或腹侧肌肉团块的松散组织的轴突片中，以及肌肉神经从神经干分出的地方和肌肉神经内部。其中一些可能是准备分支的过渡性生长锥，因为在这些区域也观察到了复杂和分支的生长锥。当轴突首次投射到肢芽时，在丛区域和丛远端同样观察到沿前后轴定向的分支轴突。在稍大一些的阶段，当基本的外周神经分支模式形成时，在共同神经干和单个肌肉神经中观察到运动生长锥，但在丛区域不再发现。分支轴突同样局限于这些外周位置。综上所述，这些观察结果表明，轴突导航的一种方式是通过生长锥变宽形式的探索，在某些情况下是末端分叉，这可能会产生轴突分支。选择最适当定向的生长锥突起和/或早熟的轴突分支可能是轴突对引导轴突进入肢芽的特定生长线索作出反应的方式之一。或者，这种早熟分支可能是对发育中肌肉的早期神经营养反应，在轴突导航中不发挥重要作用。
