Zwahlen M J, Sandri C, Greeff N G
Physiologisches Institut, Universität Zürich-Irchel, Switzerland.
J Neurocytol. 1988 Apr;17(2):145-59. doi: 10.1007/BF01674202.
In order to investigate the transglial pathways in the Schwann sheath of squid giant axons, an electron microscopic study of thin sections and freeze-fracture replicas was carried out. Hitherto the mesaxonal clefts between Schwann cells were regarded as the only pathway between the extracellular space and the periaxonal space which, like the clefts, is about 10 nm in width. The clefts were now found to be obstructed by a putative single-stranded tight junction between neighbouring Schwann cells along the entire border near the axon. The Schwann cells were found to be penetrated like a sponge by a three-dimensional tubular transglial lattice that is confluent with the periaxonal space, the mesaxonal clefts and the extracellular space. The transglial channel system (TGCS) would, therefore, serve as an alternative diffusional pathway, provided that the tubular lumen was permeable. The diameter of the tubules is about 40 nm. In freeze-fracture replicas the density of tubular openings towards the axon was estimated to be 3.3 +/- 0.72 per micron 2. In relation to the periaxonal cell surface, this constitutes a relative opening area of 0.42% as compared to the 0.15% of the mesaxonal clefts (neglecting their tight junctions). Therefore, the TGCS would provide a ubiquitous access for ionic flow between axolemma and extracellular space. The fact that the TGCS has only recently been observed in squid, but has been described for some time in the giant nerve fibres of crayfish and lobster, can be explained by the use of different fixation methods. The TGCS system is preserved in aldehyde fixation as used in the present study, whereas osmium tetroxide was applied in earlier work on squid. The comparison with the results obtained in other species suggests strongly that the TGCS is permeable and constitutes a transglial pathway for rapid ionic flow.
为了研究鱿鱼巨轴突施万鞘中的跨神经胶质途径，对薄切片和冷冻蚀刻复制品进行了电子显微镜研究。迄今为止，施万细胞之间的轴系膜裂被认为是细胞外空间和轴周空间之间的唯一途径，轴周空间与裂一样，宽度约为10纳米。现在发现，沿着轴突附近的整个边界，相邻施万细胞之间的裂被一种假定的单链紧密连接所阻碍。发现施万细胞像海绵一样被一个三维管状跨神经胶质晶格穿透，该晶格与轴周空间、轴系膜裂和细胞外空间相连通。因此，如果管状腔是可渗透的，跨神经胶质通道系统（TGCS）将作为一种替代的扩散途径。小管的直径约为40纳米。在冷冻蚀刻复制品中，朝向轴突的管状开口密度估计为每平方微米3.3±0.72个。相对于轴周细胞表面，这构成了0.42%的相对开口面积，而轴系膜裂的相对开口面积为0.15%（忽略其紧密连接）。因此，TGCS将为轴膜和细胞外空间之间的离子流动提供普遍的通道。TGCS最近才在鱿鱼中被观察到，但在小龙虾和龙虾的巨神经纤维中已经被描述了一段时间，这一事实可以通过使用不同的固定方法来解释。本研究中使用的醛类固定可保留TGCS系统，而早期对鱿鱼的研究中使用的是四氧化锇。与其他物种获得的结果进行比较强烈表明，TGCS是可渗透的，并构成了快速离子流动的跨神经胶质途径。
