Matise M P, Lance-Jones C
Department of Neurobiology, University of Pittsburgh School of Medicine, Pennsylvania 15261, USA.
Development. 1996 Feb;122(2):659-69. doi: 10.1242/dev.122.2.659.
When 3-4 segments of the chick lumbosacral neural tube are reversed in the anterior-posterior axis at stage 15 (embryonic day 2.5), the spinal cord develops with a reversed organization of motoneurons projecting to individual muscles in the limb (C. Lance-Jones and L. Landmesser (1980) J. Physiol. 302, 581-602). This finding indicated that motoneuron precursors or components of their local environment were specified with respect to target by stage 15. To identify the timing of this event, we have now assessed motoneuron projections after equivalent neural tube reversals at earlier stages of development. Lumbosacral neural tube segments 1-3 (+/- one segment cranial or caudal) were reversed in the anterior-posterior axis at stages 13 and 14 (embryonic day 2). The locations of motoneurons innervating two thigh muscles, the sartorius and femorotibialis, were mapped via retrograde horseradish peroxidase labeling at stages 35-36 (embryonic days 9-10). In a sample of embryos, counts were made of the total number of motoneurons in the lateral motor columns of reversed segments. The majority of motoneurons projecting to the sartorius and femorotibialis were in a normal position within the spinal cord. Segmental differences in motor column size were also similar to normal. These observations indicate that positional cues external to the LS neural tube can affect motoneuron commitment and number at stages 13-14. Since these observations stand in contrast to results following stage 15 reversals, we conclude that regional differences related to motoneuron target identity are normally specified or stabilized within the anterior LS neural tube between stages 14 and 15. To examine the role of the notochord in this process, neural tube reversals were performed at stages 13-14 as described above, except that the underlying notochord was also reversed. Projections to the sartorius and femorotibialis muscles did not differ significantly from those in embryos with neural tube reversals alone, indicating that the notochord is not the source of cues for target identity at stages 13-14.
在第15阶段（胚胎第2.5天），当鸡胚腰骶部神经管的3 - 4个节段在前后轴上反转时，脊髓发育出的运动神经元向肢体中各肌肉的投射组织发生了反转（C. 兰斯 - 琼斯和L. 兰德梅塞尔（1980年）《生理学杂志》302卷，581 - 602页）。这一发现表明，运动神经元前体或其局部环境的成分在第15阶段时已根据靶标被确定。为了确定这一事件的发生时间，我们现在评估了在发育早期进行等效神经管反转后运动神经元的投射情况。在第13和14阶段（胚胎第2天），将腰骶部神经管节段1 - 3（±一个头侧或尾侧节段）在前后轴上反转。在第35 - 36阶段（胚胎第9 - 10天），通过逆行辣根过氧化物酶标记法绘制了支配两块大腿肌肉（缝匠肌和股胫肌）的运动神经元的位置。在一组胚胎样本中，对反转节段的外侧运动柱中的运动神经元总数进行了计数。投射到缝匠肌和股胫肌的大多数运动神经元在脊髓内处于正常位置。运动柱大小的节段差异也与正常情况相似。这些观察结果表明，腰骶部神经管外部的位置线索在第13 - 14阶段可影响运动神经元的定向和数量。由于这些观察结果与第15阶段反转后的结果形成对比，我们得出结论，与运动神经元靶标身份相关的区域差异通常在第14和15阶段之间在腰骶部神经管前部被确定或稳定下来。为了研究脊索在这一过程中的作用，按上述方法在第13 - 14阶段进行神经管反转，但脊索也同时反转。投射到缝匠肌和股胫肌的情况与仅进行神经管反转的胚胎相比没有显著差异，这表明脊索不是第13 - 14阶段靶标身份线索的来源。
