Abstract：
Due to its proximity to the axon initial segment (AIS), the paranode of the first myelin segment can influence the threshold for action potentials and how a neuron participates in a neuronal circuit. Using serial section electron microscopy, we examined its three-dimensional (3D) organization in the ventral horn of the mouse spinal cord. The myelin loops of postnatal day 18 mice resemble those at the node of Ranvier. However, in 3-month-old mice, 13 of 22 para-AIS showed 4 types of alteration: (A) A cytoplasmic foot process, with ultrastructural characteristics of an astrocyte, was interposed between the axolemma and the myelin loops. (B) A thin extension of the inner tongue was present between the foot process and axolemma. (C) The foot process was absent. The inner tongue extension was a broad lamella from which a thin extension reached beyond the loops and spiraled around axon. (D) One set of loops was adjacent to the axon, and another was further back and underlain by compact myelin. We suggest that (A)-(C) are steps in a progression toward (D). In this progression, a glial process displaces the original loops, the inner tongue reactivates and extends beneath the foot process, then wraps around the axon to form a new set of loops. This is the first study of the 3D organization of myelin at the AIS and provides evidence for glia-mediated age-dependent remodeling at this critical region.
摘要：
由于它靠近轴突初始部分（AIS），第一髓鞘段的paranode可以影响动作电位的阈值以及神经元如何参与神经元回路。使用串行切片电子显微镜，我们检查了小鼠脊髓腹角的三维（3D）组织。出生后第18天小鼠的髓磷脂环类似于Ranvier节点的髓磷脂环。然而,在3个月大的老鼠身上，22个para-AIS中的13个显示4种类型的改变：（A）细胞质足过程，具有星形胶质细胞的超微结构特征，介于轴膜和髓鞘环之间。（B）在足突和轴突之间存在内舌的细延伸。(C)足突不存在。舌头的内部延伸部分是一个宽阔的薄片，从薄片中延伸出线圈并围绕轴突盘旋。(D)一组环与轴突相邻，另一个在后面，被致密的髓鞘覆盖。我们建议（A）-（C）是朝着（D）发展的步骤。在这一进程中,神经胶质过程取代了原来的循环，内舌重新激活并在脚部下方延伸，然后围绕轴突形成一组新的循环。这是对AIS处髓磷脂的3D组织的首次研究，并为该关键区域的神经胶质介导的年龄依赖性重塑提供了证据。
