Fraher J, Cheong E
Department of Anatomy, University College, Cork, Ireland.
Acta Anat (Basel). 1995;154(4):300-14. doi: 10.1159/000147781.
This study describes for the first time the central (CNS)-peripheral (PNS) nervous system transitional zone (TZ) of a member of the vertebrate class Agnatha, namely, the sea lamprey. It is concerned in particular with the glial tissue contribution to the TZ and the Schwann cell-CNS interface. Plastic-embedded specimens of spinal cord and nerve roots were examined using light and electron microscopy. At the TZ of each root a glial barrier, continuous with and similar in form to the surrounding glia limitans, stretches across the nerve bundle. In possessing such a barrier the lamprey TZ follows the general vertebrate pattern and differs from the Cephalochordata and invertebrates where a corresponding barrier is absent. The glial barrier is of similar thickness to the glia limitans generally, unlike in the mammal where it is thicker at the TZ than elsewhere. All lamprey axons are unmyelinated peripherally and most of them traverse the glial TZ barrier singly in individual tunnels. In this they resemble mammalian axons of similar calibre (which are, however, myelinated), but differ from unmyelinated mammalian axons, which generally traverse it in bundles. The lamprey TZ has specialisations not found in mammals. These include prominent, multiple interconnected strata of microfilament bundles which are continuous deeply with glial filament bundles and which lie under the surface plasmalemma of the glia limitans, to which they are connected by hemidesmosomes. Features of the mature lamprey TZ resemble those of developing mammalian TZs. For example, slender Schwann cell processes extend below the cord surface and become closely apposed to glial processes. In addition, together with the axon, the TZ glial processes and the basal lamina, they bound an extensive periaxonal network of spaces at the TZ, into which fine Schwann cell processes project. Accordingly, the networks could represent a primitive form of node gap.
本研究首次描述了脊椎动物无颌纲成员海七鳃鳗的中枢神经系统（CNS）-外周神经系统（PNS）过渡区（TZ）。特别关注神经胶质组织对过渡区的贡献以及施万细胞与中枢神经系统的界面。使用光学显微镜和电子显微镜检查了脊髓和神经根的塑料包埋标本。在每个神经根的过渡区，一个与周围胶质界膜连续且形态相似的胶质屏障横跨神经束。七鳃鳗的过渡区具有这样的屏障，遵循一般脊椎动物的模式，与头索动物和无脊椎动物不同，后者不存在相应的屏障。胶质屏障的厚度通常与胶质界膜相似，这与哺乳动物不同，在哺乳动物中，过渡区的胶质屏障比其他地方更厚。所有七鳃鳗轴突在外周都是无髓鞘的，并且它们中的大多数在单个通道中单独穿过胶质过渡区屏障。在这方面，它们类似于类似口径的哺乳动物轴突（然而，这些轴突是有髓鞘的），但不同于无髓鞘的哺乳动物轴突，后者通常成束穿过过渡区。七鳃鳗的过渡区具有哺乳动物中未发现的特化结构。这些包括突出的、多个相互连接的微丝束层，它们与胶质丝束在深部连续，并位于胶质界膜的表面质膜下方，通过半桥粒与之相连。成熟七鳃鳗过渡区的特征类似于发育中的哺乳动物过渡区的特征。例如，细长的施万细胞突起延伸到脊髓表面以下，并与胶质突起紧密相邻。此外，它们与轴突、过渡区胶质突起和基膜一起，在过渡区界定了一个广泛的轴周间隙网络，细小的施万细胞突起伸入其中。因此，这些网络可能代表了节点间隙的原始形式。
