Pastor A M, Delgado-García J M, Martínez-Guijarro F J, López-García C, de La Cruz R R
Laboratorio de Neurociencia, Facultad de Biología, Universidad de Sevilla, 41012-Sevilla, Spain.
J Comp Neurol. 2000 Nov 20;427(3):370-90. doi: 10.1002/1096-9861(20001120)427:3<370::aid-cne5>3.0.co;2-m.
The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.
展神经核间神经元向动眼神经核的内直肌运动神经元发出的高度特异性投射，构成了研究中枢神经系统轴突切断效应的理想模型。我们分析了成年猫展神经核间神经元的细胞体和横断轴突因这种损伤所诱导的形态学变化。通过切断内侧纵束进行轴突切断术。对尼氏染色材料和钙视网膜蛋白免疫染色的展神经核间神经元进行细胞计数，结果显示轴突切断术后3个月无细胞死亡。在轴突切断术后6、14、24和90天对这些细胞进行超微结构检查，显示细胞学特征正常。然而，与对照组相比，轴突切断的神经元表面膜上的突触小体极少。通过测量细胞体的突触覆盖百分比和突触小体的线密度对这种变化进行了量化。轴突切断术后这两个参数均显著降低，在损伤后90天达到最低值（减少约70%）。我们还通过生物素顺行标记探索了轴突再生以及重新支配新靶点的可能性。在所有分析的时间间隔内，观察到标记的轴突在损伤的尾侧边界处中断；在任何情况下，它们都未穿过瘢痕组织到达神经束的远端部分。尽管如此，在损伤部位的尾侧出现了明显的轴突发芽。发现了提示轴突生长的结构，如大型终末棒，从中经常伸出短的丝状分支。在小白蛋白和钙视网膜蛋白免疫染色后也得到了类似的结果。在电子显微镜水平上，源自芽的生物素标记的突触小体周围要么是细胞外间隙，在损伤部位细胞外间隙极度扩张，要么是神经胶质突起。因此，所检查的绝大多数标记突触小体都没有神经元接触，这表明没有重新支配新靶点。总之，这些数据表明成年猫展神经核间神经元在轴突切断术后存活，并表现出某种形式的轴突再生，即使在没有靶点连接的情况下也是如此。
