Remahl S, Hildebrand C
Department of Anatomy, Karolinska Institute, Stockholm, Sweden.
J Neurocytol. 1990 Dec;19(6):883-98. doi: 10.1007/BF01186817.
Axo-glial relations in the ventral funiculus of the spinal cord (SC) and in the corpus callosum (CC) of the cat were examined by electron microscopy during initial myelination. In addition to random transverse and longitudinal sections from several stages, two series of sections were studied. As a first step in myelination the axons become ensheathed by one to three uncompacted glial lamellae (E-sheaths). E-sheaths present a length range from less than 5 microns to 149 microns (SC) or to 93 microns (CC). E-sheaths are more frequent along SC-axons than CC-axons, and the mean E-sheath is 3.3-fold longer in the former compared to the latter. In both areas naked axon portions occur between successive E-sheaths, but these gaps are insufficient to allow elongation of all short E-sheaths into long ones. Sheaths composed of mixed compacted (M-sheaths) and uncompacted segments have a length range of 66-212 microns in the SC and 66-171 microns in the CC. In relation to the undifferentiated terminations of E-sheaths or mixed E/M-sheaths, undercoated axolemmal domains are always lacking. Fully compacted sheaths were not found in the series from the SC. In the CC, 141-212 microns long compact sheaths were found, with tight axoglial junctions at their terminations. Axolemmal domains with a 'nodal' undercoating occur in relation to some of these terminations. In both areas, individual developing axons present a chaotic mixture of naked, ensheathed and myelinated portions; bulges with clusters of vesiculotubular profiles are frequent along naked and ensheathed axonal portions, particularly in the SC. The axon diameter is clearly larger in myelinated than in naked portions of the same axon. On the basis of these results, we propose that the early glial sheaths of developing CNS axons actively elongate and undergo extensive remodelling before compaction. The maximal length of uncompacted E-sheaths, and the sheath length at which axoglial junctions and nodes of Ranvier form, are markedly different in the two areas.
在髓鞘形成初期，通过电子显微镜对猫脊髓腹侧索和胼胝体中的轴突 - 神经胶质关系进行了研究。除了来自几个阶段的随机横切片和纵切片外，还研究了两个系列的切片。作为髓鞘形成的第一步，轴突被一到三个未紧密包裹的神经胶质薄片（E 型鞘）所包裹。E 型鞘的长度范围从小于 5 微米到 149 微米（脊髓）或到 93 微米（胼胝体）。E 型鞘在脊髓轴突上比胼胝体轴突上更常见，并且前者的平均 E 型鞘长度比后者长 3.3 倍。在这两个区域中，连续的 E 型鞘之间都存在裸露的轴突部分，但这些间隙不足以使所有短的 E 型鞘延伸成长的 E 型鞘。由混合紧密（M 型鞘）和未紧密部分组成的鞘在脊髓中的长度范围为 66 - 212 微米，在胼胝体中的长度范围为 66 - 171 微米。相对于 E 型鞘或混合 E/M 型鞘的未分化末端，总是缺乏包被不足的轴膜区域。在脊髓的切片系列中未发现完全紧密包裹的鞘。在胼胝体中，发现了长度为 141 - 212 微米的紧密鞘，其末端有紧密的轴突 - 神经胶质连接。与其中一些末端相关的轴膜区域出现“节点”样的包被不足。在这两个区域中，单个发育中的轴突呈现出裸露、被包裹和髓鞘化部分的混乱混合；沿着裸露和被包裹的轴突部分经常出现带有囊泡管状轮廓簇的凸起，特别是在脊髓中。同一轴突的髓鞘化部分的轴突直径明显大于裸露部分。基于这些结果，我们提出发育中的中枢神经系统轴突的早期神经胶质鞘在紧密包裹之前会积极伸长并经历广泛的重塑。未紧密包裹的 E 型鞘的最大长度以及形成轴突 - 神经胶质连接和郎飞结的鞘长度在这两个区域中明显不同。
