Greenberg M M, Leitao C, Trogadis J, Stevens J K
Playfair Neuroscience Unit, Toronto Western Hospital, Ontario, Canada.
J Neurocytol. 1990 Dec;19(6):978-88. doi: 10.1007/BF01186825.
Axons have generally been represented as straight cylinders. It is not at all uncommon for anatomists to take single cross-sections of an axonal bundle, and from the axonal diameter compute expected conduction velocities. This assumes that each cross-section represents a slice through a perfect cylinder. We have examined the three-dimensional geometry of 98 central and peripheral unmyelinated axons, using computer-assisted serial electron microscopy. These reconstructions reveal that virtually all unmyelinated axons have highly irregular axial shapes consisting of periodic varicosities. The varicosities were, without exception, filled with membranous organelles frequently including mitochondria, and have obligatory volumes similar to that described in other neurites. The mitochondria make contact with microtubules, while the other membraneous organelles were frequently found free floating in the cytoplasm. We conclude that unmyelinated axons are fundamentally varicose structures created by the presence of organelles, and that an axon's calibre is dynamic in both space and time. These irregular axonal geometries raise serious doubts about standard two dimensional morphometric analysis and suggest that electrical properties may be more heterogeneous than expected from single section data. These results also suggest that the total number of microtubules contained in an axon, rather than its single section diameter, may prove to be a more accurate predictor of properties such as conduction velocity. Finally, these results offer an explanation for a number of pathological changes that have been described in unmyelinated axons.
轴突通常被描绘为直的圆柱体。解剖学家对轴突束进行单个横截面观察，并根据轴突直径计算预期的传导速度，这种情况并不罕见。这假定每个横截面代表一个完美圆柱体的切片。我们使用计算机辅助连续电子显微镜检查了98条中枢和外周无髓鞘轴突的三维几何结构。这些重建结果显示，几乎所有无髓鞘轴突都具有高度不规则的轴向形状，由周期性的膨体组成。膨体无一例外地充满了膜性细胞器，通常包括线粒体，并且具有与其他神经突中描述的类似的必需体积。线粒体与微管接触，而其他膜性细胞器经常游离于细胞质中。我们得出结论，无髓鞘轴突本质上是由细胞器的存在而形成的曲张结构，并且轴突的管径在空间和时间上都是动态的。这些不规则的轴突几何形状对标准的二维形态计量分析提出了严重质疑，并表明电特性可能比单截面数据预期的更加异质。这些结果还表明，轴突中所含微管的总数，而非其单截面直径，可能被证明是传导速度等特性的更准确预测指标。最后，这些结果为无髓鞘轴突中描述的一些病理变化提供了解释。
