Bullock T H, Moore J K, Fields R D
Neurosci Lett. 1984 Jul 27;48(2):145-8. doi: 10.1016/0304-3940(84)90010-7.
Modern views of agnathan phylogeny consider Petromyzoniformes and Myxiniformes to belong to distinct classes that diverged from a common ancestor at a remote period, perhaps in the lower Cambrian, greater than 600 million years ago. Both are more primitive than elasmobranchs, holocephalans and bony fishes. Myelin is well developed in elasmobranchs and other fishes but was reported to be lacking in the spinal cord of lampreys. In order to search further for possible early myelin in some part of the nervous system of one of the agnathan stems, or for further evidence that it first appeared in chondrichthians, we extended the sampling to many parts of the brain and cord of hagfish. Transmission electron microscopy was used as a nearly ideal criterion. We find no trace or forerunner of the spiral, multilaminate glial wrapping. Many axons are embedded within one or more glial cells, like unmyelinated fibers in other vertebrates, or lie contiguously in bundles without an obviously complete glial investment. True myelin must be presumed to have been invented within the vertebrates, in ancestors of the living cartilaginous fishes after the agnathans branched from the vertebrate stem.
现代对于无颌类系统发育的观点认为，七鳃鳗目和盲鳗目属于不同的类别，它们在远古时期，可能是在6亿多年前的寒武纪早期，从一个共同祖先分化而来。两者都比板鳃亚纲、全头亚纲和硬骨鱼类更为原始。髓鞘在板鳃亚纲和其他鱼类中发育良好，但据报道在七鳃鳗的脊髓中缺乏。为了进一步探寻无颌类某一支系神经系统的某些部分可能存在的早期髓鞘，或者寻找更多证据证明髓鞘最早出现在软骨鱼类中，我们将采样范围扩大到盲鳗的大脑和脊髓的许多部位。透射电子显微镜被用作近乎理想的标准。我们没有发现螺旋状、多层胶质包裹物的任何痕迹或前身。许多轴突嵌入一个或多个胶质细胞内，就像其他脊椎动物中的无髓鞘纤维一样，或者成束相邻排列，没有明显完整的胶质包绕。必须假定真正的髓鞘是在脊椎动物中发明的，是在无颌类从脊椎动物主干分支出来之后，现存软骨鱼类的祖先中出现的。
