Brown E R, Abbott N J
Marine Biological Association Laboratory, Plymouth, UK.
J Neurocytol. 1993 Apr;22(4):283-98. doi: 10.1007/BF01187127.
The ultrastructure of the Schwann cell layer surrounding the giant axon of the squid Alloteuthis subulata is described, and the permeability of extracellular compartments assessed by exposure to electron-dense tracers. Morphometric analysis is used to deduce the number, size and shape of the Schwann cells, and the routes for ion flux across the Schwann cell layer. Axons (mean diameter 233 microns) were surrounded by a 1-2 microns thick layer of Schwann cells which were approximately 1 micron thick, approximately 70 microns long and approximately 23 microns wide. There were around 62,000 Schwann cells per cm2 axon surface. The outer (abaxonal) surface of the Schwann cells was invaginated, with evidence for a covering of fine Schwann cells processes; the inner (adaxonal) surface of the Schwann cells was less folded. The percentage area occupied by mesaxonal cleft openings to the axon and to the basal lamina was 0.02% and 1.09% respectively. A system of tubules, the glial tubular system, occupied 3.9% of the Schwann cell volume, and opened to both axonal and basal lamina surfaces, with more elaborate lattice-like clusters towards the basal side of the cell. Tubule openings accounted for 0.26% of the surface area facing the axon and 0.37% of the area facing the basal lamina (where there was greater clustering of openings). The electron dense tracers horseradish peroxidase, ionic lanthanum and tannic acid filled mesaxon clefts, glial tubular system and periaxonal space. If ion flux occurred via the mesaxonal clefts, a theoretical series resistance (Rsth) of > 20 omega cm2, would be predicted, whereas if it occurred via the tubular system, the figure would be < 2 omega cm2, closer to physiological estimates. The results presented show that the glial tubular system is likely to be the major route for ion flux into and across the Schwann cell layer, and for clearance of K+ from the periaxonal space during periods of axonal stimulation. The implications for K+ homeostasis in the axonal microenvironment are discussed.
描述了围绕剑尖枪乌贼（Alloteuthis subulata）巨大轴突的施万细胞层的超微结构，并通过暴露于电子致密示踪剂来评估细胞外区室的通透性。形态计量分析用于推断施万细胞的数量、大小和形状，以及离子穿过施万细胞层的途径。轴突（平均直径233微米）被一层1-2微米厚的施万细胞所包围，这些施万细胞大约1微米厚，约70微米长，约23微米宽。每平方厘米轴突表面约有62000个施万细胞。施万细胞的外（轴突外）表面内陷，有证据表明有精细的施万细胞突起覆盖；施万细胞的内（轴突内）表面折叠较少。轴突和基膜的中轴裂开口所占面积百分比分别为0.02%和1.09%。一个小管系统，即神经胶质小管系统，占施万细胞体积的3.9%，并向轴突和基膜表面开放，在细胞基侧有更复杂的格子状簇。小管开口占面向轴突表面面积的0.26%，占面向基膜表面面积的0.37%（此处开口聚集更多）。电子致密示踪剂辣根过氧化物酶、离子镧和单宁酸填充了中轴裂、神经胶质小管系统和轴突周隙。如果离子通量通过中轴裂发生，则预测理论串联电阻（Rsth）>20Ω·cm²，而如果通过小管系统发生，则该数值将<2Ω·cm²，更接近生理学估计值。所呈现的结果表明，神经胶质小管系统可能是离子流入和穿过施万细胞层以及在轴突刺激期间从轴突周隙清除K⁺的主要途径。讨论了其对轴突微环境中K⁺稳态的影响。
