Waxman S G, Black J A, Ransom B R, Stys P K
Department of Neurology, Yale School of Medicine, New Haven, CT 06510.
Brain Res. 1993 Jun 18;614(1-2):137-45. doi: 10.1016/0006-8993(93)91027-p.
Since CNS white matter tracts contain axons, oligodendrocytes and astrocytes but not synapses, it is likely that anoxic injury of white matter is mediated by cellular mechanisms that do not involve synapses. In order to test the hypothesis, that anoxic injury of white matter is mediated by an influx of Ca2+ into the intracellular compartment of axons, we compared the ultrastructure of axons in rat optic nerve exposed to 60 min of anoxia in artificial cerebrospinal fluid (aCSF) containing normal (2 mM) Ca2+, and in aCSF containing zero-Ca2+ together with 5 mM EGTA. Optic nerves fixed at the end of 60 min of anoxia in 2 mM Ca2+ exhibit extensive ultrastructural alterations including disruption of microtubules and neurofilaments within the axonal cytoskeleton, development of membranous profiles and empty spaces between the axon and the ensheathing myelin, and swelling of mitochondria with loss of cristae. Bathing the nerves in zero-Ca2+ aCSF during anoxia protected the axons from cytoskeletal changes; after 60 min of anoxia, optic nerve axons retained normal-appearing microtubules and neurofilaments. Membranous profiles were rare, and empty spaces between axons and myelin did not develop in anoxic optic nerves bathed in zero-Ca2+ aCSF. Disorganization of cristae in axonal mitochondria was observed in anoxic optic nerves even when Ca2+ was omitted from the medium. Because Ca(2+)-mediated injury is known to disrupt the axonal cytoskeleton, these results support the hypothesis that anoxia triggers an abnormal influx of Ca2+ into myelinated axons in CNS white matter.
由于中枢神经系统白质束包含轴突、少突胶质细胞和星形胶质细胞，但不包含突触，白质的缺氧损伤很可能是由不涉及突触的细胞机制介导的。为了验证白质缺氧损伤是由Ca2+流入轴突细胞内区室介导的这一假说，我们比较了在含有正常（2 mM）Ca2+的人工脑脊液（aCSF）中以及在含有零钙和5 mM乙二醇双四乙酸（EGTA）的aCSF中暴露于60分钟缺氧环境的大鼠视神经轴突的超微结构。在含有2 mM Ca2+的环境中缺氧60分钟结束时固定的视神经表现出广泛的超微结构改变，包括轴突细胞骨架内微管和神经丝的破坏、膜性结构的形成以及轴突与包绕髓鞘之间的空隙，还有线粒体肿胀和嵴的丧失。缺氧期间将神经浸泡在零钙aCSF中可保护轴突免受细胞骨架变化的影响；缺氧60分钟后，视神经轴突保留了外观正常的微管和神经丝。膜性结构罕见，在浸泡于零钙aCSF中的缺氧视神经中，轴突与髓鞘之间没有形成空隙。即使培养基中不含Ca2+，在缺氧视神经中也观察到轴突线粒体嵴的紊乱。由于已知Ca2+介导的损伤会破坏轴突细胞骨架，这些结果支持了缺氧触发Ca2+异常流入中枢神经系统白质中有髓轴突的假说。
